Life Expectancy of Piston-Type Pumps and Motors
FLUID POWER - Design Data Sheet 29
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
- 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
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 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
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
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
- 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:
||3 to 5
||2 to 3
|Vacuum, In. Hg.
||6 to 10
||4 to 6
- 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.
Download a PDF of Fluid
Power Design Data Sheet 29 - Life Expectancy of Piston-Type
Hydraulic Pumps and Motors.
© 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