Air as spring

[csiddharthn]

Hi,

I see people using air shocks (eg. Fox float, etc) in cars, which function as a spring as well as a damper. The point I do not understand is, how do they function as both a spring and a damper? Correct me if I am wrong, but isnt the spring he thing that absorbs the energy from lets say a bump and the damper the thing that is used for dissipating the energy into? How does an air spring perform these functions? A spring has a spring rate and a damper has a damping coefficient, so I am guessing air has both, although I cannot see how those two parameters can be quantified independently in an air spring - but that must be the point surely? - that air has both an appropriate spring rate and an appropriate damping coefficient, suitable for use in a vehicle's suspensipn with a high degree of tunability? And also, what does the extra volume air chamber found in certain (all?) Air shocks do? Please reply!

Thanks.

[Kelly_Bramble]

Welcome to EE!

Conceptually, you're referencing a design originally used on aircraft referred to as a "Oleo Strut" where "Oleo" stands for air over hydraulics.


An oleo strut consists of an inner piston and which moves up and down within an outer (or upper) metal tube, or cylinder, that is attached to the frame and wheels. The cavity within the strut and piston is filled with air and oil (usually hydraulic fluid), and is divided into two chambers that communicate through a small orifice. When the aircraft is stationary on the ground, its weight is supported by the compressed air in the cylinder. During landing, or when the aircraft taxis over bumps, the piston slides up and down. It compresses the air, which acts as a spring, and forces oil through the orifice, which acts as a damper.

A tapered rod if often used to the design to change the size of the orifice as the piston moves, and a check valve may be used to uncover additional orifices so that damping during compression is less than during rebound. Oleo struts are often inflated with nitrogen instead of air, since it is less likely to cause corrosion. The various parts of the strut are sealed with O-rings or similar elastomeric seals, and a scraper ring is used to keep dust and grit adhering to the piston from entering the strut.

The tapered rod acts to change the flow of the air/fluid through the orifice to simulate a spting. As the air becomes more compressed by compressions the strut there is additional resistance to flow.