Principles of Mechanical Devices


All machines, however basic or complex, are made up of simple mechanisms. A mechanism is a device which changes an input motion and force, into a desired output motion and force.

As in electronic systems a simple mechanical systems diagram contains three stages:

  • Input: the input begins the system, such as the lever shown below for the car jack.
  • Process: this is the thinking or operating part of the system, which begins to raise the jack.
  • Output: this is the system's response which lifts the vehicle.

The car jack uses a screw and nut mechanism.

This changes the turning motion (or rotary motion) at the screw into a straight line motion (or linear motion) at the nut, and changes a small force at the screw into a large force at the nut.

In this way, only a small effort is required by the motorist, to raise the heavy load through the action of the scissor mechanism.

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Although designed to do different jobs, all mechanisms have some things in common:

  • They involve some kind of motion.
  • They involve some kind of force.
  • They make a job easier to do.
  • They need some kind of input to make them work.
  • They produce some kind of output.