LIMITED TORQUE BOLT MECHANISM FIELD OF THE INVENTION
This invention relates to a limited torque bolt mechanism, which can enable tightening of the bolt until a predetermined amount of torque has been applied to tighten the bolt; thereafter the mechanism does not allow further application of torque greater than the predetermined torque. More particularly, the mechanism does not allow tightening of the bolt at a torque greater than the predetermined torque. The limited torque bolt mechanism proposed herein has numerous applications where it is desirable to control the amount of torque so that bolt can be tightened to a desired level. One possible application of such bolt mechanism is in pipe restraining mechanism for a pipe joint involving PVC pipes or ductile iron pipes. Other application could be use in a pipe coupling secured on the pipe by set screws, which if tightened beyond a degree may damage the pipe surface.
BACKGROUND OF THE INVENTION
The conjunction of two circular pipes is necessary for a variety of purposes, i.e. in laying sewage lines, aqueducts and the like. At the junctions, the joining mechanisms should have sufficient strength to withstand the high fluid pressure exerted on the pipes. It is also necessary to avoid critical stress concentration on the surface of the pipes, which might lead to premature fatigue failure on the surface of the pipe. Hence, the junction mechanism should be equipped with a limiting stress feature, which would provide a safeguard against excessive stresses on the pipe. This can be achieved using a torque wrench, but with the risk of severe operator error. To make the mechanism 'fool-proof, an inbuilt failure mechanism is proposed, to automatically prevent high stress concentrations on the junction by limiting the torque applied by the bolt mechanism on the junction.
A few potential applications of the proposed limited torque fastening of pipe joints are recited in prior art. US Patent No. 5,071,175 discloses a pipe restraining

mechanism for use with a pipe retainer gland to secure the gland on a pipe. It also proposes a wedge operator member on the top wall of the housing forming a part of the retainer gland, and a pipe gripping wedge in the housing pocket having inside teeth for engagement with the outer surface of a pipe. The engagement with the pipe can be carried out using a torque wrench or by the proposed limited torque nut-bolt assembly.
U. S. Patent Nos. 4,779,900 and 4,896,903 teach a segmented retainer gland assembly, including at least two gland segments in the shape of a circular arc forming together a circular gland for securing around a circular pipe. The circular pipes are secured with the help of flanges, clamping brackets and set screws or bolts and nuts. In all these applications, it is desirable that torque applied on set screws/nuts and bolts is limited so as to avoid damage to pipes or flanges, otherwise this may result in failure of joints or pipes with disastrous results.
PRIOR ART
US Patent No. 4,627,774 discloses a limiting torque nut-bolt assembly including a bolt shank having a threaded first end portion and a non-threaded second end portion, a secondary drive head on the second shank portion and a primary shearable drive head on the secondary drive head, and a hollow nut secured over both drive heads and a portion of the non-threaded shank section. The hollow nut drives the primary drive head which rotates relative to the secondary drive head until the primary drive head shears off at a predetermined torque releasing the nut and primary drive head. The secondary drive head remains on the bolt shank for adjustment and removal of the bolt.
Another such limited torque nut-bolt assembly which includes a nut having an outer shell which shears when the torque applied to the nut exceeds the desired torque is disclosed in U.S. Patent No. 3,667,339, U.S. Patent No. 3,854,372, and US Patent No. 4,068,555. Yet another type of limited torque bolt taught in prior art includes a driving head which shears from the bolt when the applied torque reaches a

predetermined level. U. S. Patent No. 3,498,174 recites such a limited torque bolt having a single driving head. U.S. Patent Nos. 3,595,124 and 3,812, 757 disclose a limited torque bolt having two driving heads, one of which shears when applied torque reaches the predetermined level. U. S. Patent No. 4,627,774 teaches a limited torque bolt-nut assembly including a bolt shank having a threaded first-end portion, a non - threaded second-end portion, a secondary drive head on the second shank portion and a primary shearable drive head on the secondary drive head and a hollow nut secured over both drive heads and a portion of the non-threaded shank section. The hollow nut drives the primary drive head which rotates relative to secondary drive head until the primary drive head shears off at a predetermined torque releasing the nut and primary drive head.
U. S. Patent No. 5,772,252 proposes a nut and bolt assembly having a lower threaded end and opposing upper threaded end. A first flange seats above the lower threaded end and an upper flange seats below the upper threaded end. A weak point having a known torque limit is provided below the upper flange. A nut is mounted on the upper threaded end, such that when it is turned beyond the torque limit of the weak point, it causes the bolt to break at the weak point.
One of the problems associated with the prior art limited torque bolts and nuts relates to complicated structural design and desired uniformity of material characteristic which result in high costs of these items. Another problem with such bolts and nuts is that these are not fool proof.
These nuts or bolts get structurally altered when the shearable portions break off, thus the nuts or bolts or components thereof are not reusable.
SUMMARY OF THE INVENTION
It is the principal object of the present invention to provide a limited torque bolt mechanism with an inbuilt torque limiting feature using shearable means to obviate

