Description
Box Protector for a Threaded Joint for Pipes
Technical Field
This invention relates to a box protector for protecting a box of a threaded
joint for pipes which is used to connect oil country tubular goods (OCTG) and which
is constituted by a pin and a box until the joint is used. A box protector according
to the present invention is particularly suitable for protecting a box of a threaded
joint for pipes which is previously lubricated by a solid lubricating coating and
which does not require application in the field of a viscous liquid lubricant to the
threaded joint before connecting oil country tubular goods.
Background Art
Oil country tubular goods such as tubing and casing used for exploitation of
crude oil or gas oil are connected with each other by threaded joints for pipes. A
typical threaded joint for pipes used for connecting oil country tubular goods has a
pin-box structure. A pin is a joint component having male (external) threads, and a
box is a joint component having female (internal) threads. Typically, a pin is
formed on the outer peripheral surface of both ends of a steel pipe used as an oil
country tubular good, and a box is formed on the inner peripheral surface of both
sides of a coupling, which is a separate member. In special threaded joints having
excellent gas tightness, an unthreaded metal contact portion having a seal surface and
a shoulder surface (also referred to as a torque shoulder) is formed at the end of the
male threads of a pin and at the base of the female threads of a box. The threaded
portions and the unthreaded metal contact portions form the contact surface of a
threaded joint for pipes. With this type of threaded joint for pipes, excellent gas
tightness is obtained by inserting one end of an oil country tubular good into a
coupling and threadingly engaging the male threads and the female threads until the
unthreaded metal contact portions of the two members contact each other to form
metal-to-metal seals.
Figure 6 is an explanatory view schematically showing the assembled
structure of a typical threaded joint for pipes and showing the state of a steel pipe for
an oil country tubular good and a coupling at the time of shipment. As shown in
this figure, a pin 1 having a male threaded portion 3a is formed on the outer
peripheral surface of both ends of a steel pipe A, and a box 2 having a female
threaded portion 3b is formed on the inner periphery of both sides of a coupling B.
The coupling B is previously connected to one end of the steel pipe A. As shown in
Figure 6, a coupling-type threaded joint for pipes is normally shipped in a state in
which the coupling B is connected to the steel pipe A.
Accordingly, at the time of shipment, of the two pins and two boxes, one pin
and one box are connected to each other. Although not shown in this figure, on the
other pin and box which are not used for connection to each other, namely, the lefthand
pin and the right-hand box in the figure, a pin protector and a box protector,
respectively, are mounted prior to shipment in order to protect the contact surfaces of
the pin and box against rusting, scratches, and infiltration of foreign matter. These
protectors are removed before use.
A pin protector and a box protector each typically comprise a tubular body
which is somewhat longer than the threaded joint and which is normally made of a
resin or a metal-reinforced resin. These protectors are typically closed at one end or
in the vicinity of one end, but there are also protectors which are open at both ends.
A pin protector has female threads on its inner peripheral surface which
threadingly engage with the male threads of a pin, and it is mounted on a pin by
thread engagement. Similarly, a box protector has male threads on its outer
peripheral surface which threadingly engage with the female threads of a box, and it
is mounted on a box by thread engagement. At this time, the pin protector is
sufficiently tightened until its female threads interfere with the male threads of the
pin so that the protector will not fall off even if it receives an impact during transport
or handling after shipment. Similarly, the box protector is sufficiently tightened
until its male threads interfere with the female threads of the box. A pin protector
and a box protector including their threads are normally manufactured by injection
molding, and the threaded portions of a pin protector and a box protector are formed
of the same resin as the main body of the protectors.
In order to guarantee galling resistance and gas tightness at the time of
makeup of oil country tubular goods, a viscous liquid lubricant referred to as a
compound grease or dope and containing a heavy metal powder has been applied in
the field to the contact surfaces (the threaded portions and the unthreaded metal
contact portions) of threaded joints. Such a compound grease is prescribed by API
BUL 5A2. Compound grease also has a corrosion-preventing function which
prevents rusting of the contact surface to which it is applied.
In the past, when mounting a pin protector or a box protector on a pin or a
box, compound grease or other lubricating grease (such as a lubricant referred to as
green dope which does not contain a heavy metal powder) has been applied to the
contact surface. The lubricating grease has fluidity and can fill the space between a
protector and a pin or a box. Therefore, even if a protector is not specially provided
with a seal mechanism, the contact surfaces of the pin and box are isolated from the
exterior, and a corrosion-preventing function and a function of preventing infiltration
of foreign matter are achieved.
However, in recent years, environmental regulations are becoming stricter
around the world, and there is a demand for threaded joints for pipes which can be
made up without using compound grease since compound grease contains a large
amount of heavy metal powder which can have an adverse effect on humans and
living organisms. It is also desired that lubricating treatment of threaded joints for
pipes in the field be made unnecessary in order to increase the operating efficiency.
A typical example of such a threaded joint for pipes is one like that disclosed
in Patent Document 1, for example, in which the contact surface of one or both of a
pin and a box is coated with a solid lubricating coating having a lubricating powder
(such as molybdenum disulfide or graphite) dispersed in a resin.
Patent Document 2 discloses a threaded joint for pipes in which the contact
surface of at least one of a pin and a box is coated with a two-layer coating having a
viscous liquid or semisolid lubricating coating and atop it a dry solid coating. The
dry solid coating may be a coating of a thermosetting resin such as an acrylic resin or
a coating of an ultraviolet curing resin.
Patent Document 3 discloses a threaded joint for pipes in which a thin, nontacky
lubricating coating comprising a lubricating powder dispersed in a solid matrix
exhibiting plastic or viscoplastic rheological behavior (flow properties) is formed on
the surfaces of the threads of a pin and a box. The matrix preferably has a melting
point in the range of 80 - 320° C, and the coating is formed by hot melt spraying,
flame coating using a powder, or spray coating of an aqueous emulsion.
Patent Document 4 discloses a threaded joint for pipes in which the contact
surface of at least one of a pin and a box is coated with a two-layer coating having a
solid lubricating coating comprising a lubricating powder and a binder, and atop it a
solid corrosion-preventing coating which does not contain solid particles.
Similarly, a protector which is mounted on a threaded joint for pipes having
such a lubricating coating and particularly a solid lubricating coating formed thereon
and which is made up without using a compound grease is preferably mounted on a
pin or a box without using a lubricating grease. However, in this case, a seal is not
provided by a lubricating grease, so it is necessary for a pin protector or a box
protector itself to have sealing properties. There have been some proposals in this
respect in the prior art.
