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Fundamentals of Mechanical-Press Design

Monday, June 01, 2009
 
 
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Force and work are two terms that can be related only by means of a third variable, distance. When force is applied over a certain distance, work is performed. This corresponds in the force-distance graph to the area of the rectangle below the force curve. When work is performed, the distance over which it is performed determines the magnitude of the generated force.

These principles apply to mechanical presses, but rather than a raised weight moving over a distance to perform work, energy is stored in the rotating mass of the press flywheel. Since the energy in the rotating flywheel is only partially used during a press stroke, the electric motor driving the flywheel is not overloaded and does not need to have a large power capacity. In continuous operation, a flywheel slowdown of 15 to 20 percent is estimated to be the greatest speed drop permissible. However, this gives no indication of the resulting forces and of the stress exerted on the press components.

Press Load

Double-action presses equipped with mechanical or hydraulic drives
Double-action presses (above) equipped with mechanical or hydraulic drives find use for deep drawing. On mechanical presses, a knuckle-joint system retains the blankholder and a multi-link drive executes the drawing process. Blank-holder and drawing slide are driven from one main shaft, so the two function in a fixed
geometrical relationship. Deep drawing also is increasingly being performed in single-action presses (right) with drawing cushio

Consider a press with rated force FN0 = 1000 kN = at 30 deg. before bottom dead center (BDC); usable energy/ press stroke during continuous operation WN = 5600 Nm; continuous stroking rate n = 55/min.

Assuming a slowdown of 20 percent during continuous stroking, the usable energy is 36 percent of the total energy available in the flywheel. The overall energy stored in the flywheel:

W = Wn/0.36 = 5600Nm/0.36 = 15,600 Nm.

The given nominal load FN0 in a mechanical press indicates that this value is based on the strength calculations of the frame and the moving elements located in the force flow—crankshaft, connecting rod and slide. Nominal load represents the greatest permissible force in operating the press, defined on the basis of the permissible level of stress or by the deflection characteristics. In most cases, stress on the frame is kept low to achieve maximum possible rigidity. The nominal load is specified at 30 deg. before BDC to indicate that from here to BDC, the drive components that transmit the power—driveshaft, clutch, etc. —also have been designed for the torque corresponding to the nominal press force. Therefore, between 90 and 30 deg. before BDC, the movable parts must be subjected to smaller stresses to avoid overloading. The force-vs.-crank angle curve indicates that the press in question may be subjected to a nominal load of 1000 kN between 30 deg. before BDC and at BDC, while at 90 deg. before BDC a slide load of only FN0/2 = 500 kN is permissible.

If a metalformer uses a press with the parameters specified above to apply a constant force of 1000 kN over a distance h of 5.6 mm, the energy used during forming is W = F x h= 5600 Nm. The press then is being used to the limits of its rated force and energy. If the same force were to act over a distance of only 3 mm, the energy expended is 3000 Nm, and the force of the press then is fully used, while the available energy is not completely used.

The situation is much more unfavorable if the rated flywheel energy of 5600 Nm is applied over a working distance of h = 3 mm. In this case, the effective force of the slide will be:

F = Wn/h = 5600 Nm/0.003m = 1867 kN.

As the maximum permissible press force is only 1000 kN, in this case the press is being severely overloaded. Although the flywheel slowdown is within normal limits and gives no indication of overloading, all elements subjected to the press force may be damaged, such as the press frame, slide and connection rods. Serious overloading often occurs when press forming is conducted using high forces over small distances, such as during blanking or coining. The danger is that such overloading may go undetected, which requires the use of overloading safety devices to protect the press.

Another form of overloading results from taking excessive energy from the flywheel, which can cause extremely high press forces to develop if the displacement during deformation is too small. However, if the energy is applied over a large displacement, this type of overload is much less dangerous. For example, if the press described above is brought to a stop during a working distance of h = 100 mm, the entire flywheel energy of W = 15,600 Nm is utilized. Assuming that no peak loads have occurred, the mean press force exerted is only F= w/h = 15,600 Nm/0.1 = 156 kN.

The press is, therefore, not overloaded although the flywheel has been brought to a standstill. In this case, only the drive motor suffers a large slowdown and it is necessary to use a press of a larger flywheel-energy capacity, although the permissible press force of 1000 kN is more than adequate. Overloads of this type occur more frequently if deformation is done over a large distance—during deep drawing, for example.

Types of Drive Systems

Eccentric or crank drive—For a long time, eccentric or crank-drive systems, were the only drive mechanism used in mechanical presses. The relatively

modified knuckle joint
Modified knuckle-joint drive systems can be either top- or bottom-mounted. Particularly for solid forming, the modified top-drive system, shown here, 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 the curved path, as-illustrated. This results in a change of the stroke-vs.-time characteristic of the slide, compared to the largely symmetrical stroke-time curve of an eccentric drive system.
high impact speed on die closure and the reduction of slide speed during forming are drawbacks that often preclude the use of this type of press for deep drawing at high stroke rates.

However, in presses with capacities to a nominal force of 5000 kN, such as universal or blanking presses, a crank drive is still the most effective system. This is especially true when using automated systems where the eccentric drive offers a good compromise between the required processing time and the time required for part transport. Even with the latest crossbar-transfer presses, eccentric-drive systems used in subsequent processing stations—after drawing—satisfy the requirements for system simplification.

Linkage drive—Attempting to maximize stroke rates using crank- or eccentric-drive systems requires increasing the slide speed. However, when deep drawing, ram speed typically must not exceed 0.4 to 0.5 m/sec. during deformation. A linkage-drive system can be designed for mechanical presses so that slide speed during drawing can be reduced by as much as half compared to an eccentric drive. The slide in a double-acting deep-drawing press, for example, is actuated using a specially designed linkage drive. This kinematic characteristic offers ideal conditions for deep drawing. The slide hits the blank softly, allowing it to build up high press forces right from the start of the drawing process, forming the part at a low, almost constant speed. In addition, this system ensures smooth transitions between the various portions of the slide motion. During deformation, the drive links are stretched to an almost extended position; clutch torque, the gear load and the decelerating and accelerating gear masses are between 20 and 30 percent less than with a comparable eccentric press. In the case of single-acting machines, for instance, reducing impact increases the service life of the dies, the draw cushion and the press itself.

This offers a number of important benefits in production. For the same nominal press force and slide stroke, a link-drive press can be loaded substantially earlier in the stroke, because the press-force displacement curve is steeper within the deformation range. Therefore, a linkage press has a more favorable force-displacement curve than does an eccentrically driven press. Without increasing the impact speed, it is possible 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.

 

 
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See also: Schuler Incorporated

Related Enterprise Zones: Presses

 


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