FIELD OF INVENTION
This invention relates to a pressure compensated tractor hitch valve.
BACKGROUND OF THE INVENTION
The hitch valve is an essential component of the agricultural tractor that maintains the depth
or the height of the implement below or "above the ground level, respectively, with reference
to the datum set by the user by means of the appropriate setting lever. The setting is
referenced automatically by the implement through the hydraulic system, of which the hitch
valve is the controlling member. This valve supports the implement, holding its weight from
falling down, as well as modulates the hydraulic fluid flow from the pump, resulting in the
positioning of the implement in a dynamic manner. The implement may be supported during
its operation in association with the agricultural tractor in any one of the following three
modes.
(i) In the position mode of operation, the position of the implement is directly set by the
valve in accordance with the setting of the position lever on the tractor. Then, the draft lever
is withdrawn from the setting.
(ii) In the draft mode, the position of the implement is set by the valve to ensure that the
draft or the force encountered at the tip of the implement becomes proportional to the angle
set on the draft lever of the tractor. Then, the position lever is withdrawn from the setting.
(iii) In the mixed mode, both levers are set appropriately. The depth of the implement is
governed by the draft lever setting subject to the maximum depth obtained from the setting of
the position lever.
It is obvious that the implement must react with the soil and generate some thrust if it is
required to work in either the draft mode or the mixed mode.
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PRIOR ART
The hitch valve is a hydraulic device consisting of a compendium of individual hydraulic
valves housed within one or more housings, with galleries drilled through or cast inside these
housings to convey the hydraulic fluid. This enables the valves contained therein to function
in accordance with the requirements of the hydraulic circuit embedded in the housings. The
hitch housings may be provided with appropriate ports to ensure the connection of the pipe
couplings and hose terminals, leading to the other circuit components, such as pump,
cylinder, etc.
The hitch valve may be actuated mechanically through a set of mechanical linkages that
perform some computations in order to maintain mathematical relationships among the
various parameters of closed-loop motion control, or electrically, by means of electrical
actuators receiving commands from an electronic controller capable of analogue or digital
computation. In both these systems, the feedback signals from the implement hitch system
play the important role of closing the control loop. These feedbacks may take the form of
force signals or displacement signals obtained from the implement.
The present invention concerns the mechanically operated hitch valve. In this case, the
position of the implement is fed back to the control loop by means of the displacement of a
cam follower in relation to the rotation of the rockshaft that finally lifts the implement
through a pair of lift arms and rods. This feedback is active in the position mode and the
mixed mode of the operation of the implement. Similarly, the draft force is sensed by the
draft sensor spring, through the displacement of the draft spindle, both in compression and in
extension of the spring. This displacement is fed back to the control loop, being active in the
draft mode and the mixed mode of the operation of the implement.
All hitch systems incorporate at least one hydraulic cylinder in order to develop the thrust
required to lift the implement load. These cylinders may be located internally within the
tractor housing or externally, close to the hitch. The hitch valve may be piped directly to the
cylinders. In some cases, the hitch valve may be fitted with a pipe junction, along with a
selection valve, so as to supply hydraulic fluid temporarily to some other external cylinder,
configured to lift an external load, such as a dumping trailer.
The cylinders may be single acting or ram type. Double acting cylinders, called piston type,
may also be used.
On the other hand, the inlet port of the hitch valve is normally connected to atleast one pump
outlet, so that hydraulic power is transferred from the pump(s) to the valve. In the neutral
condition of the hitch valve, the hydraulic fluid is required to dump on to the sump as no
work is being done. The pressure developed at this stage should be low, so that only small
power is consumed while in neutral. This feature, which is called unloading, is generally
provided in all hitch valves.
The hydraulic Circuit as per the Prior art is shown in fig 1 A.
The hitch valve broadly incorporates a few essential valves that are commonly found in all
hydraulic systems. One such valve ensures the protection of the system against overpressure
by opening at a preset pressure and passing the hydraulic fluid back to the sump. This valve is
called Pressure Relief Valve (PRV).
A Directional Control (DC) valve is an essential part of the hitch valve that performs lifting
and lowering of the cylinder. A Ram cylinder may be operated up and down through a three
way three position (3/3) DC valve. A double acting piston type cylinder calls for afour way
three position (4/3) DC valve. However, even a ram cylinder circuit may also be designed
with a 4/3 DC valve or a more elaborate version.
The middle position in a three position DC valve affects holding of the implement without
moving it. One of the two end positions of the valve is available for lifting the implement,
which is then lowered when the valve goes to the other end position.
