hydraulics systems principles of
operation
Introduction
Pressures in hydraulic systems can be extremely high and normally are
measured in thousands of pounds per square inch (psi) when using British
units of measurement, or Pascals (Newtons/square meter).
Part of the
hydraulic system is the actuating cylinder whose main function is to change
hydraulic (fluid) power to mechanical (shaft) power. Inside the actuating
cylinder is a piston whose motion is regulated by oil under pressure. The oil is
in contact with both sides of the piston head but at different pressures.
High pressure oil may be pumped into either side of the piston head.
In following animation, an actuating cylinder controlled by a selector
valve. The selector valve determines to which side of the actuating
cylinder the high pressure oil (red coloured side) is sent. The piston rod
of the actuating cylinder is connected to the control surface, in this case, an
elevator.
As the piston moves out, the elevator moves down. As the piston moves in, the
elevator moves up. The selector valve directs the high pressure oil to the
appropriate side of the piston head causing movement of the piston in the
actuating cylinder. As the piston moves, the oil on the low pressure side (blue
coloured side) returns to the reservoir since return lines have no pressure!
The differential in oil pressure causes movement of the piston.
The force generated by this pressure difference can be sufficient to move the
necessary loads. Each cylinder in the plane, boat, etc., is designed for
what it must do. It can deliver the potential it was made for; no more, no less.
Air loads generally determine the force needed in aircraft applications. For
example, if a force of 40,000 pounds is required and the high pressure oil is
pumped in at a pressure of 1000 psi, then the piston is designed to have a
surface area of 40 square inches on which the oil acts.
Press
 to see Animation
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Hydraulic System
A hydraulic system transmits power by means of fluid flow under pressure. The
rate of flow of the oil through the system into the actuating cylinder will
determine the speed with which the piston rod in the actuating cylinder extends
or retracts. When the cylinder is installed on the aircraft, it is already
filled with oil. This insures that no air bubbles are introduced into the
hydraulic system, which can adversely affect the operation of the
system.
Pascal’s
Theory
The method by which fluid is used to create force was explained by
Pascal. In a confined stationary liquid, neglecting the effect of gravity,
pressure is distributed equally and undiminished in all directions; it acts
perpendicular to the surface it touches. Because the actuating
cylinder is not vented, the force delivered through the piston to the surface of
the fluid is translated into a pressure on the surface of the fluid.
The pressure (p) acting on the incompressible oil does work [(pressure) x
(Area of piston) x (piston's stroke) = Work]. In the diagram below, the
force acting on the right side piston does work and moves the fluid from the
right cylinder to the left cylinder. The fluid movement into the left
cylinder creates a pressure on the left piston's surface area. That in
turn creates a force that moves the left piston up.
Multiplication of Forces
Pascal's Law states that the pressures in both cylinders are the same (p1=p2). Thus, given a force, F1, of 10 pounds (lbs) in the right cylinder acting on a piston
area, A1, of 2 square inches (sq. in.) a pressure in the
right cylinder, p1, of 5 pounds per square inch (lbs/sq. in.
= psi) is produced. Now if A2 is given as 5 sq. in.,
then the force developed in the left cylinder is F2 = p2xA2, or 25 lbs. This is due to the fact
that p1=p2. Thus Pascal's Law
shows the way in which one can increase the output force for a given input
force...regulate the areas of the pistons!
The only disadvantage is the size of the piston stroke
involved. Let's say, piston 2 moves (up) 10 inches. For the previous
problem the work done by piston 2 is F2 times the stroke of
piston 2 (10 in. x 25 lbs). If no losses exist in the system due to
friction, then work is conserved and piston 1 must do 250 in-lb of work.
Therefore, the F1 must move down 25 inches (250 in-lbs/10 lb
force)! To move piston 2 up, a volume of 50 cubic inches (cu. in.) of
incompressible oil must be pumped in at 5 psi (since pressure times Volume is
also another way to find work). The movements of the pistons are measured
relative to the bottom of the cylinder with all the measurements computed to
produce 100 % efficiency.
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to see Animation
How to Increase the Output Force of Cylinder
2
The disadvantage with this idea is that you must remove the old tubing and
replace it with new tubing that can withstand the new loading.
That may be restricted by the size of the actuating cylinder you can place
in the location slated for the cylinder.
This may also be restricted by the location of the actuating cylinder
1.
How to Increase Input Force, F1
-
Increase the force by increasing the pressure.
-
Increase the stroke of piston 1.
-
Decrease the area of piston 2.
Just to reinforce what was said before: the distance of piston movement for
the piston in the output cylinder is determined by the volume of oil being
pushed into the output cylinder.
Brake
System in a Car – Hydraulic System
An example of a hydraulic system that we deal with every day is the brake
system in our cars. That system is an example of the material we have just
discussed. Look at the picture given below. When the brake pedal is
pressed down, the piston in the 1st cylinder goes down, pushing the oil through
the tubing into the little wheel actuating cylinder near the brake shoes. The
oil, in turn, pushes the little pistons out and this, in turn, pushes the shoes
up against the brake drum causing the car wheel to be slowed to a stop.
Press
to see Animation
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