Wheel and Axle-Connections of parts

 A wheel and axle is a simple machine, and a very useful one at that. However, not all forms of a wheel and axle are equal. Let's look at the four cases:

Wheel is fixed, Axle is fixed, Body is fixed

                      

In this case, the wheel, axle, and body are all fixed. This case will never work, because the wheel, being attached to the body, cannot rotate at all. This case is similar to, say, pushing a round pair of headphones across a table. The earphones, despite being circular and looking like they are a wheel, will not rotate and will merely slide.

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Wheel is not fixed, Axle is not fixed, Body is not fixed

                 

In this case, everything is free moving and does not have solid connections. When you push this, it will spin, but it will be highly unstable. The reason for this is because without the wheels being fixed with the axle, it may still spin, but it will not have consistent contact and control over its speed and vectors, making it highly unstable.

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Wheel is not fixed, Axle is fixed, Body is fixed

          

Similar to the last case, the wheel is not fixed to the axle, and as such it will still spin, but will not have control over itself and will be unstable.

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Wheel is fixed, Axle is fixed, Body is not fixed 

             

This is the ideal case. The wheel is fixed to the axle, so it has control over itself, but it is NOT fixed to the body, so it can rotate. This creates a system where the wheel is balanced, and rotates without issue, and it is what is used in the vast majority of Wheel and Axle systems.

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Table:  + - fixed, O - not fixed

Cases	Wheel	Axle	Body	Result
Case 1:	+	+	+	Never works
Case 2:	O	O	O	Uncontrollable
Case 3:	O	+	+	Uncontrollable
Case 4:	+	+	O	Controllable

Video:

This video, created by Teacher Krithik Prasad, is a demonstration of how these four cases work, and may help you understand the concept of Wheel and Axle connections better.

Link for Video