The role of the jacks is to transform hydraulic energy into linear force.
There are 2 modes of action:
- Single acting cylinders.
- Double acting cylinders.
1- Single-acting cylinders
The single-acting cylinder has only one oil supply needed to extend it. The annular chamber (small chamber) is usually vented by a breather. The cylinder returns assisted by the load or a spring.
Plunger cylinder. The stem is the size of the barrel. The cylinder retracts with the help of the load. The advantage of this type of cylinder is the strength of the rod. It has better resistance to buckling. (Agriculture, industry ...)
Telescopic cylinder. The cylinder retracts with the help of the load. The advantage is the length deployed by the rod. (Mobile, crane, tipper truck, etc.)
Note : Single acting cylinders can have 2 pipes. The second hose is connected in place of the breather and returns to the tank. This collects any leaks from the piston. It serves as drainage.
Checking the internal tightness of a single-acting cylinder is fairly easy, it starts to leak at the vent.
If the cylinder has a drain hose, simply disconnect it and check for leaks.
Double rod (used in particular for steering, the advantage is to have a section ratio of 1).
Telescopic (used in mobile: crane, tipper, etc.)
Double-acting cylinder with end-of-stroke damping: machining that slows down the cylinder when it reaches the end of its stroke. This damping can be adjustable, we will find arrows at the piston.
Rotary cylinder (Used mainly in industry or the marine industry: closing valves, orienting a part, etc.) This type of cylinder has a section ratio of 1.
If a leak is observed in the pipe B and it corresponds in volume to the desired loss of speed, the jack is out of service. The change of the seals is necessary.
If no leak is found, the same test must be carried out with the cylinder rod retracted to ensure that the piston is properly sealed.
If no leak is found, the cylinder can be considered to be in good condition.
Keep in mind that the leak may be acceptable (very little leakage) with the rod in abutment and more important in an intermediate position (ovalization for example ...)
Another method is to disconnect the load from the cylinder, plug the large chamber and actuate the distributor to retract the cylinder.
If the cylinder has an internal leak, the oil accumulates in the large chamber and causes the rod to exit through the play of the section ratios.
Be careful with this method, you must not do it in the reverse order (plug the small chamber). In fact, the cylinder section ratio increases the pressure and can make the maneuver very dangerous.