The DC valve incorporates a sliding valve spool fitted with a close sliding fit within a finely
finished hole executed in the valve housing. In some cases, a hardened liner bush is fitted in
the housing to facilitate finishing of the hole and to achieve a higher hardness, leading to a
long life. The porting design of the DC valve requires undercuts to be provided in the housing
around the spool to eliminate unbalanced radial loads. In valve housings made from castings,
these undercuts have been traditionally provided by coring. The face of the undercuts may
need machining with an undercutting tool to achieve land width sizing to a close tolerance.
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This design may lead to a shorter life and inconsistent quality owing to the lower hardness
and non-homogeneity of cast housing materials, as well as difficulty in machining and
inspection.
When a hardened liner bush is fitted in the housing to run the valve spool, these undercuts
have often been provided externally, matching with the port galleries. The external undercuts
on the hardened bush may be machined and inspected easily to a greater accuracy in a
conventional manner.
However, an effective sealing between the liner bush and the housing is needed to reduce the
oil leakage, the resulting power loss and oil heating. Efforts to fit the liner bush with
interference may lead to distortion of the bush and poor sealing.
In addition to the DC valve, the hitch valve is provided with another valve, generally called
the Unloading Valve (UV), which unloads the hydraulic pump when lifting of the implement
does not take place. Thus, hydraulic fluid is effectively diverted to the sump at low pressure
while the DC valve is in the neutral or in the lowering positions. The ram cylinder retracts
inward as the implement descends due to gravity.
The unloading valve may take the form of a spool type or a ball/poppet type of valve. It is
normally operated by pilot pressure, which is made available by an auxiliary port in the main
DC valve spool. The pilot may be used to open or close the unloading valve, depending on
the actual design. The sizing of the unloading valve is done to ensure the passage of the full
flow of the pump to the sump at a reasonably low pressure.
The very important function of holding the load is performed by the Check Valve (CV),
which is a very common valve type found in all hydraulic systems. It essentially consists of a
ball or a poppet sitting on a valve seat and held on to it by a light spring. In order to hold the
load pressure, both the valve seat and the ball/poppet must have a very accurate circular
geometry. It is customary in tractor hydraulics to use hardened seat and ball/poppet in order
to ensure a long life. The stiffness of the spring contributes to the opening pressure, or the socalled
cracking pressure, of the valve. However, a higher cracking pressure results in a loss of
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running power due to the pressure drop in the valve. On the other hand, the hitch linkage may
start rising spontaneously if the cracking pressure is less than the neutral pressure
encountered in the unloading valve.
Another valve located in the return line from the cylinder to the lowering valve is the socalled
Response Valve (RV), which is a throttle valve set by a hand knob, accessible from
the driver's seat. RV's generally found on the tractor are not pressure compensated. In other
words, the flow through the RV, though set by the knob, varies with the pressure drop
encountered across it. The purpose of the RV is to control the rate of the return oil flow from
the ram cylinder to the sump manually, so that the desired lowering speed or the rate of drop
of the implement may be set at the time of operation.
The return oil flow during lowering of the implement, which drops by gravity in a ram type
cylinder, needs to be controlled or modulated in a dynamic manner, in the return line through
some drop in oil pressure. This is determined by the spool position of the lowering valve. In
many basic low-cost designs of the hitch valve, the DC valve spool is configured to act as the
lowering valve. However, all sliding spool valves allow some oil leakage through the
clearance between the spool and its housing. Thus, the implement develops self-dropping
characteristics in the hold position, equivalent to the leakage flow inherent in the DC valve.
Tractors are normally allowed an acceptable dropping rate.
However, the advanced hitch valve comprises of a separate Lowering Valve (LV), which
incorporates a hydraulically balanced poppet. This poppet ensures nearly leak-free closure of
the valve with the contact of a hardened poppet with a hardened seat during the lifting and the
holding of the implement. The poppet may have an integrated spool which ensures hydraulic
pressure balancing. The land of the spool may be sealed with a piston seal.
The LV is mechanically connected to the DC valve with an adjustable one-way mechanical
coupler. This enables the adjustment of the width of the dead-band defining the hold position
or, in other words, the backlash of the control spool between the end of lifting and the start of
lowering positions, and vice versa, in terms of the spool movement. The return of the LV
spool is performed by means of a light spring.
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OBJECTS OF THE INVENTION
The primary object of the present invention is to provide a pressure compensated tractor hitch
valve which overcomes disadvantages associated with the prior art(s).
