FORM 2
THE PATENTSACT, 1970 (39 OF 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
1. Title of the Invention: "AN IMPROVED HITCH VALVE FOR HYDRAULIC SYSTEM
OF AGRICULTURAL TRACTORS"
2. Applicant MAHINDRA & MAHINDRA LIMITED
Name, Nationality FES, SWARAJ DIVISION,
Address : Registered Office:-
MAHINDRA & MAHINDRA LIMITED, Gateway Building, Apollo Bunder, Maharashtra - 400001 Mumbai., An Indian Company
COMPLETE : The following specification particularly describes the
invention and the manner in which is to be performed.

FIELD OF THE INVENTION
The present invention relates to an improved hitch valve for hydraulic system of agricultural tractors.
BACKGROUND OF THE INVENTION
In hydraulic systems of agricultural tractors, a hitch valve provided has to maintain the desired depth or the height of an attachable implement, either below or above the ground level respectively, with reference to a datum set each time by an user with a mechanical lever. The datum may be set with reference to the position of the tractor implement or the draft force encountered by the tractor during the use of the implement, or a combination of both.
The hitch valve is a hydraulic valve assembly consisting of a plurality of individual hydraulic valves in a cartridge form, disposed within one or more housings with passages laid out, by drilling or casting. Hydraulic fluid is conveyed through the housings, so as to enable the valves contained therein to function in accordance with the requirements of at least one hydraulic circuit embedded in the housings.
In a mechanical embodiment, a Hitch Valve is normally actuated by mechanically displacing a hydraulic spool through a set of mechanical linkages. This spool, termed as Control Spool, connects the fluid passages to direct hydraulic oil from a pump into a ram cylinder in order to lift the implement. The spool also relieves oil from the ram cylinder back to the oil sump during a lowering motion of the implement by connecting the passage when the implement shift original position. In the neutral position of the control spool, the implement remains held up, as the spool does not permit any flow of oil from or to the ram cylinder by closing/opening the required passage.

The hydraulic ram cylinder normally preferred in the tractor hydraulic systems is a single-acting. This means that oil needs pumping into the ram cylinder for lifting, whereas the ram cylinder retracts due to self weight of the implement/linkage during lowering.
In the prior art structure of hydraulic system, the hydraulic pump remains virtually ineffective when the implement load is being held or lowered. The pump oil flow at this time is required to dump in the sump at a negligible pressure, called "Unloading", rather than to pass through by blowing a relief valve, which wastes energy. To carry out the unloading of oil effectively at a low pressure, while holding or lowering the implement load, an unloading valve is provided.
A known mechanically operated hitch valve consists of two basic valve elements, such as, a Control Valve (CV) and an Unloading Valve (UV). The shortcomings encountered in these valves are Leakage, Spool sticking, higher than permissible Dropping Rate and Undue pressure drop, in addition to spring wear, erratic performance etc. All such phenomena limit the performance of known Hitch Valves.
The Unloading Valve element in a known valve in the form of a hydraulic pilot-operated shut-off valve, where the pilot pressure is used to close the valve. The pilot pressure is derived from the pump pressure when the implement is lifting. The basic component in the pilot operated Unloading Valve is a spool which is provided with a conical shape at the end to work as a poppet which may sit on a valve seat and close the flow passage of oil to a sump. The spool slides within the finished bore of a bush, which is permanently fitted within a valve housing.
The construction of the Unloading Valve (UV) of a known hitch valve, is illustrated in Figure 1. A Bush [2] is inserted into a Body [3], to house a sliding

Unloading Spool [6]. The bush [2] is provided with a slim head of larger diameter, which is accommodated in a corresponding undercut in the body. A Valve Seat [5] is fitted inside the Bush [2] having a close fit and sealed with a first O-ring seal [4]. A conical end of the Spool [6] is designed to close the oil passage through the Valve Seat [5]. The Spool [6] is normally pressed on to the valve seat [5] by a Compression Spring [7], The spring [7] is guided at the rear end by a Spring Seat [8]. The leakage of oil is prevented by a second 0-ring seal [9]. A first cover [10] retains the spring seat [8] at the rear end. A second cover . [11] retains the valve seat [8] at the front end. The second cover [11] also contains a return passage of oil back to a sump.
The functioning of the unloading valve (UV) may take place in one of the two modes. During unloading, the spring chamber of the valve is relieved to the sump by means of a CV spool [6] displacement to unloading position. The pump oil then pushes the unloading spool, thereby compressing the spring [7], The passage to the sump for the pump oil is thus opened and the unloading takes place at the pressure set by the spring force on the area of the spool. During lifting, when sufficient pressure develops as required by the load, the pump oil is admitted to the spring chamber through resetting of the earlier displacement of the CV spool, while shutting its connection to the sump. As the oil pressure is now available on both ends of the unloading spool, it resets on the valve seat, aided by the spring thrust and closing the sump passage.
The shortcomings in the performance of this prior art construction are:
i. Due to a poor alignment between the end of the unloading spool and the valve seat, the sealing is not effective and the valve seat leaks even if unloading valve is closed (lifting mode). This results in a