the risk of operator error with torque wrench and also to provide an option of using the torque wrench should such need arise.
It is another object of the invention to provide a limited torque bolt mechanism, the limiting torque of which can be increased or decreased by increasing or decreasing the number of shearable members or replacing the shearable members with those of different shear value.
It is another object of the invention to provide a limited torque bolt mechanism which is suitable for various applications including the pipe obviate mechanism disclosed in U.S. Patent No. 5,071175, U.S. Patent No. 4,779,900 and U.S. Patent No. 4,896,903.
It is yet another object of the invention to provide a limited torque bolt mechanism which is easy to manufacture and is economical.
Further objects and advantages of the invention will be more apparent from the ensuing description.
According to the present invention there is provided a limited torque bolt mechanism comprising:
a bolt with threaded shank portion at one end, an optional non-threaded flange adjoining the threaded portion, a non-threaded bolt head at the other end, the top face of bolt head being provided with one substantially coaxial hole for slidably receiving a pivot pin; and
a non-threaded adaptor having a head with the bottom face being provided with a pivot pin, said pivot pin being substantially coaxial with said head, wherein
said bolt head and said adaptor are each provided with shearable means for engaging the bolt head with adaptor to enable rotation of the bolt by corresponding

rotation of the adaptor and which means shear and thereby disengage bolt from adaptor when torque on the adaptor reaches a predetermined value.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
Fig. 1 shows a limited torque bolt mechanism in accordance with the preferred
embodiment.
Fig. 2 shows the bolt.
Fig. 3. shows the adaptor
Fig. 4. shows the shear pin.
Fig. 5 shows the limited torque bolt mechanism with wedge shaped members.
DETAILED DESCRIPTION WITH REFERENCE TO DRAWINGS
At the outset of the description which follows it is to be understood that the ensuing description only illustrates a particular form of this invention. However, such a particular form is only an exemplary embodiment without intending to be understood as examplary teaching of the invention and not to be taken restrictively.
In accordance with the present invention there is provided a limited torque bolt mechanism comprising:
a bolt with threaded shank portion at one end, an optional non-threaded flange adjoining the threaded portion, a non-threaded bolt head at the other end, the top face of bolt head being provided with one substantially coaxial hole for slidably receiving a pivot pin; and
a non-threaded adaptor having a head with the bottom face being provided with a pivot pin, said pivot pin being substantially coaxial with said head, wherein said bolt head and said adaptor are each provided with shearable means for engaging the bolt head with adaptor to enable rotation of the bolt by corresponding

rotation of the adaptor and which means shear and thereby disengage bolt from adaptor when torque on the adaptor reaches a predetermined value.
The present specification describes two non-limiting embodiments which teach different shearable means for engaging the bolt.
An embodiment teaches a shearable means comprising one or shear pins ends of which are received in sockets in the bolt head adaptor.
Another embodiment teaches wedge shaped members affixed onto the top surface of bolt head and cooperating wedge shaped members affixed onto the bottom surface of the adaptor.
According to a preferred embodiment of the invention the limited torque bolt mechanism (1) of the present invention comprises a bolt (2), a non-threaded adaptor (3) and at least one non-threaded pin (4) which are assembled as illustrated in Fig. 1. Bolt (2) of the preferred embodiment is provided with a threaded shank portion (5) at one end. The threaded shank portion is adjoining an optional non-threaded flange (6), which in turn has a non-threaded head (7) adjoining it. The head (7) is provided with one or more holes (8) having depth and diameter suitable for slidably accommodating first portion (14) of shear pin (4).
The head (7) has a substantially coaxial hole (9) for slidably accommodating major portion (14) of pivot pin (13) of adaptor (3). The bolt (2) is having a particular type of threads, with pitch and nominal diameter corresponding to the female threads in the part in which it is to be fitted, for example the corresponding female threads in the flanges of pipe joint or pipe restrainer.
The adaptor (3.) comprises a head (10), the lower face (11) of which has at least one hole (12). The axis of each of the hole (12) on the lower face (11) of the adaptor (3) and on the head (7) of the bolt (2) is located substantially along circumference of a notional circle of a predetermined diameter, the circle being coaxial with the head

