Working with Linear Bushings Pt. 2: Bushing Comparisons

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 with comparisons of [1] Linear Bushings, [2] Linear Guides, and [3] Plain Bushings.

Comparison of Linear Bearing Characteristics

Three linear guiding types are roughly compared below.

∆- good

O- insufficient

bushings

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

Relationship Between Characteristics and the Structures of Linear Bushings

(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 load (Photo 1).
For vertical motion applications where the load is not applied 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.

(Photo 2)

photo1andphoto2

 

(2) 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 = Converts Rotary motion to Linear motion
b) High Friction Resistance = Large Fiction force = Requires large torque/force = Push directly with linear cylinder

friction

Proceed with Caution

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 easily controllable for acceleration and deceleration velocities. But 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 Accuracy

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 “Pre-load (-3~0μm).
c) Guiding accuracy of plain bushings are lower since they are used with shaft clearances larger than linear bushings.

 

Bearing Load Contact Types

Bearing contact arrangements of linear bushings and linear guides are different. Linear bushings have Point Contact arrangement and localized contact pressures are high. This design is not suitable for heavy loads.  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 which allows for heavier loads. The contact arrangement differences reflect on the load capacities of these systems ([Fig.1] & [Fig.2]).

load

Environmental Resistance Characteristics and Maintainability

This is determined by the construction material.

a) Linear bushings and linear guides depend on the lubrication used for long term reliability thus they cannot be used in environments exceeding the environmental resistance of the lubrication used because debris may get into the bearing balls.

b)Plain bushings can be used without any lubrication and has higher environmental adaptability and better maintainability.

 

There are a variety of characteristics of bearings and bushings to consider when designing your application.  They are a versatile mechanical part which can be used in many ways.  For long lasting designs, it is best to stay within the parameters of each part or keep spares on hand for easy replacement in the field.  Seeing side by side comparison of each type will help determine what will work best in your design.  Thus ends the second series of working with linear bushings.  Happy Designing!


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