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)
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.
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]) |
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. 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). |
Characteristics and Application Examples of Materials and Surface Treatments
Construction materials and surface treatments of linear bushings are as follows.
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Characteristic Comparison of Surface Treatments
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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!
