Elements of Motion

Linear Bushings, Linear Motion Products

Linear motion products are the most commonly used motion elements in the automation of transfer, locating, and assembly systems. Here, three types of linear motion components will be compared and explained: [1] linear bushing, [2] linear guide, and [3] oil free bushing.

Comparison of linear motion products characteristics

The characteristics of the three types of linear motion products are shown in the table below.

1. The difference in performance of load capacity

a) Linear bushing and oil free bushing - A unit using linear bushings or oil free bushings that moves on a shaft where both ends are supported; a heavy load can elastically deform the shaft. In the case of vertical-directional linear motion mechanism, the shaft does not need to support the load of the unit, thus load capacity can be ignored.

b) Linear guide - Excellent load capacity because unit moves on rails mounted to the base plate.

A linear bushing and oil free bushing have the shaft supported at both ends and can support a light to medium load. A linear guide has the rail mounted on base and can support a light to heavy load.

2. The difference in performance of friction coefficient

The difference of performance depends on how motion is achieved. In linear bushing and linear guides, rolling steel balls are accurately guided by retainer so low frictional resistance is achieved, whereas in oil free bushings, two surfaces are sliding against each other, which results in higher friction.

a) Low friction = low frictional force = low turning torque = rotary motion can be turned into linear motion

b) High friction = high frictional force = high turning torque or thrust force is required = linear cylinder is recommended

Note: The value of friction coefficient can influence the capacity of the actuator and heat generation during movement. Oil free bushings are inappropriate because of heat dissipated by continuous high-speed operation. Since the flow control units are not accurate, it is extremely difficult to control acceleration/deceleration of the system with an air cylinder. Shock absorbers will help achieve a higher speed of the cylinder system in the middle of the stroke since they help soften the impact at the end of it.

3. Performance difference of guide accuracy

The performance depends on the clearance of bearing and rail/shaft.

a) For a shaft used in conjunction with a linear bushing, the fit between the shaft and bushing is a clearance fit - when g6 tolerance shaft is used, there is normal clearance; when h5 tolerance shaft is used, there is smaller clearance fit.

b) A linear guide uses a profile rail (track rail) and a bearing block (slide unit.) The fit ranges from 0µm to 3µm for clearance fit types and from -3µm to 0µm for preload types.

c) An oil free bushing is used with a shaft, where the clearance linear bushing therefore guide precision is lower.

Note: Because of the raceway design, steel balls inside linear guides can have two or four contact points. This allows even distribution of complex load. Steel balls inside linear bushing have only one - or single - contact point with the shaft, which results in centered load distribution. (See Figure 1 and Figure 2)

A linear shaft has one point contact; concentrated vertical load distribution; and is not applicable to heavy loads.

A linear guide has surface contact; distributed vertical load; and is applicable to heavy loads.

4. Environmental conditions and ease of maintenance

The performance difference depends on the materials used.

a) Linear bushings and linear guides maintain long term reliability with the use of lubricating grease. Therefore they should not be used in an environment that exceeds the envi ronmental performance specifications. Always check the grease per formance information offered by the supplier.

b) Oil free bushings provide higher performance because they do not require the use of lubricating grease.

Straight and Flange Type Bushing

1. Bushing structure and features

Both straight and flanged bushings follow similar structural design. The main advantage of a flanged linear bushing lies in its compact design.

2. Using straight and flanged bushings

The following should be considered when making linear bushing selection:

a) Decide whether force will be applied to the linear bushing. Choose a flanged-type if the linear bushing must bear the force.

b) Decide how much space is available on the surface to which the linear bushing is to be attached. Refer to step 3 - Installing linear bushing.

As shown in figure 3, depending on the design, linear bushings can either move while shafts are stationary, or be stationary while shafts are in motion.

X-Y-Z-drive table for figure 3

a) X-axis: linear bushing moves - flanged used*

b) Y-axis: linear bushing does not move*

c) Z-axis: linear bushing does not move in Z direction

* Use straight bushing with retaining ring or stopper plate.

The linear bushing in component A shown in figure 4 receives inertia force from the moving component; therefore the linear bushing must be firmly mounted to the housing. As for component B in figure 4, an air cylinder moves the shaft inside the linear bushing. The retaining ring constraining Figure 4: Examples of straight linear bushing installations Figure 5: Bearing rows and dynamic load rating the linear bushing only receives the frictional force from the shaft. Therefore, a compact design using a straight bushing is acceptable. The same can be said for C, seen in figure 4 as well.

3. Installing linear bushings

a) Constraining a straight linear bushing and using a retaining ring or stopper plate (fixing plate) is shown in figure 5.

b) Notes on installation angle: The load rating of linear bushings varies according to the load position on the circumference. Linear bushings usually have four to six rows/ball tracks that are set on an even angle. When installing, if possible, avoid positioning linear bushings so that one of the ball tracks is under direct load (see figure 5), otherwise that row will directly bear the load. (See figure 5a).

For example, figure 5 shows a linear bushing with five rows. The variance of dynamic load rating is as follows: (right figure ÷ left figure). Therefore, the angle should be installed as in the right picture.

Static load rating (right figure b ÷ left figure a) = 1.46

Dynamic load rating (right figure b ÷ left figure a) = 1.19