For example, as shown in Figure 7, Patent Document 5 discloses a box
protector 4 for a threaded joint for pipes having sealing properties in which the
protector has a first seal portion 4b and a second seal portion 4c each in the form of
an elastic annular projecting body made of the same material as the protector body 4a
in the positions opposing the shoulder stoppers 5a and 5b of a box 5.
As shown in Figure 8, Patent Document 6 discloses a box protector 6 for a
threaded joint for pipes having a seal of increased reliability by using an elastic seal
ring 6c which is mounted on the outer peripheral surface 6b of the protector body 6a
opposing the end surface 7a of a box 7 as a first seal portion and using the end
surface 6d opposing the torque shoulder 7b of the box 7 as a second seal portion.
Patent Document 1: JP 09-72467 Al
Patent Document 2:WO 2006/1 0425 1
Patent Document 3:WO 2007/0423 1
Patent Document 4:WO 2006/75774
Patent Document 5: JP 2003-240188 Al
Patent Document 6:WO 201 1/027433
Summary of the Invention
As shown in Figures 7 and 8, a conventional box protector for a threaded
joint for pipes having a solid lubricating coating provides desired sealing properties
by a first seal portion in the position opposing the end surface of a box or in the
vicinity thereof, and a second seal portion in the position opposing the end surface of
the inner rear portion of the box (a torque shoulder) or in the vicinity thereof, both
seal portions sealingly (intimately) contacting the opposing surfaces of the box.
In actual practice, however, it is not easy to maintain a state in which both
the first seal portion and the second seal portion sealingly contact the box, and there
is the problem that water, oil, or the like infiltrates the space between the protector
and the box through a seal portion which does not sealingly contact the box, and as a
result, the solid lubricating coating which covers the contact surface of the box
deteriorates.
Namely, the body 4a of the box protector 4 disclosed in Patent Document 5
and shown in Figure 7 is made of a resin material such as nylon having elasticity.
Even though the first seal portion 4b and the second seal portion 4c which are made
of the same material as the protector body 4a have a certain amount of elasticity,
there is a limit to the sealing ability which can be obtained by relying on their
elasticity. Therefore, it is not easy to prevent the occurrence of a gap at the end
surface at the inner rear portion of the box (the torque shoulder) or in its vicinity with
certainty. In addition, the acute front tip corner of the protector indicated by 5c
tends to easily chip away.
In the case of the box protector 6 disclosed in Patent Document 6 and shown
in Figure 8, if the environmental temperature rises, the protector body 6a expands
and the length of the protector body 6a in the axial direction increases. As a result,
when the box protector 6 is mounted on a box 7, the tip 6d of the protector intimately
contacts the torque shoulder 7b of the box 7 before its elastic seal ring 6c intimately
contacts the end surface 7a of the box, and a gap forms between the elastic seal ring
6c and the end surface 7a. Even if this gap is very small such as around 0.03 mm,
water or the like can pass through the gap and infiltrate the space between the box
and the protector.
In this manner, a conventional box protector for a threaded joint for pipes
develops a gap at the end surface of the box or at the end surface at the rear inner
portion of the box (the torque shoulder) when the box protector is mounted on the
box, and it is difficult to reliably prevent water or oil or dust from infiltrating through
the gap which develop.
The object of the present invention is to provide a box protector for a
threaded joint for pipes which can intimately contact both the end surface and the
inner rear portion of a box with certainty, thereby preventing the infiltration of water,
oil, or dust into the space between the box and the protector and preventing
deterioration of a solid lubricating coating formed on contact surface of the box and
corrosion of the contact surface.
The present invention is a box protector for a threaded joint for pipes which
is mounted on a box of a threaded joint for pipes having a pin-box structure in order
to protect a contact surface of the box, the contact surface of the box having a female
threaded portion including complete threads and an unthreaded metal contact portion
including at least a shoulder surface, the shoulder surface sloping so as to recede
towards the central axis of the threaded joint by an angle Qwith respect to the
direction perpendicular to the axis of the threaded joint (Q> 0) or being parallel to
the direction perpendicular to the axial direction on the entrance side of the box (Q=
0), wherein the box protector has a tubular body made of a resin with an axial length
which is longer than that of the contact surface of the box and having a structure
which can form a first seal portion and a second seal portion which contact the
surface of the box on both sides of the contact surface of the box, and the tubular
body has on its outer peripheral surface a male threaded portion which threadingly
engages with at least a portion of the complete threads of the female threaded portion
of the box, the box protector being characterized in that:
the tubular body of the protector has a shoulder surface which forms a first
seal portion which sealingly contacts the shoulder surface of the box, the shoulder
surface of the protector slopes to the rear in the direction of insertion of the protector
towards the center axis of the joint by an angle QR with respect to the direction
perpendicular to the pipe axial direction, and the angle of slope QR of the shoulder
surface of the protector is larger than the angle of slope Qof the shoulder surface of
the box;
the tubular body of the protector has a circumferential groove in its outer
peripheral surface in the vicinity of the shoulder surface of the protector, and the
position and the cross-sectional shape of the circumferential groove are such that
under a torque applied when mounting the protector, a portion of the tubular body in
the vicinity of the circumferential groove elastically deforms so as to reduce the
distance in the axial direction at the opening of the circumferential groove, thereby
ensuring the formation of the first seal portion; and
the second seal portion is formed by contacting an elastic material attached
to the protector with the box surface.
In preferred embodiments, the box protector according to the present
invention satisfies at least one of the following conditions :
- the circumferential groove has an axial cross-sectional shape selected from
a V shape, an arc shape, a U shape, a trapezoid shape, and a combination of these
shapes;
- the elastic deformation of the tubular body causes the distance in the axial
direction at the opening of the circumferential groove to decrease by at least 0.2 mm;
- the second seal portion is formed by an elastic seal ring which is disposed
on the outer peripheral surface of the tubular body of the protector so as to sealingly
contact the box end surface when the protector is mounted on the box;
- the entire contact surface of the box or a portion thereof including at least
the female threaded portion of the box is covered by a solid lubricating coating;
- the thread height (HI) of the male threaded portion of the tubular body of
the protector and the thread height (H2) of the complete threads of the female
threaded portion of the box satisfy HI > H2, and the difference [HI - H2] between
HI and H2 is larger than the maximum coating thickness of the solid lubricating
coating on the thread crests of the female threaded portion of the box;
- the male threaded portion of the protector has 3 - 5 threads; and
- the male threaded portion is disposed at a position opposing the female
threads closest to the box entrance of the complete threads of the box.
