FIELD OF INVENTION
This invention relates to a self centered direct acting pressure relief valve for tractor hydraulic system.
BACKGROUND OF THE INVENTION
Tractor Hydraulics an essential part of Agricultural Tractor, consists of a number of hydraulic devices assembled in one or more housings, to regulate and control the flow of hydraulic fluid in order to get the desired output from the hitch system.
One of the most important components of the above system is the relief valve, which may be located within the Hitch Valve Block or attached to it through a hydraulic connection.
The function of the Relief Valve is to offer a protection to the hydraulic circuit through the hydraulic mechanism contained within the valve. This mechanism is normally loaded with a spring which allows fluid passage to open when the pressure in the hydraulic system reaches the pre-set value. Fluid under pressure then passes through the passage to the fluid reservoir, thereby limiting the pressure close to the pre-set limit. The safe pressure is determined with a consideration to the system requirement, as well as the capacity of the hydraulic component, viz. the housing(s) and valves, to withstand the pressure. The pre-setting of the pressure is normally done with shims or with a nut screw mechanism which compresses the spring to the desired level of force required for that pressure.
The Relief mechanism may be of various types. However, the simplest type is called "direct-acting" where the fluid in the main flow passage directly acts on the valve element, which displaces to pass the full flow. As the spring, which is normally of helical compression type, is a linear device where the load rises linearly with the compression, the pressure exhibited by the passing fluid rises to some extent when the spring compresses further, to allow a higher flow to pass.
This phenomena results in a difference between the pressure at which the valve starts to open and the pressure at which the rated flow passes.
Therefore, the objective of the valve designer is to reduce this pressure difference as much as possible. Thus, the direct-acting type of relief valve exhibits a higher pressure difference compared to other advanced designs.
In the context of agricultural tractor, the relief valve cost needs to be kept low in order to achieve an overall economical design.
PRIOR ART
The construction of the conventional Tractor Relief Valve is shown in Fig. 1, wherein the Body (1) is designed to be fit in the Hitch Valve housing by means of external screw thread. The sealing is achieved by means of a sealing element, viz. an elastomeric seal or O-Ring or soft metal ring (not shown).
The Body is provided with a stepped oil entry hole and the valve seat. The valve spool (2) has a conical form on the valve end and a cylindrical form
as well as a spigot on the spring end. The conical end sits on the valve seat and the cylindrical end slides within the Body with a small clearance.
The spigot at the end of the valve spool serves as a guide for the spring (3) which is held by the Adjusting Screw (4).
The conventional design leads to the following drawbacks in the working of the Relief valve.
The hardness of the valve seat, which is an integral part of the Relief Valve, cannot be increased to the high value required due to the size of the part, unless an expensive material is substituted. This affects the working life of the valve seat adversely.
The difference between opening pressure and the full-flow pressure of the valve is found to be high.
The valve spool may get stuck due to the offset in the line of the spring force, which takes place on account of the partial contact between the end of the spring and the valve seat.
Hence, there is a requirement to develop a relief valve so as to address the aforesaid drawbacks.
OBJECTS OF THE INVENTION
The primary object of the present invention is to provide a relief valve for tractor hydraulic system which overcomes disadvantages associated with the prior art.
Another object of the present invention is to provide a relief valve for tractor hydraulic system with a lower rise of pressure between the opening and the full-flow conditions of the valve.
Further object of the present invention is to provide a relief valve for tractor hydraulic system with a longer working life.
Further object of invention is to provide a relief valve for tractor hydraulic system with no offset in the spring force 8s the valve always at center.
Further object of the present invention is to provide a relief valve for tractor hydraulic system which avoids sticking of the moving parts.
STATEMENT OF INVENTION
According to this invention, there is provided a self centered direct acting pressure relief valve for tractor hydraulic system comprising of valve seat with ball having required hardness, a spool guided by the adapter having a cavity matching the head of the valve seat and a clearance provided to reduce the difference between the opening pressure and the full-flow pressure; and a spring seat that transfers the load of the spring on the valve spool through a ball to eliminate the offset between the load line of the spring and the valve centerline.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Further objects and advantages of the invention will be more apparent from the ensuing description when read in conjunction with the accompanying drawings and wherein:
Fig 1 shows the Relief valve according to the prior art.
Fig 2 shows the fitting of the valve seat onto the Adapter Body.
Fig 3 shows the Arrangement of the Valve Spool along with the Valve Ball, Valve Seat with the Adapter Body as well as the Spring Seat with the Load Ball.
Fig 4 shows the complete assembly of the Relief Valve
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS
The present invention discloses a self centered direct acting pressure relief valve for tractor hydraulic system. The Relief Valve according to the present invention is shown in Fig. 4, wherein the Adapter Body (1) is designed to fit in the Hitch Valve housing by means of external screw thread. The sealing is achieved by means of a sealing element, viz. an elastomeric seal or O-Ring or a soft metal ring (not shown), in a conventional manner.
The Adapter Body is provided with the inner hole for fitting the valve seat and the upper cavity for fitting the spool. The axis of the spool cavity has good concentricity with that of the inner hole.
The Valve Seat (2) is designed with the shank machined to result in a transition fit with the Adapter Body and the head dimensioned to match with the skirt of the spool with a clearance fit.
A groove is turned on the head of the valve seat. A lip is also turned at the bottom of the cavity of the Adapter Body. This lip is crimped around the head of the valve seat, so as to enter the groove and lock it effectively.
The Spool (4) sits over the Valve Ball (3) and the skirt of the spool covers the head of the Valve Seat with a small clearance.
