Written by Camilo H Rueda Wednesday, 23 November 2011 11:09
This is a question I've received from clients and colleagues for a long time. Unfortunately it is not as easy to answer, especially because
Considering the purpose of the manufacturer of the pump, the pump was built to last infinitely. The manufacturer guarantees that the fluid will be sent to lubricate abundantly every moving part inside the pump and therefore will not generate wear. So how can one ensure that the hydraulic pump will last effectively infinite number of years? What factors affect the life of the pump?
Next, we list the main factors that decrease the life of the pump:
1. RPM and maximum flow. Constant work near maximum RPM (revolutions per minute) and peak flow can shorten the life of the pump due to the fact that the pump has the suction and discharge connections with sizes that cannot be changed. When working near these limits the fluid enters the pump at very high speeds that can generate cavitation and therefore internal damage due to lack of lubrication. Also keep in mind that factors such as temperature, viscosity of the fluid used and entry of air into the intake can lower the speed limit (rpm) and peak flow of the pump.
2. Change in the properties of the fluids used. A new hydraulic fluid will provide all necessary lubrication for internal pump parts and prevent wear of the same, but depending on the type of fluids (oil from petroleum, synthetic, biodegradable or water - oil mixtures) are degraded or additives lost over time, some more than others and of course, factors such as temperature, humidity and pollution can accelerate degradation. The end result is that the oil does not lubricate as before and cannot avoid in the same way that metal parts rub against each other and wear. The periodic change of the fluid is required to maximize the life of the hydraulic pump.
3. Control of contamination. As we know, any element other than hydraulic fluid is treated as contamination, e.g. air, water and solid particles of all kinds. This is perhaps one of the most important facts and one of the least known. The installation of filters suitable for the application and its placement within the hydraulic system are vital for controlling contamination; tank vent, suction filters, return and pressure filters are essential, as well as good selection of micronage. But the filters get clogged doing their work after a while, so it is imperative to change the filters elements regularly to extend the life of the pump.
4. Bearings. All pumps have some bearings to support the drive shaft and some other internal parts. There are thrust bearings, radial ball bearings, cylindrical roller bearings, tapered roller and so on. These elements are responsible for supporting the loads generated against the housing and covers of the pump by the movement transferred from the drive shaft to the rotating group which in turn generates the pumping of the fluid. The bearings are subjected to cyclic loads that are repeated with each shaft revolution, or at a rate of thousands of cycles per minute (RPM) which when multiplied by the hours, days and years of work generate millions of cycles in their life. The bearing load in each cycle varies according to the working conditions of the hydraulic system such as pressure, displacement and temperature among others, creating a phenomenon called 'material fatigue' which reduces its mechanical strength with the number of work cycles or over time. The load, this factor and the friction between the rolling elements (balls, rollers and tracks) are what determine the bearing life.
Obviously, the bearings used in pumps vary tremendously depending on the type of pump. For example, the bearings loads in an axial piston pump or bent axle piston pump are gigantic; however the case is very different in a balanced vane pump where the loads are minimal.
5. Material fatigue. This is a condition that affects all the parts of the pump that are subject to fluctuating or cyclic loads as explained in the bearings. Vanes that pass from the zone of high pressure to the suction zone with each turn of the shaft; or the piston in a piston pump that pumps fluid under pressure in a half of a turn of the shaft and sucks fluid in the other half; or the teeth of the gear pump as they move from the suction to the pressure side.
These cyclic loads generate cracks starting at a microscopic level and will get larger over time and the number of cycles until the part gets broken. A determining factor in the onset of fatigue is how close is the value of the load on the piece to its maximum resistance, the closer to this limit; the fewer cycles are needed to break it. Finally, the original ultimate strength of the part decreases with the number of cycles as well.
In conclusion, the pump working under extreme pressure conditions that are very close to the maximum recommended by the manufacturer makes the real life of the pump to be reduced dramatically.
Taking into account all the factors listed, the life of the pump is determined from the start with a good selection of it at the time of design, good working conditions without reaching extremes, using the proper fluid at the proper temperature and a good maintenance policy (changing fluid and filters regularly).
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