Another object of the present invention is to provide a pressure compensated tractor hitch
valve which is efficient and reliable.
Further object of the present invention is to provide a pressure compensated tractor hitch
valve which is not complicated in construction.
STATEMENT OF INVENTION
According to this invention, there is provided a pressure compensated tractor hitch valve
comprising of a direction control valve with a two-land spool wherein bush of the direction
control valve and lowering valve are accommodated in valve body with a sealing there
between; an unloading valve provided with a two-land spool and a flow control valve with a
two-land spool disposed between the flow passage from the cylinder and entry to the
lowering valve.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Further objects and advantages of this invention will be more apparent from the ensuing
description when read in conjunction with the accompanying drawings and wherein:
Fig. 1 shows: hydraulic circuit of hitch valve.
Figure lA shows: hydraulic Circuit of Hitch Control Valve of prior art.
Fig. 2 shows: direction control & lowering valve assembly.
Fig. 2A shows: spool of direction control valve.
Fig. 2B shows: bush of direction control valve.
Fig. 2C shows: spool of lowering valve.
Fig. 2D shows: bush of lowering valve.
Fig. 3 shows: unloading valve assembly.
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Fig. 3 A shows: spool of unloading valve.
Fig. 3B shows: bush of unloading valve. .
Fig. 4 shows: flow control valve assembly.
Fig. 4A shows: spool of flow control valve.
Fig. 4B shows: bush of flow control valve.
DETAIL DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING DRAWINGS:-
The present invention discloses a pressure compensated tractor hitch valve with a new and
novel configuration of each of the UV, DC, LV and FC valves. These valves are integrated
within the hydraulic circuit as shown in Figure 1. The hitch valve assembly comprising of
inlet port, delivery port and return port.
The Pump, which is directly driven by the tractor engine, sucks oil from the sump and pumps
it into the hitch valve inlet connection. While the oil flows into the P port of the DC valve, it
also reaches simultaneously the P port of the pressure relief valve (PRV), as well as the P
port of the unloading valve (UV). The T ports of all these three valves lead to the sump. The
cylinder (C) port of the DC valve, which is configured as a basic 3/2 valve, is connected to
the inlet of the check valve CV through the delivery port. The check valve (CV) is located in
a'separate housing beyond the hitch valve assembly. The exit port of the check valve is
connected to the ram cylinder. The C port of the DC valve is also internally connected to the
Pilot (Pi) port of the unloading valve (UV) through a pilot passage. A small throttle with
reverse check valve (TC) is located in this pilot line.
Another passage from the ram cylinder connects the response valve (RV), which is in the
other housing along with CV, to the return port of the hitch valve assembly. It is finally
connected to the return port R of the lowering valve LV, though the flow control (FC) valve.
The T port of the LV is connected to the sump. The flow control valve (FC) maintains the
flow as modulated by the lowering valve (LV) with pressure compensation.
Each of the above four valves, as configured in the present invention is described in detail
hereinbelow:-
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The direction control valve DC, as well as the lowering valve LV, is configured as shown in
Figure 2. The valve body (9) is bored to accept the bush (7) of the DC valve and the bush
(12) of the lowering valve. As both the ends are open, it is easier to finish the two segments
of the hole. Both the bushes are designed with an identical outside diameter. The bores
holding the two bushes are finished to a closer tolerance. The bushes are fitted with transition
fit. The clearance is filled with an anaerobic adhesive during assembly to prevent leakage of
hydraulic oil. A gap is provided between the two bushes to accommodate the tank port T to
dump the oil to the sump. Both sides are covered with side plates, (2) and (11), on the back
side and the operating side of the hitch valve, respectively. Two snap rings, (10) and (14),
positioned into the grooves cut in the side plates (11) and (2) respectively, retain the two
bushes in position. The DC spool (8) slides within the bush (7), while the LV spool (13)
slides within the bush (12). The stroke of the DC spool is limited by the side plate (11)
located on the operating side plate in the outward motion and the LV spool (13) in the inward
motion. A coupler screw (6) is connected to the inner end of the DC spool (8) by means of a
screwed gland (5). This passes through the bore of the LV spool (13), which is hollow and
emerges from the outer end of the LV spool. The dead band adjustment nut (I) is mounted on
the threaded end of the coupler and is housed within the counter bore at the end of LV spool.
