GB2157400A - Reversing valve - Google Patents

Abstract

A valve for controlling a hydraulic cylinder (26) for reversing the work stroke, comprises housing (1) containing a sliding spool (3) with a through bore (4, 4') containing a spring-loaded valve body (6) which co-operates with a seat (7) formed in the bore (4, 4') of the sliding spool (3), said slide (3) having recesses (5, 9, 22, 23) to co-operate alternately with channels in the housing when the sliding spool (3) has assumed one or the other end position. The valve seat (7) and/or the co- operating valve body (6) is formed to provide an imperfect seal, there being a relatively narrow open passageway between the two members, such that a flow of fluid under pressure through bore (4, 4') of the sliding spool (3), produces a pressure drop causing slide (3) to be moved from one end position to the other end position. Sliding spool (3) co-operates with a spring- loaded (16) piston (13) having a through bore (14) of substantially smaller cross section than the slide's bore (4, 4'), to act as a throttling passageway, the open end thereof in one end position of the piston (13) seating against a shoulder (24) on the valve housing (1) for closing off the throttling passage orifice, which causes the slide (3) to be returned to its starting position as soon as the flow of pressure fluid past the valve body (6) ceases. The valve is flow- controlled, requiring only a slight drop in pressure (2-3 bar) over the seat (7) in the through bore (4, 4') of the sliding spool (3) for holding the sliding spool (3) in one end position. <IMAGE>

Description

SPECIFICATION Reversing valve The present invention relates to a reversing or changeover valve for controlling a hydraulic actuator for reversing the work stroke, comprising a slide valve housing having a system of internal flow paths including separate channels for connecting a pressure fluid tank and a pump as well as supply/return flow lines to the actuator, and provided with a sliding spool having a through bore containing a valve body which co-operates with a seat formed in the slide bore, and wherein the sliding spool is formed with channels, grooves or the like intended to co-operate alternately with the aforesaid channels in the valve housing when the sliding spool assumes one or the other of its two end positions.

A primaryfunction of valves of this type is to cause the hydraulic actuator, e.g. a hydraulic cylinder, rotary actuator or other hydraulic motor, to execute a complete work stroke from one extreme position to the opposite position and back again, for example for the purpose of reversing a connected work tool such as a reversible plough. If the actuator is in one extreme end position to begin with, the valve will cause the actuator to move to its other extreme end position, then reverse the actuator to return it to the original starting position. No matter which end position the actuator is in when the work stroke is initiated, the valve should cause the actuator to execute a complete work stroke back and forth and then stop in its original starting position. The actuator is then continuously supplied with oil through one and the same passage.

The most common changeover valves of this type are pressure controlled, i.e., they will cause the piston to reverse direction when the oil pressure reaches a predetermined value.

Therefore, part of the available working pressure must be reversed for control of the valve, and this reserved pressure actually represents a direct loss of working pressure. If, for example, an oil pressure of 1 50 bar is available, it is not unusual to reserve about 30 bar for control of the valve, and taking into account flow losses through the valve, this means that perhaps no more than 100-110 bar will be available for executing the work stroke.If the changeover valve is utilized for reversing the piston stroke in a cylinder which serves to pivot a reversible plough, it may be difficult to force the plough past the vertex of the arc of pivot, especially if the tractor is positioned slightly obliqueiy. If a pressure controlled valve is utilized in such cases, it will not reverse the piston direction until the oil pressure has risen to a predetermined value, and this means that the pivoting movement necessarily stops momentarily until the pressure has increased sufficiently to actuate the reversal.

It is the object of the present invention to alleviate the drawbacks of the prior art pressure controlled changeover valves, and to provide a valve which in addition to having a simpler construction than the prior art pressure controlled reversing valves, also reacts more smoothly and more quickly than these.

The reversing valve of the invention, as opposed to the prior art valves for similar purposes, is not dependent on the build-up of a high oil-pressure for causing the sliding spool to change position.

