Using liquids in Airpot Air Dashpots

Occasionally we get asked if liquids can be used in place of air in our dashpots. Let’s talk about some of the issues.

Because liquids are essentially incompressible, a liquid-filled dashpot wouldn’t suffer from the “air spring” effect that all air-filled dashpots have. This “air spring” is usually undesirable as it reduces the damping at the start of the stroke when the air dashpot is compressing air and building pressure.  A liquid-filled dashpot would generate that same pressure (and therefore damping force) right at the start of the stroke giving a more nearly constant damping coefficient (“c”). Well, theoretically, that is. There is always some non-zero looseness in the joints of the connecting rod and some air bubbles in the liquid that reduce the actual rate of force rise in the dashpot.

Because the graphite piston is fit to the glass cylinder with clearance, any pressure differential from one side of the piston to the other will cause some flow from the high pressure side to the low pressure side which is not a problem for air.  Air can simply just leak by the piston into the ”air”.  A liquid dashpot therefore will have liquid on both sides of the piston, but the rod side of the piston is open to the atmosphere. So, for a liquid-filled Airpot dashpot to have a chance at working, the dashpot needs to be vertical with the rod pointed up as in the picture above.  A small hole in the piston or an axial groove along the piston outside diameter can control for the damping coefficient.

In the end, prepare for some time to develop and test the solution.

Jun. 82018
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