November 2009 Archives

Simple automation machinery (LCA) are comprised of the following.

The tutorial explanations to this point have been centered around usage points on mechanisms and machine element components. From this point forward, mechanism designs on 1. LCA that can operate at higher speeds, and 2. LCA that can repetitively operate programmed movements over long time durations will be explained.

(1) Motion mechanism designs and related engineering

The mechanisms introduced as "Essence of Karakuri fixtures (LCA Tutorial #79~)" are encompassed within the concerns of Mechanism Engineering. There, the design tasks can be carried out with only concerns on "balancing of forces", without any considerations for high speed operation related matters. For designing mechanisms capable of complex controlled motion at high speeds, the designer should be well oriented with Mechanics and Machine Vibration. If machined part strength or actuator selections and such are evaluated on these grounds failures can be reduced, and even the failures can be reengineered for further enlightenments.

[Fig.1] shows the engineering disciplines related to motion mechanism design.

When designing automated assembly systems using a various fixtures, careful considerations for shaft/bore fits along with locating pin/hole shapes and configurations are needed.

Principles of locating feature designs

	*	Adequately chamfer the pins holes to facilitate smooth pin entry.
	*	Two constraint points cannot simultaneously exist per one insertion action.
	*	Extraction action must also be considered when designing.

[Fig.1] shows typical configurations of locating pins and locating holes. Subjects related to the locating parts are as follows.

	1.	Required position locating accuracy.
	2.	Assembler/apparatus accuracy. (Degree of work difficulty or ease, etc.. Machine accuracy.)
	3.	Assembled parts accuracy. (Case of high accuracy parts, or variable accuracy parts, etc.）
	4.	Workability of parts such as contacting surface length, etc. (Relationship between produceability and accuracy, etc)

Locating pins / Example of concentricity error and pin shapes" name="image" width="527" height="240" />

Locating pin/hole design in [Fig.1] is to be proceeded with the following.

Locating pins

Locating holes

Rotary bearings are machine elements that support rotating shafts accurately while providing low friction condition. In order to obtain stable bearing performances over prolonged duration, appropriate bearing fit design considerations are required to avoid any negative effects ([Chart 1]) that may arise from bearings' rotary operation.

[Chart 1] Designs for bearing fit and issues to be solved

Design examples of rotary bearing application.

An example used here is based on "Inner race rotation : Outer race retained" arrangement shown in [Fig.1].

Selection of bearing fits based on shaft diameter

Transition fit or interference fit is selected to ensure the shaft and the inner ring will always rotate together.

[Chart 2] Selection example of fits (case for radial bearings)

Selection of fits for bearing holder bores

Clearance fit should be chosen for ease of assembly while preventing any deformation of bearings' outer/inner races. In general, [Clearance fit] (H7) is used. In case of light loading applications (H8) can be used, or (G7) when large temperature variations are to be considered.

[Chart 3] Fit selection examples for bores (case for radial bearings)

Proximity sensors can be: magnetic (reed switch type), high frequency oscillation, capacitive types. The sensors are offered in a various form factor such as rectangular, cylindrical, through-hole styles, and the popular cylindrical types are threaded on the outer body to facilitate easy adjustments.

Proximity sensor mounting

(1) Characteristics and detection distance of proximity sensors

(2) Cylindrical proximity sensor mounting examples

[Fig.1] shows an example of proximity sensor mounting using a shaft collar (4-hole type) and a mounting bracket.

[Fig.2] is an example using a post clamp.