Page 270 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 255

Fatigue failure analysis of a leg press exercise machine

P.J. Vernon, T.J. Mackin"
Unicersity of' Illinois at Urbana-Champaign, Department of Mechanical and Industrial Engineering, 1206 West Green SI.,
Urbana, IL 61801, U.S.A.
Received 21 August 1998; accepted 9 September 1998

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Abstract
The following paper is an engineering failure analysis of an adjustment pin used in a leg press exercise
machine. The pin is used to allow adjustment of the machine for people of different heights. It was modeled
as a cantilever beam subjected to varying forces depending on the weight stack setting. The fracture occurred
at a point of localized stress concentration. The analysis shows an elastic stress concentration of 1.67 which
would cause plastic deformation and a fatigue stress concentration of 1.38 that led to the eventual fatigue
failure. Based on actual user data. a block loading analysis was used to calculate a pin lifetime of approxi-
mately one year. The pin actually failed after 1; years of use. Design changes are recommended to reduce
the size of the stress concentration and subsequently increase the life of the adjustment pin. 1998 Elsevier
Science Ltd. All rights reserved.

Keywords: Fatigue; Fatigue design; Sports equipment failures

1. Background

1.1. Machine description

The following engineering analysis concerns the fatigue failure of an adjustment pin used within
a piece of weight training equipment. The particular piece of fitness equipment is called a leg
press/squat machine. It is specifically designed to exercise the quadricep muscles of the upper leg,
Fig. 1. The machine consists of a moveable sled mounted to a set of linear bearings that travel
along a pair of hardened steel shafts. The two steel shafts are mounted to a steel support frame
and are inclined at an angle of 10" to horizontal. The user lies on the moveable sled with his/her
shoulders squarely positioned against a set of padded shoulder stops and places his/her feet against

* Corresponding author.

Reprinted from Engineering Failure Analysis 6 (I), 1-12 (1999)