These and other objects are obtained with a reversing valve of the type specified above, wherein the seat in the through bore of the sliding spool and/or the co-operating valve body is formed in such a way as to prevent the valve body from sealing completely against the seat, there being a relatively narrow open passageway between the two members, whereby when a flow of fluid under pressure is introduced into the through bore of the sliding spool the valve body will ensure that a pressure drop occurs which will cause the spool to be moved from one end position to the other end position, and the spool is adapted to cooperate with a spring-loaded piston having a through bore of substantially smaller cross-section than the through bore of the spool such that the piston bore acts as a throttling passageway, the open end of the piston bore in one end position for the piston being adapted to seal against a suitably formed shoulder on the valve housing so as to close off the opening of the throttling passageway and cause the slide to be returned to its original starting position as soon as the flow of fluid under pressure past the valve body ceases.

Unlike the prior art pressure controlled reversing valves, which are dependent on a high oil pressure to function, the valve of the invention is flow-controlled and requires only a slight drop in pressure (2-3 bar) over the seat in the through bore of the sliding spool to hold the slide in one end position. Therefore, the available working pressure may be optimally utilized. The valve of the invention need not be set for a predetermined working pressure, either, because it will function equally well independent of the pressure up to the maximum pressure for which it is designed.

Other preferred features of the invention, which represent further refinements of the invention as outlined above, are claimed in the dependent claims.

The invention will be described in greater detail in the following with reference to an exemplary embodiment shown in the accompanying drawings, wherein: Figure 1 shows a reversing valve according to the invention in axial cross section, in connection with a hydraulic cylinder, Figure 2 shows a portion of Fig. 1 on a larger scale, and Figures 3 and 4 show the valve in axial cross section as in Fig. 1, but without the cylinder and with the valve's sliding spool/piston in different positions than in Fig. 1.

The reversing or changeover valve shown in the drawings, which is adapted to influence a hydraulic cylinder 26 for reversing the work stroke of the cylinder, for example for the purpose of pivoting an associated reversible plough (not shown), comprises a slide valve housing 1 having a system of internal flow paths including separate channels 18-20, 2, 17, 21 for connecting the pressure fluid tank T and pump P as well as the supply/return flow lines A, B to the cylinder. The sliding spool 3 has a through bore 4, 4' in which a valve body is provided, for example in the form of a spring-loaded 10 sphere 6, which co-operates with a valve seat 7 formed in the bore 4, 4'.The sliding spool 3 also contains channels 5, 9 and grooves 22, 23 at its periphery, adapted to co-operate alternately with the channels in the valve housing 1 in the two end positions of the slide 3.

The cylinder 26 has a piston 27, a piston rod 28 with an end 30, and a pivot point 29.

The longitudinal axis of cylinder 26 pivots between three positions: a1, in which the piston is in the fully extended innermost position (the starting position shown in Fig. 1); a2, in which the piston is in the fully retracted outer position; and a3, the changeover position in which the piston has returned to the original inner position. The longitudinal axes of the cylinder in each of the three positions is indicated by the respective broken lines in Fig. 1. The end 30 of the piston rod is articulated to a guide rod 31 having a fixed pivot point 32, and the guide rod is adapted to abut against an associated one of two contact surfaces 33, 34 in the respective two end positions a, and a3 of the cylinder 26 (in which the piston 27 is fully extended to its extreme inner position).The end 30 of the piston rod is connected in a manner not shown to, for example, a reversible plough, and serves to reverse the plough.

The margins of the valve housing 1 are indicated by broken lines. A pressure flow path 2 communicates with the sliding spool 3 via a section of passageway 25 of larger cross-sectional area than the pressure channel 2, but smaller than the channel 25' in which the slide 3 moves. The internal through bore of the sliding spool 3, in the illustrated embodiment, consists of two sections 4, 4' of somewhat different diameters, thereby forming a valve seat 7 at the interface of the two sections for a valve body in the form of a sphere 6 that is spring-loaded 10. The seat 7 as seen in Fig. 2 is formed with two symmetrical grooves 8 which prevent the sphere 6 from sealing completely against the seat 7.

