HYDRAULIC SYSTEM WITH RESTRICTED INLET FLOW

Abstract: A hydraulic system comprised of a pump (10), a first valve (15) in the inlet (11) to the pump (10), and a second valve (18) which activates or deactivates a third valve (20) which permits the loading and unloading of the pump discharge. A motor (16) is provided to operate the first valve (15) for opening or closing the pump inlet (11). This motor (16) also actuates the second valve (18) in concert with the operation of the first valve (15) to open or close a vent port to the third valve (20) which opens or closes the passage (19) from the pump discharge to tank, located away from the pump inlet (11). This permits unloading the pump discharge during the restriction (23) of the pump inlet (11) and allows the loading of the pump discharge after the opening of the pump inlet (11). With the pump inlet (11) restricted (23), the system provides a low restrictive passage (23) for circulating the prescribed small volume of flow used to lubricate and cool the pump.

Description: This invention relates to hydraulic circuits utilizing a pump and an inlet valve which restricts the flow to the pump inlet according to the first part of claim 1 In a known system of that kind (US-A-3,935,917) the pump outlet has a check valve which is closed when the inlet valve is closed having a small amount of residual oil in the pump to provide lubrication. A flow regulator valve is connected to the output of the check valve and admits a regulated fluid flow to the working or load line, surplus fluid being bypassed to tank. When the inlet and the check valves are closed, such bypass is also closed. In such system, the residual oil in the pump is heated up and can attain high temperatures so that utilizing an inlet valve which does not close the pump inlet completely would be advantageous, that is, a small opening to admit a prescribed small volume of fluid to lubricate and cool the pump should be left free. This however is not compatible with the check valve feature of the known system. If, on the other hand, in such a hydraulic circuit, the pump outlet will be on load pressure, and pump spaces (as between the gear teeth) travelling back from outlet to inlet will carry pressurized fluid the resulting sudden decompression of the pressurized volumes into the inlet space and the ensuring cavitation will cause damage to the pump. In addition, the noise attributed to cavitation is extremely loud and usually intolerable. In addition, in such a hydraulic system, it is necessary that when the inlet valve is closed, the amount of flow permitted to enter the pump and its pressure be controlled to certain minimum limits so as to reduce the conditions for cavitation while the pump is operating. If this amount of lubricating/cooling flow and its pressure exceed a certain minimum, the pump will operate with noticeable cavitation. Accordingly, among the objectives of this invention are to provide a hydraulic system wherein excessive cavitation due to internal unloading the pump when the hydraulic system is loaded is obviated; wherever the amount of hydraulic fluid which is permitted to pass through the pump when the inlet valve is closed is controlled to reduce cavitation and to prevent high levels of noise; and wherein the pressure of the fluid which is circulated through the pump when the inlet valve is closed is controlled to assure quiet operation. In accordance with the invention, a hydraulic system comprises an inlet valve, said inlet valve has a feature for restricting the hydraulic fluid flow into the pump inlet when it is in its second position; a follow valve having a closed and an open position and being mechanically and hydraulically coupled to said inlet valve so that said first admitting position of said inlet valve corresponds to said closed position of said follow valve and said second closing position of said inlet valve corresponds to said open position of said follow valve; an unloading valve connected to the pump outlet and having a closed and an unloading position, said unloading valve being hydraulically connected to said follow valve and additionally allowing venting of said pump outlet directly to tank when said follow valve is in its open position. With the pump inlet restricted, the system provides a low restrictive passage for circulating the prescribed small volume of flow used to lubricate and cool the pump. It is observed here that in hydraulic systems it is not unusual to have a pressure relief valve. Such valves however maintain a certain preset pressure in the system and are therefore different as compared to the unloading valve according to the invention. Figs. 1, 2 and 2a are schematic diagrams showing the hydraulic system in different operative positions; Fig. 3 is a longitudinal sectional view of a valve arrangement utilized with