A box protector for a threaded joint for pipes according to the present
invention can reliably sealingly contact both the end surface of the box and the
shoulder surface on the inner rear portion thereof to form a seal portion in both
positions, thereby preventing infiltration of water, oil, or dust into the space between
the box and the protector with certainty. As a result, corrosion of the contact
surface of a box is prevented, and when the contact surface is covered by a solid
lubricating coating, deterioration of the coating can be prevented.
Brief Explanation of the Drawings
Figures 1A, IB, and 1C are an explanatory view of a box of a threaded joint
for pipes, a box protector according to the present invention, and the protector
mounted on the box, respectively.
Figure 2 is an explanatory view showing various cross-sectional shapes in a
pipe axial direction of a circumferential groove of a box protector according to the
present invention along with a box.
Figure 3 is an explanatory view schematically showing a cross section in the
axial direction of a solid lubricating coating formed on a female thread of a box.
Figure 4A is an axial cross-sectional view schematically showing a portion
of a box on which a box protector according to the present invention is mounted, and
Figure 4B is an axial cross-sectional view schematically showing a portion of the
complete threads of the female threaded portion of a box threadingly engaged with
the male threaded portion of a box protector according to the present invention.
Figures 5A - 5C are axial cross-sectional views schematically showing a
portion of the complete threads of the female threaded portion of a box threadingly
engaged with the male threaded portion of a box protector.
Figure 6 is an explanatory view schematically showing the assembled
structure of a typical threaded joint for pipes showing the state of a steel pipe for an
oil country tubular good and a coupling at the time of shipment.
Figure 7 is an explanatory view showing a box protector disclosed in Patent
Document 5.
Figure 8 is an explanatory view showing a box protector disclosed in Patent
Document 6.
Modes for Carrying Out the Invention
A box protector according to the present invention can be applied not only to
the box of a coupling-type threaded joint for pipes like that shown in Figure 6 but
also to the box of an integral threaded joint for pipes in which one end of a steel pipe
is made a pin and the other end is made a box. A coupling-type threaded joint
typically makes the outer peripheral surface of an end of a steel pipe a pin and makes
the inner peripheral surface of a coupling a box, but the opposite combination is also
possible.
In the following explanation, the present invention will be explained with
respect to a box protector which is mounted on a special threaded joint having
excellent gas tightness, with the threads of the joint being tapered threads and the
unthreaded metal contact portions having a seal surface. However, the threads of a
threaded joint for pipes need not be tapered. In addition, a protector according to
the present invention can be applied to a threaded joint for pipes in which the
unthreaded metal contact portions do not have a seal surface and are constituted only
by a shoulder surface. Furthermore, there is no particular limitation on the thread
shape of a threaded joint for pipes. The threads can be buttress threads (trapezoidal
threads) specified by API standards, or they can be so-called hook threads in which
the load flank angle of the threads has a negative value.
An explanation will be given of an example of a box protector of the type
which is closed slightly before its front end. Here, the front end means the end on
the front side in the direction of insertion of the protector when mounting the
protector, and it is also referred to as the leading end. However, both ends of the
tubular body forming the protector may be open, and in the case of a closed type, the
location where the tubular body is closed is not limited. In addition, a protector
may also be of the type in which the rear end is closed by a removable lid.
The box of a threaded joint for pipes which is protected by a box protector
according to the present invention preferably has the below-described solid
lubricating coating on at least the threaded portion of the box and preferably on the
entirety of the contact surface including the threaded portion and the unthreaded
metal contact portion. However, the present invention can also be applied to a
threaded joint for pipes which does not have a lubricating coating or which has a
viscous liquid lubricating coating on the contact surface of the box.
Figures 1A, IB, and 1C are explanatory views showing a box 11 of a
threaded joint for oil country tubular goods, a box protector 2 1 according to the
present invention, and the state in which the protector 2 1 is mounted on the box 11,
respectively. Figure 2 is an explanatory view showing various cross-sectional
shapes in the axial direction of a circumferential groove of a box protector according
to the present invention together with a box.
Below, these components will be explained in sequence. A threaded joint
for pipes is constituted by a pin and a box which are threadingly engaged with each
other, but the present invention relates to a box protector, so an explanation will not
be given of a pin.
Box 11
The box 11 typically has a cylindrical outer peripheral surface 11a. On its
inner peripheral surface 1lb, it has a female threaded portion 12 with tapered threads,
a seal surface 13, and a shoulder surface (a torque shoulder) 14 on its rearmost
portion. As shown in the drawings, the seal surface 13 is typically formed between
the threaded portion 12 and the shoulder surface 14. The seal surface 13 of the box
contacts an annular seal surface disposed on the outer peripheral surface of an
unillustrated pin with a predetermined amount of interference and forms a metal-tometal
seal. In addition, the shoulder surface 14 of the box contacts a shoulder
surface formed on the end surface of the unillustrated pin with a prescribed amount
of interference. The female threaded portion 12 of the box is threadingly engaged
with the male threaded portion of the pin.
In this manner, the female threaded portion 12, the seal surface 13, and the
shoulder surface 14 of the box 11 form the contact surface which contacts
corresponding portions of a pin at the time of makeup of a threaded joint. The
female threads formed on the female threaded portion 12 may all be complete
threads, but there are also cases in which the threads in the vicinity of one or both
ends of the female threaded portion 12 are incomplete threads.
In the illustrated example, the shoulder surface 14 of the box 11 is provided
in a position on the rearmost portion of the inner peripheral surface 1lb of the box 11
(in the position of the front end of the pin) so as to contact the shoulder surface
provided on the front end of the pin. However, the shoulder surface 14 can be
provided on the open end 1l c of the box 11 or on both the front end of the pin and
the front end of the box, and the present invention can also be applied to these cases.
The shoulder surface 14 of the box 11 may be constituted by a surface which
is perpendicular to the axial direction of a joint (the pipe axial direction), but
preferably it is constituted by a sloping surface which projects in the direction of the
open end (the entrance) 11c of the box (namely, towards the rear with respect to the
insertion direction of the protector 21) from the inner peripheral surface l i b towards
the center of the joint. Specifically, the shoulder surface 14 slopes rearwardly in the
insertion direction at an angle Qwith respect to the direction perpendicular to the
pipe axis (the radial direction of the joint, which is the vertical direction in Figure 1),
but as mentioned above, it is also possible for it to be a vertical surface for which Q=
0°.
