US2377278A

US2377278A - Apparatus for controlling fluid pressure

Info

Publication number: US2377278A
Application number: US417401A
Authority: US
Inventors: William T Stephens
Original assignee: HYDRAULIC CONTROL ENGINEERING
Current assignee: HYDRAULIC CONTROL ENGINEERING; HYDRAULIC CONTROL ENGINEERING Co
Priority date: 1941-10-31
Filing date: 1941-10-31
Publication date: 1945-05-29
Anticipated expiration: 1962-05-29

Description

May 29, 1945. w. T. STEPHENS APPARATUS FOR CONTROLLING FLUID PRESSURE Filed Oct. 31, 1941 3 Sheets-Sheet l r z ffl ATTORNEYS May 29, 1945- w. T. STEPHENS APPARATUS FOR CONTROLLING FLUID PRESSURE 3 Sheets-Sheet 2 Filed Oct. 31, 1941 lNVENTOR zz/mwf. 1% BY M W ATTORNEYS May 29, 1945. w. T. STEPHENS APPARATUS FOR CONTROLLING FLUID PRESSURE Filed Oct. 31, 1941 3 Sheets-Sheet 3 INVENTOR ATTORNEYS Patented May 29, 1945 APPARATUS FOR CONTROHJNG FLUID PRESSURE William '1. Stephens, Cleveland, Ohio, aslignor to Hydraulic Control Engineering C pany,

Cleveland, Ohio, a corporation of Ohio Application October 31, 1941, Serial No. 417,401

12 Claims.

The present invention relates to apparatus for controlling fluid pressure such as may be employed in hydraulic systems, comprising a hydraulic piston, a fluid pressure pump, and a fluid reservoir.

In the operation of certain types of road machinery and other equipment where double-acting hydraulic cylinders are used, it is necessary to control the lowering as well as the raising. That is to say, it is desirable to have the blade or other implement lower at the same speed at which it is raised. In order to accomplish this, the problem to overcome is that of keeping the piston from falling away from the pump and sucking a void on the pressure side of the hydraulic cylinder. This faulty operation is especially objectionable with a bulldozer when the control valve is put in the lowering position. Sometimes the blade at first will fall almost to the ground, and then it will hesitate in that position for a few seconds until the pump fills up the void in the hydraulic cylinder. Obviously, this causes jerky operation and prevents the attainment of a smooth fine grade which can be had with equipment of this type.

There are numerous ways of acquiring this desired positive control. The best previously known way was by use of the familiar type of closed system wherein the oil or other pressure fluid was pumped from one side of the hydraulic cylinder to the other, and a check valve was used in the line from the suction side of the pump to the reservoir tank so that it was impossible for any fluid to get back to the reservoir, with the result that the blade would not lower any faster than the speed with which the pump was operating. The inherent characteristics of the closed system are such that in the neutral position, oil

was by-passed practically from one side of the pump to the other. Because of this, and in view of the fact that a bulldozer is operated a good many minutes at a time in the neutral position, an enormous amount of heat was generated because of the relatively small quantity of oil being circulated. Another known way of controlling the lowering of the blade or implement is by throttling the lowering orifices to build up a back pressure that would hold the blade back. The objection to this procedure is that it requires some two hundred pounds per square inch pressure, thus requiring that the whole hydraulic unit do work by forcing oil through an orifice. Again, the great amount of heat generated in this manner is highly objectionable.

control with various types of flow control valves which, for the most part, are complicated and have been found unsuccessful.

In the present invention the advantages of two systems have been combined. The closed system is used in which in either of the power positions the fluid is taken from one side of the hydraulic piston to the other through the pump, but in the neutral position, fluid is withdrawn from the i0 reservoir tank and carried back to that tank, as

trol with much less heat than had been previously attainable.

One of the objects of this invention is to provide eflicient control mechanism for producin the novel functions which have been described.

Another feature of the invention resides in the positioning of a check valve on the delivery side of the pump through which valve pressure fluid is supplied to either end of the hydraulic cylinder.