the pump; Fig. 4 is a fragmentary sectional view on the enlarged scale of Fig. 3, when the inlet valve is in its second, flow restricting position; Fig. 5 is a view showing the inlet valve in a partially open position; Fig. 6 is a view showing the inlet valve in a fully open position; Fig. 7 is a fragmentary sectional view of a modified form of valve arrangement; Fig. 8 is a fragmentary sectional view of a modified hydraulic system; Fig. 9 is a fragmentary sectional view similar to Fig.8 in a different operative position. Referring to Fig. 1 which is a schematic of a hydraulic system embodying the invention, the hydraulic system comprises a hydraulic pump 10, which may be of various types such as a vane pump, piston pump, gear pump or the like, having an inlet 11 and an outlet 12 extending through a unidirectional valve 13 to a load line 14. An inlet valve 15 is provided at the inlet 11 and is operated by a motor 16, such as a pneumatic actuator, through a shaft 17 to move the valve 15 from its normally closed position, which really is a restricted position, to an open position corresponding to whether there is no load or a load on the system, respectively. A follow valve 18 is normally open and also operated by shaft 17. A fluid line 19 extends from outlet 12 to a normally closed unloading valve 20. A line 21 extends from unloading valve 20 to follow valve 18. An internal bypass forming a restrictor 20a is provided within unloading valve 20 so that there is restricted communication between lines 19 and 21 even when the unloading valve 20 is closed. A line 22 extends from follow valve 18 to inlet valve 15 so that when follow valve is opened, fluid will flow through line 19, restrictor 20a and lines 21, 22, mix with the cooler fluid from the reservoir or tank T and enter the pump through a restrictor 23 in valve 15. This restricted fluid cools and lubricates the pump even though inlet valve 15 is closed. Restrictor 23 may be in the form of a clearance in valve 15 or a restricted passage or both. Referring to Fig. 2 when the hydraulic system is to be operated and there is no load on the system, pressurized air from a line 24 operates the motor 16 to open the valve 15 permitting the full flow of fluid through the inlet line 11 to the pump 10 and, in turn, to the outlet line 12 and unidirectional valve 13 to the load line 14. Simultaneously, the valve 18 is closed preventing flow through restrictor 20a and line 21. Since there will be no pressure drop across the valve 20, the spring of unloading valve 20 will maintain the valve 20 in the closed position. So the pump 10 can be run in its normal manner to feed fluid into the load line 14. When during such condition the decision is made to discontinue providing pressure fluid, the motor 16 is operated to close the valve 15 and to open the follow valve 18 (see Fig. 2A). There is still a load on outlet 12 so that the system pressure in line 19 will provide a pressure differential across unloading valve 20 to open valve 20 permitting the fluid to flow to the reservoir T and thereby immediately unloading the pressure in line 19. Also the follow valve 18 which is moved to its open position allows a flow through lines 21, 22 to tank T. The inlet valve 15 is not completely closed in that the restriction 23 is effective thereby avoiding the excessive cavitation that would result in the operation of the pump with no fluid provided thereto and, in turn, the possible damage to the pump. Referring to Fig. 3, in a preferred embodiment, the valve 15, motor 16, follow valve 18 and unloading valve 20 are preferably provided in a single unit, shown schematically in broken lines in Figs. 1 and 2. As shown in Fig. 3, the valve 15 includes a body 25 which has a surface 26 adapted to be mounted adjacent the inlet 11 of the pump 10. The valve further includes an inlet opening 27 (forming line 22) and a passage 28 extending to the inlet 11 of the pump 10. A valve element 29 is provided in the passage 28 and functions to restrict the flow in the normally-closed position by a small clearance as at 23a permitting fluid to flow into the pump 10 for lubrication and cooling. An orifice 23b in element 29 can be used to supplement the clearance. Valve element 29 is mounted on shaft 17 fixed to a piston 31 operating whithin a cylinder 32 of motor 16 and yieldingly urged by a spring 33 to the left as viewed in Fig. 3, to close the passage 28. Cylinder 32 includes a head 32a having a passage 34 extending to one side of the piston 31 to which pressurized air is supplied through the line 24 for moving the piston to the right as viewed in Fig. 4. The follow valve 18 and the unloading valve 20 comprise a partially common body 35 having a bore 36 in which a spool 37 is positioned and yieldingly urged by a spring 38 downwardly against a stop 37a as viewed in Fig. 3. Spool 37 