Power transmission in a hydraulic system is a product of flow and pressure. Therefore, the power rating of a hydraulic pump or actuator is a product of its ability to produce or absorb flow, as well as its maximum pressure rating. But what happens to hydraulic components if that pressure control device is screwed in an extra quarter turn and operating pressure exceeds design limits?

Even though the system won't necessarily blow up with a bang, damage can be caused by overpressurization that manifests itself in a number of ways, including:

  • Loss of lubrication
  • Valve plate separation
  • Mechanical damage

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Figure 1. Separation of Cylinder Barrel and Valve Plate due to Overpressurization (Bosch Rexroth)

Loss of Lubrication
One variable that impacts the creation and maintenance of hydrodynamic films is load. Hydrostatic force and therefore load is the product of pressure and area (F = P x a). This means that increasing operating pressure increases the load on lubricated surfaces. If operating pressure exceeds design limits, loads can increase causing boundary lubrication and two-body abrasion to occur - all other things equal.

Cylinder Barrel and Valve Plate Separation
In axial and bent axis piston pump and motor designs, the cylinder barrel is hydrostatically loaded against the valve plate. To maintain full-film lubrication between the rotating cylinder barrel and the stationary valve plate, the hydrostatic force holding them in contact is offset by a hydrostatic force acting to separate the parts. This is achieved by making the effective area of half the total number of piston bores slightly larger than the effective area of the pressure kidney in the valve plate.

The higher the operating pressure, the higher the hydrostatic force holding the cylinder barrel in contact with the valve plate. However, if operating pressure exceeds design limits, the cylinder barrel will separate from the valve plate (Figure 1).

Design geometry prevents a perfect alignment of the opposing hydrostatic forces. This misalignment creates a twisting force (torque) on the cylinder barrel. During normal operation, torque is supported by the drive shaft (axial designs) or center pin (bent axis designs). If operating pressure exceeds design limits, the magnitude of the torque created causes elastic deformation of the drive shaft or center pin. This allows the cylinder barrel to tilt, bearing hard against the outlet side of the valve plate and separating from the inlet side. Once separation occurs, the lubricating film is lost and the resulting two-body abrasion damages (scores) the sliding surfaces of the cylinder barrel (Figure 2) and valve plate (Figure 3). Erosion of the kidney area of the valve plate can also occur as high-pressure fluid escapes into the case at high velocity. This surge of flow into the case can cause excessive case pressure, resulting in shaft seal failure.

Note that separation can also occur at operating pressures within design limits due to distortion (loss of flatness) of the valve plate, overspeeding or excessive wear of the cylinder barrel drive-spline in axial designs.

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Figure 2. Scoring of Cylinder Barrel Caused by Separation

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Figure 3. Scoring of Valve Plate Caused by Separation

 

Mechanical Damage
Overpressurization can result in hose and seal failure, and mechanical failure of certain parts in pumps, motors, cylinders and valves. Excessive pressure in axial pump or motors has the same effect as excessive vacuum at the inlet. Both conditions put the piston-ball and slipper-pad socket in tension during intake (Figure 4), which can cause the piston retaining plate to buckle and/or the separation of the slipper from the piston, resulting in catastrophic failure.

In radial design motors, high case pressure can cause the pistons to lift off the cam while operating in the outlet cycle. The pistons are then hammered back onto the cam during inlet, destroying the motor. If residual case pressure remains high when the motor is stopped, loss of contact between the pistons and cam allows the motor to freewheel, resulting in uncontrolled machine movement.

High pump or motor case pressure also results in excessive load on the lip of the shaft seal. This causes the seal lip to wear a groove in the shaft, eventually resulting in leakage. If case pressure exceeds the shaft seal's design limits, instantaneous failure can occur. The subsequent loss of oil from the case can result in damage through inadequate lubrication.

Symptoms
Overpressurization can occur in operation without any detectable symptoms. Valve plate separation is often characterized by vibration or pulsation in the pressure line of the pump or motor combined with erratic machine movement as the cylinder barrel separates and re-couples to the valve plate.

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Figure 4. Effect of Excessive Case Drain Pressure on Axial Design Piston

Causes
Faulty or incorrectly adjusted pressure control devices, such as pressure-relieving and reducing valves, and variable-pump pressure compensators cause overpressurization. Uncontrolled decompression, thermal expansion (in load-holding circuits) or back flow through check valves or logic elements can result in pressure transients (spikes). In systems experiencing rapid changes in load, the reaction time of the pressure control devices may not be fast enough to prevent pressure transients that exceed design limits. Problems associated with excessive case pressure can be avoided by connecting piston pump and motor case drain lines directly to the reservoir through dedicated penetrations.

Prevention
In most systems, examining the operation and adjustment of pressure control devices at regular intervals will prevent overpressurization.

To avoid problems associated with overpressurization of pump or motor cases, drain lines must not be connected to the return filter. Case drains should be returned to the reservoir through dedicated penetrations. These penetrations must be higher than the unit's case port and connected to a drop-pipe inside the reservoir that extends below minimum fluid level. Filters are not recommended on case drain lines. While this does allow a small percentage of fluid to return to the reservoir unfiltered, in most applications, the contamination risk is low and can be effectively managed using oil analysis and other condition-based maintenance practices.