Altan T. Metal Forming Handbook

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case deformation takes place over large displacements, the potential

energy may be insufficient, resulting in a need for repeated hammering

or several blows.

These principles, illustrated here in the example of the forging ham-

mer, also apply to mechanical presses. Here, instead of a raised weight,

the energy is stored in the rotating mass of the flywheel (Fig. 3.2.8).

However, while all the energy stored in the hammer must be expended,

the energy in the rotating mass of the flywheel is only partially used

during a press stroke. Thus, 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 between 15 and 20% 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

components of the press.

This is illustrated more clearly by the example below (Fig. 3.2.2). The

press is assumed to have the following parameters:

Mechanical presses

Fig. 3.2.1 Force-stroke diagram showing the relationship between force and stroke for

constant work

distance

s[m]

0,2

0,4

0,6

0,8

1,0

forceF[N]

0 1000 2000 3000 4000 5000 6000 7000 8000

W=const.=750Nm

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

– rated press force: F

= 1,000 kN = 1,000,000 N at 30° before bottom

dead center (BDC)

– usable energy per press stroke during continuous operation:

= 5,600 Nm

– continuous stroking rate: n = 55/min

Assuming a slowdown of 20 % during continuous stroking, the usable

energy is 36 % of the total energy available in the flywheel. In the exam-

ple given, therefore, the overall energy stored in the flywheel is:

The given nominal load F

in a mechanical press indicates that this val-

ue is based on the strength calculations of the frame and the moving

elements located in the force flow such as crank shaft, connecting rod

and slide. The nominal load is the greatest permissible force in operat-

ing the press. This limit can be defined on the basis of the permissible

level of stress or by the “deflection” characteristics. In most cases, the

Fundamentals of press design

Fig. 3.2.2 Permissible press force of an eccentric press as a function of crank angle

permissible pressing force [kN]

crank position above BDC (bottom dead center) [°]

500 1,000 1,500 2,000 2,500 3,000

permissible torque

permissible frame load

above BDC

W W Nm Nm

== ≈/. , /. ,0 36 5 600 0 36 15 600

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

stress on the frame is kept low to achieve the maximum possible rigid-

ity. The nominal load is specified at 30°before bottom dead center to

indicate that from here to the bottom dead center, the drive com-

ponents that transmit the power, such as the driveshaft, the clutch,

etc., have also been designed for the torque corresponding to the

nominal press force. Therefore, between 90°and 30°before the bottom

dead center, the movable parts must be subjected to smaller stresses to

avoid overloading. The force versus crank angle curve, Fig.3.2.2, indi-

cates that the press in question may be subjected to a nominal load

of 1,000 kN between 30°before bottom dead center and at bottom

dead center, while at 90°before bottom dead center a slide load of only

/2 = 500 kN is permissible.

If a press with the parameters specified here is used to perform an

operation in which a constant force of F = 1,000kN is applied over a

distance of h = 5.6mm, the energy used during forming is:

The press is then being used to the limits of its rated force and energy.

If the same force were to act over a distance of only h = 3mm = 0.003 m,

the energy expended amounts to:

The force of the press is then fully used, while the available energy is

not completely used.

The situation is much more unfavorable if the rated flywheel energy

of 5,600Nm is applied over a working distance of h = 3mm. In this

case, the effective force of the slide will be:

As the maximum permissible press force is only 1,000kN, the press is

being severely overloaded. In spite of the fact that the flywheel slow-

down is still within the normal limits and gives no indication of over-

loading, all elements that are subjected to the press force may be dam-

aged, such as the press frame, slide, connection rods, etc. Serious over-

loading often occurs when press forming is conducted using high forces

Mechanical presses

WNmNm=⋅=1000000 0003 3000,, . ,

FWh Nm m N kN

== ≈ =/, /. ,, ,5600 0003 1867000 1867

WFh N m Nm=⋅= ⋅ =1000000 00056 5600,, . ,

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

over small distances, for instance during blanking or coining. The great

danger is that such overloading may go undetected. For this reason, over-

loading safety devicesmust be used to protect the press (cf.Sect. 3.2.8).

Another form of overloading results from taking too much energy

from the flywheel. Such overloading can result in extremely high press

forces 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 = 100mm, the entire

flywheel energy of W = 15,600Nm is utilized. Assuming that no peak

loads have occurred, the mean press force exerted is only

The press is therefore by no means overloaded although the flywheel

has been brought to a standstill. In this case, only the drive motor

suffers a very large slowdown and it is necessary to use a press of a larger

flywheel energy capacity, although the permissible press force of 1,000 kN

is more than adequate. Overloads of this type occur more frequently if

deformation is done over a large distance, e. g. during deep drawing, open

or closed die extrusion.

