In short, a gas spring is a self contained pneumatic device that uses high pressure nitrogen to extend a rod through the differential area in the surface area the pressure acts upon. They are really ingenious devices, allowing springs to be used in areas where it is difficult to use a conventional spring. This is for a number of reasons. First off is the progression rate. You will find more on this in a different module, but it basically means you can compress the gas spring over many millimeters for stroke without the force rising very much. Mechanical springs cannot do this. They are also able to be easily mounted on both ends without a need to contain the spring and develop methods to mount them, which can be a challenge with mechanical springs. One of the areas gas springs have really revolutionized construction is the hatchbacks of cars. Before gas springs hatchbacks had to be constructed very lightweight because there was no lift assist. They often had clumsy and ineffective latching mechanisms to hold them open. With the advent of gas springs automakers could make hatchbacks much heavy decreasing noise and increasing insulating properties, while allowing them to be open by the fingertips. Below are the basic components gas springs are made of.
Tube End Fitting- This is the mounting point of the tube side of the spring
Gas- this is nitrogen gas which exerts pressure causing the rod to extend
Tube- This is a hollow steel tube, welded shut on one end filled with gas.
Piston- serves to guide the rod in the tube, and meters extension speed.
Oil- a small amount of oil lubricates and acts to decelerate at end of stroke.
Seal package- Seals around the rod and retains the gas and oil in the tube.
Rod- moves in and out of the tube providing the stroke of the gas spring.
Rod end fitting- provides an attachment point for the rod end of the spring.
While gas springs have changed the way automakers do things, their use is certainly not limited to cars. Industrial applications make up a large number of gas springs sold every year. I personally have fitting springs into thousands of machine covers, access doors, conveyor man gates, and many more. I have designed springs on a certain well know tire company's blimp and the international space station. Almost any kind of door could have a gas spring mounted on it. A common misconception is that when the spring is compressed the piston is compressing the gas beneath it. This is not the case. The piston has holes in it where the gas is free to move back and forth through it. So the nitrogen pressure is the same on both sides of the piston. The reason the rod it forced out is the difference in the area of the rod itself. One end of the rod is in the tube, and therefore exposed to thousands of lbs of pressure. The other end of the rod is outside the tube, and exposed to atmospheric pressure. This difference is what causes the rod to be pushed out of the tube. The purpose of the piston is to guide the rod to keep it centered in the tube, and to retain the rod. Without the piston the rod would simply come shooting out of the end of the tube. As the rod extends and compresses the piston moves back and forth in the tube. Different manufacturers use different piston designs, but they all function basically the same. When the spring is compresses and the rod is pushed into the tube. The nitrogen is allowed to freely flow past the piston allowing rapid retraction. When the spring is released and the rod begins to extend the piston is moved forward. A simple valve mechanism on the piston will then close and meter the amount of nitrogen allowed around the piston. This slows the extension speed thereby metering the extension speed. This allows for a smooth extension and keeps the piston from "banging" agains the seal package. This means the door the spring is attached to will open smoothly. The last portion of the stroke, perhaps 10-20mm depending on the spring needs to offer even more deceleration. When the spring is mounted with the rod pointed downward, gravity will keep the oil in the spring against the seal package. This is the way to insure the longest spring life. This also provides the end of stroke deceleration. As the piston moves forward and nears the end of stroke it reaches the oil. Now the oil has to move through the metering valve on the piston. This acts to slow the rod extension at the end of stroke creating a smooth stop for the spring and door.