Womack Data Sheet 29: Life Expectancy of Piston-Type Pumps and Motors
Information in this issue applies only to piston-type hydraulic pumps and motors. Additional information on gear and vane-type pumps and motors will be presented in a later issue. Most of the information applies to both pumps and hydraulic motors, but for the sake of convenience, the term "pump" will be used throughout.
When selecting a hydraulic pump for a specific application, a designer should arrive at a good balance between cost of the pump and its expected life. Every pump has a certain number of "operating hours" built into it, and when these hours are "used up", the pump can be expected to fail, and must be replaced. The number of operating hours built into a pump is directly related to its cost, and the user gets pretty much just what he pays for.
"Cheap'' pumps are designed for applications where the anticipated pump life will equal or surpass the operating life of the machine on which the pump is used. It would be unwise to purchase an expensive pump for these applications. But on machines designed to operate reliably for many years, or in situations where a pump breakdown would be very costly in terms of lost time or production output, it would be foolish to purchase a "cheap" pump. On these applications, a better quality pump should be selected, and while such a pump would cost more, it would save a great deal of money during the life of the machine.
It is the purpose of this sheet to consider the factors which influence pump life, to aid the designer to make a good balance between pump cost and life expectancy.
Life Rating of a Pump
To use the information in this data sheet, the pump manufacturer should be required to furnish a life rating for the pump under consideration. This will be a statement of expected operating hours at a pressure of (so many) PSI, and a speed of (so many) RPM. This information is based on shaft bearing life expectancy, and is as far as the manufacturer can go. The rest is up to the designer, and it is his responsibility to see that circuit conditions which prolong pump life are observed. These are enumerated on the back side of this sheet.
These two factors have the greatest influence on shaft bearing life:
- Speed. Bearing life is in approximate inverse proportion to shaft RPM. For example, by reducing shaft speed to one-half, pump life expectancy is doubled.
- Pressure. Pump life varies inversely as the cube of side load on the shaft bearing, and this is directly related to hydraulic pressure, PSI, on the pump outlet port.
- For example: If system pressure is reduced to one-half, bearing life will be increased by the cube of 2, or 8 times.
- Another example: If system pressure were to be raised from 4,000 to 5,000 PSI, a factor of 1.25 times original pressure, bearing life would be reduced by the cube of 1.25, or by 51%.
- Therefore, to increase life expectancy, both speed and pressure must be kept to moderate values.
Life Expenditure of a Pump
Life of a pump, based on bearing life expectancy, is figured on the actual time during which the pump is running at the pressure and speed on which the rating is based. The time during which the pump is running unloaded or at, say less than half rated pressure, as when advancing or retracting a cylinder, is not counted as life expenditure. For example, if pump life rating is 3000 hours, this means 3000 hours running under manufacturer's specified conditions.
Estimating Pump Life Expenditure
Example: Calculate the number of hours expended in the life of a pump over a year's operation consisting of 8 hours a day, 252 days per year, if the pump is operating on a duty cycle of 5 seconds under full pressure, followed by 25 seconds running unloaded or at low pressure advance and retract of a cylinder.
Solution: Each cycle is 5 + 25 = 30 seconds. Life expenditure each cycle is 5 ÷ 20 = 0.167 or 16.7% of total running time.
Running time in a year is 8 × 252 = 2,016 hours, of which 16.7% is life expenditure. Pump life used up in a year's operation is 2,016 × 0.167 = 336.67 hours.
Manufacturer's Life Rating
As previously stated, the life rating of any pump can be obtained from the Engineering Department of its manufacturer. This will be stated as (so many) hours operation at a stated pressure and speed. This rating can then be adjusted for other speeds and pressures following the rules already given which pertain to speed and pressure.
Example: A certain piston pump is rated for operation at speeds to 3,000 RPM and pressures to 5,000 PSI. These are catalog maximums. But its rated life is 10,000 hours at 2,000 RPM and 3,000 PSI. Find its life expectancy at other speeds and pressures.
The chart below has been calculated for this particular pump using the rules previously stated:
The chart shows that pump life will be reduced to 1,440 hours if operated simultaneously at maximum pressure and maximum speed. But if run at reduced conditions of 2,000 PSI and 1,000 RPM, it would have the fantastic life rating of 67,500 hours. Similar charts can be prepared for any pump.
Factors Which Affect Pump Life
The factors affecting pump life listed on this page will serve as a checklist. Most of them are well known to designers and do not need lengthy comment.
If circuit conditions are otherwise ideal, most piston pump (and motor) failures are because the shaft bearings have reached the end of their natural life. The pump manufacturer has stated the expected bearing life under specified conditions of speed and pressure. Beyond this, he has no control of operating conditions in the system. It is then up to the system designer to provide favorable operating conditions. If he does not, then it is not the fault of the pump if it prematurely and unexpectedly fails. For maximum pump life the following factors should be considered:
- Oil cleanliness. Oil filtration should not be limited to a 150 uM pump suction strainer, but should also include a pressure or return line filter of 10 uM rating or better. Recent tests have shown that by using a filter of 3 uM absolute rating rather than 10 uM, pump life is significantly increased. Whether to go to the expense of extra fine filtration depends on how much it will cost to replace the pump, and how much a production shut-down may cost while the pump is being replaced.
- Side Load on the Pump Shaft. No matter whether or not the pump is catalog-rated for shaft load, any appreciable side or end load will always reduce pump bearing life to some extent. No general rules can be given for the effect of these shaft loads; this information must be obtained from the pump manufacturer. On some applications the natural life of the bearings is so long that additional side or end loading can be tolerated and a satisfactory pump life still obtained.
- To minimize side load against the shaft, observe these rules when installing pumps with side drive:
- Mount the gear or sheave on the pump shaft as close as possible to the front face of the pump case, and install with hub facing AWAY from pump case, to reduce the amount of flexing or bending of the shaft.
- The gears or sheaves on the pump shaft should be of as large diameter as practical. The larger the diameter, the less the side load at the same torque.
- 3. Oil Temperature. The harmful effects of excessive oil temperature are pretty well known. Heat produces contamination, premature wear or degeneration of rubber seals, excessive mechanical wear in the pump, etc. Where possible, oil temperature should be controlled with a heat exchanger if necessary.
- 4. Cavitation.The harmful effects of pump inlet cavitation are also pretty well known - pump wear due to wire drawing, mechanical wear, heat, etc. The next chart shows the maximum inlet vacuum which is permitted by most pump manufacturers:
|Vacuum, PSI||3 to 5||2 to 3||2|
|Vacuum, In. Hg.||6 to 10||4 to 6||4|
- 5. Misalignment of Pump Shaft. When direct-driving a pump from an engine or electric motor shaft, even a small amount of uncorrected misalignment can very quickly ruin the pump bearings. The obvious remedy here is to very carefully align the two shafts.
- 6. Pump Relief Valve.The relief valve, especially in systems using a series-connected flow control valve, should be set to the lowest relieving pressure which will serve the circuit. Excessive pressure, during the feed cycle, reduces pump life. When using pressure compensated pumps, unload them to near zero pressure in valve neutral rather than deadheading them to zero flow at maximum pressure. Operation at maximum pressure, even though not pumping a flow of oil, counts as running time when estimating pump life. With fixed displacement pumps, they should be unloaded to low pressure when the system is not actively working.
© 1990 by Womack Machine Supply Co. This company assumes no liability for errors in data nor in safe and/or satisfactory operation of equipment designed from this information.