Working with Linear Bushings Pt 1: Linear Guide Mechanisms

Engineers have many options when it comes to designing for linear motion.  The need for compact and efficient designs evolved the variety of linear guide mechanisms.  Rails and sliders are the simplest form of linear guide mechanisms, which are a combination of shafts and linear bushings (or oil free bushings).

Here are the limits of using shafts and linear bushings.

Design Limits of Shafts and Bushings Solutions Design Evolution of Linear Guide Mechanisms
Linear bushings cannot be used for rotary motion along the shaft circumference. Two shafts are used to constrict the rotary motion Ball spline + Spline shaft configuration.
End supported shafts configuration does not provide enough rigidity against high downward loads. Use of larger diameter shaft and double linear bushings. Shaft guide rails + shafts + linear bushings

Advances in Linear Motion Guide Devices

Simplified depiction of the evolution is as follows.

Figure 1

Linear Motion Guide Devices Explained…

a) A ball spline has ball rolling grooves on the shaft that are free to move only in a linear direction. (See [Fig.1]). This enables the circulating motion of evenly spaced balls to be maintained and high speed response to be achieved. Other benefits are that it eliminates collision, mutual friction between balls, and produces low noise.

figure-2

b) A guide rail support is provided under the entire length of the shaft, increasing the shaft rigidity. The mating linear bushing has a lower section removed to account for the guide rail. (See [Fig.2])
c) Since a free moving ball retainer is the bearing element, there are no movement restrictions in either rotation or linear motion.
 Figure 3
d) Another linear guide design which is principally similar in construction to b), but have ball rolling grooves allow for higher load carry capability as the rail design increases bearing contact surface. (See [Fig.3]).

Figure 3

As technology evolves, design components must evolve with it.  Hence, the need for linear motion devices to handle more load, speed, be compact, and have long life were required.  This also allows for diversity in the design process.  Depending on your application parameters and requirements, there may be a linear motion device for your design.

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