As schematically shown in the axial cross-sectional views of Figures 3 and 4,
the female threaded portion 1 of the box may be covered by a solid lubricating
coating 15. Preferably the entirety of the contact surface of the box (namely, the
female threaded portion 12, the seal surface 13, and the shoulder surface 14) has a
solid lubricating coating 15. In order to prevent rust, the inner peripheral surface
other than the contact surface of the box and the end surface 1l c of the open end may
also be covered with a solid lubricating coating.
The solid lubricating coating is typically a coating containing a lubricating
powder (a solid lubricant) dispersed in a suitable binder. There have been many
proposals from in the past concerning solid lubricating coatings for threaded joints
for pipes, and these coatings can be suitably employed. In a typical solid
lubricating coating, the binder is an organic resin having relatively excellent heat
resistance such as an epoxy resin, a polyacrylate resin, a polyimide resin, or a
polyamide-imide resin, but a solid lubricating coating which uses an inorganic filmforming
component such as silica sol, a hydrolyzable silane compound, a titanium
alkoxide, an alkali metal silicate, or a phosphate is also known.
It has been proposed to form a solid lubricating coating as two or more layers
or to laminate a liquid lubricating coating or a sold corrosion preventing coating atop
a solid lubricating coating, and it is possible to employ such a layered coating
structure.
A solid lubricating coating which is particularly suitable for at least the
threaded portion of a box in the present invention is a coating like that disclosed in
Patent Document 3 which contains a lubricating powder in a matrix which exhibits
plastic or viscoplastic rheological properties, and preferably a coating formed by
spray coating (namely, hot melt coating) of a composition in which its matrix is in a
molten state. Since a box is typically formed on a short coupling, hot melt coating
on the surface of a box is easier than on the surface of a pin which is typically
formed on the end of a long steel pipe.
Preferably, a solid lubricating coating of this type comprises 70 - 95 mass %
of a matrix and 5 - 30 mass % of a lubricating powder. Since the proportion of a
lubricating powder is small, the coating as a whole can exhibit plastic or viscoplastic
rheological behavioral properties, which are the properties of the matrix. A matrix
which exhibits plastic or viscoplastic rheological behavior preferably has a melting
point in the range of 80 - 320° C.
This matrix preferably comprises a thermoplastic polymer, a wax, and a
metal soap. More preferably, it further contains a corrosion inhibitor and a waterinsoluble
liquid resin.
The coating thickness (the average thickness) of the solid lubricating coating
is frequently in the range of 10 - 100 m i and preferably in the range of 25 - 60 m .
The structure of portions of the box 11 other than those described above may
be the same as for a usual box of this type. Such a structure is known to those
skilled in the art, so a further explanation concerning the box 11 will be omitted.
Box Protector 2 1
The box protector 2 1 is a protector for protecting the box 11, and it is
designed to be mounted inside the box 11 by threads. In the illustrated example, the
box protector 21 comprises a tubular body 21 which is closed at a location to the
rear of the front end 21b in the direction of insertion of the protector and which is
open at the other end (the rear end 21c). The tubular body 21a has an axial length
which is longer than that of the contact surface of the box 11 which it protects, and
on its outer peripheral surface, it has a male threaded portion 22 having male threads
which threadingly engage with the female threads of the female threaded portion 12
of the box 11.
The tubular body 2 a of the protector 21 is made of a resin. It is typically
manufactured by injection molding of a resin including the threaded portion on its
outer peripheral surface. Resins which have been used in the past for manufacture
of protectors for threaded joints for pipe can be used. Specific examples of such a
resin include low density polyethylenes and high density polyethylens.
A tubular metal member 24 which is open at both ends is often fit inside the
tubular body 1a which is made of a resin. Accordingly, the tubular body 21a of
the protector is made of a resin reinforced by metal or a resin alone. By forming the
portion of the protector which contacts the box from a resin which is softer than
metal, it is possible to prevent damage to the surface of the box when mounting the
protector on the box.
The external shape of the tubular metal body 24 can be any shape which can
fit onto the inner peripheral surface of the tubular body 2 1a of the protector. In the
illustrated example, a recess 2Id is formed in the rear end surface 21c of the
protector 21, the rear end portion of the tubular metal body 21a is formed with a bent
portion which is bent into a circular shape by press working, and the tubular metal
body 24 is secured to the protector 2 1 by the spring action of the end bent portion
which is inserted into the recess 2 d. Other securing methods can also be
employed. When such a tubular metal body is not fit into the protector, the closed
portion 25 of the protector can be located at the rear end surface 21c or in the vicinity
thereof.
The box protector 2 1 forms seal portions by contacting the box 11 on both
sides of the contact surface of the box 11protected by the protector. These seal
portions are respectively referred to as a first seal portion and a second seal portion.
Preferably, the first seal portion contacts the shoulder surface 14 of the box 11 while
the second seal portion contact the open end surface 1l c of the box. The outer
surface of the protector 2 1 preferably has a shape in which it does not contact the
surface of the box 11 (the inner surface of the coupling) except for the first seal
portion, the second seal portion, and the male threaded portion 22. In order to
ensure that the protector does not contact the seal surface 13 of the box 11, it is
preferred that the gap between the seal surface 13 of the box and the opposing
portion of the box protector 2 1 be set to a large value. When the female threaded
portion 12 on the inner peripheral surface of the box 11 has tapered threads as shown
in the figures, the outer peripheral surface of the tubular body 21a of the protector 1
preferably has a tapered shape with substantially the same slope. The shape of the
inner peripheral surface of the protector 2 1 is not limited, and it can have a roughly
cylindrical shape as shown in the figures or a tapered shape.
The number of threads of the male threaded portion 22 of the box protector
21may be set to be the same as the number of complete threads of the box 11 so as
to threadingly engage with all the complete threads of the female threaded portion 12
of the box 11. In this case, the male threaded portion 22 of the box protector 2 1
extends the entire axial length of the portion of the protector opposing the complete
threads of the female threaded portion 12 of the box 11.
