Carrying Out Effective
Repairs To Hydraulic Cylinders
By Brendan Casey
As a product group, cylinders are almost as common as pumps
and motors combined. They are less complicated than other
types of hydraulic components and are therefore relatively
easy to repair. As a result, many hydraulic equipment owners
or their maintenance personnel carry out cylinder repairs
themselves.
The following is a guide to carrying out effective repairs
to 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 (see exhibit 1). Position the
rod so that the distance between the rollers (L) is as large
as possible and measure the run-out at the mid-point between
the rollers (L/2).
Exhibit 1
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 specialist 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.
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 aluminum 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 caliper 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 – 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 specialist 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 aluminum 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 caliper and give this information to your
seal supplier.
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 16 years
experience in the maintenance, repair and overhaul of mobile
and industrial hydraulic equipment. For more information
on reducing the operating cost and increasing the uptime
of your hydraulic equipment, visit his Web site: http://www.InsiderSecretsToHydraulics.com
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