There are many methods used to assemble to retain sleeve bearings in an assembly to prevent movement under rotation and load in service. Some of these methods include bolting the bearing with a retainer, a lugged end plate, set screwing, knurled or coarse threading the sleeve OD, key retention or retained by cap screw, press fitting and shrink fitting.
Although the latter two methods are the most popular and give the most positive, efficient, economical and simplest means with little or no specialized equipment being necessary, each method will be described briefly.
FIGURE 1: BOLTED:
The sleeve bearing is slip-fitted into the housing
against the shoulder in the housing bore. The
bolted plate is counter bored to permit the
bearing to be in contact with it; the bearing
length tolerance should not be greater than .005"
and ends must be parallel and square.
FIGURE 2:
Bearing pressed or slide fit into housing and
retained by lugged end plate. Slot is milled in
end of bearing to a depth of slightly below
bottom surface of the lug.
FIGURE 3:Headless setscrew tightened against flat on
bearing. Be careful not to deform bearing. The
flat on the bearing is not necessary but the setscrew
will form a burr on the bearing surface
and make removal difficult. The setscrew may
be locked in by another screw or by locking
compound. Bearing can be press or slip fitted.
FIGURE 4:Knurled or coarse-threaded outer surface used
where a die casting is to be made around the
bearing. Located one end surface of the bearing
flush with surface of housing.
FIGURE 5:
Bearing dimensioned for key retention. Key seat
depth 112 wall thickness or less on small and
medium bearings. Length is specified same as key
length but milling-cutter overrun is shown. Finish machine
inner surface after key is in place.
FIGURE 6:
Bearing pressed or slide fit into housing. Retained
by cap screw. Hole is drilled through wall of bearing.
End of cap screw is below inner surface of bearing.
Press-fit Method: The receiving hole or bore must be within certain described limits for a given method of installation or assembly. Maximum surface contact for heat dissipation are of the utmost importance in high-speed applications, therefore a housing bore tolerance of .001" with a finish of approximately 32 RMS is recommended.
A housing bore of. 002 tolerance would generally have a finish of 63 RMS and a . 003 tolerance bore
would indicate a 125 RMS finish. This latter tolerance bore would be suitable for slow or negligible speeds with high loads.
The receiving hole or housing bore should always have an entry chamfer to facilitate bearing alignment and entry without excess shearing. The chamfer should never be greater than 40 degrees.
For press fitting, a shouldered arbor with a piloted shaft should be used with an arbor press to prevent
eschewing or tilting the sleeve bearing at press-fit operation.
Since the sleeve bearing ID will "close in" the bore after press-fit assembly, certain allowances or
adjustments may be required to the sleeve-bearing bore.
If the sleeve-bearing ID is to be used as is, then the ID should be adjusted for the "close in" depending upon the interference fit. (Refer to the calculation sheet attached to this chapter.)
If the bearing is to be used in a high-temperature service, less OD interference fit should be considered since the bronze will expand at a greater rate than the steel or cast iron housing. Further the shaft expansion must be calculated and the final bearing bore size must be adjusted. (Refer to the temperature calculation sheet attached to this chapter.)
If the bearing is to be used without machining, a thin-walled bearing (1/8" wall thickness in a 2" diameter bearing) will"close in" the ID approximately 80% to 100% of the interference fit.
A heavier-walled bearing (1/4" wall thickness in a 2" diameter bearing) will "close in" the ID 60% to 80% of the interference fit. (For full particulars, refer to the calculation sheets at the end of this chapter.)
A press fit should retain a bearing rigidly in a housing within the bronze alloys elastic limit. Excessive interference fit will prevent the intended results by deforming the bearing OD and giving it a permanent set which could result in the bearing working loose in the assembly under load and speed service.
Since bronze alloys have a higher coefficient of linear thermal expansion rate, i.e. leaded bronze (1 0. 8 X 10 -6) than a steel or cast iron housing (6.3 X 10 -6) operation from normal temperatures will require additional adjustment to the ID for these differences in expansion of mating parts. (Refer to the calculation sheet at the end of this chapter.)
