EP0160265B1

EP0160265B1 - Hydraulic control system

Info

Publication number: EP0160265B1
Application number: EP85104920A
Authority: EP
Inventors: Vinod Kumar Nanda; Henry Delano Taylor
Original assignee: Vickers Inc
Current assignee: Vickers Inc
Priority date: 1984-04-30
Filing date: 1985-04-23
Publication date: 1989-01-11
Also published as: IN164362B; AU4177085A; CA1234529A; AU583504B2; DE3567509D1; EP0160265A2; EP0160265A3

Description

This invention relates to a hydraulic control system according to the preamble of claim 1.
Such hydraulic systems are found, for example, on mobile equipment, such as excavators and cranes, and are used to control an actuator, such as a hydraulic cylinder or hydraulic motor. The actuator normally has two openings or ports to be used alternately as inlet or outlet.
A known system of that kind (US-A-4,201,052=GB-A-2,044,366) has several valves housed in a valve body designed to be mounted directly on the actuator. The valves comprise a hydraulically operated meter-in-valve, a pair of load drop check valves, a pair of hydraulically operated, normally closed meter-out valves, a pair of load pressure responsive valves, and a pair of anti-cavitation valves. The meter-in valve functions to direct fluid flow to one or the other of the actuator ports. Each a meter-out valve is associated with a pertinent actuator port and functions as a variable orifice metering fluid between the appropriate actuator port and a return line to tank. Each load pressure responsive valve acts on the respective meter-out valve to which it is associated and that in response to load pressure to enable the meter-out valves to also provide pressure relief protection. The anti-cavitation valves are associated with each of the actuator ports and are adapted to open the appropriate port to the return line.
The valve body is directly mounted to the actuator port manifold and is supplied by one full flow high pressure line, a pair of pilot pressure lines, and a load sensing line. The operation of the valves is controlled through the pilot lines from a manually operated hydraulic remote control valve which provides high or low pilot pressure. At low pilot pressure, the meter-in valve assumes a centered or neutral position with the check valves, the meter-out valves, the pressure responsive valves and the anti-cavitation valves all in closed position so that the valve system prevents uncontrolled lowering of loads, and in the case of overrunning loads, prevents fluid flow from the high pressure fluid source to the actuator even in the . event of a ruptured line. Due to the normally closed meter-out valves, such a hydraulic control system does not lend itself to free float, swing or coast the actuator.
In many applications, the need arises for such characteristics. In these applications the implement at the end of the cylinder or a swing device for a boom are allowed to coast to a stop due to frictional forces in the system.
Accordingly it is an object of the present invention to provide a hydraulic control system of the afore-mentioned type which is operable with free floating, swinging or coasting actuators.
This problem is solved in accordance with the teaching in claim 1.
Fig. 1 is a schematic drawing of a hydraulic control system for an actuator having two openings;
Fig. 2 is a sectional view of an embodiment of a modified hydraulic control system.
Referring to Fig. 1, the hydraulic control system embodying the invention comprises an actuator 20, herein shown as a linear hydraulic cylinder, having openings A, B and an output shaft 21 that is moved in opposite directions by hydraulic fluid supplied from a variable displacement pump 22 which has load sensing control 79 through 82 as is fully described in EP 85,962 A3. The hydraulic control system further includes a manually operated controller 23 that directs high or low pilot pressure through pilot port C1 or C2 to a valve system 24 for controlling the direction of movement of the actuator 20. Fluid from the pump 22 is directed through supply line 25 and port P to intake passage 26 and to a meter-in valve 27 that functions to direct and control the flow of hydraulic fluid to one or the other openings A, B through working lines having sections 32 and 33 which are connected to the openings A, B through load drop check valves 37 and 38, respectively.
The load drop check valves 37, 38 are formed as spring loaded poppet valves and separate the upstream section 32 or 33 from the downstream opening A or B of the working lines. Further spring-loaded anti-cavitation valves 39, 40 are opened when pressure in a return passage 36 to tank is higher than in the downstream opening A or B.
The pilot ports C1 and C2 are extended by pilot control lines 28, 30 and pilot control lines 29, 31, respectively, to the opposed ends of the meter-in valve 27. Depending upon the direction of movement of the meter-in valve 27, hydraulic fluid passes through working lines 32, A or 33, B to one or the other end of the actuator 20.
