Copyright Goodheart-Willcox Co., Inc.
440 Hydraulic Systems for Mobile Equipment
PQ Curve Differences
Some manufacturers have their engineers graph a PQ curve, which con-
sists of graphing the hydraulic pump’s fl ow rate (Q) at different operating
pressures (P). To understand the effects of low-pressure standby on LSPC
system performance, it is helpful to compare a PQ curve with a low value for
low-pressure standby and a PQ curve with a higher value for low-pressure
standby. Figure 18-18 is a graph that students charted on an LSPC hydraulic
system in a Pittsburg State University laboratory setting. The fl ow rate (Q) is
listed on the left side of the graph, and the operating pressure (P) is on the
bottom of the graph. Notice that the performance improved when the low-
pressure standby (LPSB) was set at 550 psi (38 bar) versus 200 psi (14 bar).
When charting a PQ curve, be sure to follow all of the test procedures, as
performance can drastically change based on many factors, especially pump
speed. Manufacturers might provide correction factors to aid the diagnostic
process.
A Low-Pressure Standby Problem
In a laboratory setting, instructors can demonstrate to students how a mis-
take in setting the low-pressure standby can affect a hydraulic system. After
an instructor set LPSB pressure close to 100 psi (7 bar) on a LSPC variable-
displacement hydraulic pump, no oil reached the DCVs. The students working on
the project later determined that the steering priority valve was biased at a spring
value of 150 psi (10 bar). Therefore, a low-pressure standby value of 100 psi
(7 bar) was too low to shift the steering priority’s spool valve and would not allow
any oil to flow to the DCVs. This type of priority valve is found on 9300 Case IH
Steigers 4WD tractors and Caterpillar 420D loader backhoes.
The solution involved setting the LPSB to a value slightly above the pressure
value of the steering priority spring. Once this adjustment was made, oil was able
to reach the DCVs.
Adjusting the Flow Control Spool
Some manufacturers might focus more on setting margin pressure than
low-pressure standby. The reason is that low-pressure standby is a static test,
whereas the margin pressure test is a more dynamic test.
Low-pressure standby is a measure of the pressure the spool is set at when
the DCV is in a neutral position. Margin pressure tends to have a little lower
value than low-pressure standby. Margin pressure values have a tighter tolerance
(for example, ± 15 psi) than low-pressure standby pressure values (for example,
± 100 psi). The margin pressure value is based on the system fl owing oil and the
spool balancing between pump output pressure and signal pressure.
For example, a low-pressure standby specifi cation might be 410 psi ± 105 psi,
while the margin pressure specifi cation might be 305 psi ± 15 psi. If a techni-
cian accurately adjusts margin pressure, low-pressure standby is typically also
within its specifi cation. Some manufacturers might only provide a specifi cation
for low-pressure standby or margin pressure and not both. Other machines will
have both specifi ed.