April 2011 Archives

Linear bushings and shafts are used quite frequently. See "LCA Mechanism Examples".

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Shafts and bushings each require some fabrication for installation, but the costs could be reduced by adopting the mounting designs for standardized products such as MISUMI FA Mechanical components.

Friction forces are generated when solid objects in contact make relative motion. These friction forces must be managed properly with lubrication and etc, or may result in troubles shown below.

1.Friction of solid objects resulting in seizures.
2.Wear begins and accelerates.
3.Increased power losses.

It is important to manage the friction and wear.

(1) About Friction

There are to types of friction.
　a）Sliding Friction
　b）Rolling Friction

The sliding friction is the type seen with Gib Slides (previous Vol.), skies and sleds. The [Fig.] shown below is an ancient Egyptian mural. It depicts a scene of transporting a large stone statue. A man can be seen at the center pouring friction reducing lubrication.

Show below are friction coefficients of various material combinations.

The rolling friction can be represented by Linear Motion Guides (previous Vol.), miniature ball guides, and etc. where friction occurs with balls and rollers. If the sliding friction is to be defied as the "Sled Era", the rolling friction can be dabbed as the "Rolling Cart Era".

Rolling friction coefficient examples are shown below. (Sliding friction coefficient and rolling friction coefficient cannot be compared equally.)

(2) About Wear

When solid objects are in contact without any lubrication between them, minute surface features are in directly coupled states. Friction occurs when this coupled features are decoupled by sliding motion, and this is the friction resistance (force). When the decoupling occurs, minute sections of material is ripped away where contacting. This is the wear. When the friction increases, the wear simply increases also.

The sliding guides are fundamentally comprised of guides (also called the shafts, rails) that constrict the motion trajectories and moving members (also called the bearings), and the following characteristics are required.

	1.	Support the moving loads.
	2.	Small and stable moving force.
	3.	Provide assured long term stable accuracies under high speed/acceleration operations.

The items 1 and 2 above are static performances (not associated with motion), and 3 is a dynamic performance (associated with motion). Sliding guide selection and structural designing will require basic knowledge shown below.

Here, representative linear sliding guides, rotary guides, and curve guides are explained.

(1) Linear Sliding Guides

Linear sliding guide is a mechanism allowing linear movements with accuracy and with little friction of linear motion units such as air cylinder driven units.

Linear Sliding Guide Types Characteristics MISUMI FA Catalog Products Linear Guides with Round Shafts ([Fig] a, b) - Single shaft config. if rotation regulation not needed - Large DIA guides for load support - Friction reduction mechanism can be added - Linear Bushings - Oil-free Bushings - Jig Bushings - Miniature Ball Guide Sets Linear Motion (LM) Guides (Guides) ([Fig] c) - Selectable performance criteria: Friction reduction, High rigidity, High accuracy - Linear Guides - Slide Way - Slide Tables Gib Guides (Flat gibs, Triangle rails) ([Fig] d, e) - Wide variety of performance levels such as low cost and ultra high performance (#) achievable due to low parts counts. - Linear Guides - Guide Rails (#: Air bearings, etc.)

(2) Rotary Guides

Similar thoughts as above are also applicable to rotary shaft guides (ball bearings). Inertia applicable on linear motion does not apply to rotary motion, and advanced ball bearing technologies are used to obtain high speed guiding characteristics.

(3) Curve Guides

For curve (circular-arc, etc.) guides, linear motion guides formed into curvature driven by motors and ultrasonic motors are used.