US3584538A

US3584538A - Hydraulic system with improved changeover control unit

Info

Publication number: US3584538A
Application number: US797468A
Authority: US
Inventors: Alex Petersen
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1968-02-09
Filing date: 1969-02-07
Publication date: 1971-06-15
Anticipated expiration: 1988-06-15
Also published as: DK121958B; NL6901449A; GB1256601A; SE350569B; FR2001595A1; CH485117A; BE374653A; DE1628119A1; BE726259A

Abstract

A hydraulic system in which the reversal of direction of a hydraulic motor therein is accomplished by a changeover control unit which comprises a four-way, changeover valve having changeover passages connecting the motor with a fluid pressure supply line and a discharge line. A restrictor provided for controlling flow in the discharge line is variably opened and closed in response to operation of the changeover valve. A metering valve in the control unit is responsive to pressure drops across the restrictor and operates to bypass supply fluid to the discharge line downstream of the restrictor to change over without damage to the system.

Description

United States Patent [72] Inventor AlexPetersen Sonderborg, Denmark [21] Appl. No. 7974468 [22] Filed Feb. 7, 1969 [45] Patented June 15, 1971 [73] Assignee Danioss A/S Nordborg, Denmark [32] Priority Feb. 9, 1968 1 Germany P 16 28 119.9

[54} HYDRAULIC SYSTEM WITH IMPROVED CHANGEOVER CONTROL UNIT 9 Claims, 8 Drawing Figs.

[52] U.S.Cl 91/421, 137/117 [51] lnt.Cl ..F15bll/08, F15b13/04 [50] FieldofSearch 91/421; 137/117 [5 6] References Cited Primary ExaminerMartin P. Schwadron Assistant Examinerlrwin C. Cohen Attarneys- Robert E. Burns and Emmanuel J. Lobato ABSTRACT: A hydraulic system in. which the reversal of direction of a hydraulic motor therein is accomplished by a changeover control unit which comprises a four-way, changeover valve having changeover passages connecting the motor with a fluid pressure supply line and a discharge line. A restrictor provided for controlling flow in the discharge lineis variably opened and closed in response to operation of the changeover valve. A metering valve in the control unit is responsive to pressure drops across the restrictor and operates to bypass supply fluid to the discharge line downstream of the restrictor to change over without damage to the system.

PATENTEU JUN 1 5 mm SHEET 1 BF 2 INVEN'IOR. Alex Pefarsezz YP ZZ 1? I I k .L

PATENTED JUN: 5197i SHEET 2 BF 2 INVENTOR,

HYDRAULIC SYSTEM WITH IMPROVED CHANGEOVER CONTROL UNIT This invention relates generally to hydraulic motors and more particularly to a hydraulic system with a new and improved changeover control unit.

Hydraulic systems are known wherein the changeover valve for effecting motor reversal and the metering valve and restrictor are separate components The restrictor in these systems is located in the pressure line leading to the changeover valve. To effect a change of direction of rotation of the motor the restrictor must be first so adjusted that the quantity of fluid supplied to the motor is very small, before the changeover can be carried out. If care is not taken to do this, the system can be damaged. Furthermore, it is difficult to keep the speed of the hydraulic motor constant, for example when it is loaded rather heavily, during uphill travel, and considerable leakage occurs. The braking of the motor is also accompanied by problems, since, due to the inertia of the moving parts, the motor acts as a pump and may produce a reduced pressure between the closed restrictor and itself. The control arrangement for such systems is very expensive; it requires considerable space, numerous components and connecting passages.

A principal object of the invention is to eliminate the above disadvantages partially or completely.

According to the invention, this is achieved by connecting an adjustable part of a discharge flow restrictor to an adjustable part of a changeover valve, for reversing the motor, and arranging a metering valve in the changeover valve. In this way, joint adjustment of the restrictor and the changeover valve is achieved, so that not only is operation simpler, but no changeover can take place without a prior reduction of the quantity of fluid supplied to the motor. Furthermore, by combining the changeover valve, the metering valve and the restrictor, considerable spaceand expense are saved. In particular, many of the connecting passages extending from one device to another can be dispensed with.

It is particularly advantageous if the adjustable restrictor is connected in the discharge line of the hydraulic motor. When the restrictor is closed, heavy forced braking of the motor results because the line through which the motor seeks to discharge fluid, as a result of its pumping action is blocked. Furthermore, the motor has a very constant speed because the restrictor in the discharge line measures the fluid actually doing work in the motor but does not measure leakage fluid.

