Common Modes of Failure for Hydraulic Piston Pumps

This article will discuss the most common modes of failure for variable displacement piston pumps. Understanding these modes of failure, the causes, warning signs and the prophylactic means of monitoring will help you to reduce unscheduled downtime.

The four most common modes of failure in piston pumps result from oil contamination, fatigue that results from transient pressure spikes, blocked or restricted pump inlet, and pump case over pressurization. These four modes account for the vast majority of all failures seen in the thousands of units inspected and repaired each year by the service technicians working at Motion Industries.

1.Oil Contamination: Oil contamination can result from water ingress, environmental contaminants, or wear elements. Signs of contamination include valve plate scoring, piston barrel scratches, pistons’ face damage, excessive wear in saddle bearings, plugged orifices, stuck compensator spools, and pistons seized within the barrel’s cylinder bore. Monitoring techniques such as regular oil sampling/analysis and sensor-based water saturation and particle count monitoring aid in detecting oil contamination. Live monitoring alongside periodic oil analysis offers a comprehensive understanding of the pump and system health.

2.Fatigue from Transient Pressure Spikes: Identifying fatigue caused by transient pressure spikes is challenging through electronic monitoring. This issue may manifest as broken pistons (at the neck), damaged shafts, sheared control pins, and cracks between the kidneys of the cylinder barrel. Swift scan rate devices are necessary to capture spikes that form and dissipate within milliseconds. Hand-held monitoring devices with quick scan rates, available from various OEMs, are valuable additions to any reliability team’s troubleshooting tools when dealing with transient pressure spikes suspicion.

3.Blocked or Restricted Pump Inlet: A blocked or restricted pump inlet can lead to cavitation on the valve plate, characterized by a “marble sound.” Detecting this condition involves using limit switches on the suction ball valve (common interlock), pressure transducers installed into the suction line, or vibration monitoring through accelerometers mounted to the pump.

4.Pressurization of Pump Case: Pump case pressurization may cause half-moong gouging or full-moong scoring of the swashplate, damaged shoe retainers, bent seal retainers, protruding shaft seals, and shoe damage like rounded edges, loose fit on the ball, and excessive wear on the backs of the shoes. This condition results in shoe lift or rolling due to increased case pressure. Pressure transducers in the case line and flow monitoring for case drains are essential to observe this condition. Monitoring flow provides insight into pump efficiency (e.g., 10% leaving the case and 90% going into the system indicates excessive wear). It’s important to note that this method is not applicable to pumps with internal checks between the case and suction.