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#074 Working with Linear Bushings - 1: Linear Bushings and Other Linear Bearings

Category : Linear Motion Components
May13, 2011

Linear guiding devices are the most frequently used motion elements on automation equipment for palletizing, transferring, positioning and assembling. Here, linear bushing application tips will be explained along comparisons of [1] Linear Bushings, [2] Linear Guides, and [3] Plain Bushings.

(1) Characteristic Comparisons of Linear Bearings

Three linear guiding types are roughly compared below.

TypeLoad CapacityFriction CoefficientGuide AccuracyEnvironmental ResistanceMaintain abilityCost
Linear BushingsLow
Linear GuidesHigh
Plain BushingsMedium

Relationship of the characteristics above and the structures are explained below.

(2) Relationship of the characteristics and the structures of linear guidings

(1) Differences in Load capacities
Linear Bushings vs. Plain Bushings
a)Linear motion units with linear bushings and plain bushings have suspended guide shafts supported on the ends and the shafts would deform under high loadings ([Photo 1]).
(For vertical motion applications where the load do not apply horizontally on the shaft, this problem does not exist and a compact design is possible)

Linear Guides
b)Excellent load capacity due to the rails directly mounted on bases.

Linear Bushings/Plain Bushings >> Move on end supported shafts >> For light ~ med. load applications.

[Photo 1] [Photo 2]

(2) About Friction Coefficient Differences

These differences are due to the differences in sliding methods (rolling motion or sliding motion). These differences will directly affect the selection of driving actuator.

a)Low Friction Resistance = Little Fiction force = Possible to drive with small motors = Rotary motion to Linear motion Conversion
b)High Friction Resistance = Large Fiction force = Requires large torque/force = Push directly with linear cylinder


Cautionary Points on Usage
1. The high friction coefficients affect driving actuator selections and operation heat generation. The plain bushings are not suitable for frequent motion at high speeds.
2. Air cylinders are not well controllable for acceleration/deceleration velocities thus some soft-stop mechanisms such as shock absorbers will need to be installed to achieve high speed operations and vibration control.

(3) Differences in Guide Accuracies

This parameter is mainly determined by clearances between the rails and bearings.

a)Linear bushings are used with round shafts, and operate with small amount of clearances such as "Clearance Fit: g6" or "Medium Fit: h5"
b)Linear guides are used as pairs of dedicated rails and precision bearings in "Slight Clearance (0~3 μm) or "Preloaded (-3~0μm).
c)Guiding accuracies of plain bushings are lower since they are used with shaft clearances larger than linear bushings.

Cautionary Points on Usage

Bearing contact arrangements of linear bushings and linear guides are different. Linear bushings have Point Contact arrangement and localized contact pressures are high. Linear guides use rails with concave grooves that are shaped to fit the spherical shape of the bearing balls and the contact arrangement is a Surface Contact. Applied loads are distributed over the contact areas. The contact arrangement differences reflect on the load capacities of these systems ([Fig.1] & [Fig.2]).


Linear Bushings >> Point Contact >> Localized Vertical Load Distribution >> Not suitable for heavy loads
Linear Guides >> Surface Contact >> De-centralized Load Distribution >> Able to bear heavy loads

(4) Environmental Resistance Characteristics and Maintainability

This is determined by the construction material.

a)Linear bushings and linear guides depend on lubrications used for long term reliability thus they cannot be used in environments exceeding the environmental resistance of the lubrications used.
b)Plain bushings can be used without any lubrication and has higher environmental adaptability and better maintainability.