In this connection, it is particularly expedient if the restrictor is fully closed at that portion of two operating positions directly adjacent a neutral position. This causes a braking action to occur before the actual neutral position is reached and the neutral position itself can thus be free of any braking action. In particular, the two working passagescan be interconnected in the neutral position. This allows the motor to run light and this is very advantageous for many purposes.

For the mode of operation described hereinafter it is advantageous if two working passages of the control unit are each connected by a relief pressure valve, to the discharge line downstream of the restrictor. Thus when the motor is in the braked position, the pressure which occurs in the particular return line of the motor can be limited to a predetermined value by the pressure relief valve.

To enable the restrictor to better accommodate the leakages, it is advantageous for it to be influenced by the pump output pressure in such manner that its opening increases with rising pressure.

Furthermore, the changeover valve according to the invention comprises two concentric sleeves, rotatable relatively to each other, one of which sleeves forms the restrictor and has at least one orifice and the other of which sleeves forms a cover for the orifices in a peripheral or circumferential direction. This results in a particularly simple combination of a restrictor and a changeover valve. The cover in the circumferential direction is somewhat wider than the orifice so that a braking position is obtained on both sides of the neutral position at which the orifices are covered.

By using a sleeve normally employed for changeover valves and having changeover orifices that cooperate with longitudinal grooves in the other sleeve, the orifice in the restrictor is advantageously formed by extending at least one such longitudinal groove.

In a preferred embodiment, a piston, axially displaceable against a spring force, is disposed inside the inner sleeve. The end faces of the piston are acted upon by the pressure on the two sides of the restrictor and, together with an axially immobilized part, form the metering valve. Here, the longitudinal grooves connected to the restrictor carry out an additional function if they extend beyond the piston and are connected, by way of a transverse passage, to the end face chamber, beyond the piston.

Also, the longitudinal grooves connected to the restrictor can communicate with the pressure line in the neutral position of the changeover valve. This results in the metering valve being exposed to the full pressure of the system in the neutral position and accordingly it opens fully, i.e. directs the entire quantity of fluid delivered by the pump past the changeover valve.

In another form of the invention, provision is made for the free cross section of the orifice of the restrictor to increase continuously with the angle of rotation of the changeover valve and for the outer sleeve of the changeover valve to be axially displaceable so that the maximum free cross section of the orifice of the restrictor is adjustable. It is known to adjust the maximum free cross section of the orifice of a restrictor in order to adjust it relative to the pump output. In the present case, the outer sleeve of the changeover valve is made adjustable for adjusting relative to the pump output. A cam can be used as the adjusting element.

It has proved expedient for the outer sleeve of the control unit to be of increased diameter near an annular passage communicating with the pressure line, and. for a resilient element to be interposed between the adjusting element and the sleeve. This resilient element consists of, for example, a resilient ring fitted round an eccentric pin on the adjusting element. As a result of the increase in diameter, the sleeve is subjected to the influence of part of the entry or applied fluid pressure. Depending upon the return force of the resilient element, a condition of equilibrium is established which brings about a certain axial displacement of the sleeve and thus suits the cross section of the restrictor to the applied fluid pressure for compensation of leakage with the result that the speed of the fluiddriven motor remains constant.

Other features and advantages of the piston assembly in accordance with the invention will be understood as described in the following specification and appended claims, in conjunction with the following drawing in which:

FIG. 1, is a schematic diagram of a hydraulic system according to the invention;

FIG. 2, is a longitudinal section view through a control unit according to the invention illustrating a. combined changeover and metering valve and taken on section line ll-Il of FIG. 4;

FIG. 3, is a fragmentary section view taken on section line llI-III of FIG. 2;

FIG. 4, is a fragmentary cross section view taken on section line IV-IV of FIG. 2;

FIG. 5, is a fragmentary cross section view taken on section line V-V of FIG. 2;

FIG. 6, is a fragmentary cross section view taken on section line VI-VI of FIG. 2;

FIG. 7, is a fragmentary cross section view taken on section line VII-VII of FIG. 2; and

FIG. 8, is a fragmentary cross section. view taken on section line VIII-Vlll of FIG. 2.

