Low Cost Automation Tutorial

#060 Working with Linear Bushings - 3: Single/Double/Long Bushings and Surface Treatments

Category : Linear Motion Components

October29, 2010

(1) Linear Bushing Lengths and Guiding Performances

There are three lengths for MISUMI ball bushings, [1] Single, [2] Double, [3] Long, and [4] Tandem Use Singles where the designer can freely design the layout of two bushings. The bushing length affects the following guide performance parameters.

a）Load Capacity Performance
b）Guiding Accuracy

a) Length and Load Capacity Relationship

When the bearing length increases, the number of contacting balls increase thus the load applied on each ball in contact is reduced. This effect can be seen as the load capacities increase as the bushings [1], [2], and [3] lengths increase. The longer bushings can be expected to have improved load bearing performances (= longer life and better reliability) ([Fig.1]).

[Fig.1] Relationship of Bearing Length and Load Capacity

b）Relationship of Bearing Length and Guiding Accuracy

Following accuracy improvements can be expected as the bearing length increases.

	a）	Shaft errors are averaged and accuracy is improved (Averaging Effect: see Notes)
	b）	Effects of shaft/bearing clearances are reduced and accuracy is improved ([Fig.3])

※	Averaging Effect: Errors are reduced to 1/2 or less by increased bearing length averaging the surface error elements (surface roughness, bumps, undulations).

[Fig.2] Explanation of Bearing Averaging Effect

Therefore, load capacity and accuracy can be improved by increasing the bearing length. The [4] Tandem Use Single Bushings are sometimes used for relatively high accuracy applications for this reason.

[Fig.4] Example of

(2) Shaft Deformation Calculation

Shaft deformation value for linear bushing based mechanism can be obtained as follows.

δ＝Ｗ・a³・b³/３・E・I・L³

　　a：Shaft support span distance
　　b：Load point to the other end distance
　　L：Shaft support span distance
　　E：Young's modulus
　　I：Shaft cross section moment of inertia
　　I＝π・d4/64≒0.05d4
　　d：Shaft radius

Since when: a=b=L/2, δ=W・L3/0.96・E・d4, increase the shaft diameter (effective by power to the 4th) or reduce the support span distance (effective by power to the 3rd) to reduce the shaft deformation.

[Fig.5] Diagram Explanation of Shaft Deformation Calculation

(3) Characteristics and App. Examples of Construction Materials and Surface Treatments

Linear bushing materials, surface treatments and app. examples are shown below.

Outer Cylinder Material	Surface Treatments	Retainer Material	Ball Material	Application Examples
SUJ2	-	Resin/SUS440C Equiv.	SUJ2	General guiding application requiring wear resistance
SUJ2	Low Temp. Black Chrome	Same as Above	SUS440C Equiv.	Optical app. requiring non-glare. Clean room. High precision apps.
SUJ2	Electroless Ni-P	Same as Above	Same as Above	Clean room, Chemical resistance, Wear resistance
SUS440C Equiv.	-	Same as Above	Same as Above	Light load clan room, Food, Medical apps.

Performance Comparisons of Surface Treatments

Outer Cylinder Material	Surface Treatments	Characteristics
SUJ2	-	SUJ2 steel will rust
Same as Above	Low Temp Blk. Chrome	Low friction coefficient, good wear characteristic Forms thin and uniform layer Black plating layer is non-glare, good heat absorption
Same as Above	Electroless Ni-P	Used for clean room apps for good chemical/corrosion resistance Hard and glossy plating layer Plating layer is non-magnetic

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