Repairing Hydraulic Cylinders
The following is provided by Brendan Casey, a Tractor By Net member. Also, see link to his website, for other hydraulic info.
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REPAIRING HYDRAULIC CYLINDERS
by Brendan Casey
Copyright 2003 by InsiderSecretsToHydraulics.com
All rights reserved.
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This article is free to distribute, provided that it
is distributed in its full form with no changes and
the author credit is included at the end of the article.
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INTRODUCTION
The following is a general guide to repairing hydraulic
cylinders. The extent of the repair work that can be carried
out in-house depends on the extent of wear or damage to the
cylinder and how well equipped your repair shop is. As with
any repair, the economics of proceeding with a repair on a
cylinder are ultimately dependent on the cost and availability
of a new one.
DISASSEMBLY AND INSPECTION
Typically, a cylinder will have been removed for repair due to
either external or internal leakage. Close inspection of the
parts of the cylinder after disassembly, particularly the seals,
can reveal problems that may not otherwise be obvious.
Piston seal - If the piston seal is badly distorted, eroded or
missing completely, this indicates that the barrel is oversize
or has bulged in service. In this case, the barrel or the
complete cylinder should be replaced. Replacing the piston seal
without replacing the barrel is a short-term fix only.
Rod seal - If the rod seal is badly distorted, this usually
indicates that either the guide bush is excessively worn or the
rod is bent. In both cases this results in the weight of the rod
riding on the seal, which causes it to fail. Replacing the rod
seal without identifying and rectifying the cause of the problem
is a short-term fix only.
Rod - Check the rod for cracks at all points where its
cross-section changes. Dye penetrant is ideal for this purpose.
It is easy to use and readily available from industrial hardware
merchants.
Inspect the chrome surface of the rod. If the chrome looks dull
on one side and polished on the opposite side, this indicates that
the rod is bent. Rod straightness should always be checked when a
cylinder is being repaired. This is done by placing the rod on
rollers and measuring the run-out with a dial gauge. The rod
should be as straight as possible, but a run-out of 0.5mm per
linear meter of rod is generally considered acceptable.
In most cases, bent rods can be straightened in a press. It is
sometimes possible to straighten rods without damaging the
hard-chrome plating, however if the chrome is damaged, the rod
must be either re-chromed or replaced.
If the chrome surface of the rod is pitted or scored, the
effectiveness and service life of the rod seals will be reduced.
Minor scratches in the chrome surface can be polished out using
a strip of fine emery paper in a crosshatch action. If the chrome
is badly pitted or scored, the rod must be either re-chromed or
replaced. Machining a new rod from hard-chrome plated round bar,
which is available in standard sizes from machinist's steel
merchants, is usually the most economical solution for small
diameter rods.
Before a rod can be re-chromed, the existing chrome plating has
to be ground off. Each time a rod is ground, the diameter of the
parent metal is reduced and therefore the thickness of the chrome
layer required to finish the rod to its specified diameter
increases. If the chrome layer is too thick, the chrome will
stress crack, resulting in premature failure of the rod seals.
Therefore, when the thickness of the chrome plating on a cylinder
rod reaches 0.008" the rod must be scrapped.
Chrome thickness can be measured using a coating-thickness gauge.
A coating-thickness gauge uses the magnetic properties of the
substrate material to determine the thickness of a non-magnetic
coating. For more information on instruments for measuring chrome
thickness, go to www.HydraulicSupermarket.com/chrometest
Head - It is common, in cylinders used in light-duty applications,
for the rod to be supported directly on the head material, which
is usually aluminium alloy or cast iron. A metallic or
non-metallic guide bush (wear band) is fitted between the rod and
the head, in applications where there are high loads on the rod.
If a cylinder is fitted with a bush between the rod and the head,
it should be replaced as part of the repair.
