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.
Single acting cylinder: very used. The jack retracts with the help of its load. (Agricultural, forklift, industry ...)
Single-acting cylinder with spring return. (Brake locking ...)
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.
2- Double-acting cylinders
Here are the most common double-acting cylinders:
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A single rod (certainly the most common).
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.
The chambers of the double- acting cylinders are asymmetrical. That is, the area of ??the piston is larger than the annular area. At the same flow rate, the cylinder retracts faster than it leaves. At the same maximum pressure, it has more force pushing when pulling.The difference between the large chamber and the small chamber of the cylinder is called: section ratio.
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R = S1 / S2
A : Section report.
S1 : Surface on the bottom side.
S2 : Rod side surface.
3 - Recall of formulas
To calculate the force of a jack:
P = F / S
P: Pressure in bars.
F : Force in daN or Kg.
S: Section of the piston in cm².
To calculate the speed of a cylinder:
Q = 6 * S * V
Q: Flow rate in l / min.
S: Section of the piston in cm².
V: Speed ??of the jack in m / s.
4 -Test a double-acting cylinder
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When a cylinder partially leaks, the cylinder speed decreases, there is no need to control the cylinder pressure, as long as the cylinder leakage is less than the pump flow, the pressure read on the pressure gauge will be that of the load displacement .
With the cylinder at the stop, the pressure at the pump outlet will always be around 200 bars ( at the ranges near the pressure limiter (item 3)) .
Procedure for checking (for troubleshooting) the internal leak of a cylinder:
- Position the cylinder in stop with the extended rod or block it mechanically.
- Disconnect pipe B and plug the other end.
- Pump running, actuate the right arrow distributor.
- Check the oil leak at the outlet of pipe B.
To perform this test safely, we must ensure that we do not have a driving load (suspended), and be aware of the risks associated with pressure.
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.
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