However, the purpose of the male threaded portion 22 of the box protector
1 is to secure the protector 1 in a predetermined position on the box 11 and to
prevent the protector 21 from falling off the box 11when a steel pipe for an oil
country tubular good receives an impact during transport or handling. Therefore, a
high thread makeup force which is applied to the male threaded portion or the female
threaded portion of a threaded joint for pipes, which is always subjected to an
extremely high internal and external pressure, is not necessary. Therefore, in a
preferred embodiment, the number of threads of the male threaded portion 22 of the
box protector 2 1 is preferably kept to the minimum number necessary for securing
the protector and preventing it from falling off. This number is 3 - 5 threads,
preferably 3 - 4 threads, and most preferably 3 threads. With two threads, it is not
possible to form a tight threaded connection, and it is not possible to secure the
protector and prevent it from falling off.
In this case, as shown in Figure 4A, the male threaded portion 22 of the
protector 2 1 is preferably located opposite the threads closest to the entrance (the
open end) 1l c of the box in the complete threads of the female threaded portion 12 of
the box 11. In this manner, because the complete threads of the box 1 which
interfere with the threads of the male threaded portion 22 of the protector 2 1 are
limited to the several threads closest to the entrance, it is possible to minimize
damage to the female threaded portion of the box 11 due to interference with the
threads of the male threaded portion 22 of the protector 2 1 and damage to a coating
when the female threaded portion 12 is coated with a solid lubricating coating 15.
The box protector 2 1 has a shoulder surface 21b, which forms a first seal
portion by sealingly contacting the shoulder surface of the box. In the embodiment
shown in Figure 1, the protector 2 1 has a shoulder surface 21b at its front end in the
direction of insertion (in this case, the shoulder surface 21b being also referred to
below as the front end surface), and this shoulder surface 21b forms the first seal
surface by contacting the shoulder surface 1 at the rearmost portion of the box 11.
The front end surface 21b of the box protector 2 1 which forms the first seal
portion is a sloping surface which slopes in the same direction as the shoulder
surface 14 of the box 11, namely, backwards in the insertion direction of the
protector towards the center of the joint with respect to the direction perpendicular to
the pipe axis so as to contact the shoulder surface 14 of the box 11, and the angle of
slope QR of the front end surface 21b of the protector 2 1 is made larger than the angle
of slope Qof the shoulder surface 14 of the box 11 which it contacts (i.e., QR > Q).
Accordingly, the shoulder surface 21b of the protector is made a sloping surface even
when the shoulder surface 14 of the box 11 is vertical. The reason why is discussed
below.
The tubular body 21a of the box protector 2 1 has on its outer peripheral
surface a circumferential groove 30 which extends entirely around its outer
peripheral surface in the vicinity of its front end surface 21b. The cross-sectional
shape of the circumferential groove 30 in the axial direction is not limited to the V
shape shown in Figure . As shown in Figure 2, it can have a cross-sectional shape
which is a circular arc, a U shape, a trapezoidal shape, or a combination of these
shapes, or it may have a cross-sectional shape other than these. A U shape includes
a horseshoe shape in which the opening of the U is narrowed.
The position and the cross-sectional shape of the circumferential groove 30
are such that it is possible to form the first seal portion by elastic deformation of the
portion in the vicinity of the groove 30 of the protector body 2 1 by the torque applied
at the time of mounting the protector. Namely, due to elastic deformation of the
protector body 21a in the periphery of the groove 30 when the front end surface 21b
of the protector 2 1 contacts the shoulder surface 14 of the box 11 at the time of
mounting of the protector, the portion 26 of the protector body 21a ahead of the
groove 30 (the portion between the groove 30 and the front end surface 21b) can
bend backwards so as to decrease the width of the groove in the axial direction. As
a result, the angle of slope of the front end surface 21b of the protector 2 1 decreases.
Accordingly, by inserting the protector into the box (by threadingly engaging the
threads) until the angle of slope QR of the protector 21, which is larger than the angle
of slope Qof the shoulder surface 14 of the box 11, decreases to the same angle of
slope as the angle of slope Qof the box, the front end surface 21b of the protector 2 1
is made to intimately contact the shoulder surface 14 of the box 11 in the form of an
areal contact rather than a line contact, and the sealing properties of the first seal
portion can be improved.
As shown in Figure 1A, for threaded joints for pipes having the same outer
diameter, the inner diameter Dl of the box 11 varies as the wall thickness varies. In
addition, the angle of slope Qof the shoulder surface of the box sometimes varies
from box to box. By making the angle of slope QR of the front end surface of the
protector larger than Q, it becomes easy to make box protectors 2 1 of the same shape
applicable to multiple types of boxes 11 having different wall thicknesses or different
angles of slope Q. The difference between QR and Qis preferably in the range of
from 5°to 20°.
A second seal portion is provided in the vicinity of the opposite end from the
first seal portion (the front end surface 21b) of the box protector 21, whereby the
contact surface of the box 11 are isolated from the exterior when the protector 2 1 is
mounted on the box. In this manner, the contact surface of the box is prevented
from contacting water, oil, dust, foreign matter, and the like. As a result, when the
contact surface is covered by a solid lubricating coating, dust or foreign matter is
prevented from adhering to the coating, and deterioration of the coating is
suppressed. When the contact surface is not covered by a solid lubricating coating,
corrosion of the contact surface is suppressed. These phenomena are all causes of a
decrease in galling resistance, so the present invention can improve the galling
resistance of a box.
The second seal portion is disposed in a location outwards of the contact
surface of the box when the protector 21 is mounted on the box 11. Accordingly,
the axial length of the protector 2 1 becomes longer than the axial length of the
contact surface of the box on which the protector is mounted.
There are no particular limitations on a means of forming the second seal
portion of the protector 1 as long as a reliable seal portion can be formed between
the protector 2 1 and the box outwards of the contact surface of the box (to the rear in
the illustrated example). Considering the ease of forming a seal, it is preferable to
form the second seal portion by contacting the end surface of the opening of the box
11. The second seal portion preferably has a seal structure utilizing an elastic
material which has an excellent sealing effect.
As shown in Figures IB and 1C, in a preferred embodiment, an elastic seal
ring 28 is disposed on the outer peripheral surface of the tubular body 21a of the
protector 2 1 so as to contact (abut) the open end surface 1l c of the entrance of the
box when the protector 21 is mounted on the box 11. As a result, the entire inner
peripheral surface of the box and the surface of the open end of the box can be
isolated from the exterior. The elastic seal ring 28 preferably has elasticity such
that it can be displaced (shrunk) by at least 0.5 mm in the axial direction. The
thickness of the seal ring in the axial direction is preferably at least 5 mm.
