On the surface, a child’s toy water gun and the hydraulic car lift you have in
your service bay may not seem to have much in common. But, what if we told you
that they operate using the same basic principle: Transmitting force via an
The working mechanics of the water gun seem simple enough. When you pull the
trigger, you are exerting force against all the water within the tube. And this
force is what sends the water shooting out the nozzle. But, something else
happens when you pull the trigger. The initial force used to pull the trigger is
multiplied in the process — which is why the water ends up shooting out of the
gun with a much higher force than you used to pull the trigger.
Your Hydraulic Car Lift Works Just Like That Toy Water Gun
Just like the water gun, the hydraulic car lift works because force applied at
one end is transmitted, via hydraulic fluid, to another point in the system. The
beauty of the system lies in the fact that the force is multiplied as the fluid
travels from the first endpoint to the other endpoint. This is why a small
amount of pressure can end up lifting a tremendous amount of weight.
Trading Force for Distance
Most hydraulic car lifts rely on a multiple piston system. The first piston is
used to apply the force and the last piston is used to lift the car. However,
the distance between that first piston and that last piston plays a major role
in the amount of weight the lift can handle. Just like a traditional mechanical
lever, the greater the distance between the two endpoints, the more weight it
The cool thing about the hydraulic system it that adding (or decreasing) force
is often as simple as changing the size of the first piston in relation to the
last piston. For example, using a mechanical principle commonly referred to as
“Trading Force for Distance,” a narrow cylinder can be connected to a wider
cylinder via the hydraulic lines. When the oil is compressed into the wider
cylinder, the distance the force needs to travel is reduced, but it still gets
multiplied in the process.
Why Does an Air Bubble Cause Trouble In a Hydraulic Lift?
The oil, or (in the example of the toy gun) the water, used in a hydraulic
system is what is known as an incompressible fluid. That basically means the
fluid’s density remains consistent regardless of how much pressure is applied.
Air, on the other hand, compresses easily and does not distribute force evenly.
(Think about what happens when you squeeze a balloon) Because of this, when a
hydraulic system gets an air bubble in the line, the whole system will become
corrupted. Instead of the force being evenly distributed across the oil, the air
bubble winds up absorbing the majority of the force. The result is that the
second piston barely moves, and the car lift struggles to get off the ground.
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