In press fitting, a light coating of oil will generally prevent galling or shearing. The pressure in press
fitting must be applied uniformly and in one continuous motion. Do not use any driving means such as a hammer which tends to peen the end of the bearing and distort the ID.
The press-fit stress put on a cast bronze bearing for every .001" of interference fit is approximately
12,500 PSI. Therefore, a .001" minimum press fit allowance should be minimally sufficient for bearings up to 3" OD.
For bearings above 3" OD, use .002 minimum through 6" OD.
Click here to get an idea of how press-fitting works
Shrink fitting: If an arbor press is not available or its capacity is insufficient for the assembling force
necessary, the bronze bearing can be assembled by the shrink-fit method. This method requires chilling reduction of the bronze bearing or by causing a temperature increase of the housing bore by use of an induction heater. However, the plug induction heater may not be practical because of
the housing bulk. In lighter housings, a plug induction heater can be used to an advantage.
The shrink-fit method is used commonly, is economical and simple. It requires the sleeve bearing to be packed in dry ice or chilling it in a commercial deep-freeze, or more desirable, by direct immersion of the bearing in liquid nitrogen. Click here to see a video.
Liquid nitrogen is particularly suitable since it is inert and free of corrosion, fire or toxic hazard.
However, precautions should be taken to avoid contact with an employee's skin and it is advisable to have a ventilated space or area.
The time for chilling by immersion varies between 4 minutes for a 1" diameter bearing to contract for a .001" of interference fit to an hour for a 10" diameter bearing to contract for a .005" interference fit.
Calculation for Temperature
Adjustment of Bearing ID
Maximum Operating Temperature = __ (-) 70° F = __ .AT° F
Bushing OD & Housing Bore Expansion:
LB= 10.8 X 10-6 (Leaded Bronze)
AB= 9.0 X 10-6 (Aluminum Bronze)
S= 6.3 X 10 -6 (Stl. Housing & Shaft)
Maximum Bushing OD_____X______AT°F X _______ = _______
B
Minimum Housing Bore ______ X ______ AT° F X_______ = (-)_______
S
Net ID Restriction From Press Fit & Temperature .................. = ________
Shaft Diameter Expansion:
Shaft Diameter ______ X ______ AT° F X ________=________
S
Net Total Expansion Adjustments(+) ....................................... __________
Running Clearance Allowance:
* . 0015 per inch of diameter (graphite lube)
* . 0025 per inch of diameter (or other solid lube)
* Allowance based on size and temperature
Shaft diameter at room temperature _____ X _____ = _____
Bushing ID adjustment for "close in" and temperature:
Shaft diameter@ room temperature = ________
Net total expansion adjustment = _________
Running clearance allowance = _________
Net bushing ID adjustment = _________Minimum ___________Maximum
Calculations for determining bearing ID
Before press fit "close in" adjustment
Interference fit:
Bushing OD: ________Maximum ________Minimum
Housing Bore: ________Minimum ________Maximum
Interference Fit: ________Maximum ________Minimum
"Close in" Calculations:
Thin wall approximately 80 % to 100%
Heavy wall approximately 60% to 80%
Maximum amount of "close in" expected: ______ % X _______ = Int. Fit ______
Mininimum amount of "close in" expected: ______% X _______ = Int. Fit ______
Running Clearance Allowance:
* .001" per inch of diameter (oil)
* .0015" per inch of diameter (grease)
* .0020" per inch of diameter (graphite)
Shaft diameter: _________ X ________ Clearance allowance = _______
Bushing ID Before Press Fit:
Shaft Diameter ________Maximum ________Minimum
"Close in" ________Minimum ________Maximum
Running Clearance ________--------__________--------
Bushing ID ________Minimum ________Maximum
Who would've thought there were so many different ways to place a bearing into a machine. We take so many things for granted these days and don't realize the hard work that goes behind it. It sure opened my eyes and taught me to appreciate things more in all aspects of my life.
Well, I say goodbye for now. Until next time my metal loving friends...
Next Up: Week Seven, Chapter 7:Oil Holes and Oil Grooves
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