I was recently engaged by a client to conduct failure analysis on a large (and expensive) hydraulic cylinder off an excavator. This hydraulic cylinder had been changed-out due to leaking rod seals after achieving only half of its expected service life.

Inspection revealed that apart from the rod seals, which had failed as a result of ‘dieseling’, the other parts of the hydraulic cylinder were in serviceable condition. The ‘diesel effect’ occurs in a hydraulic cylinder when air mixes with the hydraulic oil and explodes when pressurized.

When a double-acting hydraulic cylinder retracts under the weight of its load, the volume of fluid being demanded by the rod side of the cylinder can exceed the volume of fluid being supplied by the pump.… continue reading »

In most hydraulic applications there are essentially two ways to get the required power output: high flow at (relatively) low pressure; and low flow at high pressure. This is because power is a product of flow and pressure; increase pressure and you can reduce flow proportionately but still get the same power output. For example, 100 liters/min at 200 bar equates to the same power output as 50 liters/min at 400 bar.

Higher pressures mean higher force and torque is available from smaller components. And because these components are smaller in displacement, higher speeds are possible from smaller flows. Smaller flows mean pipework, valving and even the hydraulic tank can be made smaller.… continue reading »

1. Abrasive wear is the scuffing and scoring of a lubricated surface. It can be further divided into two-body abrasion and three-body abrasion. Two-body abrasion occurs when two lubricated surfaces come into direct contact with each other–usually as a result of loss of the lubricating oil film. Three-body abrasion occurs when the clearance between two lubricated surfaces is bridged by one or more hard particles. In this case it’s the clearance-sized hard particles that are responsible for scoring the lubricated surface(s).

2. Adhesive wear is usually a progression of two-body abrasion. If the oil film between two lubricated surfaces moving relative to each other is lost, the two surfaces begin to scuff (two-body abrasion).… continue reading »