Further novel features of improvement contributing to ease and accuracy in operation will be perceived and understood from reading the following detailed description of an illustrative embodiment of the invention in connection with the accompanying drawings, in which Fig. 1 is a side elevation showing in somewhat diagrammatic form the entire hydraulic system; Fig. 2 is a vertical, longitudinal section of the control mechanism in hold position Fig. 3 is a transverse vertical section taken on the line III-111 of Fig. 1;

Fig. 4 is a further transverse vertical section taken on the line IVIV of Fig. 2

Fig. 5 is a plan view of the control mechanism as seen on the line V-V of Fig. 1;

Fig. 6 is an additional transverse vertical section taken on the line VI-VI of Fig. 2; I

Fig. 7 likewise is a transverse vertical section at line VIIVII of Fig. 2;

Fig. 8 is a partial horizontal section showing a 4 check valve as viewed on line VIII-VIII of Fig. 3;

Fig. 9 is a vertical, longitudinal section of the controlmechanism in the raise position;

Fig. 10 is a vertical, longitudinal section of the control mechanism in lower position; and

Fig. 11 is a similar vertical, longitudinal section of the control mechanism in float position.

Referring to Fig. l, the system is shown to comprise a double-acting hydraulic ram 20 having a vertical cylinder 22 enclosing a piston 24 It has also been proposed to obtain the desired 56 carryinga piston rod 26 on its upper or inner side.

The cylinder 22 has upper and lower connections 2| and 66 which extend to a chambered valve housing 62 surmounted by a fluid reservoir tank 34. A conventional gear pump 66 has an inlet or return connection 66 with housing 62 and an outlet or supply connection 46 thereto. An operat ing lever 42 pivoted upon an extension 44 of valve housing 62 is provided with a link 46 pivotally connected to a valve stem 46.

Hold position In the neutral or hold position corresponding to the vertical, H location of operating lever 42, the valve parts will have the relative arrangement of Fig. 2. The valve stem 46 has moved its integral piston valve 66 through cylinder bore 6| so that the delivery end of pump 66, which has supply connection 46 to the upper right end portion 01' housing 62 and discharges to passageway 52, feeds through annular piston groove 64 to a passageway 66 opening directly into the oil reservoir tank 34. At the same time. oil from tank 34 is flowing through a housing port 66 into one end of an upper passageway 66 whence it is carried to return connection 36 leading to the inlet side of pump 66. The sequence of this circuitous flow is: pump 66 to chambered valve housing 62, through reservoir tank 24, back to housing 62, and finally again to pump 66. In the meantime, interconnection between the upper and lower end connections, 26 and 66' respectively, of cylinder 22 is blocked by a piston or land portion 62 of the valve 56; the intermediate passageway port 64 for supplying pressure fluid from the pump 36 to the lower connection 66 of ram cylinder 22 is closed by the same piston portion 62; and

the passageway port 66 which returns fluid to the pump 26 from the upper or rear end cylinder connection 26 is closed by an extreme left end piston portion 61 of valve plunger 66. The right end of passageway port 66 is likewise closed by a piston portion 66. p

The absence of upward acting pressure from port 66 permits a vertical control plunger 66, seated across the distant upper end of passageway port 66, to remain closed; and the downward pressure of fluid going from housing port 56 back to the pump 36 aids in maintaining the closed position.

It will be understood that the system is functioning as an open system in the hold position which has been described.

Raise position Fig. 9 illustrates the valve condition when operating lever 42 is in its raise or dotted R position (Fig. 1). A reduced connection section forming annular piston groove 16 adjacent piston portion 61 of valve plunger 50 afl'ords passage between upper or rear cylinder connection 26, passageway II, and passageway port 66 for returning fluid to pump 36 through control plunger 66. A still further annular piston groove 12, to the right of piston portion 62 admits pressure fluid from passageway port 64 for movement lengthwise of the valve cylinder bore 6| into passageway port 14 and delivery direct to the lower or front end connection 66 of hydraulic cylinder 22, to raise the bulldozer blade, or other implement.

This supply of pressure fluid to port 14 is by way of a large non-return check valve yieldable to pimp pressure, and designated generally as I6 in Fig. 5. The check valve 16 comprises a tapered hollow head 16 resiliently urged against a seat 66 formed in the valve housing 62 near the left end of a rearward passageway 62 leading from fluid pressure supply connection 46. A compressionsp f l 64 is received in the valve head 16 and may be adjusted by means of a threaded retaining plug 66 projecting externally of the chambered valve housing 62.

It is to be further observed that a valve piston portion 66 on the right end ohpiston valve 66 has moved inwardly from its hold position to close the supply passageway 62 from communi;

cation with passageway 66 leading to fluid reservoir tank 64.

