Page 58 - Engineering Plastics Handbook

32 Introduction

p = contact surface pressure between hub and shaft, MPa (psi)
c
F out = pull-out force, N (lb)
In some applications, torsion is applied to a press-fit assembly.
Transmitted torque M is calculated by [6]
2
in
M = F R s = 2πµhR s p c = F out R s
Snap-fit designs
A classical beam theory applied to snap-fit designs uses cantilever beams
with an overhang at the free end. Depth of overhang determines the
amount of deflection, which has an entrance side angle a and a retrac-
tion side angle b. The mating force is a function of a and b.
Here are suggestions for excellent snap-fit performance:
1. Wall thickness should be uniform.
2. Place snap-fit in area where the undercut section can expand easily.
3. Circular geometric shape is ideal.
4. Avoid weld lines in the area of the undercut.

References

1. M. A. Dorgham and D. V. Rosato, eds., “Designing with Plastics and Advanced
Composites—Technical Advances in Vehicle Design” Proceedings of the International
Association for Vehicle Design. Also D. V. Rosato, “Designing with Plastics: A Guide
to Product Design,” Interscience Enterprises Ltd., Geneva-Aeroport, Switzerland,
1986.
2. “Bending Condition Options,” eFunda Inc. (engineering fundamentals), online pub-
lisher in Sunnyvale, Calif., USA.
3. Roy Beardmore, RoyMech, see http://www.roymech.co.uk.
4. SP Systems, “Core Materials in Polymeric Composites,” Isle of Wight, UK.
5. D. V. Rosato and M. G. Rosato, Plastics Design Handbook, Kluwer Academic
Publishers, Dordrecht, The Netherlands, 2001.
6. Paul A. Tres, Designing Plastics Parts for Assembly, 4th ed., Hanser Gardner, 2000.
7. Eric W. Weisstein, “World of Plastics,” see www.scienceworld.wolfram.com.
8. George S. Brady, Henry R. Clauser, and John A. Vaccari, eds., Materials Handbook,
15th ed., McGraw-Hill, New York, N.Y., USA, 2002.
9. Mahendra D. Baijal, ed. Plastics Polymer Science and Technology, Wiley Interscience,
New York, N.Y., USA, 1982.
10. John J. Aklonis and William J. MacKnight, Introduction to Polymer Viscoelasticity,
Wiley, New York, N.Y., USA, 1983.
11. Stephen L. Rosen, Fundamental Principles of Polymeric Materials, Wiley, New York,
N.Y., USA, 1982.
12. Solomon Benjamin Bezaleel, Structural Design with Plastics, Van Nostrand Reinhold,
New York, N.Y., USA, 1982.
13. J. D. Ferry, Viscoelastic Properties of Polymers, Wiley, New York, N.Y., USA, 1970.
14. University of Leeds, Polymer Science and Technology, Leeds, UK.
15. University of Missouri-Rolla, Materials Research Center, Advanced Materials
Characterization Laboratory, Rolla, Mo., USA.