Low Cost Automation Tutorial: June 2011 Archives

June 2011 Archives

#077 Working with Linear Bushings - 4: Linear Bushing Application Example on Simplified Automation

By システム管理者 on June 17, 2011 12:00 AM | No Comments | No TrackBacks

Linear bushing characteristics explained so far are reorganized as shown below.

[1]	Linear bushings are low cost and well balanced linear guide bearings. (Good Cost vs. Performance)
[2]	Easy to select actuators for. (Low priced cylinder driven or medium priced motor driven)
[3]	Quiet and light drive mechanism can be realized by use of synchronous belts.
[4]	For vertical direction guidance, drive center can be placed at guide center enabling simple and compact mechanism structure.

Usage and characteristics of linear bushings are explained through application examples on simplified automation machinery.

(1) Synchronous belt and stepping motor drive

Synchronous belt drives have various advantages such as quietness, light-weight, low cost, as well as not requiring lubrication maintenance. In 2-axes XY table designs, top Y axis weight is lightened to reduce the load on the bottom X axis. For this, synchronous belt drive is often used for Y-axis design.

a) [Fig.1] shows a typical 3-axes XYZ drive mechanism

Linear guides are used on X-axis, and linear bushings are used on Y and Z axis. For driving, synchronous belts and a ball screw are used.

[Fig.1] Typical 3-axes XYZ Drive Mechanism

b) [Photo 1] is a IC chip pallet transfer machine Y-axis application example. The Y-axis motion is converted into reciprocation motion with a synchronous belt.

[Photo 1] IC Chip Pallet Transfer Machine and Belt Drive Mechanis

c) [Fig.2] is an application example for a single axis robot with following features.

	1.	Widely spaced dual linear bushing guides for good load capacity and accuracy.
	2.	Traveling pulley arrangement ([Fig.3]) is used for high motor power efficiency and good positioning precision.
	3.	Lightweight and quiet operational characteristics from synchronous belt drive.
	4.	Vertical inline orientation of the shaft and belt prevents the bushings from rotating, even for one shaft structure.

[Fig.2] A Structure Example of Improved Motor Power Efficiency and Precision Using Pulleys

Principle of Traveling Pulley

[Fig.3] Principle of Traveling Pulley

#076 Working with Linear Bushings - 3: How to Use Single/Double/Long and Surface Treated Products

By システム管理者 on June 3, 2011 12:00 AM | No Comments | No TrackBacks

(1) Linear Bushing Lengths and Guide Performances

There are four length types of MISUMI Linear Bushings [1] Single, [2] Double, [3] Long, and [4] Singles (exclusively designed for dual use). The bushing length difference affects the following guiding performances.

a）Load Capacity
b）Guiding Accuracy

a) Relationship of Bushing Length and Load Capacity

Longer bushing has more ball bearings, and the load on each ball bearing in contact will be smaller. This effect can be confirmed from the fact that the load ratings increase as the lengths of bushings [1], [2], and [3] increase. Therefore, selecting a linear bushing with longer length improves the load capacity (Extended life, and increased reliability) (Fig.1)

[Fig.1] Relationship of Bushing Length and Load Capacity

b) Relationship of Bushing Length and Guiding Accuracy

As the bushing length increases, the accuracy improvements can be expected as follows.

	A)	Accuracy improvement by averaging effect of guide rail (shaft) errors (Averaging Effect: see * below) ([Fig.2])
	B)	Accuracy improvement by reducing errors due to clearances ([Fig.3])

*	Averaging Effect on Shafts: By increasing the bushing length and the number of ball bearings, error elements such as unevenness and swells on shaft surface are averaged, and the effects of the error elements are reduced to less than half.

[Fig.2] Graphical Explanation of Averaging Effects

[Fig.3] Relationship of Bushing Length and Clearance Errors

Therefore, load capacity and guiding accuracy can be improved by increasing the length of linear bushings. For this reason, [4] Singles (exclusively designed for dual use) are used for higher accuracy applications in some cases. ([Fig.4])

[Fig.4] Example of Single <a href= Linear Bushings in Dual Use" name="image" width="527" height="250" />

(2) Explanation of Rail (Shaft) Deflection Calculations ([Fig.5])

Deflection of shafts for linear motion mechanism composed of linear bushings and shafts are calculated as follows.

δ＝Ｗ・a³・b³/３・E・I・L³

　　a：Distance from supported end to the load point.
　　b：Distance of supported end opposite of a to the load point.
　　L：Shaft support span distance.
　　E：Young's Modulus
　　I：Shaft Sectional Moment of Inertia
　　I＝π・d4/64≒0.05d4
　　d：Shaft Radius

When a＝b＝L/2, δ＝W・L3/0.96・E・d4. In order to reduce the shaft deflection, design with increased shaft diameter (affects by 4th power), or reduce the supporting span distance (affects by 3rd power).

[Fig.5] Graphical Explanation of Shaft Deflection Calculation

(3) Characteristics and Application Examples of Materials and Surface Treatments

Construction materials and surface treatments of linear bushings are as follows.

Outer Cylinder Material	Surface Treatment	Retainer Material	Ball Bearing Material	Application Example
SUJ2	-	Resin/SUS440C Equiv.	SUJ2	General slides requiring wear resistance
SUJ2	Low Temp. Black Chrome	Same as Above	SUS440C Equiv.	Glare intolerant optical equipment Precision positioning for clean room use
SUJ2	Electroless Nickel Plating	Same as Above	Same as Above	Slides for clean room use with chemical resistance Slides requiring wear resistance
SUS440C Equiv.	-	Same as Above	Same as Above	Low loading clean room use and food/medical related equipment

Characteristic Comparison of Surface Treatments

Outer Cylinder Material	Surface Treatment	Characteristics
SUJ2	-	- SUJ2 is steel and will rust
Same as Above	Low Temp. Black Chrome	- Low friction coefficient superior in wear resistance - Formation of thin and uniform layer - Black plating color does not reflect light, and has good heat absorption
Same as Above	Electroless Nickel Plating	- Often used in clean rooms for good chemical resistance and corrosion resistance - Hard plating layer retains glossy finish - Plating layer is non-magnetic