How to use linear bearings-1: linear bearings and other linear guide parts

Linear guide parts are most commonly used in motion mechanisms in automation fields such as transfer, transportation, positioning, and assembly.
Here, we will focus on the use of linear bearings by comparing and explaining three types of linear guide parts: [1] linear bearings, [2] sliding guides, and [3] oil-free bushings.
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(1) Comparison of characteristics of linear motion bearings
A rough comparison of the characteristics of the three types of linear guide parts is summarized in the following table. \n
The following introduces the above characteristics and structural relationships.
(2) Relationship between characteristics and structure of linear guide parts
1) Performance differences in load-bearing characteristics
■ Linear bearings and oil-free bushings
a) Movable elements assembled with linear bearings or oil-free bushings are generally assembled on shafts (guide rails) with support structures at both ends to achieve their motion functions. When a large load is loaded, the shaft is easily deformed ([Photo 1]).
(In addition, when direct-motion guidance is performed in the vertical direction, since the shaft does not need to support the load of the movable component, a simple structure can be adopted that ignores the load problem.
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■Linear guide rail
b) The movable component moves on a guide rail fixed to the base and has excellent load-bearing characteristics ([Photo 2])
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Linear bearings, oil-free bushings ⇒ Moves on a shaft (guide rail) fixed at both ends ⇒ Linear motion with light to medium loads
Linear Guide ⇒ Movement on the guide rail fixed to the base ⇒ Linear motion with light to heavy loads\n2) Performance differences in friction coefficient
Here, the difference in guide sliding method (ball sliding or surface sliding) determines the performance difference. The difference in friction coefficient is directly related to the selection of drive actuator.
a) Small friction resistance = small friction force = can be driven by a small torque motor = convert rotary motion into direct motion
b) Large friction resistance = large friction force = large torque or thrust drive required = direct drive with a direct-acting cylinder\n■Points to note when using
1. The size of the friction coefficient will affect the ability of the driving equipment and the heat generated during movement. Oil-free bushings are not suitable for continuous high-speed motion where the working conditions are high heat generation.
2. When using a cylinder, the initial/stop speed cannot be controlled like a motor. High-speed movement and vibration suppression can be achieved by installing flexible braking mechanisms such as shock absorbers.
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3) Performance differences in guidance accuracy
The performance is basically determined by the gap between the bearing and the guide rail.
a) The linear bearing uses a cylindrical shaft as the guide rail. The gap between the bearing and the guide rail adopts "clearance fit: g6" or "transition fit: h5" to maintain a small "clearance" state for sliding.
b) The linear guide adopts a dedicated guide rail, and high-precision bearings and guide rails such as small gap type (0~3μm) or pressurized type (-3~0μm) are used in pairs.
c) Compared with linear bearings, oil-free bushings have a larger gap with the guide rail (shaft) and lower guiding accuracy.
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■Precautions on use
Linear bearings and linear guides have different contact states between their respective balls and guide rails. Linear bearings are in a point contact state, and the contact part locally bears a large load. The contact part between the guide rail and the slider ball of the linear guide adopts a groove shape, so that the ball and the guide rail surface are in surface contact, and the load on the contact part is dispersed. There are also differences in the load-bearing characteristics between the two in terms of the contact state of the sliding part. ([Fig. 1][Fig. 2])\nLinear bearing ⇒ Point contact state ⇒ Local vertical load distribution ⇒ Not suitable for large load conditions
Linear guide ⇒ Surface contact state ⇒ Dispersed vertical load distribution ⇒ Can withstand larger loads
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4) Regarding environmental resistance and maintainability
This difference in performance is determined by differences in the materials of construction.
a) Linear bearings and linear guides can achieve long-term reliability due to the effect of lubricating oil (grease), so the environmental resistance index of the lubricating oil cannot be exceeded in the working environment.
b) Oil-free bushings are generally used in working environments where their performance can be achieved without lubricating oil. They have good environmental resistance and maintainability.