Forging presses are of either mechanical or hydraulic design. Presses are rated on the basis of force developed at the end of the stroke.
1. Mechanical presses:
Next to hammers, mechanical presses are most commonly used for closed die forging. Most mechanical presses utilize an eccentric crank to translate the motion of the motor into a reciprocating linear motion of the press slide. The ram rotary stroke is shorter than in a hammer or hydraulic press and hence the mechanical presses are best suited for low-profile forgings.
Maximum load (or pressing force) in these presses is attained when the ram is about 3 mm off the bottom dead centre position. Knuckle joint presses and screw presses are also available. Because of the linkage design, very high forces can be applied in knuckle joint presses. Screw presses derive their energy from a flywheel. The forging load is transmitted through a vertical screw, and the ram comes to a stop when the flywheel energy is dissipated. If the dies do not close at the end of the cycle, the operation is repeated until the forging is completed.
Screw presses are used for various open die and closed die operations with their capacity ranging from 1.4 to 280 MN. The initial cost of mechanical presses is higher than the hammers.
Schematic of (a) knuckle press and (b) screw press.
2. Hydraulic presses:
These are load-restricted machines wherein hydraulic pressure moves a piston in a cylinder. The full load is available in hydraulic presses at any point during the full stroke of the ram. The ram velocity can be controlled and varied during the stroke. Hydraulic presses are relatively slow speed machines (velocity range from 0.06 to 0.3 m/sec) which leads to slow squeezing action with close dimensional tolerances on forgings.
These presses are much slower than the drop hammer presses. In press forging, the pressure or squeeze is applied to the blank and the intensity of this pressure increases as the plastic metal resists deformation. Due to the greater pressures available, hydraulic presses are made to have very large capacities. These presses are available in capacity of 500 to 1800 tonnes, although 50,000-tonne presses are also available.
Showing the flow of metal under (a) slow pressure (squeeze) and (b) impact pressure.
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