In the drawings FIG. I illustrates a hydraulic system having a pump 1, which supplies fluid under pressure from a source 2 to a control device 4 through a pressure line 3. A reversible hydraulic motor 7 is connected to the control device through two

working passages

5 and 6.

A discharge line 8 connects the control device to the source 2,,through a return hydraulic fluid filter 9. A pressure relief valve 10 interconnects the pressure line 3 and the discharge line 8 and opens when a set pressure is exceeded. A metering valve 1 l is arranged in parallel with the relief valve and is controlled by the pressure drop at a regulatable restrictor 12. The metering valve 11 is connected to discharge line 8

portions

8a and 8b upstream of and downstream of the restrictor 12, by two stepping

lines

13 and 14 so that it will be controlled by the pressure drop across the restrictor 12.

A changeover valve 15 is provided for reversing the motor and is shown schematically. While the changeover valve is shown as axially operable it is constructed as a rotary valve as later described. It has five main positions, designated a through e, into which it can be positioned by outside actuating means, for example a hand lever or control 16. In a neutral position 0, the pressure line 3 is connected to the two working

passages

5 and 6, and a connecting passage 17, while a blocking element 18 blocks the discharge line 8. In two operating positions, a and e, the pressure line 3 and the discharge line 8 are each connected with one of the two working

passages

5 and 6. The changeover valve has two braking positions b and d. In these last two positions pressure line 3 is connected to one of the two working

passages

5 or 6, and blocking

elements

19 and 20 block the discharge line. In case excess pressure occurs in one of the working

passages

5 and 6 when the discharge line 8 is blocked, there is associated with each of these passages a

pressure relief valve

21, 22 which opens at a predetermined excess pressure and passes fluid under pressure to the discharge line 8, downstream of the restrictor 12, through a line 23.

The restrictor 12 is connected, through a mechanical coupling 24, to the movable part of the changeover valve 15 and is therefore likewise adjusted upon movement of the hand lever 16. Furthermore, the pressure applied in the pressure line 3 acts upon the restrictor through a stepping line 25. Finally, the restrictor 12 also forms the blocking

elements

18, 19 and 20 in the neutral position c and in the two braking positions, b and d, of the changeover valve 15.

This arrangement operates in the following manner:

The two working

passages

5, 6 are interconnected in the neutral position. The motor 7 can therefore run light. The restrictor 12, as the blocking element 18, keeps the discharge line 8 closed. All the fluid delivered by the pump 1 flows through the metering valve 11, forcibly opened in this condition, or through the pressure relief valve 10.

In the two operating control positions, a and e, the quantity of fluid supplied by the pump is divided by the metering valve; the rest flows through the motor. The opening position of the metering valve 11 is controlled by the pressure drop at the restrictor 12, through which all the fluid passing through the motor flows. The farther the lever 16 is moved towards one of its end or operating positions, the more the restrictor 12 opens and the smaller is the amount of fluid flowing past the motor through the metering valve 11. The increase in cross section of the restrictor is in linear relationship with the adjustment of the changeover valve. When the motor 7 is loaded more heavily, and the pressure in the line 3 therefore increases, increased opening of the restrictor 12 by way of the stepping line 25 occurs, so as to compensate internal leakages in the motor.

In the two braking positions, b and d, one side of the motor is still under pressure from the pump 1, but the other side is closed by the clocking

elements

19 and 20. Consequently, the motor is braked. If the increase in pressure is too great, fluid can flow into the discharge line 8 by means of a pressure relief valve, 21, 22, bypassing the restrictor 12.

In the embodiment illustrated the valve is actually a rotary valve and the neutral position c is limited to a rotary angle of 3 on each side of the zero line, and the next 0.5 corresponds to the braking position. Further rotation of the valve up to 20 leads gradually to the restrictor 12 being fully opened.

An embodiment of some of the arrangement is shown in FIGS. 2-7. In these figures the relief pressure valve 10, which is also housed in the control device 4, cannot be seen. In a housing 26 a through bore is closed by a cover 27. Within the bore a plurality of sleeves is arranged with the sleeves disposed concentrically one within the other. A first sleeve 28 is nonrotatable but is axially displaceable. A pin 29, which is mounted eccentrically on a screw 31 provided with an adjusting nut 30 and which has a resilient ring 32, engages in a complementary recess in the sleeve to preclude rotary motion but allows axial movement of the sleeve. A second sleeve 33 is axially fixed between the cover 27 and a retaining element 34, but it can be rotated. It is provided with a toothed portion 35 for receiving a toothed spindle for rotation thereof. A third sleeve 36 is axially held against the second sleeve 33 by means of a resilient spring 37 and a bearing edge 38. It can rotate with the second sleeve but need not do so. A fourth sleeve or piston 39 is pressed against a circlip 42 by a spring 40, which abuts against a plate 41 near the toothed portion 35. It can move axially towards the plate 41 as far as the clearance between the end of a pin 43 and the plate 41 will allow.