continuous leakage flow, which brings down the efficiency of the pump and results in oil heating.
ii. Due to poor geometrical integrity of the spool end and the valve seat, similar leakage occurs.
iii. Spool jamming occurs due to low contact length/diameter ratio of the spool.
iv. Undue friction may occur between the outer diameter of the spring and the bore of the bush or between the inner diameter of the spring and the spigot of the spool guiding the spring. While the spring wire touches the bore or the spigot during movement of the spool, it gets eventually worn out and fails before the expected life expires.
The construction of a prior art Control Valve (CV) is illustrated in Figure 3. A Bush [2] is inserted into a Body [3], to house a sliding Control Spool [4]. The bush is provided with a slim head of larger diameter, which is accommodated in a corresponding undercut [10] in the body. A first cover [5] the bush at the rear end, which is the operating side. The end of the control spool projects beyond the cover [5] to be operated by a mechanical linkage system. This cover [5] also contains the passage of the return oil back to the sump. A second cover [1] retains the bush at the front end.
The functioning of the Control Valve (CV) takes place in one of the three modes.
During the lifting mode, the CV spool moves to connect the pump port to the spring chamber of the UV, thereby closing the passage to the sump in the UV. The pump oil then develops the pressure required to lift the implement. It may

be noted here that the CV and the UV together effect the closure of the pump delivery oil passage to the sump. The oil flow to the ram cylinder passes through a gallery located elsewhere.
During the lowering mode, the CV spool displaces to unloading position. CV spool configuration is designed so as to connect the Pilot oil port to the sump through a concentric passage drilled within the spool. Simultaneously, the pump port of the CV is closed, owing to the spool configuration. With the fall of pressure in the CV pilot port, connected to the spring chamber of the UV, the UV spool shifts due to pump delivery pressure, developed by the closure of the pump port. This . opens the pump port of the UV to the sump, which results in dumping of pump oil directly to the sump, causing unloading. This position also marks the end of the lifting mode, described earlier in this paragraph.
The above sequence of unloading is also applicable to the neutral mode of the valve.
The lowering action takes place due to the opening of the oil return passage from a ram cylinder (R) to the sump connection located at the end (T) of the CV spool. By the design of the land widths on the CV spool, a dead band is provided between the start of the unloading action and start of the lowering action. This enables an effective neutral mode, when the implement is held up without any motion. On the other hand, larger width of the dead band adversely affects the sensitivity of control of the implement. It is, therefore, carefully set to reach a compromise between the sensitivity and the stability of the system.
In order to initiate the lowering motion at a slower speed and to progressively, gain speed, a pattern of holes are drilled in the bush at the R port. This pattern is designed to execute a rudimentary ramping in the lowering speed profile.

It is thus apparent that holding of the implement in the neutral mode with an acceptable dropping rate is related to the leakage rate through the end of the CV spool, where resistance to leakage is offered by the dead band width. Though wider dead band reduces leakage oil flow, it cannot be implemented due to its adverse effect on the sensitivity.
The leakage rate during load-holding may initially be found to be higher as the return oil ports are closed. But, with a limited load drop, the CV spool creeps out to overlap the return oil ports owing to the action of the linkage. This slows down the dropping rate to a certain extent.
The shortcomings in the performance of this construction are:
(i) Due to the lack of cylindricity at the end of the CV spool and the corresponding region of the CV bush, leakage and the resulting dropping rate are often found to be high. In spite of the distributed pattern of holes, a moderate dead band cannot achieve the required low dropping rate. The leakage takes place from the R port to the sump through the pattern of hole and the dead band.
(ii) There occurs considerable leakage from the return oil port R to the pressure port P, due to the fall of pressure in the pressure port to the unloading pressure level. The leakage is aggravated by the wide undercuts provided in the CV spool between the R port and P port, as well as the presence of four entry hole of larger diameter at the P port. It may be noted that the oil passage to the spring chamber of the UV from via the P port of the CV need not be generously proportioned. To perform the pilot operation duty, small ports are adequate. This

