Hydraulic

A serious consequence of over-pressurizing your hydraulic system

When a hydraulic system sees a spike in pressure it won't necessarily blow up with a bang. But damage can occur in a number of ways. In fact, a single pressure spike of sufficient magnitude can render a piston pump or motor unserviceable. Here's how:

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.

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, this 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 (exhibit 1).

valve plate separation

Exhibit 1. Separation of cylinder barrel and valve plate
due to overpressurization
(Bosch Rexroth)

Once separation occurs, the lubricating film is lost and the resulting two-body abrasion damages (scores) the sliding surfaces of the cylinder barrel and valve plate. 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 also that separation can also occur at operating pressures within design limits due to distortion (loss of flatness) of the valve plate, over-speeding or excessive wear of the cylinder barrel drive-spline in axial designs.

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