In your last hydraulic maintenance email, I exploded
the myth about pump suction strainers - they do more harm
than good. And installing them to 'protect' the pump is a
contradiction because they can actually destroy it.
Here's another big myth ...
Myth #2. Creep in a double-acting cylinder is caused by a
leaking piston seal.
A popular misbelief about hydraulic cylinders is that
if the piston seal is leaking, the cylinder can creep down.
Fact is, if the piston seal is completely removed from a
double-acting cylinder, the cylinder is completely filled
with oil and the ports are plugged, the cylinder will hold
its load indefinitely - unless the rod-seal leaks.
What happens under these conditions - due to the unequal
volume either side of the piston, is fluid pressure equalizes
and the cylinder becomes hydraulically locked. Once this occurs,
the only way the cylinder can move is if fluid escapes from
the cylinder via the rod seal or its ports.
If you grasp the theory at work here, you'll probably realize
there are a couple of exceptions. The first is a double-rod
cylinder - where volume is equal on both sides of the piston.
And the second is when a load is hanging on a double-acting
cylinder. In this arrangement, the volume of pressurized fluid
on the rod side can be accommodated on the piston side.
In this case a vacuum will develop on the piston side and
depending on the weight of the load, this may eventually
result in equilibrium that arrests further creep.
For more on these and many other hydraulics tips,
point your browser to:
http://www.hydraulicsupermarket.com/technical.html
Yours for better hydraulics knowledge,
Brendan Casey
Author of 'Insider Secrets to Hydraulics'; and
'Preventing Hydraulic Failures'.
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Australia
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