Hydraulic Component Inspection

The Art Of Inspection

When components are broken down and inspected for damage it requires many years of experience and more than just a basic understanding of hydraulics to determine the cause(s). It does require more than a basic understanding of the component its self.

A recent Poclain radial piston motor experienced the damage you see in these pictures. We looked for signs of cavitation and or aeration in and around the areas exposed to high pressure. Why there? Because high-pressure fluid would have given us the signs of implosion damage due to air bubbles. All the critical areas, pistons, and wear-plates showed no signs of implosion damage nor any signs of contamination from dirty oil.

All the damage was contained in the motor case area holding the cam ring, piston rollers and piston support shoes. Prior to removal, case pressure was measured at less than 30-psi.

Closed loop hydraulic pumps drive the system with four motors in parallel, two motors the same size at the rear drive and two motors the same size (but smaller) for the front drive. The damaged motor was one of the rear drive motors. Looking at the damage tells us that there were some high forces involved to produce the damage on the hardened surfaces of the parts.

A study of the circuit showed us that there was no interlock valve to prevent wheel-slip, and in fact there was wheel-slip being seen during operation, (although not severe). The choice of not using an interlocking valve (differential valve) was due to the valve prematurely engaging during normal operation. Although we felt that this problem could be overcome, the customer did not agree and would not use the valve.

We are in the process of installing pressure transducers and strip recorders on this unit to monitor the system pressures and forces, we do have an idea of what we think is causing the problem. We suspect that when wheel-slip occurs the pressure in the wheel that is slipping goes down considerably, and the pressure from the wheel still engaged is rising. When the slipping wheel then engages the ground again, a pressure spike is seen in the topside of the pistons, which translates to increased force in a downward motion against the piston roller bearing, cradle and cam ring.

This force is great enough to produce "impact" damage to the parts shown in the pictures. Once particles of metal break off, they become contamination in the case, which begins to grind the surfaces of all the parts.

Proper instrumentation of high-speed sensors should tell us if our theory is correct. .




	The Art Of Inspection When components are broken down and inspected for damage it requires many years of experience and more than just a basic understanding of hydraulics to determine the cause(s). It does require more than a basic understanding of the component its self. A recent Poclain radial piston motor experienced the damage you see in these pictures. We looked for signs of cavitation and or aeration in and around the areas exposed to high pressure. Why there? Because high-pressure fluid would have given us the signs of implosion damage due to air bubbles. All the critical areas, pistons, and wear-plates showed no signs of implosion damage nor any signs of contamination from dirty oil. All the damage was contained in the motor case area holding the cam ring, piston rollers and piston support shoes. Prior to removal, case pressure was measured at less than 30-psi. Closed loop hydraulic pumps drive the system with four motors in parallel, two motors the same size at the rear drive and two motors the same size (but smaller) for the front drive. The damaged motor was one of the rear drive motors. Looking at the damage tells us that there were some high forces involved to produce the damage on the hardened surfaces of the parts. A study of the circuit showed us that there was no interlock valve to prevent wheel-slip, and in fact there was wheel-slip being seen during operation, (although not severe). The choice of not using an interlocking valve (differential valve) was due to the valve prematurely engaging during normal operation. Although we felt that this problem could be overcome, the customer did not agree and would not use the valve. We are in the process of installing pressure transducers and strip recorders on this unit to monitor the system pressures and forces, we do have an idea of what we think is causing the problem. We suspect that when wheel-slip occurs the pressure in the wheel that is slipping goes down considerably, and the pressure from the wheel still engaged is rising. When the slipping wheel then engages the ground again, a pressure spike is seen in the topside of the pistons, which translates to increased force in a downward motion against the piston roller bearing, cradle and cam ring. This force is great enough to produce "impact" damage to the parts shown in the pictures. Once particles of metal break off, they become contamination in the case, which begins to grind the surfaces of all the parts. Proper instrumentation of high-speed sensors should tell us if our theory is correct. .