3.2.2Types of drive system

Eccentric or crank drive

For a long time, eccentric or crank drive systems were the only type of

drive mechanisms used in mechanical presses. The sinusoidal slide dis-

placement of an eccentric press is seen in (Fig.3.2.3).The relatively high

impact speed on die closure and the reduction of slide speed during the

forming processes are drawbacks which often preclude the use of this

type of press for deep drawing at high stroking rates.

However, in presses with capacities up to a nominal force of 5,000kN,

such as universal or blanking presses, eccentric or crank drive is still the

most effective drive system. This is especially true when using automat-

ed systems where the eccentric drive offers a good compromise between

time necessary for processing and that required for part transport. Even

in the latest crossbar transfer presses, eccentric drive systems used in

Fundamentals of press design

FWh Nmm N kN== = =/,/. ,15600 01 156000 156

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

subsequent processing stations, after the drawing station, satisfy the

requirements for system simplification. Stroke lengths of up to 1,300 mm

are achieved using eccentric gears with a diameter of 3,000mm. The

eccentric gears are manufactured from ductile iron in accordance with

the highest quality standards (cf. Sect. 3.7).

Linkage drive

Requirements for improved economy lead directly to higher stroking

rates. When using crank or eccentric drive systems, this involves

increasing the slide speed. However, when performing deep-drawing

work, for technical reasons the ram speed should not exceed 0.4 to

0.5m/s during deformation. The linkage drive system can be designed

such that in mechanical presses, the slide speed during the drawing

process can be reduced, compared to eccentric drive by between a

half and a third (Fig.3.2.3). The slide in a double-acting deep drawing

press, for example, is actuated using a specially designed linkage drive

(Fig.3.2.4 and cf. Fig.3.1.8).

Mechanical presses

Fig. 3.2.3 Displacement-time diagram: comparison of the slide motion performed by an

eccentric, a knuckle-joint and a link-driven press

180

270

360

bottom dead center (BDC)

crank angle[

100

200

300

400

500

600

700

900

eccentric

drive system

eight-link

drive system

modified

knuckle-joint

drive system/

six-link

drive system

stroke [mm]

800

approx. 180

approx. 170

approx.110

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

This kinematic characteristic offers ideal conditions for the deep-

drawing process. The slide hits the blank softly, allowing to build up

high press forces right from the start of the drawing process, and forms

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. Thus, the clutch torque, the gear load and the decel-

erating and accelerating gear masses are between 20 and 30 % smaller

than in a comparable eccentric press. In the case of single-acting

machines, for instance, reducing the impact increases the service life of

the dies, the draw cushion and the press itself.

Fundamentals of press design

Fig. 3.2.4 Slide (eight-link drive system) and blank holder slide drive (toggle joint drive

system) in a double-acting deep drawing press

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

This offers a number of important benefits in production: for the

same nominal press force and the same nominal slide stroke, a link-

driven press can be loaded substantially earlier in the stroke, because

the press force displacement curve is steeper within the deformation

range. Therefore, the linkage press has a more favorable force-displace-

ment curve than on an eccentrically-driven press (Fig.3.2.2). Without

increasing the impact speed, it is possible to achieve an appreciable

increase in the stroking rate and output. Due to the improved drawing

conditions, a higher degree of product quality is achieved; even sheet

metal of an inferior quality can be used with satisfactory results. In

addition, this system reduces stress both on the die and the draw cush-

ion and also on the clutch and brake. Furthermore, noise emissions are

reduced as a result of 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 determined by deformation

conditions, the part transport 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 system is frequently used,

since this represents the best possible compromise between optimum

press geometry and manufacturing costs (cf.Sect.4.4.7). In the case

of crossbar transfer presses, either an eight-element linkage drive or a

combination of linkage and eccentric drives are used in order to opti-

mize the die-specific transfer movements (cf.Sect. 4.4.8). The linkage

elements are made of ductile iron, the linkage pins are from steel

with hardened contact surfaces and the bearing bushes are from

highly stress-resistant non-ferrous alloys. A reliable system of lubrica-

tion and oil distribution guarantees safe operation of these highly

stressed bearings (cf. Sect. 3.2.11).

