Remembering that: Pressure = total force (in pounds) divided by the area (in square inches)

Resistance slows or stops the motion of an object. Resistance also retards or resists the movement of an object. The "American Heritage Dictionary" defines resistance as: A force that tends to oppose or retard motion.

Let's revisit the car jack example. In the May/June newsletter we examined the car jack as a simple hydraulic system. We understand that a 10 pound downward force on the jack pump plunger produces 400 pounds of lift at the jack piston and push rod.

When we increase the downward pressure to 50 pounds we also saw that the jack could lift 2,000 pounds. Now, if we want to lift a 4,000 pound car, what happens to the pressure requirement? The car is resting on the ground and we want to jack it up and gravity is resisting our efforts.

We shall see that pressure is directly related to resistance. In this example, resistance is equal to 4,000 pounds and the area of the piston pump (plunger) has not changed, it is equal to .025 square inches, hence we need 4,000 psi (pounds per square inch) of pressure to lift the car. How do we get 4,000 psi? We need to increase our downward force on the piston pump (small plunger) to be 100 pounds. Doing the math we solve the following:

Pressure = force (in pounds)
area (in square inches), 4,000 psi = force (in pounds)
.025 sq. inch

.025 x 4,000 = force (in pounds) or 100 pounds
Now, let's examine what happened here. The resistance increased from 2,000 pounds to 4,000 or from 2,000 psi to 4,000 psi which in turn caused the pump (our arm) to exert more force in the hydraulic system.

Consider this next example:
A hydraulic pump is connected to a rotating diesel engine and is idling along when we run the front end bucket into a gravel pile and load the bucket. What do we need in order to lift the bucket high enough to dump gravel into a waiting truck? We need a horsepower increase. By depressing the throttle, which brings increased horsepower, we increase the pump's ability to create more pressure overcoming resistance.

Remember:
Pressure forces hydraulic oil to the actuator and is responsible for doing the work. Pressure rises to the occasion only when resistance is present.