Page 16 - MetalForming June 2009

Mechanical-Press Design
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force-displacement curve than does an eccentrically driven press. Without increasing the impact speed, it is possi- ble to achieve an appreciable increase in the stroke rate and output. Due to the improved drawing conditions, a higher degree of product quality is achieved; even low-quality sheetmetal can be used with satisfactory results.
In addition, this system reduces stress on the die and the draw cushion and also on the clutch and brake. And, noise levels are reduced due to the lower impact speed of the slide and the quieter herringbone gears of the drive wheels.
In the case of large-panel transfer presses, the use of six- or eight-element linkage-drive systems is largely deter- mined by deformation conditions, part movement and the overall structure of the presses. As a result, in the case of transfer presses with tri-axis transfer system, a six-element linkage-drive sys- tem often is used, representing the best possible compromise between optimum press geometry and manufacturing costs. In the case of crossbar transfer presses, either an eight-element linkage drive or a combination of linkage and eccentric drives is used to optimize the die-specific transfer movements.
Knuckle-joint drive—This design principle is applied primarily for coin- ing. The knuckle-joint drive system consists of an eccentric or crank mech- anism driving a knuckle-joint. The fixed joint and bed plate form a compact unit. The lower joint moves the press frame and acts as a slide to move the attached top die up and down. Due to the optimum force flow and the favor- able configuration possibilities offered by the force-transmitting elements, a highly rigid design with very low deflec- tion characteristics is achieved.
The knuckle-joint drive, with a rela- tively small connection-rod force, gen- erates a large pressing force—with the same drive moment it is possible to achieve three to four times more force than with an eccentric press. Further, the slide speed in the region 30 to 40 deg. above BDC is appreciably lower. Both of these design features represent a particular advantage for coining, and
for forming in a horizontal press.
By inserting an additional joint, the kinematic characteristics and the speed- vs.-stroke of the slide can be modified. Knuckle-joint and modified knuckle- joint drive systems can be either top- or bottom-mounted. Particularly for solid forming, the modified top-drive sys- tem is popular. The fixed point of the modified knuckle-joint is mounted in the press crown. While the upper joint pivots around this fixed point, the lower joint describes a curved path. This results in a change of the stroke-vs.- time characteristic of the slide, com- pared to the largely symmetrical stroke- time curve of the eccentric drive system. This curve can be altered by modifying the arrangement of the joints, or by
integrating an additional joint.
As a rule, it is desirable to reduce the slide velocity during deformation, reducing impact and the pressing speed of the slide. Using this principle, the slide displacement available for defor- mation can be increased by three or four times than when using eccentric presses with a comparable drive torque. Blankholder drive—Sometimes used in double-acting deep-draw presses, this drive is a special case. The required standstill of the blankholder during deep drawing is achieved in this type of machine by superimposing a double knuckle-joint system with an eccentric or linkage drive of the slide. The stand- still of the blankholder represents a crank angle of between 90 and 130 deg., with a maximum residual movement of 0.5 mm. This residual movement does not, however, influence the deep-draw- ing process because the blankholder overload safeguard acts as a storage sys- tem and the elastic deflection of the entire press—a few millimeters—has an overriding effect. The slide drive, the blankholder drive and wheel gear
are integrated in the press crown.
Drive Motor and Flywheel
In larger-scale presses, the main drive system usually is powered by DC motors, primarily to provide a large stroking rate. Frequency-controlled three-phase motors offer an alternative, particularly
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14 METALFORMING / JUNE 2009
www.metalformingmagazine.com