As previously mentioned, the fluid returning to valve housing 62 from connection 26 leading to the upper end of ram cylinder 22 will flow through piston groove 16 and into passageway port 66 as piston 24 rises. As illustrated best in Figs. 2 and 6, an upper distant part of port 66 is arranged to surround the lower end or skirt 66 of the control plunger 66 in a way which permits fluid to lift it from the closed position of Fig. 2 to the open position of Fig. 9 or Big. 10. In the upward or open position, the head 62 of control plunger 66 will seat against and close housing port 66 communicating with reservoir 64, but through a. plurality of large lateral openings 64 just below the head, the returning fluid will enter the passageway 66 for discharge through connection 66 to the inlet side of pump 66. Since the volume of oil being displaced above piston 30 26 is less than that being added below the piston,

passage so left.

The system is functioning at this time as a closed system with the plunger or valve member 66 limiting the flow of hydraulic fluid from the reservoir tank 64 to the control valve means.

Lower position Referring to Fig. 10, the piston valve 66 is placed as shown for its lower position by moving operating lever 42 to the dotted L position of Fig. 1.

Valve piston portion 62 is now to the right of the point of entry of passageway port 64 into cylinder bore 6|, but still blocking communication with passageway port 14. Consequently, pressure fluid from pump delivery connection 46 into passageway 62 passes through the check valve I6 (as for raise position) and enters passageway Ii through piston groove 16; thence it is discharged through upper ram cylinder connection 26 to lcawer the blade (not shown) Joined to piston rod 2 Oil forced from the front or lower end of ram cylinder 22 enters connection 66 and then passageway port 14. From port 14 the oil or other fluid passes piston valve groove 12 and flows into the right end of passageway port 66 where it can raise and upwardly seat control plunger 66, as previously explained. In so doing the oil escapes through plunger openings 64 into pump return passageway 66.

By means of a small bore 66 through the control plunger head 62, the excess volume of returned fluid from the lower end of cylinder 22, due to the lesser fluid displacement at the upper piston rod (26) end, will be forced back into the tank 64. With piston rods on both ends of the ram piston, no such compensation would be required.

Float position In the float or by-pass position the piston valve II will have the relation to the bore 3| of chambered housing 32 which is illustrated by Fig. 11. This position is characterized by the direct communication of upper and

lower ram connections

20 and 30 through passageway II, valve plunger notch or groove I0, and passageway port I4, thus permitting oil to flow freely from one side of hydraulic piston 24 to the other.

As in the neutral or hold position, 011 enters passageway from housing port 00 communicating with oil reservoir 34 and flows to pump inlet connection 30. The flow control plunger 30 also is maintained in its downward closed position, and

Both ends of passageway port 80 are closed to the entrance of fluid by the piston ortions 01, 82, and 63, but by means of the relief valve I20, later described, excess pressure in passage 'II due to the difference in volume of the two ends of the motor cylinder will be relieved to passage 60.

Relief valves Referring to Figs. 4 and 5, it will be seen that the pressure side of the control apparatus is provided with a, relief valve I00 arranged transversely to unload predetermined and excessive pump pressure from passageway 82 through a by-passageway I02 into passageway 00, and back to the suction side of pump 36.

The relief valve I00 comprises a centrally bored seat member I04 of the same outside diameter as relief passageway I02. A screw I06 threaded vertically through housing 32, and accessible from its exterior, engages a cooperating lateral portion of seat member I04 and secures it in position within passageway M2 and adjacent passageway @2. A tapered valve head I having bore engaging guides I01 seats upon and closes the bored seat member I04 under the urging of a compression spring I08 carried on the inner end of a valve adjusting rod 0 threaded through a plug M2 threadedly secured in an outwardly opening counter bore H4 continuation of relief passageway 902. As shown in Fig. 4, a lock cap IIS and washer IIB maintain'the rod H0 in the desired position. This relief valve assembly is such as to require a minimum of working of the housing 32 and avoids concentric boring by use of the insertable valve seat member I04.

Figs. 2, 3, and 8 show a small check valve I20 which has an important function contributing to the efliciency of operation of the present controlling apparatus in the hold and float positions Figures 2 and 11 respectively.

