Metal presses are machine tools that shape pieces of metal by applying pressure. They work the metal through a force that acts over a distance -- or stroke -- and forges the metal into shape. Presses can be powered by hydraulics, electricity, compressed air (pneumatics) or steam. Hydraulic presses are built in many different sizes and are used in all types of manufacturing industry, ranging from automobile plants to jewellery making.
A hydraulic press delivers force to shape metal through fluid pressure. The fluid – usually oil or an oil and water emulsion -- is kept in a chamber and its pressure can be increased or decreased by using pumps. A typical hydraulic circuit consists of a motor, a cylinder, piston, pumps, pipes and valves. The fluid is poured into the cylinder below the piston. The fluid’s pressure increase makes the piston rise. The fluid is pumped out from below the piston decreasing the pressure in that space. Simultaneously, fluid is pumped in above the piston, increasing the pressure there and forcing the piston down. The downward force is used to work the metal.
Leaks of fluid are the most common problem associated with hydraulic presses. Even a small leak can cause a large mess. Leaks can show up in the hydraulic fluid lines and fittings that are not properly tightened. They can be caused by operator error and poor maintenance. Older presses are more prone to leaks as their components become worn. Water-based hydraulic fluids can cause erosion of metal components and affect the system’s seals.
Heat develops in a hydraulic system as the fluid is forced through pumps, tubing and valves. The system also generates heat when the fluid moves from a high pressure to a low pressure area without performing any work, such as the pressure drop. A temperature build up above 82C (180F) can degrade and oxidise the hydraulic fluid, make it more viscous and deposit sludge at the bottom of the cylinder. This lessens the system’s lubrication, blocks relief valves and can seize up operations. High temperatures will also damage seals and increase the frequency of leaks.
Hydraulic presses are not high-speed machines. The cylinder action and fluid pressure build up is a slow process – it takes about a second -- compared with mechanical or pneumatic presses whose cycles take fractions of a second. Hydraulic metal presses cannot be used for rapid production line processing of small metal components that require fast strokes.
The pumps used in hydraulic metal presses are bulky and have high energy consumption. These have to provide the high pressure for fluids in the cylinder. Insufficient pressure can cause irregularities in the metal processing. If a batch of work has to be repeated, even more energy is consumed. When combined with deterioration in machinery due to temperature and pressure fluctuations and leaks in hydraulic pipes, the energy efficiency of this system falls even further.