Simple Measurements in Hydraulic Systems
FLUID POWER - Design Data Sheet 55
Easy ways of making some simple tests on hydraulic systems are
briefly described in this issue. Some of these tests have been
described in greater detail in previous issues. Please request any
of these back issues which you do not have.
Suggested circuit locations for pressure gauging stations.
Pressure Gauging Locations
All hydraulic fluid power systems may at some time require
troubleshooting, and the most important first step in
troubleshooting is to take pressure measurements in various parts
of the circuit. When constructing the system, a little forethought
in providing gauge points will save a lot of time and trouble if
troubleshooting should be required.
Figure 1 shows gauging points which will prove
most useful. These are marked with the standard TS
symbol (test station). They need be no more elaborate than a tee in
the plumbing, with the side port plugged. They cost very little to
add to a system.
When using these gauge ports, be sure to clean off the plug
before removing it to install a gauge. An air hose, if handy, will
do an adequate cleaning job in most cases.
The gauge ports can be a little more elaborate by permanently
installing a needle valve in the port, thus permitting the
installation of a gauge without shutting down the system. Another
arrangement is to install one-half of a quick disconnect coupling
in each gauge port, with the mating half of the coupling installed
on a gauge. Normally, a pressure gauge should not be left
permanently in the system because pressure surges and machine
vibration will wear it out prematurely and it will not be ready
There are plug-in fittings marketed under the name "Reddy-Chek"
which allow instant plugging of a gauge into a test station without
the use of tools or a shut-off valve.
Checking Pump GPM
Observing the length of time which a cylinder takes to complete
its stroke will allow calculation of the GPM oil flow to the
cylinder. The pump GPM rating, its RPM, and the bore and stroke of
the cylinder must be known.
Use a stop watch or the second hand on a wrist watch to find the
number of seconds to make the extension stroke. Calculate the
actual GPM flowing into the cylinder:
GPM = [A × S × 60] ÷ [t ×
A = piston area,
S = stroke, inches;
t = time, seconds
Compare the calculated GPM with the
rated pump GPM. If it is substantially less, part of the pump oil
is being lost without producing movement of the cylinder
- Pump may be so badly worn that the internal slippage is quite
high. If so, the pump should be replaced.
- Oil in the system is running too hot. The oil has become very
thin and slips excessively in the pump.
- Pump may be cavitating and cannot pull in its full
displacement. Check the usual causes, dirty strainer, etc.
- Pump relief valve may be set too low, causing a part of the oil
to discharge to tank when system builds up high pressure. Be
cautious about raising the relief setting. It may cause an overload
on the electric motor, or may cause damage to components from
- Seals in the cylinder may be so badly worn that they should be
Measurement of Pressure
For accurate measurement of peak transient pressures an
oscilloscope is needed. However, an approximate measurement can be
obtained with a soft seal, leaktight check valve in series with a
pressure gauge. For more details of this measurement, please refer
Design Data Sheet 14.
Two low wattage (40 or 60 watt), identical light bulbs when
connected in series make a simple tester for identifying blown
fuses or circuit breakers, and for a rough test of voltage. Solder
test leads to the base of the bulbs and wrap well with insulating
tape. Two 115-volt bulbs, in series, will light to full brilliance
on 230 volts, and glow on 115 volts.
Testing a Loaded Circuit. Figure 2. If the
circuit is connected to a load, for example a 3-phase electric
motor, test on both sides of each fuse. If bulbs light up, the fuse
Testing a Non-Working Circuit.
Figure 3. If circuit is not connected to a load, test
across each pair of wires on the load side of the fuses. If there
is one blown fuse, the bulbs will light up when connected across
the two good fuses, and will fail to light on the other two pairs
Measuring Electric Motor
A loop ammeter should be in the inventory of test equipment of
every company using integral HP electric motors. Current in each
line can be quickly measured without breaking into the wiring. The
iron ring on the ammeter is simply looped around each line wire in
turn, and the current reads directly on the meter scale.
When attempting to replace an electric
motor with a hydraulic motor, any momentary HP overloads during the
cycle must be detected, and this can be done with a loop ammeter by
monitoring motor current through a complete cycle. Although an
electric motor can "ride through" short overloads, a hydraulic
motor will stall unless supplied with sufficient pressure to
develop enough torque to handle them.
The HP at which an electric motor is
operating can be determined by taking line current readings and
comparing them with the current stamped on the motor nameplate. If
more or less than nameplate rating, the chart on
Design Data Sheet 49 will show the operating
To measure temperature of the oil in a hydraulic reservoir, an
ordinary candy thermometer can be inserted through the filler
opening after removing the protective screen, if any. Or through
any other access hole which may be available. To prevent entry of
dirt, the access hole should not be left open any longer than
A complete test of oil condition can only be done in a laboratory
with precision equipment, but a few preliminary tests can be made
by the user to decide whether more extensive tests should be made.
There are several types of contamination which can be harmful to
hydraulic components such as pumps, cylinders, and hydraulic
Dirt. Solid particle
contamination may enter through the air breather on the reservoir
or through the rod seals on cylinders. Or it may be generated
within the system, primarily by the pump. If a particle count shows
more contamination than the pump can tolerate, additional
filtration should be added in the pressure line or return line.
Air. Air may enter
through worn shaft seals on pumps, cylinders, and hydraulic motors,
from leaks in the pump suction line, low reservoir oil level, etc.
While the system is operating, the air mixes with the oil causing
it to appear cloudy. Usually the oil clears a short time after the
pump is stopped. Air causes excessive wear in the pump, and its
point of entry should be discovered and closed.
Water. Water may
condense in the reservoir as a result of temperature changes in the
air inside the reservoir, or may enter through a leak in a water
cooled heat exchanger. Rust formed in the system can be very
Entrained water mixes with the oil
when the system is running, causing the oil to become cloudy. It
will usually settle in the reservoir and low places in the plumbing
a short time after the pump is stopped. Its presence may be
detected by drawing a fluid sample from the reservoir drain valve
and allowing it to settle. To eliminate condensed water, a small
amount of fluid should be tapped from the bottom of the reservoir
daily, as done in a compressed air system.
Chemicals. Oil reacts
chemically with oxygen from the air to form undesirable chemical
compounds in the oil which cannot be filtered out. These unwanted
compounds eventually form the "sludges" found in strainers and on
the bottom of the reservoir, and the "varnish" which deposits on
valve spools causing them to stick, and which close up small
orifices. Oxidation of the oil causes it to darken in color as
compared to a new sample of the same oil. If temperature is held
below 150°F, with a heat exchanger if necessary, there will be few
problems with chemical contamination. But on systems running
overheated, these compounds form rapidly, causing many breakdowns
and service problems.
Oil Testing. Draw an
oil sample while the system is running and allow it to stand
several hours. Any change in the oil after standing, oil clears,
dirt or water settle out, etc. is an indication that the oil is
unfit for use in a modern hydraulic system and should be replaced
after corrective measures have been taken, filters installed, water
leaks stopped, heat exchanger added, etc. Extremely dark coloration
compared to new oil shows that undesirable oxidation has taken
place. A sample may be sent to the lab of a major oil company for
particle and chemical analysis. However, the sample must be drawn
under the "surgically clean" procedure described in ANSI Standard
B93.19-1972. This analysis may take from 1 to 4 weeks.
Download a PDF of Fluid
Power Design Data Sheet 55 - Simple Measurements in 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