Fundamentals of

Mechanical Engineering

Mechanical Design and Analysis: Stress Analysis - Dimension Tolerances in Stress Analysis

Stress analysis is usually done using the design dimensions of a part. However, the actual dimensions may vary due to tolerances in manufacturing. To find out how to account for the differences, search on ‘tolerances and stress analysis’ and open the chapter 2.7 Drawing Tolerances in the “Airframe Stress Analysis and Sizing (3rd Edition)”:

Mechanical Design and Analysis: Components - Bearing Selection

Bearing selection is one of the most frequently performed tasks in the design of mechanical components with movable parts. To review selection guidelines covering different types and sizes of bearings, search on ‘bearing selection’ and open the section A1 Selection of Bearing Type and Form in the “Tribology Handbook (2nd Edition)”. This section contains data for bearings suitable for continuous and oscillatory movement. Then continue to the section A2 Selection of Journal Bearings and A3 Selection of Thrust Bearings. Each of these two sections contains bearing selection charts and information for selection bearings working under special environmental conditions and performance requirements. For example, for journal bearings:

For example, if you are looking for a journal bearing working at 600 rpm under the load 1000 lbf, open the interactive graph “Selection by load capacity of bearings with continuous rotation”. In it we see that 2 rolling bearings are suitable for this speed: 25 mm for loads under 774 lbf and 50 mm loads under 2709 lbf (see circle points on the chart), while rubbing plain bearings and even plain bearings (see square point) are not suitable.

(Click image to enlarge.)

Mechanical Design and Analysis: Joining - Snap Fits

Many plastic components today are designed with snap-fits to reduce the cost of assembly. Knovel has a lot of information about the design of snap-fits. Determining the deflection and the engagement forces of the snap-fit is an important task in snap-fit design. To find equations for calculating these forces for cantilever snap-fits, search on cantilever snap-fits’ and open the section 14.28.2 Cantilever Snap-Fits in the “Handbook of Plastics Joining”.

First, review calculations for permissible deflection:

Deflection force:

And finally, Engagement force:

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