What Is Pressure, Work, And Energy?

Pressure

Fig. 1-2. Determining pressure

PRESSURE is the amount of force (in pounds) exerted on an object or a substance divided by the area (in square inches) over which the force is exerted. Although pressures are measured and specified in different ways, the use of pounds per square inch (psi) is the most common. As shown in Fig. 1-2, if a 100 pound force is applied to an area of 8 square inches, the resulting pressure is 12.5 pounds per-square-inch. Force can also be calculated by multiplying the pressure by the area.

In hydraulic systems, we are concerned with two kinds of pressure-atmospheric and hydraulic. Atmospheric pressure (at 14.7 psi) is at work at all times on fluid reservoirs that are vented to the atmosphere. Hydraulic pressure is created by the fluid pump and acts on all internal passages on the discharge side. On the intake side a "negative" pressure exists. This more properly referred to as a partial vacuum, and is expressed in inches of mercury instead of psi.

Pressure gauge used in hydraulic systems measure only pressure that is higher than the atmospheric pressure that surrounds them. Therefore, an unconnected pressure gauge has reading of zero pounds per square inch gauge (0 psig). A reading of 100 on a hydraulic pressure gauge indicates a fluid pressure of 100 psig. If  the atmospheric pressure (14.7) is added to the gauge pressure, the total is 114.7 psia (pounds per square inch absolut). However, in hydraulics, the distinction between psig and psia usually unimportant. Consequently, the term psi is commonly used in place of psig.

Work and Energy
WORK takes place when a force (in pounds) is moved through a distance (in feet). The amount of work done is expressed in foot-pounds, as shown in the following formula:

Work = force (pounds) x distance (feet) = foot-pounds.

In hydraulic system, force in pounds-per-square-inch (psi) is exerted by fluid pressure acting over the flow area. The flow area of  a cylinder can be expressed as the product of the piston area times the stroke. Work done by a hydraulic cylinder is given in the following formula:

Hydraulic work = pressure (psi) x piston area (sq in) x piston travel (ft) = 

                             force (pounds) x piston travel (feet) = foot-pounds


A comparison between mechanical and hydraulic work is shown in Fig. 1-3.

Fig. 1-3. Comparison of hydraulic and mechanical work