Troubleshooting Tips for Hydraulic Pumps
FLUID POWER - Design Data Sheet 48
The pump is probably the component
most subject to wear in a hydraulic system, and the one most likely
to cause a sudden or gradual failure in the system.
Pump trouble is usually characterized
by increased noise, increased heat, erratic operation of cylinders,
difficulty or mabthty to develop full output, decreased speed of
cylinders or hydraulic motors, or failure of the system to work at
One of the following problems will
most likely be the cause if any of the above symptoms appear, if
they are indeed caused by the pump.
Cavitation is the inability of a pump to draw in a full charge of
oil. When a pump starts to cavitate its noise level increases, and
it may become extremely hot around the shaft and front bearing.
Other symptoms of pump cavitation are erratic movement of
cylinders, difficulty in building up full pressure, and a milky
appearance of the oil. If cavitation is suspected, check these
a. Check condition of pump suction
strainer. Clean it even if it does not look dirty. Use a solvent
then blow dry with an air hose. Varnish deposited in the wire mesh
may be restricting the oil flow but may be almost invisible. If you
find varnish deposits on internal surfaces of pumps or valves, the
system is operating at too high a temperature. A heat exchanger
should be added.
b. Check for restricted or clogged
pump inlet plumbing. If hoses are used, be sure they are not
collapsed. Only those hoses designed for vacuum should be used in
the pump inlet. They have an internal wire spiral to prevent
c. Be sure the air breather on top
of the reservoir is not clogged with lint or dirt. On systems where
the air volume above the oil is relatively small, the pump could
cavitate during its extension stroke if the breather became
d. Oil viscosity could "be too high
for the particular pump. Some pumps cannot pick up the prime on
heavy oil or will run in a partially cavitated condition.
Cold weather start-up is
particularly damaging to a pump. Running a pump for several hours
in a cavitated condition until the oil warms up can greatly shorten
its life. On equipment operating outdoors use an oil not only of
the recommended viscosity but also with as high as possible
viscosity index. This minimizes the viscosity change from cold to
hot oil operation and reduces cavitation on a cold start-up.
e. Check suction strainer size. Be
sure that original strainer has not been replaced with one of
smaller size. Increasing its size, where possible, may help on some
systems where the original size selection may have been
f. The use of high quality oil may
reduce formation of varnish and sludge.
g. Determine recommended speed of
pump. Check pulley and gear ratios. Be sure the original electric
motor has not been replaced with one which runs at a higher
h. Be sure pump has not been
replaced with one which delivers a higher flow which might overload
the suction strainer. Increase suction strainer size if
Air Leaking Into the System
Air which is in a new system, just assembled, will purge itself
after a short time. The system should first be cycled for perhaps
15 minutes to 30 minutes without trying to build more than very low
pressure. Entrapped air will dissolve in the oil, a little at a
time, and be carried into the reservoir, from where it will escape.
This process can be accelerated, of course, by bleeding air from
high points in the plumbing, and especially at cylinder ports.
Air which comes into the system from air leaks will cause the
oil to have a milky appearance a short time after the system is
started, but the oil will usually clear up a bout an hour after
shutdown. To find where air is entering the system, check out these
a. Be sure the oil reserve is filled
to Its normal level, and that the pump intake is well below the
minimum oil level. The NFPA reservoir specifications call for the
highest point on the suction strainer to be at least 3 inches below
minimum oil level.
Check the oil level when all
cylinders are extended to be sure it is not below the "Low" mark on
the gauge. However, do not overfill the reservoir when cylinders
are extended; it may overflow when the cylinders are retracted.
b. Air may be entering around the
pump shaft seal. Gear and vane pumps which are pulling suction oil
from a reservoir located below them, will have a slight vacuum
behind the shaft seal. When this seal becomes badly worn, air may
enter through the worn seal. Piston pumps usually have a small
positive pressure, up to 15 PSI, behind the shaft seal. Air is
unlikely to enter these pumps through the seal.
c. Check all plumbing and joints in
the pump inlet line, especially unions. Check for leaks in hoses
used in· the inlet line. One easy way to check for plumbing leaks
is to pour oil over a suspected leak. If the pump noise diminishes,
you have found your leak.
Check also around the inlet port.