In a transverse bore 44 in the housing 26, and which is sealed by a plug 45, are located two spheres 45 and 47 which are acted upon by a spring 46 and correspond to the

relief pressure valves

21 and 22. The first sleeve 28 and housing define four annular chambers. An annular chamber 49 communicates with a threaded connection passage or bore 50 for the pressure line, and an annular chamber 51 communicates a threaded connection passage or bore 52 for the discharge line. Two intermediate annular chambers, 53 and 54, lead to passages 55 and 56 for communications with and connection to the working

passages

5, 6. Three

openings

57, 58 and 59 distributed equidistantly over the periphery connect each of the

annular chambers

49, 54 and 53 to the interior of the first sleeve 28.

The second sleeve has three fairly short longitudinal grooves 60 and, between these, three longer longitudinal grooves 61. The shorter longitudinal grooves 60 can connect the openings 57 with the

openings

58 and 59 respectively. The longer longitudinal grooves 61 perform the same function, but in addition extend as far as the annular chamber 51. There, they are substantially fully covered by tabs 62 on the first sleeve 28. Furthennore, openings 63 lead from the shorter longitudinal grooves 60 to the inside of the second sleeve 33, while openings 64 lead from the longer longitudinal grooves 61 to the interior. Openings 65 are provided as extensions of the shorter longitudinal grooves, which openings connect the inner chamber 66 of the bore to the annular chamber 51.

The third sleeve 36 defines near the openings 63, an annular chamber 67 with openings 68, and, near the openings 64, an annular chamber 69 with openings 70. Moreover, openings 71 are provided on the left-hand side, and these lead into the inner chamber 66. The piston 39 has a longitudinally extending annular chamber 72, which connects the openings 68 with the openings 71 in the third sleeve 36, when the sleeve 39 is axially displaced to the left.

In a right-hand portion 73, the first sleeve 28 is of somewhat greater diameter than in its left-hand portion. It is therefore biased to the right side of the drawing by the pressure obtaining inthe annular chamber 49, the axial displacement depending upon the return force of the resilient ring 32.

The pressure in the longer longitudinal groove 61 obtains each time in a chamber 74 to the right of the piston 39, since this chamber communicates with this groove through the opening 64, the annular chamber 69 and the openings 70. The pressure in the annular chamber 51 obtains each time in the inner chamber 66, since these two chambers are interconnected by the openings 65. Consequently, the piston 39 is subjected to the effect of the pressure drop at the restrictor, which is here formed by the longitudinal groove 61 and the tab 62. Consequently, the piston 39 moves a corresponding distance to the left against the spring 40 and the annular chamber 72 opens by the uncovering of a variable portion of the openings 71. The annular chamber 72 and the openings 71 form the metering valve, which communicates with the pressure line connection 50 through the openings 60, the annular chamber 67, the openings 63, the shorter longitudinal grooves 60, the openings 57 and the annular chamber 49, and communicates with the discharge line connection 52 through the inner chamber 66, the openings 65 and the annular chamber 51.

As can be seen from FIG. 3, by turning the second sleeve 33, the cross section of the longitudinal groove 61 is uncovered at a linear rate beneath the tab 62. Consequently, the restrictor opens at a linear rate with this turning movement. Since the tab 62 has a greater width than the groove 61, opening takes place only after the neutral position of the changeover valve has been left. By turning the screw 31, the first sleeve 28 can be axially displaced. Consequently, the end of the longitudinal groove 61 can be axially covered to a greater extent (right-hand side of FIG. 3) or to a lesser extent (lefthand side of FIG. 3). Thus, by adjusting the screw 31, the restrictor can be matched to the maximum cross section of the flow passage of the particular machine or motor connected in the system. It will be seen that this does not interfere with the linear increase in the opening of the restrictor. Furthermore, as already mentioned, the outer sleeve 28 is displaced axially against the return force of the resilient ring 32, in dependence upon the magnitude of the initial pressure, so that the restrictor opening can also be varied in dependence thereon.

