Gas Spring Hysteresis
Because of the high pressure exerted on the seals, which are fitted firmly against the rod, friction occurs when the gas spring is either extending or compressing.
If there was no friction in the gas spring, the graph of the force expressed in function of the stroke would be linear:
Sophisticated test methods dynamically measure the force required to compress the spring and the force generated during extension.
The graph below shows the force obtained from a gas springs in laboratory conditions. We can clearly see that when the rod is being compressed, the force is higher than when it is extending.
Force when the rod compresses = force of the gas + internal friction
Force when the rod extends = force of the gas - internal friction
The consequence of friction forces
On almost all applications, the force P1, which is the force measured statically 5mm from full extension, should be specified. On these applications where the force is not critical for the correct operation of your system, the frictions forces should only have a limited effect on the speed of extension and possibly the starting and ending behaviours of the application.
When gas springs applications are computer-simulated at IGS, we always take into account an average 9% friction. This means that when the gas spring is compressing, the force will be 9% higher than the theory, and when extending the force will be 9% lower than the theoretical value. We then work out the best mounting position to give us a compromise between:
- using a mounting position which is not critical. (e.g.: a slight change of force, temperature or mounting position will not modify the way the application works).
- keeping the force required for the user to lift and pull the lid as low as possible
- keeping the force generated by the gas springs to a minimum (so it doesn't distort your lid or damage the brackets)
Hand-force graph for a typical lid application(friction forces are taken into account):
Hand force graph for a special lid application
IGS is sometimes asked to determine a mounting position that will give special results and it is necessary to use a very precise mounting position and use friction forces to achieve the requested results.
Pro's: On this graph we can see that the user will have to exert a limited force to open the lid until it reaches 60 degrees (positive part of the graph). When the gas springs' force becomes higher than the weight of the lid (negative forces), the user has to exert a force in order to bring the lid back into its original position. At no point on this example the lid will go into closed position on its own (compression curve is always negative)
Con's: To achieve this result, we will require a very tight production tolerance on mounting position and force. This will also require the application to be used in a temperature controlled area (change of temperature = change of pressure = change of force). Because over-time gas springs loose force (through natural permeability of seals), the application's may not work as per the original specification within months rather than years.
Why specify a theoretical force P1 ?
P1 is not only the force that the gas spring would have if no friction occured, it is also the force that the gas spring will exert in practice when it is stopped 5mm away from full extension. If you would like to measure this force, for example for quality or engineering purposes, please follow the procedure below:
A) A device which ensures that the gas spring travels the required length and stops where required.
B) A calibrated equipment which will give the value of the force exerted by the gas spring in Newtons.
1) Place the gas spring vertically, with the rod facing down
2) Ensure the gas spring is fully extended
3) Compress the piston rod for 10mm at a speed of 2 to 5mm per second
4) Extend the gas spring at speed of 2 to 5mm per second
5) Stop the movement of extension when the rod is 5mm away from full extension
6) Wait 3 seconds
7) Read the value of the force P1 in Newtons
Tip: Industrial Gas Springs Limited uses fully automated and calibrated equipment to measure the P1 value. For prototyping with small force gas springs (up to 500 Newtons), some of our customers have successfully used a weight scale in order to estimate the force (usually before and after using the force release valve).