Fluid Dynamics:Hydraulics and Pneumatics

How does a hydraulic lift work?

Pascal’s Principle states that pressure in a liquid is independent of direction. Pressure is force divided by area, so force exerted by a liquid is equal to the pressure times the area. A hydraulic lift has a small piston in a cylinder. If you exert a force on the piston, it will create a pressure in the fluid. The lift also has a second, large area cylinder and piston. The fluid creates a pressure in this cylinder that exerts a much larger force on the piston because of its larger area. Therefore the lift has a mechanical advantage. Of course, energy is still conserved, so the small piston must move much farther than the large piston.

An automobile lift, used in many automotive repair shops, allows the operator to use very little force to lift an automobile off the ground, by pushing liquid from a small-diameter cylinder and piston through a thin tube that expands into a larger-diameter cylinder and piston, which is located beneath the vehicle to be lifted. Since the liquid cannot be compressed like air, the liquid from the small cylinder is pushed into the large cylinder, forcing the large piston to move upward.


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