Working with Linear Bushings Pt 3: Length, Load, and Surface Treatment

Simple linear movement could be achieved using simple parts but as designs evolved and demand increased so did mechanical parts.  Diving into linear bushings specifically, there are a variety of types to accommodate for different loads.  Bushing length and load capacity relationships will be examined as well as bushing materials.

There are four length types of Linear Bushings [1] Single, [2] Double, [3] Long, and [4] Singles exclusively designed for dual use. The bushing length difference affects the following guiding performances.

a)Load Capacity
b)Guiding Accuracy

a) The Relationship between Bushing Length and Load Capacity

Longer bushings have more ball bearings and the load on each ball bearing in contact will be smaller. This effect can be confirmed from the fact that the load ratings increase as the lengths of single, double, and long bushings increase. Therefore, selecting a linear bushing with longer length improves the load capacity which then extends life and reliability. (Fig.1)

fig-1-bushing-length-and-load

b) The Relationship between Bushing Length and Guiding Accuracy

As the bushing length increases, the accuracy improvements can be expected as follows.

A) Accuracy improvement by averaging effect of guide rail (shaft) errors (Averaging Effect: see * below) ([Fig.2])
B) Accuracy improvement by reducing errors due to clearances ([Fig.3])
*Averaging Effect on Shafts: By increasing the bushing length and the number of ball bearings, error elements such as unevenness and swells on shaft surface are averaged, and the effects of the error elements are reduced to less than half.

fig-2-averaging-effects

fig-3-bushing-clearance

Therefore, load capacity and guiding accuracy can be improved by increasing the length of linear bushings. For this reason, [4] Singles (exclusively designed for dual use) are used for higher accuracy applications in some cases. ([Fig.4])

fig-4-dual-use

Explanation of Rail (Shaft) Deflection Calculations ([Fig.5])

Deflection of shafts for linear motion mechanism composed of linear bushings and shafts are calculated as follows.

δ=W・a3・b3/3・E・I・L3

a:Distance from supported end to the load point.
b:Distance of supported end opposite of a to the load point.
L:Shaft support span distance.
E:Young’s Modulus
I:Shaft Sectional Moment of Inertia
I=π・d4/64≒0.05d4
d:Shaft Radius

When a=b=L/2, δ=W・L3/0.96・E・d4. In order to reduce the shaft deflection, design with increased shaft diameter (affects by 4th power), or reduce the supporting span distance (affects by 3rd power).

fig-5-shaft-deflection

Characteristics and Application Examples of Materials and Surface Treatments

Construction materials and surface treatments of linear bushings are as follows.

Outer Cylinder Material Surface Treatment Retainer Material Ball Bearing Material Application Example
SUJ2(Bearing Steel) Resin/SUS440C Equiv. SUJ2 General slides requiring wear resistance
SUJ2(Bearing Steel) Low Temp. Black Chrome Same as Above SUS440C Equiv. Glare intolerant optical equipment
Precision positioning for clean room use
SUJ2(Bearing Steel) Electroless Nickel Plating Same as Above Same as Above Slides for clean room use with chemical resistance
Slides requiring wear resistance
SUS440C Equiv.(Stainless Steel) Same as Above Same as Above Low loading clean room use and food/medical related equipment

 

Characteristic Comparison of Surface Treatments

Outer Cylinder Material Surface Treatment Characteristics
SUJ2(Bearing Steel) – SUJ2 is steel and will rust
Same as Above Low Temp. Black Chrome – Low friction coefficient superior in wear resistance
– Formation of thin and uniform layer
– Black plating color does not reflect light, and has good heat absorption
Same as Above Electroless Nickel Plating – Often used in clean rooms for good chemical resistance and corrosion resistance
– Hard plating layer retains glossy finish
– Plating layer is non-magnetic

 

As you can see there are a variety of materials and surface treatments for bushings.

Now that different linear guide mechanisms, bushing comparisons, and guide performances have been examined, it is time to apply these principals to applications.  In the next two series of blogs, we will review different types of linear applications that use linear bushings in their designs.  Stay tuned!

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