September 2012 Archives

Assembling bores and rods are classified into two assembling procedures based on the dimensional relationship between the two parts.

Here, the glossary of press-fitting that requires forces for assembly is explained.

[Table 1] Glossary of press-fitting work

Explanation on press-fitting procedures

- Press-fitting is a procedure to fit two parts of a shaft diameter and a bore diameter where a targeted interference is utilized by spreading by the parts fit.
- The surface pressure and friction of spreading this interference apart is applied as the press-fitting pressure. The way to apply this press-fitting pressure classifies the press-fitting operation.
- Obtaining of this press-fitting pressure requires some preparation operations with associated difficulties such as accurate locating of two parts and managing of gripping forces, but the press-fit pressure would commensurate the resultant parts combing reliability.

Defective troubles regarding screw fastening are classified, and the countermeasures on screw fastening device troubles are explained.

(1)Screw fastening defects and the factors classified

Screw fastening defects can be classified as the following.

Of the above, below are the screw fastening device related defects.

a) Errors on screw fastening work

	・	Improper controls on the automated screw fastening unit
	・	Defective screw fastening (Misalignment, Oblique fastening, etc.)
	・	Screw positioning error

b) Inadequate screw fastening tool

	・	Wear and damages on screw fastening bits
	・	Wear on screw chuck
	・	Inadequate maintenance on fastening tool

(2)Examples of screw fastening device trouble countermeasures

1)Screw fastening device (in case of pneumatic driver) trouble countermeasures

	（1）	Since the driver receives reactive forces during screw fastening, it is likely that some functional errors may occur at some usage intervals.
	（2）	In many cases, the problems are attributed to rotational motion instabilities due to wear on rotational shaft, foreign objects adhesion, rusts, and seizure. Apply lubricant oil and repeat forward and reverse rotations.
	（3）	Check air supply pressure of the pneumatic driver.

2)Countermeasure examples on inadequate screw fastening tool

	（1）	With an assumption that the screw driving bits are subject to wear deformation, maintain spares and periodically (quantity base managed) replace the bits.
	（2）	Screw chuck related parts are the same as the (1) above.

3)Management techniques for the stable operation of the screw driving device

In order to maintain the stable operation of the automated machinery, not only proper management of the automated machinery itself, but preventative measures such as quality control maintenance of assembly parts and finished goods will be effective.

[Examples of Managed Items]

The Screw Fastening Unit is the most important mechanism of the automated screw fastening device. The representative screw fastening methods and examples of the fundamental structure of automated devices are introduced here.

(1)Screw Fastening Methods

The representative structure of screw fastening unit is as follows.

There are following screw fastening methods for the screw fastening units. Variations of the fastening forces can be improved in the order as a>b>c {high accuracy}.

	a) Torque Control Method	： A method of transmitting the power of electric motor to the shaft of the screw fastening unit as rotational torque.
	b) Angular Control Method	： A method of controlling the screw tightening angle with motors with angular encoders.
	c) Yield Point Method	： A tightening force control method by detecting the deformation behavior of tightened screws.

(2)Example of a screw fastening unit

[Fig.1] is an example of an automated nut fastening device with 6 axes of parallel equipped pneumatically operated nut fastening actuators (nut runner). Nuts are tightened after the 6 axis nut fastening actuators are moved by the cylinder on linear guides.

Useful countermeasure production technology against screw parts misfeeding, which comprises a high percentage of the troubles, will be explained.

There is a large variety of features of screws depending on fastened product characteristics such as a) screw head shapes, b) thread shapes, c) screw hole shapes, d) presence of washer or not, e) material, surface treatment differences, and many others. Therefore, the screw parts feeding systems are required to adopt a wide variety of screw types.

(1)Screw parts feed system configurations

Screw parts feed system configuration is as follows.

Screw parts feed unit - - - Device that automatically stores - Aligns - Separates the screws. See [Photo 1]

Screw distribution unit ---

A device that supplies the screws from the alignment unit to the chuck of the automated fastening device. It utilizes air pressure and etc. to transfer the screws through the distribution tube.

(2)Countermeasure examples for screw parts supply device

1)Countermeasure examples on screw parts feed units

	(1)	In many cases, screw parts feeding misses (Causes: mixed contents, out of parts, hang ups due to burrs) can be effectively handled with error detections by photoelectric sensors and abnormality determination with a timer on the screw alignment feeder.
	(2)	Feeder internals and chutes where the components travel must be kept clean in order to maintain smooth sliding of the components.

2)Countermeasure examples of screw distribution unit troubles

	(1)	Because the screw transfer is based on sliding by air, the distribution tubes should be stabilized with gradual curvatures, and detailed maintenance on air leakages due to tube wear is important.
	(2)	Proper distribution methods according to the screw shapes (head shapes, weight, length, etc.) need to be selected.