August 2009 Archives

Combinations of shafts and linear bushings (or oil-less bushings) as rails and sliders are the simplest form of linear guide mechanism.

（1）Shortcomings of linear guide with shafts and bushings.

Following shortcomings can be noted.

（2）Advances in linear motion guide devices

Simplified depiction of the advancements is as follows.

Explained by simplified pictorial

	a）	A ball spline has ball rolling grooves on the shaft, and free to move only in linear direction. (See [Fig.1])
	b）	A guide rail support is provided under the entire length of the shaft, increasing the shaft rigidity. Mating linear bushing has a lower section removed to account for the guide rail. (See [Fig.2])
	c）	Since a free moving ball retainer is the bearing element, no movement restrictions in either rotation or linear.
	d）	Principally similar in construction to b), but ball rolling grooves are provided on the rail to increase bearing contact surfaces for high load capability.

A hollow shaft has a hollow bore through the center of the shaft, and has the following characteristics.

Hollow shaft characteristics

	1.	The through bore can be utilized for various purposes.
	2.	Lighter in weight compared to solid shafts of the same diameter.
	3.	Notably, the through bore does not have high concentricity with the shaft, and cannot be used as an accurate guiding bore.

Each characteristic is explained in examples.

（1）Utilizing the through bore.

（a）Cooling of the shaft.

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In order to prevent the shaft temperature rise due to friction, fittings can be mounted on the ends to introduce various cooling medium (air, water, etc.) through the hollow bore.

（b）Guiding electrical wire and other plumbing components.

・

Motor cables and air cylinder's air supply can be run through the bore in cases where way covers and other components are obstructing the routes. (See [Fig.1])

（2）Lighter in weight compared to solid shafts of the same diameter.

For applications where the shafts reciprocate, the weight of the shafts affect the sizing of actuator's prime mover. Cost reduction, energy savings, and motor heat reduction can be gained by keeping the prime mover as small as possible. However, the hollow shafts generate louder noises when subjected to bearing motion, as compared to other shafts in general.

L shaped shaft holders are available in four shaft mounting configurations. Characteristics for the four types are organized below as selection references.

The holders in [Fig.1 Side insertion type] and [Fig.3 Separate type] have similar appearances with a difference only in shaft mounting methods. It can be seen from the figures that the shafts are inserted/removed in different manners. The difference affects the moving table assembly procedure and maintenance work. Chart below compares the differences of the four types.

Other than the types shown in the table above, there are four other shaft holders such as T-type, bottom mount type, and side mount type. These can also be selected based on the perspective shown below.

Perspective for shaft holder selection

	1.	Assembly/maintainability
	2.	Placement relationship with surrounding components
	3.	Degree of mounted part strength
	4.	Ease of standardization

A shaft collar retains itself on a shaft by clamping on it's circumference. This clamping shaft collars can be utilized as stoppers for the linear motion mechanism. Benefit of utilizing the shaft collars as stoppers is that stop position can easily be adjusted. Fig.1 shows a representative usage of shaft collar stopper mechanism, and the characteristics are listed below.

	・	A dual-shaft linear guiding system is driven by an air cylinder.
	・	Slit-type shaft collars are used for the air cylinder position stopper. Two collars are tied together with a stopper plate for increased strength.
	・	Stop position can easily be adjusted by loosening the slit-tightening screws on the collars.
	・	Two linear shafts are tied together at ends with a connecting plate for increased structural strength.

Additionally, there are two mounting options available for the shaft collars: threaded hole type, and through hole type. The collars can be fastened from either side of the stopper plate, as shown on [Fig.2] below.