The Ball (5) with a smaller diameter is located inside the conical surface machined over the spool (4). The spring seat (7), with a similar conical surface sits on the Ball (5), making it free to tilt and adjust to the end face of spring (8).
The Body (6) of the relief Valve is screwed to the large end of the Adapter Body (1) without any seal.
The spring seat (7) is also tapped on the other end. The spring (8) is provided with the required clearance around the outer diameter with respect to the Body (6) and on the inner diameter over the spigot of the Spring Seat (7) and Adjusting Screw (9).
The attachment of the Valve Seat (2) to the Adapter Body (1) is shown in detail in Fig. 2 .The valve seat (2) is designed with an end chamfer for lead-in and an undercut shank near the shoulder for easy finishing of the shank.
The head is provided with a groove which has for example a square outer edge and a beveled inner edge for easy crimping and positive retention. The shape of the lip before and after crimping is also shown. The crimping is carried out with a tubular tool through axial thrust. The profile of the lip is cut with an under-cutting form tool.
The valve set may be installed in the Adapter Body with an Anaerobic Adhesive for effective sealing.
The position of the Spool (4) with respect to the Adopter Body (1) and the Valve Seat (2) is shown in detail in Fig. 3.
When the hydraulic pressure exceeds the spring setting, a very small clearance appears between the valve seat (2) and the ball (3).The fluid begins to escape through the clearance and occupies the volume between the spool (4), the ball (3) and the valve seat (2). Fluid then passes through the gap between the head of the valve seat (2) and the skirt of the spool (4).
As the flow increases, the pressure drop through the above gap increases. This pressure, developed beneath the spool cavity generates an additional thrust, which also depends on the diameter of the edge of the cavity. The additional thrust supports the spool, thereby reducing the extra pressure developed in the fluid as the flow builds up.
Pressure equalizing grooves, both circumferential and longitudinal, are cut on the sliding surface of the spool.
The mounting of the spring is shown in detail in Fig. 4.The smaller Ball (5) is located over the Spool (4). The spring seat (7) sits on the Ball (5).However, for easy assembly, a tapping is provided on the outer side of the spring seat (7).This tapping may be used to fit a long stud which is an assembly tool. The assembly of the Spring Seat (7), Spring (8) and the Adjusting Screw (9), may be conducted outside with the help of assembly stud, to the required setting length of the spring by means of nut. The assembly then may be inserted by screwing inside the Valve Body (6) to touch the smaller Ball (5). Subsequently, the nut of the assembly may be unscrewed and the assembly tool may be removed.
In the present invention, the following features; with reference to Fig. 4, are adopted :
(a) Valve element with Ball (3) and Conical Valve Seat (2), both with very high hardness of approximately 58 HRc, made from hardened ball-race steel, without restricting scope of the invention to the same.
(b) Separate Valve Seat (2) machined according to the requirement.
(c) Valve Seat (2) inserted within the Relief Valve Adapter Body (1) with a transition fit and sealed by an anaerobic adhesive.
(d) Valve Seat (2) locked by crimping of built-in lip machined within the Relief Valve Adopter Body (1) after fitting.
(e) The ball (3) acting on the conical valve seat floats freely and is not attached to or captive within, any other part. The ball is replaceable during normal over-hauling of the Relief Valve.
(f) A spool (4) with an internal conical surface at the end bears on the ball (3). The spool slides within a close fitting cylindrical cavity machined in the Adapter Body (1).
(g) The valve end of the spool has a cylindrical cavity to match valve seat head with a small clearance, which reduces the difference between the opening pressure and the full flow pressure of the Relief Valve.
(h) The spring seat (7) transfers the load of the spring (8) onto the above spool through a ball (5), wherein the Spring Seat is free to a limited extent.
(i) The body of the relief valve (6) is separate from the Adapter Body (1), to which it is connected by means of screw thread.
Reference :-
S1. No. Description
1 Adopter body
2 Valve Seat
3 Valve Ball
4 Spool
5 Ball
6 Body
7 Spring Seat
8 Spring
9 Adjusting Screw
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 self centered direct acting pressure relief valve for tractor hydraulic system comprising of valve seat with ball having required hardness, a spool guided by the adapter having a cavity matching the head of the valve seat and a clearance provided to reduce the difference between the opening pressure and the full-flow pressure; and a spring seat that transfers the load of the spring on the valve spool through a ball to eliminate the offset between the load line of the spring and the valve centerline.
2. A self centered direct acting pressure relief valve for tractor hydraulic system as claimed in claim 1, wherein the valve seat is a separate hardened and ground insert accepting the hardened and ground ball, each having very high hardness.
3. A self centered direct acting pressure relief valve for tractor hydraulic system as claimed in claim 1 or 2, wherein the valve seat is fitted in Adapter Body with transition fit & anaerobic adhesive & locked by inward crimping of machined lip to make it integral with Adapter Body.
4. A self centered direct acting pressure relief valve for tractor hydraulic system as claimed in any of the preceding claims, wherein said valve with a hardened ball is provided on a conical seat and retained by spool with conical seat for the valve ball where the ball is not captive but floating.
5. A self centered direct acting pressure relief valve for tractor hydraulic system as claimed in any of the preceeding claims wherein the spool in provided with a plurality of circumferential and longitudinal grooves.
6. A self centered direct acting pressure relief valve for tractor hydraulic system as claimed in any of the preceeding claims wherein said spring seat is supported on ball for transfer of load to spool using internal conical seats for both the spring seat and the spool to align the load through the ball centre thereby ensures self centering.
7. A self centered direct acting pressure relief valve for tractor hydraulic system substantially as herein described with reference to the accompanying drawings.