includes the restrictor 20a as a sized passage that functions to provide a restricted passage for the lubrication/cooling flow when the valve 15 is in closed position and provide a pressure drop to position spool 37 to unload the pump displaced fluid to tank when valve 15 starts to open the pump inlet 11. The follow valve 18 comprises a tubular valve element 18a surrounding the shaft 17 and yieldingly urged by a spring 43 to the left as viewed in Fig. 3 against a shoulder 39 in the body 25. The tubular valve element 18a is spaced from the shaft 17 to define a space 22a which forms a portion of line 22 in Figs. 1, 2, whereas vent port or passage 21a is a section of line 21. Flow through spool 37 passes from opening 19a which is a section of line 19 through passages 20a, 21a, 22a to mix in inlet opening 27 with the fluid from the reservoir. Unloading valve 20 also provides a means for decompressing the discharged volume prior to closing of the pump inlet 11 by valve 15. As valve 18 opens vent port 21a, valve 20 opens and directs the pump outlet to reservoir T away from the pump inlet 11. When valve 15 closes the pump inlet 11, there is insufficient flow to keep valve 20 open and spool 37 will return to closed position. In the closed position, the passages as 20a in valve 20 are sufficient for hot fluid from lines 12, 19 to pass through lines 21, 22 into the cooler space 27 without affecting the internal pressure balance. In the normally closed position shown in Figs. 3 and 4, fluid is not permitted to flow to the pump 10 except for a small portion of fluid through the clearance 23a and/or orifice 23b for lubrication and cooling. When the motor 16 is actuated the valve element 29 is moved to the right, as viewed in Figs. 5 and 6, permitting the fluid to flow to the inlet 11 of the pump 10. When the system is without load, the pressure in line 19 is low and the follow valve 18 is moved to close the vent port 21a (Fig. 6). If the motor 16 is operated to open the valve 15 while there is a load on the system, initially the valve element 29 will move to a partially open position shown in Fig. 5. Any initial increased pump outlet will follow the route of the lubricating and cooling flow through lines 19, restrictor 20a, line 21 and space 22a of valve 20 and valve 18. Because of the increased flow and the size of passage 20a, the resulting pressure difference will cause valve spool 37 to act against the spring 38 and open the tank passage 40 for bypassing the increased discharge volume until the motor 16 completes its motion and closes valve 18. After this, the entire pump outlet is available for doing work in the hydraulic system. In the absence of valve 20, the increased flow would enter valve 18 and create a large pressure drop across valve element 18a due to the restricted flow through space 22a. Such large pressure drop would inhibit the closing of valve 18 by motor 16. During this normal operation, the valve element 29 is permitted to move to a closed position, the follow valve 18 is returned to the position shown in Fig. 4 and the unloading valve 20 functions to unload line 19 to prevent pressure build-up in the pump discharge and to prevent cavitation damage to the pump. It can thus be seen that there has been provided a hydraulic system wherein excessive cavitation due to unloading the pump when the hydraulic system is still loaded is obviated; wherein the amount of hydraulic fluid which is permitted to pass through the pump when the inlet valve is closed is controlled to prevent high levels of noise; and wherein the pressure of fluid which is circulated through the pump when the valve is closed is controlled to assure quiet operation. In the modified form shown in Fig. 7, instead of having a bore in the valve element 37, as in Fig. 3, the valve body 20b is formed with a bypass passageway 41 extending between opposite ends of said valve element 37 and having a restriction 42 therein which functions in the same manner as passage 20a. Referring to Figs. 8 and 9, in order to apply the present invention to a dual pump hydraulic system, the valve of Fig .3 is provided with a T connection 45 having passages 46, 47 connected by lines 48, 49 to dual pumps 50, 51 with common inlet and driven by common motor 52. A shuttle valve 53 in the form of a ball functions to apply the higher pressure from the discharge line 48, 49 depending upon which of the discharge lines has the higher pressure. This applies the higher pressure to the unloading and follow valves. When both pumps circuits are decompressed the ball will center itself as shown in Figs. 8 and 9 and provide passages for bypassing the cooling flow to both pumps when there is no load on either of the pumps. By this arrangement it is possible to utilize a single valve system for dual pumps with common inlet and driven by the same motor.

