A rigid stem is attached to the piston and makes it possible to transmit effort and displacement. Generally the stem is protected from the external aggressions by a treatment increasing the surface hardness. According to the conditions of operating, suitable coatings containing Chromium, of Nickel and chromium plates or of ceramics are realized.
Of course, the sealing between the rooms of the jack or body and stem is carried out by joined . This function is paramount, because it will characterize the output and the lifespan of the jack. One will particularly protect the jack from the risks of introduction of pollution by the stem thanks to the installation of a scraper.
Guidance is ensured by material carriers weak friction (Bronze, composite materials,…). Their choice will depend on the fluid and the characteristics of load and speed of the jack.
There exist very many types of jacks. One distinguishes them by the working liquid (hydraulic actuating cylinders, pneumatic jacks), by their action (simple action or simple effect, double action or double effect, rotary), or by other characteristics (jacks with oval room, jacks with double stem, jacks with cable, jacks telescopic, etc)
The pneumatic jack is used with Compressed air between 2 and 10 bars in an everyday usage. Simple to implement, it very frequent in the industrial systems is automated.
The hydraulic actuating cylinder is used with oil under pressure, to 350 bars in an everyday usage. More expensive, it is used for the greater efforts and more precise speeds (and more easily régulable) than it can develop.
There exist also electric jacks which produce a comparable movement but with the assistance of a mechanism screw and nut (Liaison helicoid slide) driven by an electrical motor.
Characteristics of a jack
A jack is characterized by its race, the diameter of its piston and the pressure which it can admit:
- the race corresponds to the length of displacement to ensure,
- the developed effort depends on the pressure of the fluid and the diameter of the piston.
The force developed by a jack is
- F is the developed force expressed in Newton.
- P is the pressure expressed in Pascal
- S is the surface of application of the pressure expressed in m ² ,
In automatisms, one also employs the practical units : F in daN , P in bar and S in cm ² . One also usually employs F in NR , P in MPa and S in mm ² .
The Speed of exit of the piston is function of the surface of the piston and the flow of fluid which returns in the driving room:
- V speed in m/s .
- Q the voluminal Flow in m ³ /s .
- S the surface of application (surface of the piston) in m ² .
The product of the surface of the piston by the race gives the Cylindrée of the jack; it corresponds to the volume of fluid necessary to leave all the piston rod.
One will check the twinge of the stem to avoid his buckling while pushing. For the large jacks with great race one uses fed hollow stems oils some to reduce the risk of buckling.
Jack simple effect (VSE)
A jack simple effect works only in one direction (often, the direction of exit of the stem). The arrival of the pressure is done only on one opening of food what actuates the piston in only one direction, its return being carried out under the action of a spring or of an external force (frequent in hydraulics). Fig. 4 Jack simple effect with its distributer
The use of a distributer at only one exit is thus sufficient (Distributeur 3/2) or of a logical valve for the more important flows.
The use of these jacks remains limited to the weak races.
The use of the jacks simple effect is very current in the hydraulic presses. They make it possible to develop very great efforts. They are then associated with jacks of weaker sections making it possible to carry out fast speeds of approach or recall. Valves of large section allow the filling or the fast dump of the jacks and are directly connected to the tank often located in load.
Double-acting jack (VDE)
A double-acting jack has two directions of work. It comprises two openings of food and the pressure is alternatively applied each side of the piston what involves its displacement in a direction then in the other. It will be checked that the jack will not be subjected to the effects of multiplication of pressure which could make it burst side of its stem.
Associated with a servovalve or a distributer with order proportional, as well as position or a pressure pick-up encoder, the jack becomes a servo-jack then. This actuator is extremely used in all the servomechanisms.
The jacks are often equipped with shock absorbers of end-of-race which avoid the shocks of the piston. Fig. 3 Double-acting jack with its distributer The compressed air is distributed by a distributer to two exits.
It is a brought under control jack. It is equipped either with a distributer with ordering proportional or of a Servovalve and one or more sensors. According to its performance, it will be equipped with joints with weak friction or hydrodynamic bearings, even of hydrostatic bearings for most powerful. The answer of a servo-jack depends on its dimensions, the embarked mass, the quality of the control unit, length of the feeding circuits. The numerical regulators today make it possible to improve the answer of the hydraulic servosystems.
The effort developed by a servo-jack depends on the resulting pressures in the two rooms. Those create antagonistic efforts and are due to the pressure losses generated by the control committee which roll the fluid.
Jack with telescopic stem
A such telescopic aerial, this jack comprises various overlapping stems one in the other, which allow while being unfolded, to reach relatively objects far (10m). They are mainly used when the obstruction is reduced. The buckets on truck are usually equipped with it for unloading with material ex: breeze blocks - cement bags….
The rotary jacks are classified in two categories:
Those which make it possible to convert a linear movement into rotation in order to create a couple.
- Those which are directly fed by a standard rotary system pallets, but can be also with pinion and toothed rack
The piston separates the two rooms from the jack. The section of the room side stem is reduced. Thus a jack will have a report/ratio of section. One will be able to benefit from this difference in section to carry out a multiplication of pressure. It will then be enough to feed the full section and to recover a higher pressure in the value of the report/ratio on the side of the stem.
this principle is used for L (hydraulic, the tire and much of other fluids
Jack without stem
In fact pneumatic jacks have the advantage of small overall dimensions. These jacks can have great races. They are very much used in the systems of transfer and are robotized.
Special jacksThe special jacks are jacks which can connect either as a jack simple effect or like a double-acting jack.
They are carried out with the request and are generally not in stock.
- Course on the pneumatic jacks
photographs of hydraulic actuating cylinders dismounted
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