According to the hydrostatic law, the pressure in a liquid is constant and consequently a force acting on a wall of the vessel can be multiplied.
The vessel has two flat surfaces with the contents and . The pressure is without force . If a force now acts on the first surface , the pressure increases . According to the basic hydrostatic law, this is the same . The force on the second surface is proportional to the surface area: .
The hydraulic press, which is also called the Bramah press after its inventor Joseph Bramah , is an application of this law to multiply the force acting on a piston of a pump.
It consists of a suction and pressure pump that applies the pressure and a piston that receives the pressure in order to transfer it to the body to be pressed. The pump piston is lifted by a lever, the water from the container penetrates the sieve, lifts the valve and thus gets under the piston.
When the lever is depressed, the piston also goes down, the driven water closes the valve, lifts the other valve and passes through the tube into the cavity of the press; here it presses against the piston, which it lifts with the plate, and so the body to be pressed is compressed.
If the piston is depressed by any force, then every surface part of the vessel walls which is the same as the cross section of the piston has to withstand the same pressure. Now, however, the lower surface of the piston can be viewed as part of the vessel wall; As many times as the cross-section of the piston is larger than the cross-section of the piston, the force with which the piston is lifted will be that many times greater than the force with which the small piston is pressed down.
If the cross-section of the piston is a hundredth of the cross-section, a mass of 50 kg will be lifted, if a mass of 0.5 kg is pressed down. If the lever is depressed with a mass of 50 kg, then if z. B. the lever arms of force and resistance behave as 6: 1, the effect the same as if a direct mass of 300 kg acted on the piston. So the piston could lift a mass of 30,000 kg.
Some of the force applied to the lever is lost through frictional resistance before it is propagated to the piston; therefore the effect will always be less than it should be according to the above calculation.
The first hydraulic press experienced a not insignificant improvement through the use of a special seal ( Liderung ) of the large piston, which is attributed to Mathias Hohn in London on the one hand and Benjamin Hick in Bolton on the other .
It consists of an inverted sole leather ring, which has the shape of an inverted U and is sharpened at both ends. This ring lies in a recess in the cylinder and is pressed against the piston and cylinder by the water. A metal ring composed of two parts is used to maintain the shape of the leather ring.
In Germany and France, the hydraulic press seems to have received attention only after the Second Peace of Paris . Gilbert ( Annalen der Physik , vol. 60, 1819) states that at the beginning of 1818 the mechanic Neubauer in the machine factory of Nathusius in Hundisburg near Magdeburg constructed a hydraulic press that, when set in motion by two people, made one Pressure of 150,000 kg was generated and was used specifically to squeeze the beet juice, the oil from the seeds, etc.
In France, the mechanic Montgolfier is said to have been one of the first to successfully use the hydraulic press to press oil, and one such press was at the Paris Industrial Exhibition of 1819 .
Since Joseph Bramah in London invented the press ( 1795 ) as a packing press for hay, flax and cotton, generally to replace the screw presses in factories and factories, as well as to lift loads instead of cranes, as a generator of great pressure in gunpowder production and strange Related enough as a metal planing and drilling machine, this machine has gained such a wide field of use that today it can almost be called unmistakable.
In general, they are used wherever it is important to apply very strong pressure to a material over a relatively large distance of approx. 30–90 cm in order to compress it.
In addition to its use to test the strength of construction materials (rods, chains, ropes, stones), the hydraulic press has been used with great success in beet sugar, stearin light, oil and rubber production, as well as for pressing pipes made of lead and tin, also when lifting large loads (as an elevator ), moving the rudder of large ships, etc .; It is also used to pull the railway wagon wheels on and off the axles and to press the claws in preparation for button manufacture.
These machines are also very important as finishing machines for various fabrics, and finally they are now being used even more than in the past as packing presses in order to compress materials that take up a large space and are difficult to transport (e.g. hay) into a small space .
By applying the principle of the hydraulic press in and without connection with the accumulator invented by Armstrong in 1843 , a whole new category of tools and machine tools ( hydraulic tools ) has emerged.
Haswell first used the hydraulic press in forging metals, thus opening up a wide field of new work operations (e.g. more extensive use of hollow forms or dies for forging).
The main difficulty that had to be overcome here was the slow movement of the plunger, during which each work piece had to cool down so much that the pressure that then took place could only have an extremely inadequate effect.
Haswell's machine, which is free from this drawback, has the advantage over other forging machines that the pressure can be regulated as desired, which now also acts evenly on the inner parts of the iron, is gradually increased and causes the molds to be completely filled without vibrations.
An Tangyes scissors is the cylinder, the fixed and movable simultaneously with the piston scissor blade is the pressure pump, the lever to the piston movement and f the vessel, from which the pressure pump, the pressure fluid drawn (water, glycerol etc.). Iron bars with a side length of 75 mm squared should be cut through in about 2¼ minutes if a man is working on lever h . These scissors are used successfully wherever only a few workers are available.
The punching machine is similarly constructed in that the pusher or punch is attached to the moving piston of the press. With such a machine, a man is able to press out a hole 25 mm in diameter in a 21.5 mm thick iron plate in about ½ minute. Hydraulic riveting machines and winches are also built according to this system and, like the punching machines, are characterized by high performance with sufficient lightness and portability. Of course, it should not be overlooked that the machines (according to the nature of the water effect) work very slowly.
Hydraulic presses today
In modern hydraulic presses, the pressure in the medium is applied by means of electric motors and high-performance pumps . Pressures of up to 300 bar in the pressure medium are common, which means that, depending on the piston diameter, pressing forces of several thousand tons (up to 800 MN or approx. 80,000 t) can be achieved. Depending on the pressing force, the presses are designed with a C or O frame, and single, double and (rarely) triple-acting designs are also available. A frequently occurring, universally applicable design is single-acting with an active draw cushion in the pressing force range of 100–2500 t.
Use of modern hydraulic presses
Hydraulic presses are used in the entire spectrum of metal forming. They are particularly suitable for pulling operations because they can apply the maximum force over the entire stroke length regardless of the ram position. They are also used for cutting. They are also used as trial, spotting and try-out presses.
In general, hydraulic presses are more flexible and easier to set up than mechanical presses, but do not achieve their cycle times. That is why they are very widespread, especially in smaller companies.
- Speed and power easily adjustable
- Drive can be installed anywhere
- Overload protection
- high forces can be generated
- China's strategic leadership in press forging - 3x 80,000-ton press forges! . In: Firebase88 , January 22, 2018. Retrieved February 5, 2019.