The spring (4) is housed within the spacer (6). The lock nut (15) is used to lock the dead band
setting by tightening it against the adjustment nut (1). The piston seal (3) is mounted in a
groove on the land of the LV spool (13) in order to reduce the dropping rate of the hitch to a
marginal value. The outside diameter of the LV spool (13) is less than the bore of the DC
valve bush (7), so that the spool can be taken out for maintenance.
To bring the hitch valve to the lifting position, the DC valve spool is moved in. This results in
the compression of the spring, thereby closing the lowering valve effectively with a gradually
increasing force. The valve land of the DC valve spool uncovers the cluster of holes on the C
port of the DC valve bus progressively. The oil flow to the cylinder builds up and the lift rises
with a smooth acceleration. Simultaneously, the small under-lap in the working land ensures
effective closure of the connection of UV external pilot to tank. The connection from the C
port of the DC valve to the external pilot opens through the check valve and closes the
unloading valve UV progressively.
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To bring the hitch valve to the lowering mode, the DC valve spool is withdrawn and moved
out. The coupler screw unseats the LV poppet positively. Simultaneously, the DC valve spool
covers the C port and connects the external pilot of the UV to the sump. This opens the UV
and passes the pump flow to the tank, causing unloading. The throttle in the miniature throttle
check valve ensures a small time delay which prevents the shock.
In the hold position, the pressure in external pilot of the UV is relieved due to the small
under-lap in the working land of the DC valve spool, which opens the UV. The LV is still
held closed due to the dead-band setting in the LV. The load on the hitch is thus held in
position, while the pump unloads.
The spool configuration of the DC valve is shown in Figure 2A. It has two lands on the same
sliding diameter. The stem of the spool is designed with end taper to meet the two inner
flanks of the two lands. This facilitates ojl flow from P port to C port and reduces pressure
drop, as well as turbulence, in the flow. The land at the inner end of the spool is the working
land for 3/2 valve, provided with a small under-lap with respect to the C port of the DC valve
bush. The length of this land is required to be controlled with close tolerance, as this
determines the amount of under-lap. The land at the outer end of the spool isolates the P side
from the sump.
The inner end of the spool is tapped to receive the gland used to retain the head of the coupler
screw. This end also serves as the seat for the compression spring between the two spools.
Reference may be made to Figure 2B, wherein the bush has a through bore, which is easy to
finish. The two external grooves correspond to the P and the C ports. The P groove has four
inlet holes to pass the full pump flow. The holes in the C groove form a cluster of holes of
various diameters and arranged in such a manner that the area of opening is broadly
proportional to the spool displacement. Each hole in the cluster has a corresponding opposite
hole with the same diameter. This balances the transverse hydraulic load on the spool and
prevents spool sticking.
-11-'
The length of the envelop over the segment of the cluster of holes in the C port is required to
be controlled with close tolerance, as this determines the amount of under-lap.
The outer end of the bush is also provided with a groove for the retaining snap ring. The outer
diameter of the bush is finished to provide a transition fit with the bore of the body. The bush
is fitted with an anaerobic adhesive.
The spool configuration of the lowering valve LV is shown in Figure 2C. It is hollow with a
through hole, so that the coupler screw can pass through it. The shape of the head of the spool
is configured as a poppet, with the contact circle with the valve seat located midway on the
conical land of the poppet. The outer diameter of the spool poppet is smaller than the bore of
the DC valve bush. Thus, the poppet may be removed without removing the DC valve bush.
The land at the tail end, which slides in the bore of the LV bush, effects hydraulic balancing
of the spool. The contact diameter of the poppet is equal to the same sliding diameter. The
land at the tail end is provided with a groove where a piston seal is installed. In addition to
the poppet head and the land on the tail end, the spool is also provided with an intermediate
land which has a cylindrical segment on the head side and a conical segment on the tail side.
This intermediate land, reacting within an intermediate land in the lowering bush, serves to
modulate the return fiow and results in a smooth stoppage of the lift at the end of a lowering
motion.
The head end of the poppet is provided with a spigot that serves as the seat of the spring.
The tail end of the spool land is provided with a counter-bore that serves as the seat for the
dead-band adjustment nut.
The bush configuration is shown in Figure 2D. It has a through bore, which is easy to finish.
The external groove corresponds to the R port. This groove has several inlet holes to pass the
full return flow. The holes terminate in an internal groove machined on the bore of the bush.
The side walls of groove are configured with a conical form having the appropriate angle
needed to pass the spool with the piston seal mounted on its land.
-12-
The oil exits from outer end of the bush around the valve seat which is designed with a
conical form. The angle of the cone is somewhat larger than that of the poppet cone, so that
the contact diameter is equal to the sliding diameter.