It has been previously noted that the fluid displacements on opposite ends of the ram piston 24 are unequal by reason of the piston rod 26 on the upper or inner end of that piston. As a consequence, when the piston 24 and its connected tool or implement are lowered positively, less oil will be pumped into the top of cylinder 22 than is removed from the bottom by piston displacement. It will be recalled that this greater volume of withdrawn oil flows to port passageway I4 01' housing 32, and freely past valve piston groove I2 into the right end of passageway 03, having free access at this time to passageway 00 connected to the inlet side of pump 30. The excess supply of pressure fluid to the piston rod side of the system is relieved through the passage 33 in the head of the flow control valve which is now in the raised position.

In the hold or float positions the passageways I I, I4, communicating at all times with the opposite ends of cylinder 22 are either completely isolated or connected together. A short transverse, spur passageway I22 is in constant communication with passageway II and emergency communication with pump return passageway 00 through a rather small connection passage I24 forming the seat for a valve ball I26 of the check valve I20. The valve ball I20 is on the end of passage I24 adjacent passageway 00 and is normally held seated by a spring I20, retained in pomtion by a threaded plug member I30 closing.

the outside end of a counterbored continuation I32 of connecting passage I24. Therefore, as is apparent from Fig. 8, check valve I20 will open in one direction to return an excess of pressure fluid in passageway II to the pump return passageway 66, whereby it has access to either the pump inlet or the tank through passage 96. Valve I20 is set to open at a pressure greater than that required to open valve I00 so that the smaller "valve will never be opened during the lowering operation.

Conventional, fluid seal, packing assemblies I34 are disposed around the piston portions GI and 83 at opposite ends of housing 32 (Fig. 2) to provide suitable scaling for the reciprocable piston valve 50. The right assembly I34 constantly receives a supply of oil from passageway 50, and the left assembly I34 is connected by a small passageway I36 (Fig. 11) to the bottom of an isolated well space I38 opening into reservoir 34 through a large port I40,

It is to be understood that the above described embodiment of the invention is for the purpose of illustration only and that various changes may be made therein without departing from the scope and spirit of the invention.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A fluid pressure system comprising, a fluid pressure motor having a cylinder and piston; a hydraulic control mechanism connected to opposite ends of the cylinder of said fluid-pressure motor; a fluid-pressure pump having its inlet and outlet connected to said hydraulic control mechanism; a fluid reservoir for said hydraulic control mechanism; and selectively operable means for actuating said hydraulic control mechanism, said hydraulic control mechanism including a valve housing, ported passageways within said housing connecting the opposite ends of the cylinder of said fluid-pressure motor, passageway means formed and arranged to supply fluid from the outlet of the fluid-pressure pump alternatively to the said ported passageways for the said opposite cylinder ends, a check valve in said passageway means constructed and arranged to prevent return flow of the outlet of the fluid-pressure pump, a passageway for connecting the outlet of the fluid-pressure pump to said fluid reservoir, a conduit for connecting said fluid reservoir to the inlet of the fluid-pressure pump, a passageway formed and arranged to withdraw fluid alternatively from the said ported passageways for the said opposite cylinder ends, a conduit connecting the pump inlet to said reservoir, said withdrawing passageway having a connection to said last mentioned conduit, a valve responsive to pressure voir to the passageway for withdrawing fluidfrom the ported passageways for the said opposite cylinder ends.

3. The combination of claim 1 in which a compensating check valve is arranged to provide restricted discharge of an excess of fluid into said withdrawing passageway from one oi said ported passageways having connection with a cylinder end.

4. The combination of claim 1 in which a packing assembly surrounds oneend of the piston valve, and a fluid connection extends between the reservoir below the level of the liquid therein and said packing assembly to insure a fluid seal.

5. A fluid pressure system comprising, a fluid pressure motor having a cylinder and piston; a hydraulic control mechanism connected to oppodte ends of the cylinder of said fluid-pressure motor; a fluid-pressure pump having its inlet and outlet connected to said hydraulic control mechanism; and a fluid reservoir for said hydraulic control mechanism, said hydraulic control including a valve housing, ported M 1 s within said housing connecting the opposite ends of the cylinder of said fluidprusure motor, eway means formed and arranged to supply fluid from the outlet of the fluid-pressure pump alternatively to the said ported eways for the said opposite cylinder ends, a check valve in said passageway means constructed and arranged to prevent return flow to the outlet of the fluid-pressure pump, a w 1 way for connecting the outlet of the fluid-pressure pump to said fluid reservoir, a i a formed, and arranged to withdraw fluid alternatively from the said ported passageways for the said opposite cylinder ends, a conduit connecting the pump inlet to said reservoir, said withdrawing passageway having a connection to said last mentioned conduit, a valve responsive to pressure in said withdrawing passageway to open the connection to the said conduit and to simultaneously restrict flow from the reservoir to the pump inlet when in one position; and to connect said reservoir through said conduit to the pump inlet while blocking the withdrawing conduit in another valve position, a piston valve formed and arranged to close and open selectively the aforesaid passageways to move the piston of said fluid-pressure motor in either direction, said piston valve having alternate lands and passages spaced thereon to cooperate with the passageways to cause reservoir to pump to reservoir circulation when the ported passageways are separated from each other and from the outlet of the pump, or when they are freely connected together, and to cause cylinder to pump to cylinder circulation when either end of the cylinder is connected to the pump outlet.

