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1.
Environment
It is important that the composition of our carbon-graphite material
be compatible with the environment to which it may be subjected.
Please provide the details of the environment to our engineering
department, so that compatible materials can be recommended. Examples
of the environment would be exposure to a chemical medium, inert
atmosphere, cryogenic service, vacuum, air under normal environmental
condition, a high temperature oxidizing environment, or a high temperature
neutral environment.
2.
Temperature
Though carbon does increase in strength with an increase in temperature,
you must not exceed the maximum operating temperature of the materials.
A minimum Rockwell C of 55 is recommended. Longer life will also
be obtained if the mating materials is ground and/or polished to
a 16 micro-inch finish or better.
3.
Mating Materials
Our materials may be run against most materials, with best results
when run against the harder surfaces. Ceramics and hard chrome plated
surfaces are excellent mating materials. We recommend a minimum
Rockwell C of 55 and that the mating material be ground to a 16
micro-inch finish or better.
4.
Load
Our materials, in dry running conditions, can withstand a maximum
Pressure Velocity (PV) of 15,000. When thoroughly lubricated, the
PV may be increased up to 150,000 or more, depending on circumstances.
The bearing load, in pounds per square inch, is calculated by figuring
the total bearing load divided by the projected area (length x inside
diameter) of the bearing. Pressure velocity (PV) is the product
of the bushing load x the shaft velocity (in feet per minute). If
the calculated PV of a dry running application has approached 15,000,
the bushing life can be improved by increasing the bushing length
and/or introducing constant and thorough lubrication.
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5. Interference Fit
Since Carbon-Graphite bearings are usually installed in metal housings
and run against metal shafts, the designer must be aware of the
difference in the coefficient of thermal expansion of the materials
and take this into consideration when designing the size of the
bushing. At maximum operating temperature, the bushing must have
at least .0015 inches/inch diameter interference fit.
6.
Close-in
When a carbon-graphite bushing is either pressed into a housing
or heat-shrunk into a housing, the inside diameter of the carbon-graphite
bearing will shrink in relation to the amount or interference fit
on the outside diameter. This shrinkage must be taken into account
when designing the inside diameter. Refer to the following chart
for approximate values.
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7.
Running Clearance
The running clearance will vary depending upon the application requirements;
however, a general guideline is to have .002 inches/inch of shaft
diameter with a minimum of .003" clearance. These clearances
apply after the bearing is installed in the housing. With high surface
speeds the clearance may be reduced, as for slow surface speeds,
the clearance may be increased.
8.
Bearing Length
The bearing length should be one to three times the shaft diameter,
with one and a half times being the preference.
9.
Preferred Design
The preferred design for the bushing is to be a straight sleeve
type without shoulders and grooves. Shoulders create stress points
and can be a cause of failure. Grooves can be used quite successfully
if they are installed with a radius tool bit and do not cut so deep
into the wall as to weaken the bearing. The minimum wall thickness
of a bushing should be 1/8", with 3/16" preferred. The
recommended wall thickness is 1/8" + 10% of the bushing inside
diameter.
10.
Tolerances
Our bushings can be machined to very close tolerances. The recommended
tolerances are listed below; though, much closer tolerances can
be machined at an additional cost.
O.D………………. +/-.001"
I.D….…………….. +/-.001"
Length……………..+/-.010"
Concentricity………002"T.I.R.
Surface finish………16 rms of finer
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