FIGS. 2 to 7 show a condition which might correspond to a clockwise running or rotation of the motor and which coincides with the position a of the changeover valve. In this position, fluid under pressure flows through the connection passage and passes through the annular chamber 49, the openings 57, the short longitudinal groove 60, the openings 58, the annular chamber 54 and the connection passage 56 to the working passage 5 of the motor. From the motor, the fluid flows through the passage 6, to the connection passage 55 and then through the annular chamber 53, the openings 59, the longer longitudinal groove 61, the restrictor, the annular chamber 51 and the connection passage 52, to the discharge line. At the restrictor, there occurs a pressure drop in dependence upon which the piston 39 is displaced to the left and the pressure-regulating

valve

72, 71, is opened a varying extent, so that part of the fluid supplied is directed past the motor. In this way, an almost constant flow of fluid through the motor is maintained. Its magnitude can be changed by turning or rotating the second sleeve 33. The same applies in the case of counterclockwise turning when the fluid under pressure flows from the shorter longitudinal groove 60 to the annular groove 53 by way ofthe openings 59 and not the openings 58.

1f the second sleeve 33 is rotated toward the neutral position, then, shortly before the changeover opening, i.e. just before connection takes place between the

longitudinal grooves

60, 61 and the

openings

58, 59 is rendered noneffective, the left-hand end of the longitudinal groove 61 will have been fully covered by the tabs 62. No fluid can therefore continue to flow to the motor by the normal path. The motor is thus braked. The fluid can then flow only from the annular chambers 53 or 541, through the associated pressure relief valves 47 and 48 respectively, bypassing the restrictor 61, 62 and into the traverse bore 44 and then to the discharge line 8.

In the neutral position the two working

passages

55 and 66 are directly interconnected, since the

openings

58 and 59 connect the associated annular chambers 53 and 54 with two

adjacent grooves

60 and 61 respectively. At the same time, however, the connection between the shorter longitudinal groove 60 and the pressure line union 50 is maintained. The motor can therefore run light; the motor system is connected to the pump pressure line.

Since, however, the pump pressure also obtains in the longer longitudinal groove 61, this pressure is also applied to the chamber 74, so that the piston 39 is fully pushed to the left and the

metering valve

72, 71 is completely opened.

A neutral position spring 76 consisting of, FIG. 8, four spring plates 75, bears against both the first sleeve 28 and the second sleeve 33 in the nonoperating position. If the second sleeve is turned, the spring plates 75 are tensioned and permit the second sleeve to return to the nonoperating position again upon completion of rotation.

While preferred embodiments of the invention have been illustrated and described it will be understood that many modifications and changes can be made within the true spirit and scope of the invention.

What I claim and desire to be secured by Letters Patent is:

1. In a hydraulic system, a reversible fluid-driven motor, a source of pressure fluid for rotationally driving said motor, conduit means connecting said source of pressure fluid and said reversible motor comprising a pressure line to said motor and a discharge line from said motor, a changeover valve connected to said conduit means for reversing direction of rotation of said motor, means defining a variable restriction restricting fluid flow in said discharge line variably opened and closed in response to change of settings of said changeover valve to different operating position, a metering valve connected to said conduit means responsive to fluid pressure changes across said restriction bypassing at least some flow of fluid from said pressure line upstream of said motor to said discharge line in dependence upon pressure changes across said restriction, and said means defining said restriction comprising means responsive to pressure changes in said pressure line increasing opening of said restriction in response to increases of pressure in said pressure line.

2. In a hydraulic system, a reversible fluid-driven motor, a source of pressure fluid for rotationally driving said motor, conduit means connecting said source of pressure fluid and said reversible motor comprising a pressure line to said motor and a discharge line from said motor, a changeover valve connected to said conduit means for reversing direction of rotation of said motor, means defining a variable restriction restricting fluid flow in said discharge line variably opened and closed in response to change of settings of said changeover valve to different operating positions, a metering valve connected to said conduit means responsive to fluid pressure changes across said restriction bypassing at least some flow of fluid from said pressure line upstream. of said motor to said discharge line in dependence upon pressure changes across said restriction, said changeover Valve, said metering valve, and said means defining said restriction comprises at least some common parts defining jointly a unitary control device, control device comprising a plurality of coaxial sleeves relatively rotatable, one of said sleeves having a plurality of grooves providing communication between said pressure line, said motor and said discharge line for driving the motor in opposite direction, said means defining said restriction comprising means on another of said sleeves variably restricting flow through one of said grooves upon relative rotation of said sleeves, and means for relatively rotating said sleeves.

