Mastering Linear Bushings - 1: Linear Bushings and Other Linear Motion Products

Linear motion products are the most commonly used motion elements in automation of transfer / locating / assembly.
Here, three types of linear guides, [1] Linear Bushing, [2] Slide Guide, and [3] Oil Free Bushing will be compared and
explained as we master linear bushings.

(1)Comparison of Linear Motion Products Characteristics

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

The characteristics and structure of each type are explained below.

(2)The characteristics and structure of linear motion

(1) About the difference in performance of load capacity

Linear Bushing and Oil Free Bushing
a) A unit using Linear Bushings or Oil Free Bushings which moves on a Shaft where both-ends are supported, a heavy
load can elastically deform the Shaft. (see Photo 1). (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.)

Linear Slide Guide
b) Excellent load capacity because unit moves on rails assembled on the base plate.
(see Photo 2)

Guides, rolling steel balls are accurately guided by retainer so low frictional resistance is achieved. Where as in Oil Free
Bushing two surfaces are sliding against each other, which result 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

Notes
Value of friction coefficient can influence the capacity of actuator and heat generation during
movement. Oil Free Bushings are inappropriate because of heat dissipated by continuous
high-speed operation. In the case of using Air Cylinders, speed control of the start / stop is
not possible. Mechanisms such as Shock Absorption Dampers need to be set to stop the speed
softly. It can shorten the cycle time.

(3) About performance difference of guide accuracy

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

a) Shafts are used for the rail with a Linear Bushing. The fit between Shafts and bushing is clearance fit
(when g6 tolerance Shaft is used we have normal clearance, when h5 tolerance Shaft is used we have
smaller clearance fit).

b) Linear Guide uses profile rail (or track rail) and bearing block (or slide unit). Fit ranges for 0-3µm for
clearance fit types to -3-0 µm for preload types.

c) Oil Free Bushing is used with a Shaft, where the clearance Linear Bushing therefore guide precision
is lower.

Notes
Because of the raceway design, steel balls inside linear ball guides can have 2 or 4 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)

(4) About Environmental Conditions and Ease of Maintenance

The performance difference depends on materials used.

a) Linear Bushings and Linear Slide Guides maintain long term reliability with the use of
lubricating grease. Therefore they are not applicable to be used in an environment that
exceeds the environmental performance of it.

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

Mastering Linear Bushing – 2: Straight and Flanged type Bushing

shing structure and features

Both straight (Photo 1) and flanged (Photo 2) follow similar structural design. Main advantage of Flanged

Linear Bushing lies in its compact design (Figure 1):


1. Value of friction coefficient can influence the capacity of actuator and heat generation during movement. Oil Free
Bushings are inappropriate because of heat dissipated by continuous high-speed operation.

2. In the case of using Air Cylinders, speed control of the start / stop is not possible. Mechanisms such as Shock
Absorption Dampers need to be set to stop the speed softly. It can shorten the cycle time.

(2) Using straight and flanged types

The following should be considered when making Linear Bushing selection.

1. Decide whether force will be applied to the Linear Bushing ⇒ Choose a flanged type if the Linear Bushing must bear to
force.

2. Decide how much space is available on the surface to which the Linear Bushing is to be attached. ⇒ Refer to part (3)

As shown in Figure 2, depending on the design, Linear Bushings can either move while Shafts are fixed, or be stationary (fixed) while Shafts are in motion.

The Linear Bushing in the component a) receives inertia force from the moving component, therefore the Linear Bushing must be firmly screwed to the housing.
As for component b), an Air Cylinder moves the Shaft in the Linear Bushing. The Retaining Ring fixing the Linear Bushing only receives the frictional force from
the Shaft. Therefore a compact design using a straight type is fine. The same can be said for c).

Fixing a Straight Linear Bushing, using Retaining Ring or Stopper Plate
(Fixing Plate) (Photo 3) is shown in Figure 3.

Notes on installation angle:
Load rating of Linear Bushing varies according to the load position on the circumference.
Linear Bushing usually has 4-6 rows/ball tracks, that are set on even angle. When installing,
if possible avoid positioning Linear Bushing so that the ball track is under direct load (Figure 4),
otherwise that row will directly bear the load (Figure 4a).

For example Figure 4 shows a Linear Bushing with 5 rows. The variance of dynamic load rating
is as follows. (right figure ÷ left figure). Therefore, angle should be installed as in the right picture.

Static Load Rating
(Right figure a ÷ left figure b)=1.46

Dynamic Load Rating
(Right figure a ÷ left figure b)=1.19

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