Knuckle-joint drive system

This design principle is applied primarily for coining work. The knuck-

le-joint drive system consists of an eccentric or crank mechanism dri-

ving a knuckle-joint. Figure 3.2.5shows this concept used in a press

with bottom drive. The fixed joint and bed plate form a compact unit.

The lower joint moves the press frame. It acts as a slide and moves the

attached top die up and down. Due to the optimum force flow and the

favorable configuration possibilities offered by the force-transmitting

Mechanical presses

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

elements, a highly rigid design with very low deflection characteristics

is achieved. The knuckle-joint, with a relatively small connecting rod

force, generates a considerably larger pressing force. Thus, with the

same drive moment, it is possible to reach around three to four times

higher pressing forces as compared to eccentric presses. Furthermore,

the slide speed in the region 30 to 40° above the bottom dead center, is

appreciably lower (cf. Fig. 6.8.3). Both design features represent a par-

ticular advantage for coining work (cf. Fig. 6.8.21) or in horizontal pres-

ses for solid forming (cf. Fig. 6.8.7).

By inserting an additional joint, the kinematic characteristics and

the speed versus stroke of the slide can be modified. Knuckle-joint and

modified knuckle-joint drive systems can be either top (cf. Fig. 4.4.10)

Fundamentals of press design

flywheel

press frame

housing

connecting rod

crankshaft

fixed press bed

knuckle-joint

Fig. 3.2.5 Knuckle-joint press with bottom drive

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

or bottom mounted. For solid forming, particularly, the modified top

drive system is in popular use. Figure3.2.6illustrates the principle of a

press configured according to this specification. 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 curve-

shaped path. This results in a change of the stroke versus time charac-

teristic of the slide, compared to the largely symmetrical stroke-time

curve of the eccentric drive system (Fig.3.2.3). This curve can be altered

by modifying the arrangement of the joints (or possibly by integrating

an additional joint).

As a rule, it is desirable to reduce the slide velocity during deforma-

tion (e.g. reduction of the impact and pressing speed of the slide).

Using this principle, the slide displacement available for deformation

can be increased to be three or four times greater than when using

eccentric presses with a comparable drive torque. This represents a fur-

ther advantage when forming from solid (cf. Linkage drive).

Mechanical presses

flywheel

driveshaft

eccentricdrivesystem

pressslide

slide

adjustment

motioncurve,

pointB

Fig. 3.2.6

Modified knuckle-

joint drive system

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

The blank holder drive, sometimes used in double-acting deep draw

presses, is a special case (Fig.3.2.4and cf.Fig.3.1.8). The required stand-

still of the blank holder during the deep-drawing phase is achieved in

this type of machine by superimposing a double knuckle-joint system

coupled with the eccentric or linkage drive of the slide. The standstill of

the blank holder represents a crank angle of between 90 and 130°with

a maximum residual movement of 0.5mm. This residual movement

does not, however, influence the deep-drawing process because the

blank holder overload safeguard acts as a storage system and the elastic

deflection of the entire press, which is a few millimeters, has an over-

riding effect. The slide drive, the blank holder drive and the wheel gear

are integrated in the press crown.

3.2.3Drive motor and flywheel

In larger-scale presses, the main drive system is usually powered by DC

motors, primarily in order to provide a large stroking rate. Frequency-

controlled threephase motors offer an alternative, particularly where a

high protection class rating is called for. The output of a press, which is

determined by force, slide displacement and speed, including lost ener-

gy, is provided by the main motor. Periodically occurring load peaks are

compensated by the flywheel which stores energy. The highly elastic

DC drive system is able to compensate for a drop of the flywheel speed

of up to 20% in every press stroke and to replace the consumed energy

by acceleration of the flywheel prior to the subsequent stroke.

Useful energy should also be available during set-up operation with a

reduced stroking rate, for example five strokes per minute. In this case,

a speed drop of 50% is admissible. An important criterion in configur-

ing the energy balance is to ensure a short run-up time. Running up

a large-panel press from its minimum to its maximum production

stroking rate generally takes less than one minute under load.

Due to the required useful energy and the permissible flywheel slow-

down of 20%, the flywheel is generally designed to operate under the

most energy-consuming condition, i. e. in the lower stroking range of

the press. Although a high flywheel speed is beneficial here, this is lim-

ited by the admissible speed of the clutch, brake and flywheel itself.

Fundamentals of press design

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998