Paper Authors

Abstract

NOTE: The first page of text has been automatically extracted and included below in lieu of an abstract

Session 3425

Self-Aligning Mechanisms, Forgotten Part of ME Curriculum

Wieslaw M. Szydlowski

University of Nebraska-Lincoln Mechanical Engineering Department

Abstract

Mechanical engineering students designing machinery are confronted with the lack of a reliable method in determining if the machinery will move after assembly, and under what conditions assembly is possible at all. Gruebler’ and Chebyshev’s formulas found in the majority of American textbooks are unreliable. A simple, though almost unknown, loop analysis developed by Ozol can solve the problem. The loop analysis allows one not only to check if the mechanism can move, but also provides a valuable insight into the design of self-aligning mechanisms insensitive to manufacturing and assembly errors.

Introduction

One of the most important tasks in designing a mechanism is checking if the proposed device constitutes a mechanism and not a rigid structure. In the language of mechanical engineers, the procedure is called checking the mobility of the mechanism. The mobility of the mechanism is defined as the number of degrees of freedom that the mechanism possesses with respect to one arbitrarily chosen link. One can determine the mobility of the mechanism by “fixing” links one by one, until the mechanism is not able to move. The number of fixed links that immobilizes whole mechanism is equal to its mobility. The Gruebler’s and Kutzbach’s formulas for the mobility of a plane mechanism and found in majority of textbooks on kinematics, [1], [2], [3], although they are known to produce misleading results.

The second task of the designer is to formulate geometric conditions (parallel axes, tight tolerances on some dimensions, etc.) to make assembly possible. The geometric conditions imposed in this stage on the links of the mechanism are also known as the redundant constraints. An example of a system with redundant constraints is a four-leg table placed on a warped floor. To avoid shaking of the table one of its legs must have a strictly defined length.

Unfortunately, parts are machined with errors. If errors are too large, the assembly may not be possible, or would require force to connect its parts together. Reshetov [4], gives examples of when bad geometry of links caused internal loads on the parts that exceeded many times the loads for which the machine was designed. To visualize the problem, compare two sets of links for two four-bar linkages shown in Fig 1. The set shown on the left can be easily assembled

• Citation

• Format
  • APA
  • APA - LaTeX bibitem
  • MLA
  • MLA - LaTeX bibitem
  • Bibtex
  • EndNote - RIS

Szydlowski, W. M. (2000, June), Self Aligning Mechanisms – Forgotten Part Of The Me Curriculum. Paper presented at 2000 Annual Conference, St. Louis, Missouri. 10.18260/1-2--8689