September 2009 Archives

Three bearing retaining components used are marked with (*) below.

Example 1 (See [Fig.1])

This is an example of rotating shaft retention using a bearing holder set (T-shaped two bearing type, or bottom mount two bearing type) and bearing nuts with anti-loosening set screws. Loosening of the bearing nut is prevented by use of a set screw and a set piece made of copper alloy. The set piece made of soft copper alloy is first inserted in a screw hole, and a set screw is tightened to crush the soft alloy piece on the shaft thread to prevent the bearing nut from loosening.

Example 2 (See [Fig.2])

This is an example using bearing holding pins. A bearing holding pin holds against the inner ring O.D. from the end side, and a collar from the inside is pressed against the inner ring O.D. of the bearing from the other side to fix the bearing to the shaft.

Example (See [Fig.1])

Here, an axial mounting method of bearings is explained using a linking mechanism with bearings mounted on two T shaped bearing holders.

Three bearing retaining components used are marked with (*) below.

	・	A metal washer is used on the tightening nut side of cantilever pin to hold against the inner ring of a radial bearing.
	・	A bearing spacer is placed between the link arm and a bearing to support the link's rotation motion.
	・	Bearing in the other bearing holder is fixed to the cantilever pin shaft by it's inner ring with a bearing end cap.

A bearing (rolling ball bearing type) needs to be retained in three directions, radial, axial, and circumferential in relation to it's housing and a shaft. The radial and circumferential direction retention is mainly based on a concern on fit tolerance selection (see Tutorial #103). The remaining "axial direction" retention cannot be solved by press fitting of the bearing. The axial retention methods will be explained here with examples.

For instance, high speed bearings such as end bearings for ballscrews will require some measures against heat expansion, but here we'll discuss bearing mounting methods for low to medium speed bearings.

(1) MISUMI's bearing mounting/retaining components

Following standard components are available.

Example 1 (See [Fig.1])

Mounting method for belt tension adjusting idler pulley bearing

	・	The idler pulley is mounted with a cantilever pin. This example uses a nut tightened cantilever type.
	・	Bearing retaining collar" is used to fix the idler pulley bearing.
	・	A washer is placed between the bearing and the collar to provide an ample access to the collar's tightening screw.
	・	The washer O.D. should match the O.D. of the bearing's inner ring.

Example 2 (See [Fig.2])

V-groove idler pulley mounting with a bearing end cap.

・ Flanged, screw mounted cantilever pin is used. ・ The bearing end cap needs to match the O.D. of the cantilever pin shaft.

Here, we'll introduce some most advanced linear motion guide technology, levels above the slide guides.

High definition displays such as PC monitors and cellular phone displays require sub-micron level accuracies (i.e. 0.3 micrometers or better). Most of these key components are manufactured using ultra precision molds. To manufacture the molds, ultra precision machines that can achieve sub-micron level accuracies are used.

Typical examples of ultra precision machine.

Linear guide mechanism construction of the ultra precision machines can be categorized into the following two types.

（a）About contacting slide guide

	・	[Photo 1] below shows a representative configuration for a contacting slide guide for an ultra-precision machine. It uses a V-groove hand scraped by highly skilled operators and ultra precision grade roller bearings.
	・	Moving body and the roller bearings are placed on the hand scraped V-groove, and repeated adjustments are made using a measurement instrument.
	・	This linear guide mechanism is driven by a ball screw. In order to negate any vertical errors that may be generated by the ballscrew deflection, the ballscrew nut is retained by parallel springs and only linear force is transmitted.

（b）About non-contacting slide guide

	・	shows an air bearing arrangement using Granite material typically used for surface plates.
	・	Since the granite material is of natural origin, changes over time can be neglected, insensitive to temperature variation caused distortion, and vibration damping in nature.
	・	The air bearing achieves ultra precise non-contacting linear guide by maintaining an air gap of approx. 6 micro meters with pressurized air.
	・	The linear guide rail is a convex shaped block, and is hand finished by polishing for the final accuracy.
	・	Air bearing rigidity and resonance avoidance must be taken in consideration for the air pad design of the moving body [Photo 4].
	・	Sliver colored rectangular objects are linear motors.
	・	Completely non-contacting linear guide mechanism is comprised of air bearings and linear motors.

In the ultra precision motion control world as shown above, controlling the environment is required such as environmental temperature stabilization (i.e. within +/- 0.1 deg. C), independent foundation design for complete isolation from any external vibrations, and dust-less environment.