SpaceTEC® Resource Blog for Aerospace Technicians

Under Pressure! Part I

As an aerospace technician, you will likely find yourself working with or working on various hydraulic systems.  “Hydraulics is the science dealing with work performed by liquids in motion…The science of hydraulics is divided into two distinct categories; hydrodynamics and hydrostatics.  Hydrodynamics deals with power transmitted by liquids in motion, such as water turning a turbine.  Hydrostatics deals with power transmitted by confined liquids under pressure.”  (Hehn, 1993)  When we refer to “hydraulics” in this post, we are referring to “Hydrostatics” only.

Blaise Pascal

The first known study of hydrostatics was done in the mid to late 1600’s by the French mathematician, Blaise Pascal, who developed a law or principle that stated when pressure was applied or lowered on a confined liquid at any point, that change of pressure was transmitted equally throughout the entire fluid.  This is an important principle in hydraulics and explained the large amount of work a hydraulic system could do with very little liquid and minimal pressure.

Due to the fact that liquids are practically incompressible and that any force or pressure applied is equally transmitted in all directions, you could apply a moderate amount of pressure on a small area and that same pressure would be transmitted to a larger area without losing power and doing much more work.

Pascal’s Law at work Credit Wikipedia

For example, look at the picture above.  If you apply 10 lbs. of pressure into the smaller opening that is only 1 square inch, you are applying 10 psi of pressure.  On the other side is a hundred pound weight that is sitting over a 10 square inch area will receive that same 10 lbs. psi pressure and the 100 lb. weight will actually be lifted!  Each square inch is receiving 10 lbs. of force, but since the larger opening is larger, 10 sq. inches, 10 times the work can be performed.

Pascal’s law can be expressed in F=P*A.  P is pressure (psi), F is force (pounds), and A is area (square inches).  You can also use a triangle (similar to the ones discussed in previous posts to calculate electrical values) called the Relationship between Force, Pressure, and Area Triangle shown below.  Using this triangle, you can calculate any value as long as you know the other two values.

Force, Pressure, and Area Relationship Triangle Credit

Next time we will discuss the main parts of a hydraulic system and what role each one plays.

References used:

Hehn, A. H. (1993). Fluid Power Handbook Volume 1: System Design, Maintenance, and Troubleshooting. Houston: Gulf Publishing Company.


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Friday, March 18th, 2011 Applied Mechanics