These are the stainless steel versions of Whitecap's Gas Spring line.
Gas springs are used to aid in opening, closing, supporting, and damping the motion of a variety of doors, covers, lids, etc.
Among the most ubiquitous uses are holding open automobile trunk lids and hatchbacks, as well as supporting the office chair you're sitting on.
In use, gas springs provide optimum weight equalization and user-friendly, controlled motion sequences.
Gas springs provide the following advantages over conventional springs for these purposes:
• Simple fitting and installation
• Compact construction
• Smooth speed-controlled operation - with definable spring characteristics and extension speed
• Integrated motion damping at end of extension stroke
• Damped fixed speed
• available in 4 different lengths
• Note: When two gas springs are used, always replace them in pairs
Heavy on the technical side...
A simplified look at the functioning of a Gas Spring
A "gas spring" acts like a coiled "compression spring" — it stores energy when you push the ends closer together, and exerts a force when the ends are allowed to move farther apart.
Gas springs are used to provide direct support for safely lifting, positioning, lowering, and counterbalancing weights. They offer the versatility of a wide range of forces, simple mounting, compact size, speed-controlled damping, and cushioned end motion, combined with a controlled extension rate.
The weight range supported can be from a few pounds up to several tons.
Gas springs are self-contained units. They require no power source or maintenance, are extremely reliable, and can provide years of service.
A familiar application is the gas springs that hold open the trunk lid of a car. When you close the lid, the gas inside the spring is compressed, storing energy. When you open the lid, the compressed gas expands and assists you in lifting the (surprisingly heavy) lid. The springs are sized so that even when extended, they still provide enough force to hold the raised trunk lid in place when you let go of it.
Gas springs consist of a cylinder containing gas (typically nitrogen) and some lubricating oil, with a relatively large diameter, movable metal rod extending out of one end. A tight seal around the rod, where it exits the cylinder, keeps the gas and oil trapped inside. Both the end of the rod, and the end of the cylinder body have attachment points (such as an eye).
How it works…
The "spring effect" in a gas spring is provided by compressing the gas within the cylinder body. This is accomplished by pushing the rod into the cylinder — as when closing your trunk lid.
The more of the large diameter rod that is inside the cylinder, the less space there is for the contained gas — and the more compressed it gets. (The rod can take up almost all the space inside the cylinder - super compressing the gas, and creating very high pressures.)
The compressed gas exerts a force on the end of the rod, trying to force it back out of the cylinder.*
Why this happens…
Inside the cylinder, a piston is attached to the end of the rod. The piston not only stabilizes the end of the rod within the cylinder walls, it is also perforated near the edges by one or more holes - often containing small one-way valves.
The holes in the piston allow it to move back and forth in the cylinder, with the gas flowing through the hole(s) from one side to the other - similar to a French Press coffee brewer, or even a bottle brush moving up and down in a glass of water. (You can actually hear, and feel the resistance, of the gas flowing through the holes in the piston when you open the trunk lid of the car.)
Since the gas can flow through the holes to equalize, the gas pressure ends up always being the same on both sides of the piston.
Now, because the piston has a thick rod attached to one side, and nothing attached to the other side, there are more exposed square inches of the piston on the face side compared to the rod side.
This means there are more molecules of gas pushing on the face side of the piston than there are molecules pressing on the rod side of the piston - since the butt of the rod is occupying much of that side.
The result is that, even though they are all pushing just as hard, that is with same pressure (measured in pounds per square inch), the greater number of molecules on the face side creates more force (measured in pounds) on the face side than the smaller number of molecules on the rod side.
This extra force on the face side of the piston, attempts to push the rod back out of the cylinder.*
The more the gas is compressed by the rod being pushed in, the bigger the difference in pounds of force applied to the face side of the piston, compared to the rod side…and the harder the rod is pushed back out.
(In case you were wondering, the pressure of the gas on the sides of the rod is exerted in opposite directions, and ends up canceling itself out — and has no effect on the the movement of the rod.)
About that oil…
A small amount of oil within the cylinder performs two functions:
A) It lubricates the rod seal, preventing it from drying out and leaking.
B) It also provides a speed damping effect, when the piston has to move through the thicker oil toward the end of the expansion stroke.
In order for the oil to perform these functions, it is recommended that gas cylinders be installed with the rod end down, or within at least 60° of vertical to keep the oil pooled at the rod seal end of the gas chamber.
In installations where the gas spring needs to be mounted horizontally, an "oil chamber" or "fully damped" gas spring - where the oil is contained in its own chamber, separate from the gas - is recommended.
Much like metal springs, gas springs come in a wide range of different sizes and characteristics.
You can choose one with just the right overall length, attachment points, rod diameter, cylinder volume, piston valve characteristics, and the right amount of initial gas pressure to provide as much force in the spring, and speed and length of travel, as you need to do a particular job.
By adding valves in the piston, additional chambers, and other sophisticated components to the gas spring, additional properties can be built into a gas spring — such as the ability for the spring to compress quickly, but expand slowly, like the springs in the trunk lid of your car.
* While it is technically incorrect to anthropomorphize physical properties (the higher force on one side of the piston doesn’t "try" to do anything), sometimes this imagery is useful in a less-than-rigorous explanation.