leakage path is visualized in Figure 3, illustrating a prior art embodiment of the Hitch Valve.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved hitch valve for hydraulic system of agricultural tractors, which eliminates the disadvantages of prior art.
Another object of the invention is to propose an improved hitch valve for hydraulic system of agricultural tractors, which prevents continuous leakage of oil through the joint between the spool and seat of unloading valve element of the hitch valve.
A still another object of the invention is to propose an improved hitch valve for hydraulic system of agricultural tractors, which eliminates frequent jamming of the valve spool due to spool's low contact length to diameter ratio.
A further object of the invention is to propose an improved hitch valve for hydraulic system of agricultural tractors, which prevents oil leakage through the control valve element of the hitch valve.
A still further object of the invention is to propose an improved hitch valve for hydraulic system of agricultural tractors, which reduces the pressure drop.
SUMMARY OF THE INVENTION
Accordingly, there is provided a improved hitch valve for hydraulic system of

agricultural tractors, the hitch valve comprising a plurality of first vaive element and second valve element housed in a common or individual housings with oil
feed passages to direct hydraulic oil into a single acting ram cylinder which lifting an implement of the agricultural tractor, the implement upon retraction of the ram cylinder lowered by self-weight, the first valve element constituting an unloading valve having at least a body, a bush, a spool assembly, a compression spring, a valve seat, a spring seat, and at least two covers, the second valve (CV) element constituting a control valve comprising a valve body having an undercut and accommodating a bush, at least two covers, the bush having at least one each pressure port and return port for hydraulic oil, both the valve elements are connected through a set of mechanical linkages, the improvement is characterized in that the spool assembly of the first valve element (UV) is constructed with at least three separate members, a first member in the form of a spool with a flat bottom socket constructed of a hardened and ground member and a third member in the form of a hardened and ground ball sitting within an internal conical surface of the valve seat with an included angle of 60°, the valve seat of the spool and having an undercut corresponding to that of the bush and fitted with an V seal positioned close to the first cover allowing a radial play; the length of sliding segment of the spool is increased with a clearance in the middle to reduce spool jamming by attaining a ratio of contact length to diameter upto 1.5, and a second '0' seal having identical size of the first '0' seal is provided between a spring seat inserted through the bush bore to the second cover; and a pattern of holes in the return segment (R) of the bush is configured to have a first row of two holes, a second row of holes having two holes with diameter larger than that of the holes in the fist row, and a third row of holes with four holes with larger diameter such that the total flow area is equivalent to that of the main return oil passage, wherein each pair of holes are located diametrically opposite; the pattern of holes in the pressure (P) segment of the

bush of the first valve element (UV) provided with two small holes to increase hydraulic resistance in the leakage flow path through the P-port; and the spool segment between the R-port and the P-port provided with a plurality of pressure distribution grooves eliminating the existing undercuts.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a first valve element of a hitch valve according to prior art
Figure 2 - shows an improved first valve element of a hitch valve according to the present invention
Figure 3 - shows a second valve element of a hitch valve according to prior art
Figure 4 - shows an improved second valve element if a hitch valve according to the invention.
DETAIL DESCRIPTION OF THE INVENTION
In the present invention, the prior art disadvantages of the unloading valve (UV), are addressed by improved configuration, enabling achievement of higher performance in conventional but cost effective production methods. The embodiment of the unloading valve (UV) according to the present invention is shown in Figure 2.
As shown in figure 2, a bush [2] is inserted into the Body [3] to house a sliding Unloading Spool assembly (6,6A,6B). The bush is not provided with a head of larger diameter but is made cylindrical, which is accommodated within the

cylindrical bore of the body. This reduces the costs of both the bush and the spool. The retention is positively ensured by the two Covers [1,10].
The Valve Seat [5], having an undercut diameter at the spool end, is fitted inside the Bush, but is sealed with an O-ring seal [4], positioned close to the Cover [1]. This modification enables a small radial play of a seating cone of the valve seat. The requirement of attaining the concentricity with a high level of accuracy between a sealing nose of the unloading spool and the valve spool is thus made redundant.
A further degree of play is achieved by constructing a three-piece unloading spool assembly, as shown. The sealing end of the Spool [6B] is designed with a socket with a flat bottom formed by a hardened and ground piece [6A], A hardened and ground Ball [6], is enclosed within the socket end of the spool and retained as captive by crimping the rim of the spool socket around it, permitting a small clearance. This enables the ball to roll and float within close limits and helps to achieve a better sealing by self-alignment. The high hardness and geometrical accuracy of the commercially available ball reduces leakage and cost and enhances life of the unloading spool.
The ball sits within an internal conical surface of the valve seat, also hardened and ground, with an included angle of 60 degrees, having a small land width. This ensures prevention of leakage and longer life. The design also leads to a damping action, or gradual slowing down of the oil flow, before its actual cut-off.
The length of the sliding segment of the spool is also increased, with a clearance in the middle of the land, thereby attaining a higher ratio of contact length to