(10) of the adaptor and the head (7) of the bolt. Each of the one or more holes on the lower face (11) of the adaptor is of appropriate size to slidably accommodate the second portion (15) of shear pin (4). Each of the one or more holes on the top face of said bolt head (7) is axially aligned with the corresponding hole on the bottom face of said adaptor head (10), when the top face of the bolt head faces the bottom face of the adaptor head. The number of holes for shear pins in each of the adapter head and bolt head may be 2 to 8.
According to another embodiment, the shear pins may be push-fitted in corresponding holes in the head (7,10) of either the bolt or of adaptor or both. Alternatively, the shear pins may be integral with either the bolt head (7) or the adaptor head (10). When assembled the gap between the top face of bolt head and the bottom face of adaptor head after fitting the shear pins may be between 0.05 mm to 5.00 mm.
The shear pins (4) may be substantially circular in cross-section and made of cold drawn mild steel or any other steel or non ferrous alloys such as brass etc.
All the components viz. bolt, adaptor and shear pins are preferably protected against corrosion by electroplating and/or passivation.
The length of shear pins and depth of corresponding holes (8,12) in the bolt head (7) and adaptor head (10) respectively may be such that there is clearance of about 0.5-3 mm or more between the top face of the bolt head (7) and bottom face (11) of the adaptor head (10), when the bolt, pins and adaptor are assembled together. The length of pivot pin (13) and corresponding depth of coaxial hole (9) in bolt head are also designed accordingly. The number of holes for accommodating shear pins in the bolt head and adaptor head can be increased/decreased to obtain higher/lower torque value while using shear pins of identical cross section/diameter and material.
In order to assemble the limited torque bolt in the pipe assembly the threaded portion of the bolt is engaged with the corresponding female threads in the flange or the restrainer of a pipe fitting. Thereafter, the shear pins (if not integral with the heads of

the bolt or the adaptor are slid into the corresponding holes on the top surface of the bolt head. The adaptor is then assembled on the top of the bolt head so that shear pins slidable fit into the corresponding holes in the bottom face of the adaptor head and the central pivot slidably fits into the pivot hole in the top of bolt head. To tighten the bolt, the adaptor head is rotated with a spanner or wrench to enable assembly of threaded portion of the bolt into the female threads of pipe flange or restrainer. The bolt is tightened till the rotational movement of bolt thread gets restrained in the mating female threads to the extent that torsional shear stress on any shear pin just exceeds the ultimate shear strength of the material of the shear pin. This pin fails and releases a force equivalent to the shear stress of the pin multiplied by the cross-sectional area of the pin. Consequently, the remaining pins are subject to greater stress and subsequently all the pins fail under shear. The applied torque thus gets limited within the desired torque value. Shearing of the pins releases the adaptor from the bolt head.
The present invention provides numerous advantages to the user. The limited torque bolt mechanism of the present invention is easier to manufacture. The torque of bolt mechanism is controlled by shear strength of the shear pins, which is much easier to control by selecting suitable material with appropriate heat treatment/cold work condition and selecting the cross sectional area of the shear pins. The present invention enables the adaptor to be reused as a part of another limited torque bolt mechanism. Shear pins can be made from commercially available cold drawn wire with fairly uniform results. The bolt of the present invention can be made from commercially available bolts. The present limited torque bolt mechanism provides better control of limiting torque as compared to the prior art limited torque nut and bolt mechanisms.
According to another embodiment the limiting torque mechanism comprises a bolt (2), a non - threaded adaptor (3). The bolt (2) is provided with a threaded shank portion (5) at one end. The threaded portion is adjoining an optional non-threaded flange (6), which in turn has a non-threaded bolt head (7) adjoining it. The bolt head

