Rotary expansible chamber devices – Moving cylinder – Rotating
Reexamination Certificate
2002-12-26
2004-06-01
Denion, Thomas (Department: 3748)
Rotary expansible chamber devices
Moving cylinder
Rotating
C418S071000
Reexamination Certificate
active
06743005
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an improved hydraulic motor, and more particularly to a gerotor hydraulic motor having balance grooves configured for controlling hydraulic forces acting on a set of gerotor gears, so as to minimize frictional losses and maximize torque delivered to a load.
2. Description of the Related Art
Gerotor hydraulic motors are well known in the art. They comprise an inner gear and an outer gear, the axes of which are offset by a fixed distance. The inner gear is disposed interiorly of the outer gear and has exteriorly facing teeth that mesh with interiorly facing teeth on the outer gear. The outer gear is sized to have a sliding fit within a cylindrical housing. The inner gear is keyed to a driven shaft and meshes with the outer gear. The inner gear has one less tooth than the outer gear. The shape of the gear teeth is such that each tooth of the inner gear is always in sliding contact with a tooth of the outer gear. The resulting geometry creates discrete, multiple chambers that change from minimum to maximum and back to minimum volume for each rotation of the shaft.
A typical gerotor motor is driven by hydraulic fluid, received into a kidney-shaped chamber known as an inlet kidney port and discharged from a kidney-shaped chamber, known as an outlet kidney port. The flow of fluid past the inlet kidney port and into the gears causes rotation of the gear set as the gear chambers transition from minimum to maximum volume. The fluid is discharged through the outlet kidney port as the gear chambers transition from maximum to minimum volume. The hydraulic pressure drop between the inlet and outlet kidney ports varies from time to time as a function of resistive shaft torque, friction and volumetric displacement of the gear set. Further information regarding the construction and operation of gerotor devices may be found in Pareja. U.S. Pat. No. 4,199,305.
Gerotors may be used in pump applications, as well as in motor applications. In fact, gerotor pumps have a proven record of reliability and performance and are employed much more commonly than gerotor motors. One reason for this is the tendency of a gerotor motor to stall at initial start-up, even when no torsional load is applied to the motor shaft. Increasing the inlet pressure may help initiate rotation, but sometimes this only causes further binding of the shaft. Usually, a motor that begins to turn will continue to do so until the next time it comes to a complete stop.
Those skilled in the art will recognize this phenomenon as “hydraulic lock-up”, characterized by an unbalanced hydraulic force acting on one or both gerotor gears, resulting in high static friction. The frictional forces often increase as pressure increases, sometimes consuming all of the torque generated by the motor. If the motor does begin to rotate, the friction from the hydraulic imbalance reduces the motor's torsional efficiency and generates undesirable heat. This problem occurs in gerotor pumps, as well as gerotor motors. In that regard reference may be made to Pareja mentioned earlier herein.
FIG. 1
shows a typical radial pressure gradient in a prior art hydraulic gerotor motor. It may be observed that the inlet and outlet pressures act on the inner gear and cause a side load on the shaft. This load is supported by the shaft bearings. Torsional friction is minimal because of the small moment arm from the shaft axis to the shaft bearings. The inlet and outlet pressures also act on the outer gear and cause a similar side load against the housing gerotor bore. This can create significantly more torsional friction due to the larger moment arm. Note that there may be a starter-groove that ports fluid between the inside and outside of the outer gear. The purpose of this groove is to help balance the net radial pressure forces acting on the outer gear. Gerotor motor and pump manufacturers often use one or more starter-grooves. While these grooves offer limited improvement, experience has shown they do not provide consistent hydraulic balance required for a motor that starts reliably.
FIG. 1
shows why starter grooves are unreliable. Note that the radial pressure gradient varies from inlet pressure on the right side of the drawing (at the starter groove) to “some” low pressure on the left side of the drawing. The exact magnitude of the pressure is not defined except at the starter groove. Thus, for about 350 degrees of rotation, the pressure on the outside of the outer gerotor depends on radial and axial clearances, temperature and surface finish. If we find the sum of the hydraulic forces acting radially on the outside of the outer gerotor and add this to the sum of the hydraulic forces acting radially on the inside of the outer gerotor, the result should be near zero. Tolerances cause variations in the outside pressure gradient and the result is some will be poor starters. This is unacceptable for automotive cooling applications that must start every day, every time, at all temperatures for every motor produced.
Hydraulic balance is well known to engineers who design hydraulic pumps or motors.
Pumps are hydraulically balanced to reduce internal wear on rubbing parts and to minimize heat generation. This improves torsional efficiency. Pumps are typically driven by an electric motor and rarely (if ever) have a no-start problem as long as the motor can overcome the initial pump torsional friction. Once a pump begins to spin, a lubrication film builds up and tends to reduce rubbing friction. Note as well that typically hydraulic pressure is not generated until the pump begins to spin.
Hydraulic motors are especially sensitive to stalling unless they are “hydraulically balanced”. Note that the generated torque increases as pressure increases but the frictional torque also increases as pressure increases. If the frictional torque is equal to the generated torque, the motor will not spin. This is called “hydraulic lock” and is eliminated by hydraulically balancing the rubbing parts. However, prior to this invention there has been no fully satisfactory method for balancing gerotor motors. Existing gerotor balancing schemes have likely been aimed at gerotor pumps, not gerotor motors.
For years, engineers who design gerotor pumps and motors have attempted to balance them with “starter grooves” in the gerotor bore. A good example is found in starter grooves 44, 46 shown Pareja U.S. Pat. No. 4,199,305. These starter grooves represent the current “state-of-the-art” in gerotor pump and motor design and are commonly used in all designs. Unfortunately, they do not reliably minimize torsional friction and motors using these grooves will often stall.
FIG. 2
illustrates a typical axial pressure gradient in a prior art hydraulic gerotor motor and shows another deficiency of the prior art. Torsional efficiency is improved when the axial pressure gradient is the same on both sides of the inner and outer gears. This is particularly true of the outer gear since its moment arm to the shaft axis is larger than that of the inner gear. Often overlooked is the effect of radial leakage between the housing and cover plate. This leakage can be due to either an O-ring groove or to a low-pressure cavity. The leakage distorts the pressure gradient acting on the outer gear resulting in an axial pressure imbalance. A large undercut can similarly distort the axial (and radial) pressure gradient and further reduce torsional efficiency.
Another deficiency of the prior art is extreme sensitivity to gerotor/bore dimensional tolerances. Small variations in axial or radial clearances can dramatically change the critical radial and axial pressure gradients. In addition, temperature and surface finish can also cause wide variations in a motor's torsional efficiency and, ultimately, ability to initiate rotation.
The invention described herein addresses these deficiencies of the prior art and offers much improved motor starting capability. While this invention is primarily directed at gerotor motors, those skilled in the a
Jacox Meckstroth & Jenkins
Trieu Theresa
Valeo Electrical Systems, Inc.
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