In your last hydraulic maintenance email,
we talked about how to avoid troubleshooting
mistakes by checking and eliminating
the easy things first.
Today I want to move onto how to determine
the condition of the hardest working component
of a hydraulic system - the pump.
As a pump wears in service, internal leakage increases
and therefore the percentage of flow available
to do useful work (volumetric efficiency) decreases.
If volumetric efficiency falls below a level considered
acceptable for the application, the pump will need
to be overhauled.
In a condition-based maintenance environment, the decision
to change-out the pump is often based on remaining
bearing life or deterioration in volumetric efficiency,
whichever occurs first.
Volumetric efficiency is the percentage of theoretical pump flow
available to do useful work. It is calculated by dividing
the pump's actual output in liters or gallons per minute by its
theoretical output, expressed as a percentage. Actual output
is determined using a flow-tester to load the pump and measure
its flow rate.
Because internal leakage increases as operating pressure
increases and fluid viscosity decreases, these variables
should be stated when stating volumetric efficiency.
For example, a hydraulic pump with a theoretical output
of 100 GPM, and an actual output of 94 GPM at 5000 PSI
and 120 SUS is said to have a volumetric efficiency
of 94% at 5000 PSI and 120 SUS.
When calculating the volumetric efficiency of
a variable displacement pump, internal leakage
must be expressed as a constant.
To understand why this is so, think of the various leakage paths
within a hydraulic pump as fixed orifices. The rate of flow
through an orifice is dependant on the diameter (and shape)
of the orifice, the pressure drop across it and fluid viscosity.
This means that if these variables remain constant, the rate
of internal leakage remains constant, independent of
the pump's displacement.
For a detailed example, which shows how costly it can be if
you don't understand this concept, read the rest of this story:
http://www.insidersecretstohydraulics.com/variable-hydraulic-pump.html
Yours for better hydraulics knowledge,
Brendan Casey
Author of 'Insider Secrets to Hydraulics'; and
'Preventing Hydraulic Failures'.
http://www.hydraulicsupermarket.com/books.html
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