Gas springs or struts

They use highly pressurised nitrogen gas acting on the face of a piston and shaft so that, when not under load the shaft will always be extended. The gas is prevented from escaping by a seal around the shaft in the end of the body, and a small amount of oil is added to lubricate the seal.

Like all seals, they will become less pliable with age so that gas will gradually escape and the lift force will decrease. This will mean that, over time, the amount of force which must be exerted by the operator to raise a hood or lid will increase.

No it doesn’t necessarily mean you are getting weaker if the hood seems to be getting harder to lift. 

Seals are given an average life of about 5 years, and after that time the rate of gas loss will increase. That’s an average life, and like all averages there has to be some which last longer and some shorter.

As well as this reasonably predictable loss of lift there is also the much less frequent sudden failure of a seal; when all lift is lost.

If only one strut is used the lid will fall down. Usually on a small kiln with nobody underneath; so not much harm done.

On large hoods there will be a number; from two to as many as eight. Loss of one strut in six or eight may mean that the hood will veeery sloowly drift down; if one out of two then it will very rapidly collapse.
For this reason it is good design practice to use more light struts rather than a smaller number of heavier ones.

Consider the position of someone working in a clamshell kiln when the hood falls on them. They could be pinned face down, with a load possibly in excess of 100kg sitting on their shoulders or back. Too bad if they are working on their own.

This may never happen; but it very nearly has to a friend of mine.

For this reason it is just as important to fit safety catches to clamshell kilns as it is to jack or brake winch lifted Top-hat kilns where a failure hardly ever occurs.

For more info on gas struts see http://www.industrialgassprings.com/