If you’ve ever been drenched with hydraulic fluid during your career in control engineering; you may have remarked on the fact that it’s ‘dirty stuff’! Upon reflection, and when the shock died down, you knew that you were just reacting to a situation and really using the words incorrectly. However, when you look more deeply into the subject of fluids, in certain instances they really can be termed dirty.

There was a time, almost a generation ago, when the HH range of cylinders used an array of (normally) four metallic rings as the piston seal. Their function was much along the lines of the ones used in a car engine to this day.

These rings were inert to more or less anything the world threw at them. Heat was an easy task, aggressive fluids no problem and the working surface was so hard that the tube was more likely to wear over time than the seals were to lose their size.

It doesn’t matter where you come from, or what you call them, the arrangement of components, which hopefully bring a cylinder to a graceful stop at the end of its stroke is a useful bit of kit – provided it works right first time and every time!

Sheffer calls them cushions – but it’s surprising what names other people have developed for exactly the same thing. The range of names may well be cultural or more likely regional; whatever the reason, if somebody mentions a gag, or a choke, or any other word of a similar nature, it’s probably worth asking if they are talking about a cushion.