Sticking with the gym equipment theme, I decided to analyse the Squat Machine which allows users to perform squats with weights, without worrying about stability. I thought this would be a good machine to look into given that this week's Brainware was on designing a precision linear axis. The squat machine has two guide rods along which the carriage slides. The carriage itself has two linear bearings on each side. The guide rods are angled away from the vertical to provide proper form to the user. 

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I was curious about how the system was designed to prevent jams along the rail. The length of the bearings were about 4 times the diameter of the guide rod. We know from Saint Venant's Principle that this is a good design. However, I realised that while performing squats, the user does not always apply force along the center of the guide rail. Thus the applied force is not always along the centre of friction. At times, the rail can experience large lateral loading. Also, the assembly of the machine might not be perfect and the rails may not be parallel. Given all these factors, I was surprised to see that the rails never jam. Knowing how the designers achieved this, would be very useful for my Rising Desk design.

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Closer inspection of the end of the guide rail and the linear bearing holder, revealed how the machine works so smoothly. As can be seen in the video and in the images below, a compliant rubbery bushing is used in between the guide rod and the frame and well as in between the linear bearings and the carriage. This bushing compresses when any lateral load is applied and the the net error of the guide rods is elastically averaged out. This prevents the carriage from jamming.

 

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