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Rabu, 22 September 2010

Air, are your cylinders hoop stressed?

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Hello again Air,

In your last hydraulic maintenance email,
I explained how to use volumetric efficiency
to correctly determine the condition of
a hydraulic pump.

Today I want to continue on this tack
and talk about testing hydraulic cylinders.

The conventional way of testing the integrity
of the piston seal in a double-acting cylinder
is to pressurize the cylinder at the end of stroke
and measure any leakage past the seal. This is
commonly referred to as "end-of-stroke bypass test"

The major limitation of the end-of-stroke bypass test,
is it generally doesn't reveal ballooning of
the cylinder tube caused by hoop stress as a result
of under designed cylinder wall thickness or reduction
of wall thickness through excessive honing.

The ideal way to test for ballooning of the cylinder tube
is to conduct a piston-seal bypass test mid-stroke.
The major difficulty with doing this is that the force
developed by the cylinder has to be mechanically resisted,
which in the case of large diameter, high-pressure cylinders
is impractical.

However a mid-stroke bypass test can be conducted
hydrostatically using the intensification effect.
The necessary circuit along with a step-by-step
procedure for conducting this test is shown at:

Yours for better hydraulics knowledge,

Brendan Casey
Author of 'Insider Secrets to Hydraulics'; and
'Preventing Hydraulic Failures'.

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