3. In a hydraulic system according to claim 2, in which said metering valve comprises a piston reciprocable in an innermost of said sleeves in response to variations in pressure drop across said restriction.

4. In a hydraulic system according to claim 3, in which a outermost of said sleeves defines said changeover valve and is axially movable relative to the other of said sleeves.

5. In a hydraulic system according to claim 4, including a manually actuated cam axially displacing said outermost sleeve.

6. In a hydraulic system according to claim 5, in which said cam comprises an eccentric pin received in a recess in said outermost sleeve, and a resilient ring about said pin.

7. In a hydraulic system, a reversible fluid-driven motor, a source of pressure fluid for rotationally driving said motor, conduit means connecting said source of pressure fluid and said reversible motor comprising a pressure line to said motor and a discharge line from said motor, a changeover valve con nected to said conduit means for reversing direction of rotation of said motor, means defining a variable restriction restricting fluid flow in said discharge line variably opened and closed in response to change of settings of said changeover valve to different operating positions, a metering valve connected to said conduit means responsive to fluid pressure changes across said restriction bypassing at least some flow of fluid from said pressure line upstream of said motor to said discharging line in dependence upon pressure changes across said restriction and said changeover valve comprising two concentric sleeves relatively rotatable, defining said means defining said restriction, one of said sleeves having a groove defining in communication with said discharge line, the other sleeve having means variably controlling flow through said groove in dependence upon the relative angular positions of said sleeves.

8. In a hydraulic system according to claim 7, in which said

Claims

2. In a hydraulic system, a reversible fluid-driven motor, a source of pressure fluid for rotationally driving said motor, conduit means connecting said source of pressure fluid and said reversible motor comprising a pressure line to said motor and a discharge line from said motor, a changeover valve connected to said conduit means for reversing direction of rotation of said motor, means defining a variable restriction restricting fluid flow in said discharge line variably opened and closed in response to change of settings of said changeover valve to different operating positions, a metering valve connected to said conduit means responsive to fluid pressure changes across said restriction bypassing at least some flow of fluid from said pressure line upstream of said motor to said discharge line in dependence upon pressure changes across said restriction, said changeover Valve, said metering valve, and said means defining said restriction comprises at least some common parts defining jointly a unitary control device, control device comprising a plurality of coaxial sleeves relatively rotatable, one of said sleeves having a plurality of grooves providing communication between said pressure line, said motor and said discharge line for driving the motor in opposite direction, said means defining said restriction comprising means on another of said sleeves variably restricting flow through one of said grooves upon relative rotation of said sleeves, and means for relatively rotating said sleeves.

7. In a hydraulic system, a reversible fluid-driven motor, a source of pressure fluid for rotationally driving said motor, conduit means connecting said source of pressure fluid and said reversible motor comprising a pressure line to said motor and a discharge line from said motor, a changeover valve connected to said conduit means for reversing direction of rotation of said motor, means defining a variable restriction restricting fluid flow in said discharge line variably opened and closed in response to change of settings of said changeover valve to different operating positions, a metering valve connected to said conduit means responsive to fluid pressure changes across said restriction bypassing at least some flow of fluid from said pressure line upstream of said motor to said discharging line in dependence upon pressure changes across said restriction and said changeover valve comprising two concentric sleeves relatively rotatable, defining said means defining said restriction, one of said sleeves having a groove defining in communication with said discharge line, the other sleeve having means variably controlling flow through said groove in dependence upon the relative angular positions of said sleeves.

8. In a hydraulic system according to claim 7, in which said one sleeve comprises other grooves providing flow paths in said changeover valve for effecting changeover of flow of fluid to said motor thereby to reverse direction of rotation thereof.

9. In a hydraulic system according to claim 8, in which said metering valve comprises a piston reciprocable internally of said concentric sleeves, and means providing communication of one side of the piston with fluid uPstream of said restriction and an opposite side of the piston with fluid downstream of said restriction variably moving said piston in response to changes in pressure drop across said restriction.