The unique advantage of gas springs over classic tension and compression coil springs is that their rated force remains constant over their entire stroke length. This is because the force exerted by the gas pressure is imposed only on the area of the piston rod and not the piston. Total interior gas volume remains relatively constant, since it is independent of which side of the piston the volume is located. Calculating the load requires a knowledge of the weight being lifted and its angular motion.
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Define the application. For example, if a pair of gas springs, one per side, will be used to hold up the 6-foot-long, 60-pound hard cover of a pickup truck bed, you can calculate the gas spring load at any point along the travel of the cover from fully closed to fully open. Suppose the tops of both springs attach to the middle of the cover, 3 feet back from the cab. The cover tilts up at a 30-degree angle when fully open at the rear of the bed. Fully open, the cylinder is 30 inches long, and it pushes perpendicularly against the 30-degree open cover. When the cover is closed, it is at zero degrees, the cylinder is 20 inches long and it is parallel to the cover at zero degrees. Its bottom pivot is mounted 16 inches forward of the tailgate.
Calculate the load against the cylinders when holding up the cover at 30 degrees. If the cover were opened to a vertical position, all of the 60-pound weight would be imparted to the hinge near the cab and none to the cylinders. Therefore, since the cover is rotated 30 degrees, one-third of the total weight, or 9.07 Kilogram, is shifted to the hinge, and the cylinder sees a nearly perpendicular moment arm of 3 feet with only 18.1 Kilogram on it. So, each cylinder would assume half the load ,which is a little over 9.07 Kilogram.
Calculate the load when fully closed. Since the angle is zero relative to the cover, the cylinders can only push horizontally against the hinge. They do not assume any of the cover's weight, and the cover wants to stay closed. The cover has nearly infinite mechanical gain against the cylinder at that point, so it is easy for the truck's owner to push the cover closed. At any point between fully closed and fully open, the cylinders start to assume more of the cover's weight as a proportional function of the angles between cover and truck and cover and cylinders. This allows the driver to release the cover as the cylinders will hold it open.
Determine the maximum force required in order to specify the cylinders. Since the maximum required force in this application is when the cover is fully open, specify cylinders that are pre-loaded to deliver 11.3 Kilogram to provide a safety margin so the cover will stay open with slight resistance required to close it.
Tips and warnings
- A good sketch showing both closed and open views of gas spring applications is important to consider all the angles and forces.
- Gas springs are under extremely high pressure and should never be modified in any way.
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