Shaft and Housing Fits

Planning for proper fit between the bearing and the shaft or housing is as important as bearing design and cleanliness. Improper bearing fits or incorrect installation can damage the bearing assembly.

The ideal mounting for a bearing is line to line on both shaft and housing. Such a fit has no interference or looseness. For random fitting the fit tolerances may need to be increased to meet the lot variances. For selective fitting the bearing inner and outer diameter should be accurately measured, then the shaft and housing machined to suit.

Interference fits should be used with care as they can distort the raceway and reduce radial internal clearance. In preloaded pairs, reducing the internal radial clearance increases the preload. If excessive, the results can be significantly reduced speed capability and higher operating temperatures which will ultimately reduce life.

Some applications require interference fits such as:

• Heavy radial loading
• Intense vibration
• Lack of axial clamping

Radial internal clearance is reduced by approximately 80% of the interference fit.

Interference fits are usually applied to the rotating ring.

Loose fits may be suggested when:

• Axial clamping is possible, such a snap rings, locking collars or adhesive
• Ease of assembly
• Axial movement is required for spring preload or thermal movements

Fits are often overlooked and is arguably the most common bearing handling mistake.

The table below is simply a guideline as there are many influencing factors to be considered such as.

• Load, speeds and temperatures
• Ease of assembly and disassembly
• Rigidity and accuracy requirements
• Machining tolerances

Therefore the appropriate fit may fall somewhere in between.

 

ROTATING RING
APPLICATION	DESIRED FIT TYPE	FIT(inches)
Low speed, or spring preload.	Loose	.0001L to .0005L
Medium speed	Line to Line	.0002L to .0002T
High speed	Light press	.0000 to .0004T
High speed, high load	Tight press	.0002T to .0006T

 

L = Loose fit, T = Tight fit, d = Bearing I.D., D = Bearing O.D.

Selecting the right bearing for a specific application requires a review of performance requirements and material operating limitations. Certain design considerations must be met in the device and bearing to address such factors as load, speed, temperature, environment, method of lubrication and fit. Often times sacrifices must be made in one area to satisfy another to achieve optimal life.

Many factors come into play when selecting a bearing or creating a new design in which a ball bearing will be employed. Most important are speed, load and temperature.

Load	Speed	Temperature	Environment	Shaft / Housing
Direction	Constant	Average	Vacuum	Fits
– Radial	Variable	Max.	Gas or Air	Geometry
– Axial	Hi or Low	Operating	Contaminants	Tolerances
– Shock	dN>	Ambient		Mounting
– Combined	250,000? *

 

*d=bore in mm, N=RPM