Power plants – Pressure fluid source and motor – Having apparatus control by timer or delay means
Reexamination Certificate
2001-01-12
2003-06-24
Lopez, F. Daniel (Department: 3745)
Power plants
Pressure fluid source and motor
Having apparatus control by timer or delay means
C060S460000, C060S464000, C091S038000
Reexamination Certificate
active
06581378
ABSTRACT:
The present invention relates to a valve device for at least one hydraulic motor suitable for driving a mass of large inertia, the motor being designed to be connected to a fluid circuit which includes two main ducts, namely a fluid feed duct and a fluid discharge duct, which ducts are suitable for being closed off when the motor is not operating, and an auxiliary duct in which the fluid pressure is lower than the feed pressure of the motor.
The mass that the motor serves to drive is referred to below as the “driven mass”.
The motor to which the valve device applies serves, for example, to rotate a turret on plant such as a hydraulic excavator, or to move in translation plant having tracks or tires of large mass.
It may be a hydraulic motor of the “fast motor” type (1000 to 2000 revolutions per minute (r.p.m.)) driving gearing, or a “slow motor” (whose rotary speed is about 100 r.p.m., for example), e.g. of the type having radial pistons.
In operation, a flow of fluid is maintained through the motor, the main ducts being connected to main orifices of the motor (serving for feed and for discharge), so that one of the main ducts is put under pressure so as to act as feed duct, while the other of the ducts is at a relatively lower pressure and is connected to fluid removal means so as to act as discharge duct.
Starting from a situation in which the motor is operating at a given drive speed, the motor is stopped by performing a deceleration stage, and then by closing off the feed and discharge ducts. During the deceleration stage, the pressure in the feed duct becomes low pressure, while the pressure in the discharge duct becomes high pressure. Finally, on closing off the main ducts of the motor, i.e. on isolating the motor, the fluid situated in the discharge duct is at a pressure that is higher than the pressure of the fluid situated in the feed duct. This phenomenon is further reinforced by the fact that, due to its large inertia, the driven mass tends to continue its initial movement.
On flat terrain, the system reaches equilibrium only when the pressures in the feed and discharge ducts are substantially equal. On sloping terrain, or when the driven mass is leaning, the system reaches equilibrium only when the difference between the pressures in the feed and discharge ducts reaches a given value (positive or negative) that makes it possible to compensate for the slope in order to hold the mass stationary.
In any event, in order for the motor and the driven mass to be actually stopped in a stable position, it is necessary for the pressure difference between the feed and discharge ducts to reach a given value, be it zero, positive, or negative.
It is indicated above that, on closing off the feed and discharge ducts, the discharge duct is at high pressure that is further increased by the inertia of the driven mass. This high pressure tends to push back the driven mass in a return movement in the opposite direction, thereby transferring to the feed duct (which is closed off) the high pressure of the discharge duct (which is also closed off).
In addition, the hydraulic fluid is slightly compressible. As a result, after the motor has been isolated, the inertia mass continues to move until the pressure in the discharge duct reaches a maximum value corresponding to the fluid present in said discharge duct being compressed. The return movement of the mass causes the pressure in the feed duct to increase until the fluid present in said feed duct is brought to a compression pressure that is substantially equal to the maximum pressure that prevailed in the discharge duct just before the return stage began.
Naturally, the return stage is followed by another movement stage in the initial direction, during which expansion takes place in the feed duct and compression takes place in the discharge duct.
Thus, after closing off the feed and discharge ducts, an oscillating movement is imparted to the driven mass, the frequency of oscillation for turrets of plant such as hydraulic excavators being about 1 Hz. Although the oscillating movement is of relatively low amplitude, and is finally braked naturally due to friction phenomena, it is clearly extremely inconvenient, in particular when the mass driven by the motor is to be placed in a very precise position by stopping the motor without mechanical braking.
Paradoxically, the oscillating motion phenomenon used to be less of a problem when the drive was provided by means of low-performance motors in which the relatively large leaks limited the compression in the feed and discharge ducts. Motors have gradually been perfected, in particular to improve efficiency, to reduce the duration of the acceleration stage, and to facilitate handling in difficult conditions, e.g. when leaning.
To limit the oscillations, i.e. to reduce their amplitude and finally to stop them, it is known that a damping system can be used, consisting in creating leaks between the feed and the discharge ducts, which leaks feed a transfer volume. After isolating the motor, it is possible to compensate, at least partially, for the pressure difference between the feed and discharge ducts by using the fluid available in the transfer volume.
Another system consists in allowing continuous leaks to take place between the feed and discharge ducts of the motor.
Those systems are not fully satisfactory insofar as they result in a reduction in the efficiency of the motor, in contradiction with the efforts that have been made to increase efficiency, and insofar as they make it almost impossible to position the driven mass accurately on stopping the motor. For example, when the motor serves to drive the turret of a hydraulic excavator, the turret actually stops with an angular offset relative to the target angular position in which the motor is to be isolated, the angular offset corresponding to the amount of fluid that is available in the transfer volume for being put into circulation.
EP-A-0 457 913 discloses a device seeking to prevent cavitation phenomena and to limit or reduce shocks when the motor driving a mass of large inertia is stopped.
The device comprises a valve which includes first and second main ducts serving to be connected to respective ones of the two main ducts of the fluid circuit, and an auxiliary duct serving to be connected to the auxiliary duct of said circuit, the device being able to have a first configuration in which, with the fluid pressure in the first main duct being greater than the fluid pressure in the second main duct, said second main duct is connected to the auxiliary duct, while the first main duct is isolated from the second main duct and from the auxiliary duct, and a second configuration in which, with the fluid pressure in the second main duct being greater than the fluid pressure in the first main duct, said first main duct is connected to the auxiliary duct while the second main duct is isolated from the first main duct and from the auxiliary duct.
The valve enables the main duct, which is at a low pressure when the motor is stopped, to be connected to the auxiliary duct, thereby preventing cavitation in said main duct. In other words, the valve serves only to select the duct which is at low pressure and to connect it to a booster pressure.
The device does not however prevent almost unbraked oscillations from occuring after the motor has been stopped. As a function of said oscillations, the valve passes alternately from its first configuration to its second configuration without rapidly limiting the pressure difference between the two main ducts.
An object of the invention is to remedy the above-mentioned drawbacks by providing a device that is simple and reliable, and that makes it possible to brake and to reduce to zero very rapidly the oscillations of the system after isolating the motor, regardless of the conditions under which the mass is being driven, in particular regardless of whether it is being driven on sloping or banking terrain, or on flat terrain.
This object is achieved by the fact that the valve device includes time delay means suit
Ladas & Parry
Lopez F. Daniel
Poclain Hydraulics Industrie
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