6. A hydraulic control mechanism, comprisll ing in combination a valve housing; a fluid reservoir for supplying fluid to the valve housing; multi-ported valve means operable within said valve housing, a fluid-pressure pump, the valve housing being formed to provide connections to the pump inlet and outlet, connections within the housing for said pump inlet and outlet to opposite ends of a fluid-pressure motor, and open communication at several spaced points with said fluid reservoir, and said multi-ported valve means being constructed and arranged to control fluid flow between said connections to pass pressure fluid from the p outlet connection alternatively to either fluid-pressure motor connection and to pass fluid alternatively from the remaining fluid-pressure motor connection to the pump inlet connection when in one position; to connect the pump inlet connection to the pump outlet connection through said fluid reservoir and to connect the fluid-pressure motor connections with each other when in another position; a check valve in the connection between the pump outlet and the fluid-pressure motor connections; and a control plunger in the connection from the pump inlet to the motor and responsive to pressure therein, one of said reservoir communications leading to the pump inlet and said plunger being arranged to close the last mentioned communication under the action of said pressure and simultaneously to open communication between the pump inlet and motor exhaust.

7. A hydraulic control mechanism, comprising in combination a valve housing; a fluid reservoir mounted above the valve housing; a multiported piston valve operable within said valve housing, a fluid-pressure pump, the valve housing being formed to provide connections to the pump inlet and outlet, connections within the housing for said pump inlet and outlet to opposite ends of a fluid-pressure motor, and open communication at several spaced points with said fluid reservoir, and said multi-ported piston valve being constructed and arranged to control fluid flow between said connections to pass pressure fluid from the pump outlet alternatively to either fluid-pressure motor connection and to pass fluid alternatively from the remaining fluid-pressure motor connection to the pump inlet when in one position; to connect the pump inlet connection to the pump outlet connection through said fluid reservoir and to connect the fluid-pressure motor connections with each other when in another position; a check valve in the connection between the pump outlet and the fluid-pressure motor connections; and a control plunger in the connection from the pump inlet to the motor and responsive to pressure therein, one of said reservoir communications leading to the pump inlet, said plunger being positioned to close the pump inlet to reservoir communication under the action of said pressure and simultaneously to open communication between the pump inlet and the motor.

8. A hydraulic control mechanism, comprising in combination a valve housing; a fluid reservoir mounted above the valve housing; a multiported piston valve operable within said valve housing. a fluid-pressure pump, the valve housing being formed to provide connections to the pump inlet and outlet, connections within the housing for said pump inlet and outlet to opposite ends of a fluid-pressure motor, and open communication at several spaced points with said fluid reservoir, and said multi-ported piston valve being constructed and arranged to control fluid flow between said connections to pass pressure fluid from the pump outlet connection alternatively to either fluid-pressure motor connection and to pass fluid alternatively from the, remaining fluid-pressuremotor connection to the pump inlet when in one position, to connect the pump inlet connection to the pump outlet connection through said fluid reservoir and to connect the fluid-pressure motor connections with each other when in another position; a check valve in the connection between the pump outlet and the fluid-pressure motor connections; a control plunger in the connection from the pump inlet to the motor and responsive to pressure therein, one of said reservoir communications leading to the pump inlet and said plunger being arranged to close the last mentioned communication under the action. of said pressure and simultaneously to open communication between the pump inlet and motor exhaust, and a relief valve for discharging fluid from the fluid-pressure motor side of the check valve to the connection to the inlet of the fluid-pressure pump.