Screwing a tapered pipe fitting into a straight thread port will
damage the thread, causing a permanent air leak which it is
difficult or impossible to repair.
d. Air may be entering through the
rod seal of a cylinder. This can happen on cylinders mounted with
the rod up, and which are not properly counterbalanced. On the
downstroke, the gravity load may cause a partial vacuum to appear
in the rod end of the cylinder. Cylinder seals are not usually
designed to seal air out, so even a good seal can leak under these
e. Be sure the main tank return line
discharges well below the minimum oil level and not on top of the
oil. On new designs it is helpful to increase the diameter of the
tank return line for a few feet before it discharges. This causes
oil velocity to decrease, reducing turbulence inside the
Water Leaking Into the System
Water leaking into the system will cause the oil to have a milky
appearance while the system is running, but the oil will usually
clear up a short time after the system is shut down as water
settles to the bottom of the reservoir. Water may enter into the
system in these, possibly other ways:
a. A leak in a shell and tube heat
exchanger may allow water to mix with the oil.
b. Condensation on the inside walls
of the reservoir. This is almost unavoidable on systems operating
in an environment where the ambient temperature changes from
daytime to nighttime. The proper solution is to daily tap off a
small quantity of fluid from the bottom of the reservoir through
the drain valve. Since water settles to the bottom, it will drain
off before oil starts coming out.
c. Be sure that any tubing or piping
which carries cooling water inside the air space of the reservoir
enters and leaves below the oil level, so water cannot condense on
Oil Leakage Around the Pump
a. Leakage Around the Shaft. On some
pumps (piston pumps or those pumps operating with an overhead
reservoir), there may be a slight pressure behind the shaft seal.
As the seal becomes well worn, external leakage may appear. This
will usually be more pronounced while the pump is running, and may
disappear while the pump is stopped.
Other pumps such as the gear and
vane type, usually run with a slight vacuum behind the seal.
Leakage may only occur after the pump has been stopped.
Prematurely worn shaft seals may be
caused by excessive oil temperature. At temperatures of 200°F and
higher, rubber seals have a very short life.
Abrasives in the oil may wear seals
out quickly, and will also produce circumferential scoring of the
shaft in the seal area. If abrasives are present, they will settle
out of a sample drawn from the reservoir if it is allowed to stand
an hour or so. Check all points where abrasives can enter. The most
common entry point is through the air breather on the reservoir. To
solve this problem, seal the reservoir air tight and maintain 1 or
2 PSI (no more) on top of the oil.
b. Leakage Around a Pump Port.
Sometimes leakage at these ports is caused from screwing a taper
pipe thread fitting into a straight thread port. Once the threads
have been damaged there is no easy way to repair the pump.
Check tightness of fittings in the
ports. If dryseal pipe threads are used, there should be no need to
use a pipe thread sealant. Beware of screwing taper pipe threads
too tightly into a pump body casting. This may cause the casting to
c. If leakage is from a small crack
in the body casting, this most likely has been caused either by
screwing a pipe fitting in too tightly, or from operating the pump
in a system where either the relief valve is set too high, or where
high transient pressure spikes are generated as a result of shocks.
It is possible that the casting may originally have been defective
but this has rarely turned out to be the problem.
Pump Delivering Too Little or No Flow
a. Shaft turning in wrong direction.
Shut down immediately. Reversed leads on a 3-phase motor are the
commonest cause for wrong rotation. Pumps must be run in the
direction marked on their nameplate or case.
b. Intake Clogged. Check suction
strainer for dirt, and check for collapsed intake hoses.
c. Low oil level in the
d. Stuck vanes, valves, or pistons,
either from varnish in the oil or from rust or corrosion. Varnish
indicates the system is running too hot. Rust or corrosion may mean
water is getting into the oil.
e. Oil too thin, either from wrong
choice of oil or from thinning out at high temperature. A system
with this problem may operate normally the first few hours after
start·up, then gradually slow down as the oil gets overheated.
f. Mechanical Trouble. Check for
broken shaft or coupling, sheared key or pin, etc.
g. Pump running too slow. Most pumps
deliver a flow at all speeds, proportional to RPM. But some vane
pumps which depend on centrifugal force to extend the vanes, will
deliver little or no flow at slow speeds such as engine idle
h. If the driving motor has been
replaced, make sure it is the correct speed for the pump.
Pump Noise Has Recently Increased
a. Cavitation of pump inlet.
b. Air leaking into the system from
low oil or other cause.
c. Mechanical noise caused by loose
or worn coupling, loose set screws, badly worn internal parts,
d. System may be running too
e. Pump may be running too fast.
Short Pump Life
a. Operation of pump above catalog
pressure rating, especially if pump must maintain this pressure for
a high percentage of total running time.
b. Oil of wrong viscosity or of poor
c. Operating the oil at excessively
d. Inadequate filtering.
e. Failure to keep suction strainer
f. Misalignment of pump shaft with
driving motor or engine. Note: When replacing a
foot mounted pump, leave the bracket and replace only the pump and
the new pump will not have to be re-aligned with the driving
g. Air or water may be leaking into
h. Running the pump too fast or too
i. Inlet cavitation from other
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Power Design Data Sheet 48 - Troubleshooting Tips for Hydraulic
© 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