Page 271 - Failure Analysis Case Studies II

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Fig. 1. Person performing a leg press.

15.5mm b------- 39mm 1
1 I

8.0 mm

Fig. 2. Diagram of adjustment pin.

a fixed footpad. To perform the exercise, the user pushes against the footpad and shoulder stops
using the upper leg muscles. The sled rolls along the shafts until the person’s legs are fully extended.
The user then slowly allows the sled to return to the original position. This constitutes one exercise
repetition. An adjustable stack of cast iron weights provides the resistance force. The weight stack
is connected to the sled by a steel cable. The steel cable travels through a series of pulleys and is
connected to the sled by a telescoping box beam. To accommodate people of different heights, the
distance between the shoulder stops and the footpad must be adjustable. This is accomplished by
the telescoping box beam connected in series between the cable and the bottom of the sled. The
inner tube is connected to the cable while the outer tube is firmly welded to the sled. The load is
transferred from the inner beam to the outer one through a spring-loaded steel pin, Fig. 2. The
pin is firmly attached to the outer box tube and locks into a series of holes drilled in the inner tube.
Adjustments are made by disengaging the spring loaded pin from one hole in the inner box tube
and engaging it into another. By engaging the pin into different holes, the sled can be moved a