Claims: A hydraulic system comprising a pump (10; 50, 51) having an inlet (11) and an outlet (12); an inlet valve (15) having a first position for admitting fluid to the pump inlet (11) and a second position for closing the pump inlet (11); a motor means (16) for moving said inlet valve (15) into said first or second positions; characterized in that said inlet valve (15) has a feature (23, 23a, 23b) for restricting the hydraulic fluid flow into the pump inlet (11) when it is in its second position; a follow valve (18) having a closed and an open position and being mechanically and hydraulically coupled to said inlet valve (15) so that said first admitting position of said inlet valve corresponds to said closed position of said follow valve and said second closing position of said inlet valve corresponds to said open position of said follow valve; an unloading valve (20) connected to the pump outlet (12) and having a closed and an Unloading position, said unloading valve (20) being hydraulically connected to said follow valve (18) and additionally allowing venting of said pump outlet (12) directly to tank when said follow valve (18) is in its open position. The hydraulic system set forth in claim 1 including means (37, 40) for diverting the unloading flow away from the follow valve (18) during the initial opening of the inlet valve (15) to prevent the over pressurization at the follow valve (18) and facilitate the closing of the follow valve (18) by the motor means (16). The hydraulic system set forth in claim 1 or 2 wherein said motor means (16), said inlet valve (15) and said follow valve (18) are constructed and arranged such that said follow valve (18) is operable by said motor means (16) to open prior to the inlet valve (15) reaching its second position. The hydraulic system set forth in any of claims 1 to 3 wherein said unloading valve (20) has restricted passage means (20a) which is provided for returning the flow back through said follow valve (18) to the pump inlet (11). The hydraulic system set forth in any of claims 1 to 4 wherein said inlet valve (15) comprises a valve body (25) adapted (26) to be connected to said pump inlet (11), said inlet valve (15) having an inlet valve element (29) which takes said first position for admitting fluid to the pump inlet (11) or said second position for closing the pump inlet (11), said follow valve (18) comprising a second body (35) mounted on said inlet valve body (25), said motor means (16) comprising a body (32, 32a) mounted on said follow valve body (35) and having a motor element (17) connected to said inlet valve element (29) for operating same, said follow valve (18) comprising a tubular valve element (18a) mounted on said motor element (17) and adapted to be moved by said inlet valve element (29) to take said closed or opened positions and means (38) yieldingly urging said unloading valve (20) into its closed position. The hydraulic system set forth in claim 5 including means (43) yieldingly urging said follow tubular valve element (18a) into said open position of said follow valve (18) and in the direction of said valve element (29). The hydraulic system set forth in any of claims 1 to 6 including a first pump (50) and a second pump (51) and means (53) for sensing the outlet pressure of said first pump (50) and second pump (51) and applying the higher outlet pressure to said unloading valve (20). The hydraulic system set forth in any of claims 1 to 7 wherein said unloading valve (20) includes a restricted passage (20a; 41, 42), a valve body (35, 20b), a bore (36) and a spool (37) sliding in said bore (36) between said closed and unloading positions depending upon pressure differential due to fluid flow through said restricted passage (20a; 41, 42) to said follow valve (18). The hydraulic system set forth in any of claims 1 to 8 including a tank line (22) connecting the outlet of said follow valve (18) to the inlet of said inlet valve (15). The hydraulic system set forth in claims 5 and 6 wherein said motor element is a common shaft (17) of said inlet and follow valves (15, 18), said inlet valve element (29) is mounted on said common shaft (17), said tubular valve element (18a) is spaced from said shaft (17) defining a passage (22a) to an inlet space (27) of said inlet valve (15). The hydraulic system set forth in claim 8 wherein said unloading valve (20) comprises a tank passage (40) in said body (35) which is directed away from the pump inlet (11, 28). The hydraulic system set forth in claim 8 wherein said restricted passage (20a) is through said spool (37). The hydraulic system set forth in any of claims 1 to 12 wherein said feature (23, 23a, 23b) for restricting fluid flow is a clearance (23a) between valving element (29) and valve body (25) and/or an orifice (23b) in valving element (29). A method of operating the hydraulic system of any of claim 1-13 wherein said inlet valve (15) and said unloading valve (20) are normally closed valves and said follow valve (18) is normally open and wherein the hydraulic system has a discharge line (19) connected to a load line (14) through a unidirectional valve (13), characterized in that, if there is no load on the system, said motor means (16) is operated t close the inlet valves (15) and to open the follow valve (18), however, when there is a load on the hydraulic system and the motor means (16) is operated to open the inlet valve (15), the follow valves (18), for the present, remains open and the inlet valve (15) in a transition position (Fig. 5) providing communication between the unloading valve (20) and tank so that a fluid flow is flowing from the discharge line (19) through the unloading valve (20) and thereby causes a pressure differential across the unloading valve (20) to open same and lower the pressure in the discharge line (19) so that the inlet valve (15) can reach its fully open position and the follow valve (18) its closed position.

Inventors: NIEMIEC, ALBIN JOSEPH
Applicants: VICKERS, INCORPORATED
Application year: 1988
Main classification: F04B49
IPC codes: F04B49