Examples of preferred materials for the elastic seal ring include nitrile rubbers and
silicone rubbers.
As shown in the figures, a circumferential groove 31 having an axial width
slightly narrower than the thickness of the seal ring 28 may be provided in the outer
peripheral surface of the protector 21 in order to secure the seal ring 28. The side
wall on the front side of the groove 31 is preferably positioned slightly forwards (by
a distance smaller than the above-described displacement) of the position of the end
surface 1l c of the open end of the box 11when the protector 2 1 is mounted on the
box 11 whereby the elastic seal ring 28 is compressed by the end surface 1l c of the
open end of the box 11 so that the thickness of the seal ring 28 is shrunk. In the
illustrated example, the thickness of the seal ring is shrunk by approximately 1.0
mm. As a result, the sealing properties of the second seal portion provided by the
elastic seal ring are improved.
The elastic seal ring 28 which constitutes the second seal portion intimately
contacts the open end surface 1l c of the open end of the box 11 as described above.
In the illustrated example, the end surface 1l c of the open end of the box is a surface
perpendicular to the axial direction, so the side surface of the elastic seal ring 28 is
also made a perpendicular surface.
In Figures 1A and IB, symbol PCL is the distance (mm) in the axial
direction from the intersection between the protector end surface 21b and the outer
peripheral surface of the protector to the second seal portion (in the illustrated
example, the side wall on the front side of the circumferential groove 31), symbol
HL is the distance (mm) in the axial direction from the corner where the front end
surface 21b of the protector intersects the outer peripheral surface of the protector to
the edge of the opening on the front side of the circumferential groove 30, symbol H
is the depth (mm) of the circumferential groove 30 in the direction perpendicular to
the pipe axial direction, symbol PA is the diameter (mm) of the outer peripheral
surface of the protector at the end surface 21b of the protector, symbol MUL is the
distance (mm) in the axial direction from the end surface 1l c of the open end of the
box 11 to the corner where the seal portion 13 intersects the shoulder surface 14 of
the box, and symbol DA is the diameter (mm)at the corner where the seal portion 13
and the shoulder surface 1 of the box 11 intersect (the outer diameter of the
shoulder surface 14).
PCL is preferably equal to MUL ± 0.5 (mm), and more preferably PCL
equals to MUL. Even if PCL equals to MUL, since the axial length of the protector
21 is reduced when mounted on the box 11 by the shrinkage of the circumferential
groove 30, the seal ring 28 can be compressed.
The separation HL in the axial direction between the circumferential groove
30 and the protector end surface 21b is preferably 0.5 - 2.0 mm and more preferably
1.0 - 1.5 mm. Due to the presence of this separation of the groove 30 from the tip
end 21b of the protector, the problem of easily chipping away at the corner of the
groove which is observed in the prior-art box protector shown in Figure 7 can be
avoided.
The depth H of the circumferential groove 30 is selected so that the
periphery of the groove can elastically deform and so that fracture of the groove does
not take place in the groove when a pressing torque is applied to the front end surface
2lb of the protector 11 by the shoulder surface of the box 11when mounting the
protector 2 1 on the box 11. As a result of this elastic deformation, the groove width
at the top of the groove 30 (the axial length of the groove opening) preferably
decreases by at least 0.2 mm and more preferably by at least 0.3 mm. In the
example shown in Figure 1C, the groove width at the top of the groove decreases by
approximately 0.5 mm when mounting the protector. The depth H of the groove
necessary for such elastic deformation varies with the structure of the protector and
the type of resin, but it is preferably 3.0 - 6.0 mm and more preferably 4.0 - 5.0 mm
when HL is 1.0 - 1.5 mm. The wall thickness of the protector at the front end
surface 21b is preferably at least 4.5 mm.
In order to form a gap between the box 11 and the protectore 2 1 which
prevents the inner peripheral surface of the box from contacting the outer peripheral
surface of the protector except for the threaded portion 22 of the protector when the
protector 2 1 is mounted on the box, PA is preferably made less than DA. As a
result, when the contact surface of the box 11 is covered with a solid lubricating
coating, the damage to the coating by mounting of the protector can be minimized.
The difference between PA and DA is preferably at least 1.0 mm.
The reason why the seal provided by the protector 2 1 becomes inadequate at
either the end surface 1l c or the shoulder surface 14 at the rearmost portion of the
box 11 as described above with respect to Patent Documents 5 and 6 is due to
unavoidable variations in the dimensions of the protector 2 1 which is manufactured
by injection molding of a resin. Taking into consideration contraction and
expansion of the material, a tolerance of around 1mm is unavoidable as a
manufacturing accuracy of a protector body 21a made of a resin.
The body of the box 11 also has variations in dimensions due to minute
tolerances. Therefore, in order to simultaneously contact the end surface 1l c and
the rear shoulder surface 14 of the box 11 with the box protector 21, it is
advantageous that the seal portions of the box protector 21 have an elastic structure
which can absorb variations in the dimensions.
Even a box protector having the shape disclosed in Patent Document 6 can
increase the sealing properties of both the end surface 1l c at the entrance of a box
and the shoulder surface 14 at the other end of the box by increasing the tightening
torque when mounting the protector on a box to above a conventional level.
However, an equally high torque becomes necessary also when removing the
protector from the box on the field, which is not desirable. Therefore, from a
practical standpoint, it is desirable to easily contact both ends of a box with a
conventional torque for securing a protector.
Considering sealing properties, an elastic seal ring provides a greater elastic
effect than a seal portion utilizing elastic deformation of the resin constituting the
protector and is advantageous for forming a seal. Therefore, it is conceivable to use
an elastic seal ring for each of the two seal portions of a protector. However, it is
necessary to take into consideration the cost required for disposition of two elastic
seal rings and the increased possibility of the seal rings falling into a well.
Therefore, in a box protector according to the present invention, an elastic
material and specifically an elastic seal ring 28 is provided as a seal only in the
portion of a protector 26 which contacts the end surface 1l c of the open end of a box
11. The other seal which contacts the shoulder surface 1 in the rearmost portion of
the box is constituted by an elastically deformable portion which is flexible in the
axial direction and which is provided by a circumferential groove 30 located in the
vicinity of the front end surface 21b of the tubular body 21a of the protector 21.