9. A hydraulic control mechanism, comprising in combination a valve housing; a fluid reservoir for supplying fluid to the valve housing; multiported valve means operable within said valve housing, a fluid-pressure pump, the valve housing being formed to provide connections to the pump inlet and outlet, connections within said housing for said pump inlet and outlet to opposite ends of a fluid-pressure motor having a double-acting pressure fluid from the pump outlet alternatively to either fluid-pressure motor connection and to pass fiuid alternatively from the remaining fluidpressure motor connection to the inlet to the fluidpressure pump when in one position; to connect the pump inlet connection to the pump outlet connection through said fluid reservoir and to connect the fluid-pressure motor connection with each other when in another position; a check valve in the connection between the pump outlet and the fluid-pressure motor connections; a control plunger in the connection from the pump inlet to the motor and responsive to pressure therein, one of said reservoir communications leading to the pump inlet and said plunger being arranged to close the last mentioned communication under the actionof said pressure and simultaneously to open communication between the pump inlet and motor exhaust, and means for compensating under any of the above connections for the unequal fluid displacements of the fluidpressure motor due to its piston rod.

10. An open and closed hydraulic system comprising in combination a double-acting hydraulic motor; a continuously operable hydraulic pump; a reservoir tank for hydraulic fluid; a control valve for the hydraulic motor; a conduit connecting the tank to the control valve; a conduit connecting the inlet side of the pump to the control valve; a conduit connecting the outlet side of the pump to the control valve; and conduits connecting the opposite ends of the hydraulic motor respectively to the control valve, said control valve including a housing, a valve member feed to provide fluid passage therepast at a plurality of points andmovable within the hous- M" said housing being formed with passages and ports arranged for selective cooperation with said valve member to place either end of the hydraulic motor in fluid communication with the. outlet side of the pump while the other end thereof is placed in fluid communication with the inlet side of the pump, and to provide for-flow from one end of the hydraulic motor directly to the other, a check valve arranged to control the entrance of high pressure fluid from the outlet side of the pump to the control valve, a fluid pressure actuated flow control valve member disposed within the housing of the control valve and constructed and arranged to automatically restrict flow of hydraulic fluid from the reservoir tank to the control valve when the hydraulic motor is being operated in either direction and to place the inlet side of the pump in communication with the reservoir tank when the control valve is in either a float position connecting the opposite ends of the hydraulic motor to each other or a hold position in which return flow from both ends Of the hydraulic motor is prevented.

11. An open and closed hydraulic system comprising in combination a double-acting hydraulic motor; a continuously operable hydraulic pump; a reservoir tank for hydraulic fluid; control valve means tor the hydraulic motor; a conduit connecting the tank to the control valve; a conduit connecting the inlet side of the pump to the control valve means; a conduit connecting the outlet side of the pump to the control valve means; conduits connecting the opposite ends of the hydraulic-motor respectively to the control valve means, said control valve means being constructed and arranged selectively to place either end of the hydraulic motor in fluid communication with the outlet side of the pump while the other end thereoi is placed in fluid communication with the inlet side of the pump, and to provide for flow from one end of the hydraulic motor directly to the other; a check valve arranged to control the entrance of high pressure fluid from the outlet side of the pump to the control valve means; and a flow control valve connected with the control valve means and constructed and arranged to operate independently of the control valve to restrict flow of hydraulic fluid from the reservoir tank to the control valve means when the hydraulic motor is being operated in either direction and to place the inlet side of the pump in communication with the reservoir tank when the control valve means is in either a float position connecting the opposite ends of the hydraulic motor to each other or a hold position in which retin'n flow from both ends of the hydraulic motor is prevented, the control valve means being arranged to place the outlet side of the pump in communication with the reservoir tank when the same is in float or hold positions.

12. An open and closed hydraulic system comprising in combination a double-acting hydraulic motor; a continuously operable hydraulic pump; a reservoir tank for hydraulic fluid control valve means for the hydraulic motor; a conduit connecting the tank to the control valve; a conduit connecting the inlet side of the pump to the control valve means; a conduit connecting the outlet side of the pump to the control valve means; conduits connecting the opposite ends of the hydraulic motor respectively to the control valve means, said control valve means being constructed and arranged selectively to place either end of the hydraulic motor in fluid communication with the outlet side of the pump while the other end theredf is placed in fluid communication with the inlet side or the pump; and a iiow.controi valve hydraulically connected with the control valve means and constructed and arranged to limit flow of hydraulic fluid from the reservoir tank to the control valve means when the hydraulic motor is being operated in either direction and to place the inlet side or the pump in communication with the