diameter. The ratio is increased from a value of 1 to the higher value of 1.5. This reduces the chance of spool jamming.
A compression spring [7], which presses the Spool on to the valve seat is so designed that the outer diameter does not touch the bore of the bush. The spring is guided at the spool end by a long spigot integral with the spool, which has a larger diameter at the root in order to match with the inner diameter of the spring. The remaining part of the spigot is undercut, so that it may not touch the spring while the sliding motion takes place. This enables the spring to operate without touching the spigot in the working zone, thus ensuring a longer life of the spring.
A Spring Seat [8] is guided by the bore of the Bush and backed by the Cover [10]. Leakage is prevented by the O-ring seal [9], which has the same size as the other O-ring [4]. This reduces the variety of the seals and helps inventory control. The design of the Cover [1], is simplified as there is no groove for an 0-ring.
The bush, [2] is provided with ends incorporating countersunk internal cone of 60 degree included angle. This enables cylindrical grinding of the outer diameter of the bush, holding it between centers. This process results in a higher geometrical accuracy of the outer dimension of the bush. This countersinking is also utilized in retaining the valve seat, item [5], and the spring seat, item [8], which are designed with matching conical heads, in a positive manner.
In the present invention, the above issues are addressed by improved design, enabling achievement of higher performance with conventional and cost effective production methods. The embodiment according to the present invention is shown in Figure 4

The pattern of holes in the return segment R of the bush [2] is modified, with three rows of holes in three adjacent planes. The first row of holes from the end of the CV spool near the R port holds only two holes of small diameter. The middle row holds two more holes of larger diameter, whereas the last row holds four holes also of the larger diameter. The total flow area is calculated to be equivalent to that of the main return oil passage. Each pair of holes are located diametrically opposite, to balance the radial load due to the hydraulic pressure acting on the spool through the holes. The magnitude of hydraulic resistance from the R port to the sump passage provided in the cover [1] increases in the new pattern of holes, as described above.
In addition to the above, the pattern of holes located in the P segment of the CV bush is also modified, with only two small holes provided for the pilot oil circuit leading to the UV spring chamber and the sump line through the axis of the spool [4]. This also increases the hydraulic resistance in the leakage flow path through the P port.
The segment of the spool [4], between the R port and the P port is also modified by eliminating the large undercuts seen in the known embodiment. Instead, a few small pressure distribution grooves are provided to eliminate chances of spool stick-up.

WE CLAIM
1. An improved hitch valve for hydraulic system of agricultural tractors, the hitch valve comprising a plurality of first valve element and second valve element housed in a common or individual housings with oil feed passages to direct hydraulic oil into a single acting ram cylinder which lifting an implement of the agricultural tractor, the implement upon retraction of the ram cylinder lowered by self-weight, the first vale element constituting an unloading valve having at least a body, a bush, a spool assembly, a compression spring, a valve seat, a spring seat, and at least two covers, the second valve (CV) element constituting a control valve comprising a valve body having an undercut and accommodating a bush, at least two covers, the bush having at least one each pressure port and return port for hydraulic oil, both the valve elements are connected through a set of mechanical linkages, the improvement is characterized in that:-
- the spool assembly of the first valve element (UV) is constructed with at least three separate members, a first member (6B) in the form of a spool with a flat bottom socket constructed with a hardened and ground member (6A), and a third member (6c) in the form of a hardened and ground ball (6) sitting within an internal conical surface of the valve seat (5) with an included angle of 60°, the valve seat of the spool and having an undercut corresponding to that of the bush and fitted with an O' Ring seal (4) positioned close to the first cover (1) allowing a radial play; the length of sliding segment of the spool is increased with a clearance in the middle to reduce spool jamming by attaining a ratio of contact length to diameter upto 1.5, and a second 'O' Ring seal (9) having identical size of the first 'O' Ring seal (4) is provided

between a spring seat (8) inserted through the bush bore (2) to the second cover (10); and
- a pattern of holes in the return segment (R) of the bush (2) is configured to have a first row of two holes, a second row of holes having two holes with diameter larger than that of the holes in the fist row, and a third row of holes with four holes with larger diameter such that the total flow area is equivalent to that of the main return oil passage, wherein each pair of holes are located diametrically opposite; the pattern of holes in the pressure (P) segment of the bush of the first valve element (UV) provided with two small holes to increase hydraulic resistance in the leakage flow path through the P-port; and the spool segment (4) between the R-port and the P-port provided with a plurality of pressure distribution grooves eliminating the existing undercuts.