These two seal portions make it possible to absorb unavoidable variations in the
dimensions of the box protector 2 1 and reliably produce sealing contact with both the
end surface 1l c of the open end of the box and at the shoulder surface 1 in the
rearmost portion of the box. As a result, corrosion and deterioration of the contact
surface of the box (and of a solid lubricating coating when such a coating is formed),
and contamination and injury of the box due to infiltration of foreign matter can be
prevented with certainty.
(Relationship between the solid lubricating coating and the threaded portion
of the protector)
The contact surface of the box including at least the female threaded portion
is preferably covered by the above-described solid lubricating coating. As a result,
it is unnecessary to apply a viscous liquid lubricant such as a compound grease to the
contact surface of the box each time makeup of a threaded joint is carried out, and
the operating efficiency of makeup of a threaded joint is increased.
The thickness of a solid lubricating coating which covers a thread is not
uniform in an axial cross section of the threads. A solid lubricating coating is
generally formed by applying a liquid coating composition to the surface of a
threaded joint followed by solidification of the applied coating by drying, heating,
cooling, irradiation with ultraviolet light, or the like. Therefore, as shown in Figure
3 for a thread having a thread crest 51, a thread root 52, a stabbing flank 53, and a
load frank 54, the solid lubricating coating 15 which is formed on the threaded
portion is thick at the center 1a of the thread crests 5 1 and the coating thickness
varies in the axial direction so as to become thinner toward both ends, and the
coating thickness becomes extremely thin at the corners 51b of the thread crests 51.
The cause of such a coating thickness distribution is thought to be that it is inherently
difficult for a liquid coating composition to adhere to the corners 51b and that the
coating often contracts at the time of curing of the coating. In contrast, on the
thread roots 52 where liquid accumulates, the coating thickness becomes a maximum
in the corners, but the variation in the coating thickness in the axial direction on the
thread roots 52 is smaller than on the thread crests 51.
The solid lubricating coating 1 easily peels off the female threaded portion
12 of the box 11 and particularly from the thread crests 51 at the time of mounting
the box protector 1 on the box 11. The reason therefor is thought to be that the
solid lubricating coating 15 is extremely thin on the corners 51b of the thread crests
51. Due to contact with the male threaded portion 22 of the box protector 21, the
solid lubricating coating 15 first peels from the corners 51b. Then, at the time of
interference with the male threaded portion 22 of the box protector 21, the solid
lubricating coating 15 peels off the entirety of the thread crests 51.
Therefore, by making the shape of the threads of the male threaded portion
22 of the box protector 2 1 a shape such that contact with the thread crests 51 of the
female threaded portion 12 of the box 11 is avoided and contact primarily takes place
with the thread roots 52 of the female threaded portion 12 of the box 11, it is possible
to prevent the solid lubricating coating 15 which covers the female threaded portion
12 of the box 11 from peeling caused by mounting of the box protector 21, leading to
a marked improvement in the galling resistance of the box 11 after removal of the
box protector 21.
For this purpose, the thread height HI of the male threaded portion 22 of the
protector 2 1 and the thread height H2 of the complete threads of the female threaded
portion 1 of the box 11 satisfy the relationship HI > H2, and the difference (HI -
H2) between HI and H2 is preferably larger than the maximum coating thickness t
(the total thickness when there are two or more layers) on the thread crests 51 of the
solid lubricating coating 15 which covers the female threaded portion 1 of the box
11. The difference between H1 and H2 is preferably in the range of 10 - 1,000 m h.
More preferably, this difference is around 1.5 - 3 times the maximum coating
thickness t.
By having the thread heights I and H2 satisfy the above-described
relationship and conditions, as shown in Figure 4B, when the box protector 2 1 is
mounted on the box 11 and the threaded portions 12 and 22 of the two members are
made to interfere, the thread crests 22A of the male threaded portion 22 of the box
protector 2 1 interfere with the thread roots 12B of the female threaded portion 12 of
the box 11, but a gap remains between the solid lubricating coating 15 which covers
the thread crests 12A of the female threaded portion 12 of the box 11 and the thread
roots 22B of the male threaded portion 22 of the box protector 1. Therefore, the
solid lubricating coating 15 which covers the thread crests 12A of the female
threaded portion 12 of the box 11 is prevented from contacting the threads of the
male threaded portion 22 of the box protector 2 1 and from being damaged.
If HI = H2 or HI < H2, the thread crests of the female threaded portion 12 of
the box 11 interfere with the thread roots of the male threaded portion 22 of the box
protector 21. Because the threads undergo relative movement in the axial direction
at this time, the solid lubricating coating 15 begins to peel off, particularly from the
corners of the thread crests where the coating thickness is extremely small, and the
possibility develops of the solid lubricating coating 15 eventually being completely
peeled off the thread crests.
Because the thread crests of the male threaded portion 22 of the box
protector 2 1 interfere with the thread roots of the female threaded portion 12 of the
box 11, damage to the solid lubricating coating 15 on the thread roots of the box 11
cannot be avoided. However, as stated above, the coating thickness of the solid
lubricating coating 15 on the thread roots is larger than the coating thickness on the
thread crests, and it is particularly large in the corners of the roots. Therefore, even
if the thread roots of the female threaded portion 12 of the box 11 interfere with the
threads of the male threaded portion 22 of the box protector 21, complete peeling of
the solid lubricating coating 15 does not readily take place, and the solid lubricating
coating 15 partially remains on the thread roots of the female threaded portion 12 of
the box 11. In addition, the solid lubricating coating 15 which is pushed away by
interference sometimes moves to the flanks of the threads of the female threaded
portion 12 of the box 11. Therefore, even if a lubricating grease is not applied at
the time of makeup of a threaded joint, due to the solid lubricating coating 15
remaining on the thread roots and the solid lubricating coating 15 which moves
around from the flanks, sufficient lubricating properties are imparted to the thread
roots of the female threaded portion 12 of the box 11. Of course, a sound solid
lubricating coating 15 is present on the crests of the threads of the female threaded
portion 12 of the box 11. Thus, galling of a threaded joint for pipes which takes
place at the time of makeup after removal of a box protector 2 1 can be effectively
prevented by just the solid lubricating coating 15 on the box 11, even if the pin of the
threaded joint does not undergo any special lubricating treatment (such as formation
of a solid lubricating coating).
