Preventing Pump Damage
- Contamination is the single largest contributor to hydraulic component failures.
Contamination is considered to be any substance found in the hydraulic oil not specified by the oil producer. - Water, dirt, dust, air, etc….
System cleanliness and regular preventative maintenance insure a long lasting, responsive hydraulic system.
- Particle counting is the most common method for determining cleanliness levels of hydraulic fluids. Powerful optical instruments are used to count the number of particles in a specified volume of fluid.
- Maximum recommended contamination levels are a function of component clearances and system operating pressure. As the system pressure increases the oil film separating the working parts become thinner making the components more susceptible to damage by contaminant.
Small particles (dust dirt, etc.) can accumulate in very small tolerances within hydraulic equipment which form a wedge between many components, increases friction, increases heat and accelerates wear. These particles accelerate wear as they move across wear surfaces and eroding the material and weakening the wear surface and causing pitting and spalling.
The pitting and spalling occurs when fatigue cracks initiate on wear surfaces or just below the surface. Usually pitting is the result of surface cracks caused by metal-to-metal contact of asperities or defects due to moisture causing low lubricant film thickness. These cracks may start at inclusions in material which act as stress concentrators, and propagate below and parallel to the wear surface. Pits are formed when these cracks break through the tooth surface and cause material separation. When several pits join a larger pit a spall is formed.
Spalling is caused by high-contact stresses possibly associated with proud areas of the wear surface. Excessive, internal stresses, from improper heat treatment, can cause spalling in surface-hardened gears. In through-hardened and softer material, spalling appears to be a massing of many overlapping or interconnected large pits in one locality. The suspected cause of pitting in most industrial applications is hydrogen embrittlement of metal due to water contamination of the lubricant, however pitting can also be caused by foreign particle contamination of lubricant. These particles create surface stress concentration points that reduce lubricant film thickness and promote pitting.
- Roller bearings are more susceptible to small particle contamination rather than bushings that are more susceptible to large contaminates.
- Commercial gear pumps require filtration of ISO 17/14 or better.
- As the gear teeth mesh and oil is forced out of the pump outlet, a small amount of the oil is trapped by the meshing teeth in the tooth space root area (circled). The trapped oil is forced at right angles into the faces of the thrust plates. Relieved areas on the plates called trapping grooves allow the high pressure oil to escape. The high velocity of the oil moving at a right angle to the thrust plate will do serious damage to the thrust plate in the trapping groove area if fine particle contamination is being carried by the oil.
When contamination is present two wear bands will develop under the ring seals on the gear shaft of the roller bearing pumps. These bronze high pressure seals are designed to reduce the high pressure oil lubricating the bearings to low pressure before reaching the shaft seal. Tiny particles, under high pressure and velocity, progressively wear the gear journal surface until the ring seal is no longer effective. High pressure jets under the ring seal pressurizing the shaft seal area. The oil pressure on the shaft seal can force the seal lip away from the journal or even dislodge the seal from the bore. In both instances an external leak would occur. Pump shaft seals are typically rated for a maximum pressure of15 to 20 psi. Shaft seals are sensitive to shaft speed, an increase in speed can cause a decrease in the maximum pressure rating of the seal.
Contamination
Cause
- Improper Filtration
- Low oil level – concentration of contaminates
- Loose or lost Breather cap
- Leaking fittings, seals and wipers
- Missing or collapsed inlet strainers
- Clogged filter and or by-pass
Effect
- Accelerated Wear
- Bearings
- Thrust/wear plates
- Housing assembly
- Bearing and Bushing Failure
- Reduction in Pump efficiency
- Reduction of equipment life
- Excessive Heat
- Internal Leaks
- Pump Failures
Cavitation and Aeration
Excessive noise in a hydraulic system can be caused by the presence of air in the hydraulic oil. Air can be introduced into the system two ways; through cavitation or aeration. “Cavitation” occurs when the pump inlet flow can no longer fill the pump
chambers created by the separation of the gear teeth. The partial vacuum vaporizes some of the oil causing air and or water to come out of solution.
Cavitation is the formation of air, and water or oil vapor bubbles in the hydraulic oil.
Aeration is a form of cavitation that occurs when outside air is drawn into the hydraulic system due to a loose fitting or low oil level in the reservoir. As the bubbles, created by cavitation and aeration, are carried around to the pump outlet, the increase in pressure squeezes the bubbles.
An implosion occurs when the pressure becomes too great and the bubbles collapse inward. When the air bubbles implode intense shock waves bombard the surface of the wear plates. The shock waves erode the plates’ surfaces on the high pressure side and trapping groove area. The sealing capability of the plate is reduced resulting in a loss of output flow.
The presence of air can also reduce the volume of oil available to carry away heat that is produced by mechanical friction. Vacuum conditions created in the pump rob oil from the bearing and thrust plate areas. The loss of oil in bearing bore upsets the loading which can result in premature bearing failure. Thrust plates, relying on the oil to maintain their balance become unbalanced, resulting in a milling of the plate surface. Physical damage can also be seen on the gear housing bore in the form of a rough surface in the gear wipe area.
Cavitation
Cause
- Clogged inlet strainer/breather
- Inlet strainer too small
- Inlet line bore too small
- Excessive engine speed
- Collapsed inlet hose
- Suction head too great
- Oil too viscous (cold weather)
Effect
- Noise
- Heat
- Accelerated wear – thrust plates / housing
- Internal leaks
- Reduction of pump life
- Erratic actuator performance
- Pump failure
Aeration
Cause
- Low oil level
- Vortexing of oil above strainer – whirlpool
- Loose inlet fittings
- Worn pump shaft seal
- Foam suspended in oil due to sloshing in the reservoir
Effect
- Noise
- Heat
- Accelerated wear – thrust plates / housing
- Internal leaks
- Reduction of pump life
- Erratic actuator performance
- Pump failure
Heat Damage
Heat is generated in a hydraulic system whenever oil dumps from an area of high pressure to low pressure without doing mechanical work. Oil blowing over a relief valve or flowing through piping, a valve, a clogged filter or strainer all are examples of sources of heat. Proper reservoir size can dissipate much of the heat generated in a must be added to sufficiently cool the oil. On some applications an oil cooler must be added to sufficiently cool the oil. Other factors, such as, contamination, cavitation/aeration, improper oil viscosity can add to the heating problems of a hydraulic system. Heat build-up causes the hydraulic oil to loose its viscosity resulting in oil that no longer meets specifications. This greatly reduces the lubricating effects of the oil on the close tolerance parts. The heated oil oxidizes, encouraging corrosion, leakage, and the development of sludge. Sludge can clog filters and strainers compounding system problems leading to cavitation and additional heating of the close running tolerances of the pump/motor components makes them susceptible to heat damage. Add the fact that the thrust plates are being squeezed against the gear faces with hydraulic pressure; a loss of lubrication between the two surfaces can friction.
Heat Damage
Cause
- Low oil level
- Cavitation/aeration/water
- Contamination
- Inlet restriction
- Relief valve
- Incorrect fluid
- Poor reservoir
- Undersized fittings, hoses and components
Effect
- Breakdown of oil/loss of lubricity
- Accelerated wear
- Reduction of efficiency
- Leakage
- Varnish/sludge
- Internal seal destruction
- Component seizure