The hydraulic control system further includes exhaust means including normally closed meter-out valves 34d, 35d, each positioned between the openings A or B and the return passage 36. The meter-out valves 34d, 35d control the return flow or fluid to tank. In addition, the meter-out valves 34d, 35d can be designed as second stage of a relief valve, see Figure 2. To that end, spring loaded poppet valves 41, 42 (Figure 2) acting as pilot valves are associated with each valve 34d, 35d.
The system also includes a back pressure valve 44 connected to the tank port T and associated with the return passage 36. Back pressure valve 44 functions to minimize cavitation when an overruning or a lowering load tends to drive the actuator down. A charge pump relief valve 45 is provided to take excess flow above the inlet requirements of the pump 22 and apply it to the back pressure valve 44 to augment the fluid available to the actuator.
Meter-in valve 27 comprises a bore in which a spool is positioned, at low pilot pressure ("normally") the spool is maintained in a neutral position by springs and blocks the flow from the supply line 26 to the line sections 32, 33. When high pilot pressure is applied to either end of the spool, the spool moves until a force balance exists among the high pilot pressure, the spring load and the flow forces. The direction of movement determines which of the line sections 32, 33 is provided with fluid under pressure from intake passage 26. The single meter-in valve 27 may be replaced by two meter-in valves as shown in DE-A-3,011,088.
When high pilot pressure is applied to either control lines 28, 30 or 29, 31 leading to the meter-in valve 27 and to meter-out valves 34d or 35d, such valve is actuated to admit flow from the return working line section A or B to the return passage 36, whereas the other meter-out valve remains closed.
As is fully described in EP-A-0085,962 and shown in Figure 2, the meter-out valves 34d, 35d are of the poppet type and have control chambers 63a and 63b, respectively, which are connected to the openings A and B through orifices 62a and 62b, respectively, and can be vented by retracting a stem 65a and 65b, respectively, each is connected to a piston 67a and 67b, respectively. When pilot pressure is admitted through control line 28, piston 67a and stem 65a are moved and control chamber 63a vented so that pressure in the opening A opens meter-out valve 34d. Similar operation is carried out with pilot pressure in control line 29 and meter-out valve 35d.
As an additional feature, the exhaust valves 34d, 35d are also controlled by the poppet valves 41, 42, which are acted upon, on one side, by load pressure in the opening A or B, and, on the other side, by the same pressure, yet delayed. To that end, a restricted passage 72 through load drop check valve 37 leads to an accumulator volume 72a and to a spring cavity 41 a of the poppet valve 41. So poppet valve 41 is sensitive for sudden pressure rise in working line A and lowers the respond pressure (Ansprechwert) of the meter-out valve 34d for a short time. This is accomplished by venting the control chamber 63a or meter-out valve 34d to low pressure in return passage 36 via a passage 73a. A similar arrangement is with poppet valve 42 including another spring cavity 42b, accumulator volume 72b, orifice 62b and passage 73b. When the pressure rise has passed, poppet valve 41 or 42 returns in its normal position shutting off the passage 73a or 73b, so that control pressure in valve 34d or 35d is again built up.
When meter-in valve 27 is centered (low pilot pressure), restricted passages 27a, 27b in the valve spool connect pilot line 30 to working line section 32 and pilot line 31 to working line section 33.
Furthermore, in accordance with the present invention, a drain passage 90 is provided to connect upstream working line section 32 to the accumulator volume 72a and therethrough to the spring cavity 41 a of the poppet valve 41. So low pilot pressure is extended to spring cavity 41a, i.e. the pressure holding the poppet valve 41 closed is reduced and allowing to open valve 41, even if a relatively low load pressure (approximately 200 psi) is developed in downstream opening A. Control pressure in space 63a is vented through passage 73a, and the pressure in the downstream opening A is allowed to open the meter-out valve 34d to the return passage 36.
Thus it can be seen that addition of a simple passage 90 to the valve system 24 provides for a float or coast characteristic of the actuator movement in one direction.
If a float or coast characteristic is desired for the other direction of movement, a similar passage (92 in Fig. 1) between the upstream working line section 33 and the accumulator volume 72b is added which reduces the control pressure of the meter-out valve 35d in the neutral position of the meter-in valve 27.
According to the requirements, one or the other or both passages 90, 92 are used to lower control pressure for the valves 34d or 35d which are thus allowed to open in response to low load pressures in the opening A or B.
Whereas the meter-out valve 34d or 35d in the embodiment of Figure 1 is controlled by a single pilot source (the controller 33), the meter-out valve 34d or 35 in Figure 2 is controlled by so as to speak "two pilot sources", firstly the controller 23 and secondly the load pressure in opening A or B.