In order to minimize damage to the solid lubricating coating 1 on the thread
flanks of the female threaded portion 12 of the box 11 caused by the box protector
21, the thread width L2 of the male threaded portion 22 of the box protector 2 1 is
preferably 0.5 - 0.75 times the thread root width LI of the complete threads of the
female threaded portion 12 of the box 11. As a result, as shown in Figure 4B, even
if the female threaded portion 12 of the box 11 is subjected to a compressive force or
a tensile force by the box protector 21, the solid lubricating coating 15 on one of the
two flanks of the threads of the box avoids contact with the threads of the male
threaded portion 22 of the box protector 2 1 and is not readily damaged. If the
thread width L2 becomes too small, the tightening force by the threads becomes
inadequate.
Figures 5A - 5C are axial cross-sectional views schematically showing a
portion of the complete threads of the female threaded portion 2 of a box 11 which
is threadingly engaged with the male threaded portion 22 of a box protector 21.
In Figures 5A - 5C, the female threaded portion 12 of the box 11 is the same
in all of these figures in which the load flank angle a of the threads is a negative
angle. In contrast, the load flank angle b of the threads of the male threaded portion
22 of the box protector 21 is different in these figures in such a manner that it is a
negative angle in Figure 5A, it is vertical (0°) in Figure 5B, and it is a positive angle
in Figure 5C. The stabbing flank angle of the threads of the male threaded portion
22 of the protector 21 is vertical (0°).
The load flank angle a of the threads of the female threaded portion 1 of the
box 1 is often nearly a vertical angle, namely, it is in the range of -3° to +3°. In
this case, the load flank angle b of the threads of the male threaded portion 22 of the
protector 2 1 is also preferably substantially a vertical angle (-2° to +2°), as shown in
Figure 5B. As a result, after the protector 2 1 is mounted on the box 11, contact
between the load flanks of the female threaded portion 12 of the box 11 and the male
threaded portion 12 of the protector 11 can be stabilized. Accordingly, the protector
2 1 is stably mounted on the box 11 even when the number of threads of the male
threaded portion 22 of the protector 2 1 is a small value of 3 - 5.
On the other hand, when the threads of the female threaded portion 12 of the
box 11 are hook shaped such that the load flank angle a is a negative angle, the load
flank angle b of the threads of the male threaded portion 22 of the protector 2 1 is
preferably substantially equal to the load flank angle a of the threads of the female
threaded portion 12 of the box 11 (within the range of a ± 2°) or smaller. For
example, when the load flank angle a of the threads of the female threaded portion
12 of the box 11 is -3°, the load flank angle b of the threads of the male threaded
portion 22 of the protector 2 1 is substantially equal to the load flank angle a when b
is -1° to -5°. As a result, as described above, the protector 2 1 is stably mounted on
the box 11.
When the load flank angle b of the protector 2 1 is smaller than the load flank
angle a of the box 11 (when a is -3°, b is smaller than -5°, such as -8°), contact
between the load flanks of the box and the protector can be concentrated in the
corners of the thread roots of the female threaded portion 12 of the box 11 or can be
restricted to just the corners of the thread roots. As a result, the region of contact of
the threads of the male threaded portion 22 of the protector 21 with the female
threaded portion 12 of the box 11 is more limited, and the solid lubricating coating
15 on the thread crests of the box can be effectively protected.
Claims
1. A box protector for a threaded joint for pipes which is mounted on a box
of a threaded joint for pipes having a pin-box structure in order to protect a contact
surface of the box, the contact surface of the box having a female threaded portion
including complete threads and an unthreaded metal contact portion including at
least a shoulder surface, the shoulder surface being sloped so as to recede towards the
center axis of the joint by an angle Qwith respect to the direction perpendicular to the
axis of the threaded join or being parallel to the perpendicular direction, wherein the
box protector has a tubular body made of a resin with an axial length which is longer
than that of the contact surface of the box and having a structure which can form a
first seal portion and a second seal portion by contacting the box surface on both
sides of the contact surface of the box, and wherein the tubular body has on its outer
peripheral surface a male threaded portion which threadingly engages with at least a
portion of the complete threads of the female threaded portion of the box,
the box protector being characterized in that:
the tubular body of the protector has a shoulder surface which forms a first
seal portion which sealingly contacts the shoulder surface of the box, the shoulder
surface of the protector slopes to the rear in the direction of insertion of the protector
towards the center axis of the joint by an angle QR with respect to the direction
perpendicular to the pipe axial direction, and the angle of slope QR of the shoulder
surface of the protector is larger than the angle of slope Qof the shoulder surface of
the box;
the tubular body of the protector has a circumferential groove in its outer
peripheral surface in the vicinity of the shoulder surface of the protector, and the
position and the cross-sectional shape of the circumferential groove are such that
under a torque applied when mounting the protector on the box, a portion of the
tubular body in the vicinity of the circumferential groove elastically deforms so as to
reduce the distance in the axial direction at the opening of the circumferential
groove, thereby ensuring the formation of the first seal portion; and
the second seal portion is formed by contacting an elastic material attached
to the protector with the box surface.
2. A box protector as set forth in claim 1 wherein the circumferential
groove has an axial cross-sectional shape selected from a V shape, an arc shape, a U
shape, a trapezoid shape, and a combination of these shapes.
3. A box protector as set forth in claim 1 or claim 2 wherein the axial
distance at the open end surface of the circumferential groove is reduced by at least
0.2 mm by the elastic deformation.
4. A box protector as set forth in claim 1 or claim 2 wherein the second
seal portion is formed by an elastic seal ring which is disposed on the outer
peripheral surface of the tubular body of the protector so as to sealingly contact the
box end surface when the protector is mounted on a box.
5. A box protector as set forth in claim 1 or claim 2 wherein all or a portion
of the contact surface of the box including at least the female threaded portion
thereof is covered with a solid lubricating coating.
6. A box protector as set forth in claim 5 wherein the thread height (HI) of
the male threaded portion of the tubular body of the protector and the thread height
(H2) of the complete threads of the female threaded portion of the box satisfy HI >
H2, and the difference [HI - H2] between HI and H2 is larger than the maximum
coating thickness of the solid lubricating coating on crests of the complete threads of
the box.
7. A box protector as set forth in claim 1 or claim 2 wherein the male
threaded portion of the protector has 3 - 5 threads.
8. A box protector as set forth in claim 7 wherein the male threaded portion
is disposed at a position opposing the female threads closest to the box entrance of
the complete threads of the box.