1. Field of the Invention
The invention is directed to an inertia load dampening hydraulic system wherein a second supply line keeps an exhaust line charged with pressurized hydraulic fluid.
2. Description of the Prior Art
Hydraulic motors in the form of linear hydraulic cylinders and rotary motors are used to move large bodies resulting in large inertial forces when the bodies are stopped. As the load is quickly stopped, oil on one side of the motor is forced over relief, and oil on the other side of the motor experiences cavitation. Fluid is directed to the cavitating side through anti-cavitation valves. In systems having closed center control valves there may be insufficient fluid to supply the cavitating side of the motor resulting in oscillation of the load as it is stopped.
One example of a machine that may experience this oscillation problem is a backhoe. A backhoe is provided with a pivotal boom which is attached to the vehicle by a swing frame. The swing frame is provided with a vertical pivot for pivoting the backhoe about a vertical axis relative to the vehicle. As the boom is quickly swung. and stopped the boom will oscillate . This oscillation is caused, by return fluid from the hydraulic swing cylinders being forced over the relief valves at high pressure as the closed center control valve closes. At the same time the supply side of the hydraulic swing cylinders experience a loss of fluid or cavitation. The high pressure developed on the return fluid side of the hydraulic swing cylinder now forces the boom back towards the cavitated side now building up pressure in that side. The newly generated pressure then pushes the hydraulic swing cylinders. This oscillating movement continues until the swing energy is dissipated and the boom oscillating motion stops.
It is an object of the present invention to provide an inertial load hydraulic dampening system for dampening the high inertial forces generated by a body being driven by a hydraulic motor.
It is a feature of the invention that pressurized hydraulic fluid is directed to the exhaust line through a pressure reducing valve to assure that the anti-cavitation circuit of the hydraulic motor is adequately supplied.
The hydraulic circuit for this system is provided with a source of pressurized hydraulic fluid that is directed through a first supply line to a control valve. From the control valve the fluid is directed to work lines to a hydraulic motor. In the example explained in the description below, the hydraulic motor is two hydraulic swing cylinders used to swing a boom on a backhoe. Exhausted hydraulic fluid from the hydraulic motor is directed through the control valve to an exhaust line having a back pressure check valve set at a first pressure level. The back pressure check valve maintains a specified amount of hydraulic pressure in the exhaust line adjacent to the control valve as directed by the set pressure level of the valve. The hydraulic motor is provided with a pressure relief valve and an anti-cavitation valve that are mounted in parallel with one another. The anti-cavitation valve is hydraulically coupled to the exhaust line. With a closed center control valve pressurized hydraulic fluid is not continually passing through the exhaust line, as such the back pressure set by the back pressure check valve may be much less than the pressure dictated by this valve. To keep the exhaust line fully charged a second supply line extends between the first supply line and the exhaust line. The second supply line is provided with a pressure reducing valve that is set at a second pressure level. The second pressure level of the pressure reducing valve is less than the first pressure level of the back pressure check valve.
In the preferred embodiment the hydraulic system is a PCLS (Pressure Compensated Load Sensing) system having a variable displacement pump used to supply pressurized hydraulic fluid. The hydraulic motor is a double acting hydraulic cylinder. In addition, the pressure reducing valve can be located in the valve stack for controlling the various operations of a machine.