How to use linear bearings −3: Differences between single lining type, double lining type, extended type and surface treatment

(1) Bearing length and guide performance of the bearing
Linear bearings can be divided into 4 types according to the bearing length: [1] Single lining type, [2] Double lining type, [3] Extended type and buyer's own design type [4] (Special design using 2 single lining types ). The difference in bearing length is directly related to the underlying guide performance.
a) Bearing performance
b) Guidance accuracy
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a) Relationship between bearing length and load-bearing performance
The longer the bearing, the more support points the bearing has and the smaller the load required for each bearing contact point. This conclusion can be drawn based on the actual situation of [1], [2], [3] types of linear bearings having different lengths and the rated load increasing sequentially.
Therefore, choosing a linear bearing with a longer bearing length can improve the product's load-bearing performance (=increased life and reliability) ([Figure 1]). \nb) Relationship between bearing length and guide accuracy
The longer the bearing length, the higher the guidance accuracy.
1) Improve product accuracy by averaging the guidance error of the guide rail (shaft) (average effect: refer to the note) ([Figure 2]).
2) Improve product accuracy by reducing the gap error with the guide rail (shaft) ([Figure 3]).
※Averaging effect of bearings: By increasing the length of the linear guide rail bearing to increase the number of bearing supports, the error factors (surface roughness and bending deformation) on the guide rail surface are averaged, and the influence of the error factor is suppressed to less than half. \nTherefore, by increasing the bearing length, the load-bearing performance and guidance accuracy can be improved. Therefore, type [4] (special design using 2 single-liner types) linear bearings are often used in certain high-precision working environments. ([Fig. 4]) \n (2) Calculation instructions for guide rail (shaft) deformation ([Fig. 5])
In a linear motion mechanism composed of a linear bearing and a shaft, the deformation of the shaft can be calculated using the following formula.
δ＝Ｗ・a3・b3/(３・E・I・L3)
a: The distance from the support end point to the load position
b: The distance from the end point of the support on the opposite side of a to the load position
L: The support spacing of the shaft
E: Young’s modulus
I: Second moment of section
​I＝π·d4/64≈0.05d4
d: shaft diameter
W: The load borne by the linear bearing (unit N)
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When a=b=L/2, δ=W・L3/(9.6・E・d4)
It can be seen from this
If you want to reduce the deformation of the shaft, you should adopt the design idea of ​​thickening the shaft diameter (4 times the effect) or shortening the shaft support spacing (3 times the effect). \n(3) Characteristics and application examples of part materials and surface treatments
The constituent materials, surface treatments and applications of linear bearings are as shown in the table below. \nComparison of surface treatment characteristics