Example of a hydraulic system.
Example of a hydraulic system.


Hydraulic systems use liquids to transmit force between pistons. Pistons are sealed containers that are used to move fluids. They usually use oil, as water can corrode the pistons. They can be used to magnify force, so that an effort force on one piston can be magnified to move a heavier load with the other piston. The mechanical advantage of a hydraulic system can be calculated this way:

(area of larger piston) / (area of smaller piston)

30cm² / 10cm² = 3, so the mechanical advantage of this hydraulic system is 3. This means that if a piston with an area of 10cm² is pressed on with a force of 10N, the piston with an area of 30cm² would be able to lift a load of 30N. However, the smaller piston would have to travel three times as far as the larger piston.

Hydraulic Rams and Valves

Since the pistons always have to be moving, a pump powered by an engine quickly applies force to a piston. It needs to apply a much smaller force than the digging power of the backhoe but at a high speed.

Unlike the example above however, the pistons in the backhoe need to move in different directions in order to move the backhoe in various directions. This is achieved by having a piston with oil on both sides of the piston head, with oil being pumped in through two openings. This kind of piston is called a hydraulic ram.

Pushing oil into bothe sides if the hydraulic ram can be done by using a valve to change the direction the oil is pumped. This is called a spool valve. The valve can be moved to direct oil from the pump into different hoses, leading to different sides of the ram.

A hydraulic ram.
A hydraulic ram.