Extending radially outward from the bore 4, 4' in the slide 3 are channels 5, 9 which open out at the outer periphery of the slide.

The opposite end of the compression spring 10 for the sphere 6 rests against a locking ring 11 which is inserted in a transverse peripheral groove in the slide 3. On the opposite side of the locking ring 11 is a second spring 12 which at the opposite end thereof bears against a piston 1 3 having a narrow through bore 14 terminating in an annular, sharp-edged end section 1 5. A spring 16, which is more powerful than the spring 12, holds the piston 13 in contact against the sliding spool 3.

As mentioned previously, the valve housing 1 has a system of flow paths leading from the slide channel 25'. One such channel 21 in Fig. 1 communicates with the valve's A-flow line, while a corresponding channel 1 7 communicates with the B-flow line. Other radial channels 18-20 in the valve housing 1 communicate with the tank flow line T.

As seen in Fig. 1, the A and B passageways are connected to the fluid supply/return line for the hydraulic cylinder 26, but the latter could be substituted by a rotary actuator or other hydraulic motor. The functioning of the reversing valve will in the following be explained in connection with a hydraulic cylinder, as illustrated in the drawings.

In the starting position as seen in Fig. 1, the cylinder and sliding spool are in one of the two main positions, namely the "right" end position. Oil under pressure is supplied through the flow path 2. The oil flows via the passageway section 25 through the bore 4 in the sliding spool 3. The oil will follow the path of least resistance, that is, flowing through the bore 4 over the seat 7, past the sphere 6 and further through the bore 14 in the piston 13, and passing from there via a radial channel 1 8 in the valve housing 1 to the tank T. When the oil flows past the seat 7 via the grooves 8, the sphere 6 will cause a predetermined drop in pressure to occur, which will be approximately constant within the pressure range for which the valve is designed. This pressure drop causes the sliding spool 3 to change position, so that the channel 9 in the slide 3 will then communicate with the channel 21 in the valve housing 1 and thus also with the A-flow path to the hydraulic cylinder. At the same time, the oil pressure will build up between the piston 1 3 and the sliding spool 3 owing to the substantially smaller cross section of the piston bore 14 compared to the bore 4, 4' in the slide 3, such that the bore 14 of the piston 1 3 acts as a throttling passageway.This pressure buildup is sufficiently great to cause the end section 1 5 of the piston 1 3 with the bore orifice to abut against a suitably formed shoulder 24 on the valve housing 1, thereby blocking the flow through the bore 14 to the tank flow path 1 8 (Fig. 3). The ensuing pressure buildup will increase until the pressure is high enough to cause the piston 27 to begin to move (be retracted) in the hydraulic cylinder 26. The oil will flow from the rod side of the hydraulic cylinder via the B-flow line through the channel 1 7 in the valve, passing via an annular peripheral groove 22 in the sliding spool 3 to the channel 20, which returns the oil to the reservoir T.As long as this piston movement within the hydraulic cylinder continues, the oil will flow past the seat 7 in the valve and maintain the necessary pressure drop for holding the sliding spool in the position shown in Fig. 3.

When the piston 27 in the cylinder 26 reaches the extreme outer (retracted) position, represented by the cylinder axis a2, the oil will cease to flow over the seat 7 and the pressure will be equalized by means of the grooves 8, so that the sliding spool 3 reaches a state of equilibrium. The spring 1 2 will then urge the slide back into its original position (Figs. 1 and 4), while the piston 1 3 will remain in its "left" end position (Figs. 3 and 4) sealed against the shoulder 24 owing to the oil pressure between the sliding spool 3 and piston 1 3. This is the position of the slide 3 and piston 1 3 shown in Fig. 4. When the slide 3 changes position, the volume between the slide and piston 1 3 increases.The small amount of oil required to fill this increased volume is supplied via the grooves 8.