If the rod is supported directly on the head, use an internal
micrometer or vernier calliper to measure the head's internal
diameter. Take measurements in two positions, 90 degrees apart,
to check for ovalness. The inside diameter of the head should
not exceed the nominal rod diameter plus 0.004". For example, if
the nominal diameter of the rod is 1-1/2" then the inside
diameter of the head should not exceed 1.504". If the head
measures outside this tolerance, it will allow the rod to load
the rod seal, resulting in premature failure of the seal.
Therefore, the head must be sleeved using a bronze bush or be
replaced with a new head, machined from a similar material.
Minor scoring on the lands of the seal grooves inside the head
is not detrimental to the function of the cylinder, as long as
the maximum diameter across the lands does not exceed the
nominal rod diameter plus 0.016". For example, if the nominal
diameter of the rod is 1-1/2" then the inside diameter of the
head, measured across the lands of the seal grooves, should not
exceed 1.516". If the seal lands measure outside this tolerance,
the service life of the rod seal will be reduced. Therefore,
the head must be replaced with a new head, machined from a
similar material.
Barrel (Tube) - Inspect the barrel for internal pitting or scoring.
If the barrel is pitted or scored, the effectiveness and service
life of the piston seal will be reduced. Therefore, the barrel
must be honed to remove damage or be replaced. On small diameter
barrels, pitting or scoring less than 0.005" deep can be removed
using an engine-cylinder honing tool. The barrel must be honed
evenly along its full length.
The maximum bore diameter for standard-size piston seals is the
nominal bore diameter plus 0.010". For example, if the nominal
bore diameter of the barrel is 2-1/2" then the maximum size after
honing should not exceed 2.510". This size should be checked at
several points along the barrel, using an internal micrometer.
If scoring or pitting is still present at 0.010" oversize, the
barrel must be honed further to accommodate oversize seals or be
replaced. Manufacturing a new barrel from honed tubing, which
is available in standard sizes from machinist's steel merchants,
is usually the most economical solution for small diameter
cylinders.
Large diameter, inch-size cylinder barrels can be salvaged by
honing either 0.030" or 0.060" oversize and fitting the
corresponding oversize piston seals. Oversize seals for
metric-size cylinders have limited availability and therefore
it is not always possible to salvage metric-size barrels by
fitting oversize seals.
Piston - The pistons of cylinders used in light-duty applications
are usually machined from aluminium alloy or cast iron and
operate in direct contact with the cylinder bore. Minor scoring
on the outside diameter of the piston is not detrimental to the
function of the cylinder, as long as the minimum diameter of the
piston is not less than the nominal bore diameter minus 0.006".
This can be checked using an external micrometer. For example,
if the nominal diameter of the barrel is 2-1/2" then the minimum
piston diameter would be 2.494". If the piston diameter measures
outside this tolerance, it must be replaced with a new piston,
machined from a similar material.
Non-metallic wear bands are fitted between the piston and barrel,
in applications where there are high loads on the rod. If the
cylinder is fitted with piston wear bands, these should be
replaced as part of the repair.
ORDERING SEALS
If you order seals from a seal supplier, avoid the common practice
of measuring the old seals. Seals can either shrink or swell in
service and in some cases, an incorrect seal may have been
installed previously. To ensure that you are supplied with the
correct seals, measure all seal grooves with a vernier calliper
and give this information to your seal supplier. For more
information on hydraulic seals, go to
www.HydraulicSupermarket.com/seals
ASSEMBLY
Thoroughly clean all parts in a petroleum-based solvent and
blow-dry using compressed air. Coat all parts with clean
hydraulic fluid during assembly. Prior to installing seals,
ensure that the seal grooves are clean and free from nicks and
burrs. Avoid using a screwdriver or other sharp object when
installing seals, as this can result in damage to the seal.
After the cylinder has been assembled, plug its service ports
to prevent ingress of moisture or dirt.
ABOUT THE AUTHOR
Brendan Casey has more than 15 years experience in the
maintenance, repair and overhaul of mobile and industrial
hydraulic equipment. For more information on increasing the
uptime and reducing the operating cost of your hydraulic
equipment, visit his web site:
http://www.InsiderSecretsToHydraulics.com
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