Claims

1. A hydraulic control system comprising

a pump (22) for supplying fluid under pressure,

a hydraulic actuator (20) having a movable element (21) and at least one opening (A) adapted to function alternatively as an inlet and an outlet for the fluid,

a line system (26, 32, 36) for connecting said pump (22) to said actuator opening (A) and to a tank and including a working line (32, A) and a return line (36);

meter-in valve means (27) positioned in said line system so as to shut off or to admit fluid to be supplied to said actuator opening (A) through said working line (32, A);

exhaust means including a (first) meter-out valve (34d) connected between said working line (A) and said return line (36) so as to admit fluid to flow to tank or to shut off such flow, and

a pilot controller (23) for applying fluid at high or low pilot pressure to said meter-in valve means (27) and said meter-out valve (34d) so as to determine the position of such valves,

said meter-in valve means (27) at said low pilot pressure being in its closed position and at said high pilot pressure in its admitting position,

said meter-out valve (34d) having a control chamber (63a) which is provided by a restricted connection (62a) with load pressure and can be vented by a pilot valve (65a, 67a) which is controlled by the pilot controller (23), said meter-out valve (34d) being of a construction so that the load pressure acts on the meter-out valve (34d) in the opening direction against the closing force exerted by the pressure in the control chamber (63a), characterized in that

a (first) drain passage (90, 27a) is provided to lower the pressure in the control chamber (63a) of the (first) meter-out valve (34d) when said meter-in valve (27) is in its neutral position, thereby allowing the meter-out valve (34d) to open.

2. The hydraulic control system according to claim 1, wherein said actuator (20) is double acting and has two actuator openings (A, B), said line system includes two working lines (32, A; 33, B) and said exhaust means includes a second meter-out valve (35d), characterized in that a second drain passage (92, 27b) is provided to lower the pressure in the control chamber (63b) of the second meter-out valve (35d) when said meter-in valve (27) is in its neutral position, thereby allowing the second meter-out valve (35d) to open.

3. The hydraulic control system according to claim 1 or 2, wherein each said working line (32, A; 33, B) includes a load drop check valve (37, 38) which, in the neutral position of said meter-in valve means (27), separates an upstream section (32, 33) of said working line from said opening (A, B) as a downstream section thereof, wherein at least a normally closed spring loaded poppet valve (41; 42) having a spring chamber (41 a, 42b) is urged in the opening direction by load pressure in the pertinent actuator opening (A; B) and in the closing direction by pressure in the spring chamber (41a, 42b), and is associated with a pertinent meter-out valve (34d, 35d) to control the pressure in the control chamber (63a, 63b) of said meter-out valve (34d, 35d), and wherein said drain passage (90, 27a; 92, 27b) has a first section (90, 92) connecting said upstream section (32; 33) to said spring chamber (41a; 42b) of said poppet valve (41; 42).

4. The hydraulic control system according to claim 3, wherein said spring chamber (41a; 42b) of said poppet valve (41; 42) is connected to an accumulator volume (72a; 72b) which is connected to said first section (90; 92) of said drain passage (90, 27a; 92, 27b).

5. The hydraulic control system according to claims 3 or 4, wherein said meter-in valve means (27), said meter-out valve or valves (32, A; 33, B) and said drain passage or passages (90, 27a; 92, 27b) are provided in a valve body (24).