(7) is has a coaxial hole (9) for receiving a pivot pin. The top face of bold head has at least one wedge shaped member (16) fixed on the top of bolt head.
The adaptor (3) comprises a head (10) bottom face (11) of which has one or more wedge shaped members (16) fixed on to it. The wedge shaped members are configured and arranged such that inclined plane of each such members abuts corresponding inclined plane of the wedge shaped member fixed on the top face of bolt head. Each of the inclined plane fixed on the bolt head and the adaptor has serrations on the surface such that when abutting, the inclined plane fixed on top face of bolt meshes with the serrations on the inclined plane of corresponding wedge shaped member fixed on the bottom face of the adaptor. The bottom face of adaptor is provided with a co-axial pivot pin (13) which is slidably received in the corresponding coaxial hole (9) in the bolt head. The serrations on the inclined plane of the wedge shaped members on the bolt head mesh with serrations on the inclined plane of the corresponding wedge shaped member and enable rotating of bolt by rotating the adaptor and the serrations shear when the rotational torque reaches a predetermined value, thereby providing a limited torque mechanism, which ensures that tightening torque does not exceed the predetermined torque value.
According to another embodiment the serrations on the inclined plane of each of the wedge shaped members fixed on the top face of the bolt head and on the bottom face of the adaptor are saw tooth shaped to enable rotation of the bolt in predetermined direction i.e. either in clockwise or in anti-clockwise direction by turning the adaptor. Such shape will result in slippage when any one tries to rotate the bolt with the help of adaptor in direction opposite to predetermined direction the teeth will not mesh and there will be slippage between wedge shaped members on the adaptor and bolt head respectively. The head of adaptor may be square or hexagonal or octagonal shaped. The wedge shaped members may be integral with bolt head and adaptor respectively or may be separately affixed. The wedge shaped member may be made of mild steel. The wedge shaped members may be preferably 1 to 10 in number fixed on each of the bolt head and adaptor.

We Claim;
1. A limited torque bolt mechanism comprising:
a bolt with threaded shank portion at one end, an optional non-threaded flange adjoining the threaded portion, a non-threaded bolt head at the other end, the top face of bolt head being provided with one substantially coaxial hole for slidably receiving a pivot pin; and
a non-threaded adaptor having a head with the bottom face being provided with a pivot pin, said pivot pin being substantially coaxial with said head, wherein
said bolt head and said adaptor are each provided with shearable means for engaging the bolt head with adaptor to enable rotation of the bolt by corresponding rotation of the adaptor and which means shear and thereby disengage bolt from adaptor when torque on the adaptor reaches a predetermined value.
2. A limited torque bolt mechanism as claimed in claim 1 wherein said means are
one or more sockets provided on the top face of the bolt head, corresponding sockets
provided on the bottom face of the adaptor and one or more shear pins, one end of
each pin being slidably received in the socket in the bolt head and the other end of
said pin being slidably recovered in the corresponding socket in the adaptor.
3. A limited torque bolt mechanism as claimed in claim 1, wherein said means
are one or more wedge shaped members fixed on the top face of bolt head and one or
more wedge shaped members fixed on the bottom face of the adaptor, the wedge
shaped members on the bottom face of adaptor being configured and arranged such
that inclined plane of each member substantially abut the inclined plane of
corresponding member on the top face of bolt head, each said inclined plane on
bottom face of adaptor having serrations which mesh with serrations on
corresponding inclined plane on the top face of the bolt head.
4. A limited torque bolt mechanism as claimed in claim 3, wherein said
serrations are of saw tooth shape to enable rotation of bolt either in clockwise or in
anti-clockwise direction by turning the adaptor, the adaptor being torqued in only one
direction.
5. A limited torque bolt mechanism as claimed in claim 3 or 4, wherein said
wedge shaped members are 1-10 in number.
6. A limited torque bolt mechanism as claimed in claim 2, wherein the number of
sockets for shear pins on each of the top face of bolt head and the bottom face of
adaptor is 2 to 8.
7. A limited torque bolt mechanism as claimed in claim 2 or 3, wherein the shear
pin or wedge shaped member is integral with the head of adaptor or bolt head or both.
8. A limited torque bolt mechanism as claimed in claim 1, wherein the head of
adaptor is square or hexagonal or octagonal shaped.
9. A limited torque bolt mechanism as claimed in claim 2, wherein the gap
between the upper face of the bolt head and lower face of adaptor head after
engaging the bolt head with adaptor is between 0.05 to 5.0 mm.
10. A method of applying limited torque to a bolt head comprising engaging the
bolt head to be tightened with a adaptor head through the shearable means as claimed
in claim 1 having pre-determined shear strength, applying increasing torque on the
adaptor till said means fail due to shearing thereby applying limited torque to the bolt
head.
11. A limited torque bolt mechanism, substantially such as hereinbefore described
with reference to the accompanying drawings.
12. A method of applying limited torque to a bolt head, substantially such as hereinbefore described with reference to the accompanying drawings.