The oil now flows through the channel 5 in the sliding spool 3 and the channel 1 7 in the valve housing 1 to the B-flow line for the cylinder 26, thereby allowing the cylinder to complete its work stroke (i.e., full extension of the piston 27 to the inner end position-long- itudinal axis a3). At the same time, the oil from the A-flow line of the cylinder 26 can flow through the channel 21 in the valve housing 1, passing through a second annular peripheral groove 23 in the sliding spool 3 and flowing via the channel 1 9 to the tank T.

When the piston 27 has returned to its inner end position in the cylinder 26, the complete back-and-forth work stroke has been executed. A new complete work stroke will not automatically be initiated because the end section 1 5 of the valve piston 1 3 with the through bore 14 is sealed against the shoulder 24 in the valve housing. Under these conditions, no oil flow over the valve seat 7 is possible, and such flow is necessary for establishing a pressure drop which will then cause the slide 3 to move. The piston 1 3 will stay in this position until the oil pressure drops in the pressure flow line 2.When this occurs, the spring 1 6 will guide the piston 1 3 back to the starting position (Fig. 1), and the valve is ready to control a new complete to-and-fro work stroke of the cylinder 26, which then moves from the a3-position to the a,-position shown in Fig. 1.

The invention is not restricted to the embodiment illustrated and described hereinabove, but can be varied, modified and/or supplemented within the scope of the claims.

It is thus an essential feature of the invention that a permanent passageway be established at the seat 7 past the sphere 6, but a passage of this type can be established by providing cooperating grooves in the seat and sphere or in the sphere 6 only, rather than forming the seat 7 with symmetrically-positioned grooves 8 as shown in the exemplary embodiment. In the drawings, the valve piston 1 3 is situated immediately behind the slide 3, which is a preferred embodiment but this is not an absolute requirement for the invention to function as it is designed to do. The piston 1 3 could be arranged in a channel at any location in the valve housing, as long as the piston 1 3 communicates with the same channels as shown in the drawings. In addition, the reversing valve of the invention can be supplemented by the inclusion of other known valves if there is a need for this. For example, a locking and/or shock valve could be made part of the valve. Because such auxiliary members have no effect on the functioning per se of the reversing valve, they are not included either in the drawings or in the description.

Claims (5)

1. A reversing or changeover valve, for controlling a hydraulic actuator for reversing the work stroke, comprising a slide valve housing having a system of internal flow paths including separate channels for connecting a pressure fluid tank and a pump as well as supply/return flow lines to the actuator, and provided with a sliding spool having a through bore containing a valve body which co-operates with a seat formed in the slide bore and wherein the sliding spool is formed with channels, grooves or the like intended to co-operate alternately with the aforesaid channels in the valve housing when the sliding spool assumes one or the other of its two end positions, wherein the seat in the through bore of the sliding spool and/or the cooperating valve body is formed in such as to prevent the valve body from sealing completely against the seat, there being a relatively narrow open passageway between the two members, whereby when a flow of fluid under pressure is introduced into the through bore of the sliding spool the valve body will ensure that a pressure drop occurs which will cause the spool to be moved from one end position to the other end position, and the spool is adapted to co-operate with a springloaded piston having a through bore of substantially smaller cross-section than the through bore of the spool such that the piston bore acts as a throttling passageway, the open end of the piston bore in one end position for the piston being adapted to seal against a suitably formed shoulder on the valve housing so as to close off the opening of the throttling passageway and cause the slide to be returned to its original starting position as soon as the flow of fluid under pressure past the valve body ceases.

2. A reversing valve according to Claim 1, wherein the sliding spool and piston are arranged in a common bore in the valve housing as extensions of each other, and are adapted to assume three main positions relative to each other, namely a "right" and a "left" end position in which the two members are in abutment, and a third main position in which the piston is in the "left" end position while the slide is in its "right" end position.

3. A reversing valve according to Claim 1, wherein the seat in the through bore of the slide is formed with grooves, preferably symmetrically disposed, for the purpose of preventing the valve body from establishing a completely tight seal against the seat.

4. A reversing valve according to Claim 1, wherein the return spring for the piston is more powerful than a return spring for the sliding spool.

5. A reversing or changeover valve substantially as hereinbefore described with reference to the accompanying drawings.