Machinery's Handbook, 31st Edition
Presses for Sheet Metal Working 1463 uses a mechanical screw to translate rotational motion into the linear motion of the ram. Briefly, the ram acts as the nut on a rotating screw shaft, moving up or down depending on the screw rotation. Energy is either delivered from a flywheel, which is usually coupled with a torque-limiting (slipping) clutch, or by a direct drive reversing electric motor. The knuckle —the knuckle press is a modified version of the crankshaft style. The crank shaft is behind the press frame in this type of press. The knuckle crankshaft runs from left to right in relation to the press frame. The knuckle is a group of three levers, one lever or connector of which joins the crankshaft throw to the other two levers. The upper lever or knuckle is fixed to a pivot point in the press crown at one end. The lower knuckle is fixed to a pivot point on the press ram at one end. The knuckle press is used for dies that coin, or when heavy sheet metal is drawn or formed. Because of the knuckle, the stroke of the press is limited. Fig. 72 schematically illustrates a knuckle drive mechanism.
Fig. 73. Available Tonnage Curve for a Mechanical Press
Fig. 72. Knuckle Drive Mechanism
Tonnage: The tonnage of a press is the force that a press ram is able to exert. In actual practice, press rams can and do exert forces greater than the rated tonnage. A safety factor is designed into the frame and drive mechanism. The tonnage of a mechanical press is calculated when the ram is near the bottom of its stroke. Thus, the tonnage of a mechanical press is constant and cannot be varied as in the hydraulic press. The tonnage of a mechanical press when the ram is not near the bottom of its stroke is greatly reduced. Most press manufacturers will supply curves showing the tonnage avail able for each inch of stroke. Fig. 73 illustrates the available tonnage curve for a mechanical press. Mechanical presses have forces that typically range from 20 to 5,000 tons. A few specially designed large capacity presses with ratings up to 6,000 tons are in operation. Stroke: The stroke of a press is the distance of ram movement from its up position to its down position. The offset on a crankshaft determines the press stroke. The stroke is con stant for the crankshaft and eccentric drives. Strokes on mechanical presses range from 0.2 to 45 inches. Strokes per Minute: Strokes per minute (press speed) is a self-explanatory term. The C-frame low tonnage presses (15 to 30 tons) have the highest stroke per minute of all press types. They run at up to 1,800 strokes per minute and typically involve light forming, such as electrical connectors; when cutting dies are operated, the speeds usually range from around 20 to 800 strokes per minute. Action: The term “action” simply means the number of rams on the press. Types of press actions include the following:
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