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) E xcellent load capacity because unit moves on rails assembled on the base plate.
(see Photo 2)
Linear Bushing, Oil free Bushing ⇒ Shaft supported at both ends ⇒Light
to medium load Slide Guide ⇒ Fixed rail on base ⇒ Light to heavy load
(2) About the difference in performance of friction coefficient
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. 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 = rotatry 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)
Linear motion ⇒ Point contact ⇒ Concentrated vertical load distribution
⇒ Not applicable to heavy load Slide guide ⇒ Surface contact ⇒ Distributed
vertical load distribution ⇒ Applicable to heavy load
(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.
MISUMI USA, INC.
1105 Remington Road, Suite B
Schaumburg, IL 60173
www.misumiusa.com
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