A second internal groove with similar conical walls is machined next to the above groove on
the R port, on the exit side of the bush. This is designed to pass the oil while the intermediate
land of the bush enters the groove during the start of the hitch lowering motion.
The outer end of the bush is also provided with a groove for the retaining snap ring. The outer
diameter of the bush is finished to provide a transition fit with the bore of the body. The bush
is fitted with an anaerobic adhesive.
S.The unloading valve UV is configured as shown in Figure 3. The valve body (9) is bored to
accept the bush (7) and the spacer (6). As both the ends are open, it is easier to finish the two
segments of the hole. The smaller bore, holding the bush is finished to a closer tolerance. The
bush is fitted with transition fit. The clearance is filled with an anaerobic adhesive during
assembly to prevent leakage of hydraulic-oil. Both sides are covered with side plates (2 and
11), on the back side and the operating side of the hitch valve, respecfively. The spool (8)
slides within the bush (7) and the stroke of the spool is limited by the stopper (1) located on
the back side plate (2). Both side plates are sealed by 0-rings, (3 and 10). A spring retainer
(5), in the form of a washer, is mounted over the external pilot end of the spool (8). The
spring (4) is housed within the spacer (6). The spool is provided with a cross-hole and a
connecting end hole to pass oil from the P port to the end of the spool. This forms the internal
pilot connection effectively on the operating side. The external pilot connection is made in
the opposite side by drilling the pilot hole in the back side plate (2). The pressure, tank and
external pilot ports are marked P, T and Pi respectively.
The spool configuration is shown in Figure 3A, which has two lands on the same sliding
diameter. The stem of the spool is designed with end taper to meet the two inner flanks of the
two lands. This facilitates oil flow from P port to T port and reduces pressure drop, as well as
turbulence, in the flow. The spring end of the spool is provided with a seat to mount the
spring retainer and a spigot to touch the stopper end.
-13-
Now, referring to Figure 3B, the bush has a through bore, which is easy to finish. The two
external grooves correspond to the P andT ports. The P groove has four inlet holes to pass
the full pump flow. The T holes form a cluster of holes of various diameters and arranged in
such a manner that the area of opening is broadly proportional to the spool displacement.
Each hole in the cluster has a corresponding opposite hole with the same diameter. This
balances the transverse hydraulic load on the spool and prevents spool sticking.
The spool, while in operation, maintains an equilibrium position under the condition of force
balance. The thrust of the internal pilot pressure equals the thrust of external pilot pressure
plus the spring force. As the spring is light, the spring force is small compared to the
hydraulic pilot forces. Thus, the spool effectively maintains a nearly constant pressure drop,
caused by the spring force acting on the area of the sliding diameter between the two pilot
pressures by keeping a floating position, which diverts a fraction of the pump flow to tank
and allows the remainder to pass through the valve.
It therefore works as a by-pass type pressure compensated flow modulator, which is normally .
closed when there is no flow due to the spring action. When the external pilot pressure is
withdrawn by positioning the spool of the DC valve in neutral or lowering positions, this
valve fully opens owing to the thrust of the internal pilot and unloads the full pump oil at a
pressure determined by the compressed spring force. Thus, it also works as an unloading
valve.
Further, this valve also allows the pump pressure to rise only marginally above the loadinduced
pressure even when the cylinder- does not consume the full flow of the pump. This
valve is active only during load lifting and opens fully during neutral and lowering.
The flow control valve FC is configured as shown in Figure 4, in which the valve body (9) is
bored to accommodate the bush (7) and the spacer (6). As both the ends are open, it is easier
to finish the two segments of the hole. The smaller bore, holding the bush is finished to a
closer tolerance. The bush is fitted with transition fit. The clearance is filled with an
anaerobic adhesive during assembly to prevent leakage of hydraulic oil. Both sides are
covered with side plates, (2 and 11), on the back side and the operating side of the hitch valve
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respectively. The spool (8) slides within the bush (7) and the stroke of the spool is limited by
the stopper (1) located on the back side plate (2). Both side plates are sealed by O-rings,
(3 and 10). A spring retainer (5), in the form of a washer, is mounted over the external pilot
end of the spool (8). The spring (4) is housed within the spacer (6). The bush is provided with
an inclined hole connecting to the end of the spool to pass oil from the R port to the end of
the spool. This forms the internal pilot connection effectively on the operating side. The
external pilot connection on the opposite side is open to sump by drilling the pilot hole in the
stopper (I). The entry and exit ports are marked C and R respectively.
