Basic Hydraulics At Work

Hydraulics is based on a very simple fact of nature - you cannot compress a liquid. Now if you put that liquid into a sealed system and push on it at one end, that pressure is transmitted through the liquid (confined/sealed vessel) to the other end of the system. The pressure is not diminished.

An example of forces applied to pressure.

A force is applied to a piston (A1) that presses on an enclosed fluid. Pascal's Principle tells us that a pressure (F2) will be applied on the much larger area (A2) piston. This allows a small force to be amplified into a much larger force.

Hydraulic Principles

Pressure is stress that is exerted uniformly (or the same way) in all directions.

Pressure is measured in units of force applied per unit of area.

Force exerted on a square inch of area of a confined liquid is transmitted at every angle to every square inch of area of the interior of the vessel. If one of the areas of the interior of the vessel is a second piston, the force would be transmitted to every square inch of its area as well.

Hydraulic / Mechanical example: If pump piston A is pushed down 12 inches, then cylinder piston B will rise 1 inch.

Pressure = 2,000 PSI
Piston A @ 1 sq. in. = 2,000 lbs.
Piston B @ 12 sq. in. = 24,000 lbs.

Basic Physics

Basic physics tells us that we can trade off force for distance in all mechanical systems. In a hydraulic system, we do this by changing the relative size of the pistons at each end of the system.

For example, a small piston moving a relatively long distance (say a foot) will exert pressure on a larger piston at the other end. The force will be enough to move a heavy weight a small distance (much less than a foot).

Engineers can calculate exactly how much distance needs to be traveled and the relative sizes of the pistons required to move a particular weight. This is the principle that allows relatively small cylinders to move extremely heavy loads.

Example

If D = 2 ft then Force = 500 lbs.

If D = 10 ft then Force = 100 lbs.

If D = 20 ft then Force = 50 lbs

Hydraulic Formulas

The basic principle behind any hydraulic system is very simple - pressure applied anywhere to a body of fluid is transmitted equally in all directions, with the pressure acting at right angles to any surface in contact with the fluid (Pascal’s Law).

The amount of force a hydraulic cylinder can generate is equal to the hydraulic pressure times the effective area of the cylinder. (Effective Area is the surface area of the piston face in square inches.)

Effective Area of Piston = Diameter x Diameter x .78 = (answer in sq. in.) Force = psi x Area of Piston = (answer in pounds) Pressure = Force in Pounds ÷ Area of Piston = (answer in psi)

FORMULAS:

• Force = psi x Area of Piston
• Effective Area = Diameter x .785
• Pressure = Force / Area of Piston (answer in psi)
• Displacement = Area of Piston x Stroke = (answer in Cubic Inches)