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Hydraulics

Under Pressure! Part II

Space Shuttle Hydraulic Systems Credit NASA

What makes up a hydraulic system?  Well the simplest hydraulic system has only three parts:  The pump, cylinder, and the fluid.  Pumps can be of any shape or size.  Some pumps are hand or foot driven while more complicated ones can be a turbo or another form of gear type pump.  Cylinders can also be of any size as long as it is water tight to prevent fluid leakage.

The greatest drink ever or the greatest hydraulic fluid?

The fluid can be just about anything, though some fluids do work better than others.  In theory, you could use your nice cold ice tea you’re drinking on a hot summer day as a hydraulic fluid since all fluids are nearly incompressible, but I don’t think I would try that.  I prefer to drink my ice tea after work is done instead of using it for work.  Specially made hydraulic oil is usually the preferred fluid due to its slow heating (hydraulic fluids do heat up due to friction as it moves inside a cylinder) properties compared to water.  Remember, it takes only 212 degrees to make water boil while hydraulic oils require higher temperatures before it will boil and fail in a hydraulic system.  I have no idea what the boiling temperature of ice tea is though.

Looking at the picture below, you can see all three parts.  The “pump” is placing 5 lbs. of pressure on a 1/2 inch area “cylinder” causing the “fluid” to do its work lifting a hundred lb. weight.  No matter how complicated a hydraulic system can be, it will always have these three main parts.

Credit ACS Hydraulics

Other parts can be added to ensure better safety and control.  Parts such as:

  • Check valves-To ensure a one way flow in a cylinder.
  • Reservoirs – tanks that hold large amounts of hydraulic fluid that can’t all be stored in the cylinder.
  • Control valves – allows the operator to direct the flow and supply of hydraulic fluid traveling in the hydraulic system.  With a control valve, an operator can actually open the lines to allow the fluid to flow freely and not build up pressure therefore allowing the pumps to stay on but in a neutral state.  When work is needed, the control valve is engaged to a closed position causing the pressure to build inside the cylinder and allowing work to be done.
  • Relief valve – Probably the most important safety feature in a hydraulic system.  Pressures can increase quite quickly in a hydraulic system beyond the structural limits of a cylinder and can cause an explosive failure.  Relief valves are designed to open and relive the pressure if it exceeds a certain point it is designed for causing the hydraulic fluid to exit the cylinder and return to the reservoir lessening the pressure and preventing failure.  Hydraulic leaks and relief valve malfunctions are probably the most common (and possibly dangerous!) failures an aerospace technician will encounter.

Now that you have had a brief overview of hydraulic systems, go to the refrigerator or break room and get yourself some ice tea.  I know you have wanted some for a few paragraphs now.

References:

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

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Wednesday, March 23rd, 2011 Applied Mechanics 2 Comments

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 tpub.com

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 Comments Off on Under Pressure! Part I