The spool configuration is shown in Figure 4A, which has two lands on the same sliding
diameter. The stem of the spool is designed with end taper to meet the two inner flanks of the
two lands. This facilitates oil flow from C port to R port and reduces pressure drop, as well as
turbulence, in the flow. The spring end of the spool is provided with a seat to mount the
spring retainer and a spigot to touch the stopper end. A groove is made on the land of the
spool near the spring side so that a piston seal may be fitted therein. This prevents oil leakage
from the C port to the sump through the external pilot.
The bush configuration as shown in Figure 4B has a through bore, which is easy to finish.
The two external grooves correspond to the C and the R ports. The C groove has four inlet
holes to pass the full pump flow. The R holes form a cluster of holes of various diameters and
arranged in such a manner that the area of opening is broadly proportional to the spool
displacement. Each hole in the cluster has a corresponding opposite hole with the same
diameter. This balances the transverse hydraulic load on the spool and prevents spool,
sticking.
The spool, while in operation, maintains an equilibrium position under the condition of force
balance. The thrust of the internal pilot pressure equals the thrust of the spring force as the
external pilot pressure is relieved to the sump. Thus the spool effectively maintains a nearly
constant pressure at the R port, caused by the spring force acting on the area of the sliding
diameter. This pressure is representative of the pressure drop across the lowering valve, as the
delivery of the lowering valve is discharged directly into the sump.
-15-
It therefore works as an in-line type pressure compensated flow modulator which is normally
open when there is no flow due to the spring action. When the oil from the cylinder is fed into
the C port, this valve tends to close to the extent the pressure at the R port is reduced to the
pressure determined by the spring load acting on the area of the sliding diameter. However,
the cluster of holes is so designed that a small flow passage is available in the bush even
when the spool reaches the end of the stroke.
It is to be noted that the present invention is susceptible to modifications, adaptations and
changes by those skilled in the art. Such variant embodiments employing the concepts and
features of this invention are intended to be within the scope of the present invention, which
is further set forth under the following claims:-

WE CLAIM;
1. A pressure compensated tractor hitch valve comprising of a direction control valve
with a two-land spool wherein bush of the direction control valve and lowering valve
are accommodated in valve body with a sealing there between; an unloading valve
provided with a two-land spool and a flow control valve with a two-land spool
disposed between the flow passage from the cylinder and entry to the lowering valve.
2. A tractor hitch valve as claimed in claim 1, wherein the inner land spool of the
direction control valve has a closely controlled width causing a small under lap with
the width of a plurality of holes provided on groove in the bush leading to the passage
to the cylinder.
3. A tractor hitch valve as claimed in claim 1 or 2, wherein the lowering valve bush and
DC valve bush are fitted in the valve body with transition fit, sealed by an adhesive.
4. A tractor hitch valve as claimed in any of the preceding claims, wherein the DC valve
and the Lowering valve are co-axial and mechanically coupled with one-way coupler
with a compression spring between the ends of the DC spool and the Lowering spool.
5. A tractor hitch valve as claimed in claim 4, wherein the coupler is provided with a
head and connected to the DC spool by a threaded gland to allow a non-rigid
connection.
6. A tractor hitch valve as claimed in any of the preceding claims, wherein the coupler is
provided with a means to enable adjustment of the distance between the DC spool and
the Lowering spool to allow setting of the width of the dead-band between lifting and
lowering, with a provision to lock the setting.
7. A tractor hitch valve as claimed in any of the preceding claims, wherein the outer
diameter of the Lowering spool is less than the bore of the DC valve bush so as to
facilitate withdrawal of the Lowering spool for maintenance without the removal of
the DC bush.
8. A tractor hitch valve as claimed in any of the preceding claims, wherein the
Lowering valve spool is provided with an intermediate land having a steep conical
part in addition to a cylindrical part and the bore of the Lowering valve bush is
provided with the corresponding groove.
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9. A tractor hitch valve as claimed in any of the preceding claims, wherein the
Unloading valve maintains a uniform lifting speed, T port of which is provided with
a plurality of holes resulting in flow area proportional to the displacement of the spool
wherein the Unloading valve bush is retained in position by a spacer housing the
spring.
10. A tractor hitch valve as claimed in any of the preceding claims, wherein the flow
control valve is provided with a bush having an inclined hole as an internal pilot
connection between the R port and the end of the spool opposite to the spring wherein
the R port leading to the Lowering valve is provided with a plurality of holes.