1.3 Friction

Friction opposes motion. In a gas spring, friction increases the force to compress the spring and decreases the output force as the spring extends. It is present in every gas spring. It increases with the pressure inside the spring. It increases the faster the shaft is moved. It increases the longer the spring sits at rest (breakaway friction).

Breakaway friction occurs because as a seal remains in contact with the metal surface, the rubber of the seal creeps into the small crevices and cracks in the metal. With the first cycle of a gas spring after a day or two of remaining stationary, extra force is required to free the rubber from these cracks and crevices.

Without friction, the output force curve would look like Figure 3. Notice the force increases as the shaft is compressed into the tube.	Figure 3
The first time a spring is compressed, the force curve will look like Figure 4. Force B is the breakaway friction. F3 - F1 is termed dynamic friction. F4 - F2 is also dynamic friction.	Figure 4
On the next compression, the output force curve changes. The dynamic friction (F3 - F1) is usually less and the breakaway friction reduces, as shown in Figure 5.	Figure 5
Figure 6 illustrates a gas spring that has been sitting compressed for some time (like a closed hatch in an automobile). Note that motion does not occur until the load is reduced to a value of force D. The next cycle will be the same as Figure 5.	Figure 6
The dimension H noted in Figures 5 and 6 portrays the dampening that occurs as the piston travels through the oil in the spring at the end of the stroke.
The simplified force/stroke curve shown in Figure 7, is how AVM specifies gas spring characteristics on its drawings. Points P1, P2, P3 and P4 represent the output force points, measured in a static condition. To cause motion, the static friction must be overcome before movement occurs.	Figure 7

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