TechArchive

Original article date: June 1999

US company Motion Industries provides a taste of what you can see hear and smell in faulty hydraulic systems!

A hydraulic system offers many potential problem areas; the most common being excessive noise excessive heat incorrect speed (flow) incorrect pressure or faulty operation. Before embarking on hydraulic troubleshooting a knowledge of basic hydraulic principles is assumed (see panel). Then the next step is train your senses (i.e. feel smell sight) in the aid of troubleshooting.

• Feel – Heat is the best indicator of a problem. Touch different areas of the system. If you cannot touch a component without discomfort there is a problem. The component with the highest temperature is faulty.
• Smell – A bitter smelling oil indicates excessive heat and a breakdown of the additive packages. This oil should be removed and replaced as soon as possible. Further system operation could cause major component damage.
• Sight – Erratic movement of actuators is a strong indication of an entrapped air problem.

Cavitating pumps generate a tremendous amount of heat and noise. Cavitaton is caused by a restriction in the inlet of the pump. These restrictions can include: high oil viscosity; dirty and/or clogged inlet filters; foreign matter; or collapsed inlet hoses. When oil flows across a restriction there must be a pressure drop. Atmospheric pressure can’t be increased so the pump inlet pressure must drop. At this lower pressure the air that is trapped in the oil (10 – 12%) vaporises in the inlet side of the pump. As these vapour bubbles enter the outlet side of the pump they are subjected to system pressure causing the vapor to be pushed back into the oil. Depending on system pressure this occurrence could generate hundreds of thousands of pounds of force on the surface areas of the outlet. These forces are the major cause of damage generated inside the pump.

Another common problem encountered with noisy pumps is air in the fluid referred to as aeration. Air can be drawn into inlet lines at connections such as unions and flanges. Flat O-rings in 2 or 4-bolt SAE flanges are major causes of aeration.

Finally as in any hydraulic component the pump could be worn beyond the manufacturer’s specifications. However change the pump only when all other troubleshooting steps have been exhausted. Often pumps are replaced only to find the original problem still exists.

Electric motor noise is usually an indication of misaligned couplings and/or worn motor bearings. A cure to the misalignment problem is to use C-face electric motors with a pre-engineered pump/motor mount. The pre-engineered pump/motor mounts are machined to allow optimal alignment. Relief valve noise may simply indicate that the valve is open. Normally closed these valves are designed to open for routing oil to the reservoir when maximum system pressure is reached. Multiple relief valves with pressure settings too close may allow a valve to open prematurely. The minimum pressure setting difference should be 125psi between valves. Wear in the pilot head poppet of pilot operated relief valves and the main poppet in direct operated relief valves may allow the valve to open at a lower than set pressure.

Over-heated pumps may signal several problem areas. Improper relief valve and/or pressure compensator settings are a common problem. Pressure compensated pumps which require a safety relief valve are piston and non-direct compensated vane pumps. These pumps and relief valves should have a pressure setting difference of 250psi minimum. If the pump compensator pressure is increased without increasing the safety relief pressure the system reverts to a fixed volume system with the excess oil dumping over the relief valve.

Over-heated electric motors could imply one of several problems. Most commonly overlooked is one of excessive load. Excessive load indicates that system pressure may be set at a higher setting than the system design allows. Another cause of motor overheating is worn motor bearings.

Over-heated relief valves signal improper pressure settings and/or worn and damaged internal parts. Don’t forget a relief valve that is staying open longer than it should is a heat generator.

Over-heated fluid could be one of several problems. The system pressure could be set too high allowing the pressure control valves to leave an open path to the reservoir. The fluid could be dirty and/or low in supply volume as well as too high a viscosity. Oil coolers can also be a source for over-heated fluid. A fan type oil cooler with clogged fins will not dissipate heat at a rate required by the system; a water type cooler with a faulty temperature regulator may function similarly. Finally eliminate the possibility of worn or damaged system components as a source for generating excessive heat.

Remember a critical element of any plant maintenance function is SAFETY. Before you approach the system be sure all loads are lowered or mechanically secure. NEVER depend on a hydraulic control valve (directional flow or pressure) to hold a load. Exhaust the pressure locked in the system (ie accumulators and lines between valves and actuators). Isolate the electrical control and power supply systems by locking out the enclosure. Lock-outs save lives!

• Motion Control Industries Inc
• www. motion-industries.com/hydromro.htm

June 1999