Altan T. Metal Forming Handbook

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In crossbar transfer presses, flywheels of up to 2,500 mm in diameter

and a weight of 25t are used. The mounting and lubrication of this

type of flywheel in roller bearings are highly demanding, as a large

quantity of lubricant and continuous temperature monitoring are

required (cf.Sect.3.2.11). The flywheel is driven by the main motor via

a flat belt or high-performance V-belt. When the main drive system is

switched off, the flywheel is brought to a standstill within a maximum

of 30 s by reverse braking of the motor and by an additional pneumatic

brake.

In isolated cases, normal car body presses and tri-axis transfer presses

are equipped with mechanical creep speed drive systems of one stroke

per minute. In this case, the drive is achieved by a threephase motor

acting on a worm gear which actuates the drive shaft via the brake

flange of the main brake. The transmitted torque generally falls short of

the nominal drive torque. However, by means of a pneumatic or

hydraulic servo system in the brake, the nominal force may be achieved

for distances of up to 10mm. This is generally sufficient for set-up

operation.

3.2.4Clutch and brake

One of the characteristics of mechanical presses is the clutchused to

transmit the motor and flywheel torque to the gear shaft and, after

clutch release, the brakewhich is used to decelerate the slide, the top die

and the gear. Particularly when working in single-stroke mode, the

masses in translational or rotational motion must be brought to a stand-

still after every stroke within an extremely short time: 200 to 300ms for

large-panel presses and 100 to 150ms in universal presses. Conversely,

after engaging the clutch, the same masses must be accelerated from

zero to operating speed.

For safety reasons, braking is generated mechanically by spring pow-

er. The clutch torque is calculated from the nominal press force and the

required working distance, generally 13 to 25mm above bottom dead

center.

Pneumatic single-disk clutch and brake combinations with minimum

rotating masses have been in successful use for decades(Fig.3.2.7).

Mechanical presses

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

Pneumatic control systems with safety valves and damping devices are

reasonable in cost and generally comply with requirements. One of the

problems of pneumatic systems, however, is the limited switching fre-

quency of single-stroke presses and the environmental damage caused

by wear to the clutch and brake. In order to eliminate these drawbacks,

hydraulic systems are being used increasingly in recent years. With the

aid of cooling systems, these permit far higher switching loads per time

unit and are practically immune to wear. In single-stroke presses, sepa-

rate units which permit switching frequencies of up to 30/min have

been used to permit more effective control and to avoid overshooting

and undershooting by the clutch and brake. Large-panel transfer press-

es use clutch-brake combinations based on sintered disk sets with up to

20 friction surfaces and transmission torque levels of up to 500,000 Nm.

The compact drive system used in smaller mechanical presses, con-

sists of a flywheel, clutch, brake and planetary gear in a complete unit.

Such a system achieves a particularly high level of efficiency with an

extremely low moment of inertia and small braking angle, coupled

Fundamentals of press design

clutch

brake

friction blocks

drive shaft

wear indicator

Fig. 3.2.7 Single-disk clutch and brake in a large-panel press

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

with a long service life and low maintenance costs (Fig. 3.2.8). The elec-

tric motor drives the eccentric shaft by means of a flywheel, a clutch-

brake combination that can be either hydraulically or pneumatically

actuated and a high-performance planetary gear (cf.Fig.4.4.4). This

eliminates the need for the reducing gear generally required for univer-

sal presses. Compact drive systems using a hydraulic clutch-brake com-

bination are particularly environmentally friendly, as they produce no

abraded particles and run at an extremely low noise level.

3.2.5Longitudinal and transverse shaft drive

The design of a mechanical press is determined mainly by the arrange-

ment of the drive shafts and axles, and by the number of pressure points

of the connecting rod on the slide (cf.Fig.3.1.3). The term longitudinal

or transverse shaft drive is defined by the arrangement of the drive

shafts relative to the front of the press, where the press operator stands.

Mechanical presses

clutch-brake disks

flywheel

planetary gears

eccentric shaft

Fig. 3.2.8

Compact drive

system used in a

universal press

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

Longitudinal shaft drive is used mainly in single-acting one- and two-

point presses with eccentric drive systems (cf.Fig.4.4.4and 4.6.29).

These are generally blanking presses and universal presses in the lower

force range of up to 8,000kN. This classical drive arrangement is also

used in long and narrow transfer presses where between two and six

pressure points or connecting rods are located in the longitudinal direc-

tion. The connecting rods are driven by a continuous drive shaft. The

transfer presses are designed with nominal pressing forces between

1,000 and 35,000 kN. They have up to four pairs of uprights and three

slides (cf. Sect. 4.4.6).

The connecting rods are arranged in the area of the uprights and tie

rods with the purpose of minimizing the bending stress on the press

crown. This leaves the top of the work stations in the slide free for ejec-

tion functions. For simpler production and assembly, in larger-scale

transfer presses, the longitudinal shaft drive is also replaced by a trans-

verse shaft drive system. Knuckle-joint presses with single-point drive,

covering a force range from 1,500 to 16,000 kN represent another spe-

cial construction.

The transverse shaft pressis designed to have one-, two- or four-point

connections and pressing forces of between 1,600 and 20,000 kN. This

design permits a greater bed depth, a longer stroke and also the use of a

joint system. Double-acting deep drawing presses are only available in

this configuration (Fig. 3.2.4).

Large-panel presses with tri-axis transfer in a range from 18,000 kN to

45,000kN press force are also based on a transverse shaft drive system

(cf. Sect. 4.4.7). To permit the distribution of pressing forces – generally

over four connecting rods per slide – extensive gear units are required

which are accommodated in multiple-walled press crowns. Thus, the

gear drive systems are simplified and improved.

The latest development in the field of large-panel presses is the

crossbar transfer press. Its functions include the production of pas-

senger car side panels in dies measuring up to 5,000 32,600mm

(cf. Sect. 4.4.8). In a five-stage configuration, five transverse shaft

drive units are joined to form a machine assembly similar to a press

line, and all six drive shafts connected by clutches (Fig. 3.2.9). This

transforms the complete press into a longitudinal shaft drive offer-

ing all the advantages of a transverse shaft drive.

Fundamentals of press design

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

3.2.6Gear drives

In contrast to blanking presses, whose continuous stroking rate cor-

responds to the speed of the flywheel, presses with one or two-stage

reducing wheel gears are used for longer working strokes. In large-panel

presses, the connecting rod and links are generally driven by a two-

stage reduction gear. Depending on the necessary power distribution

over two, four or, in the case of transfer presses, up to twelve connect-

ing rods, the gear units with wheels and links form the nerve center of

modern presses.

Eccentric wheels with a diameter of up to 3,000mm and reducing

gears of up to 1,600mm in diameter are manufactured from high-

strength ductile iron (cf.Sect.3.7). The gear teeth on the shafts and

pinions are hardened. Double helical gearing guarantees optimum

power transmission and low running noise (Fig.3.2.4). The gears and

quill pinion shafts are mounted on floating axles with friction bearing

bushing for easy assembly. At the start of the power train is always the

drive shaft with drive pinion, clutch and brake. Unlike other gear dri-

ves, roller bearings are used in the speed range from 300 to 500 rpm.

Mechanical presses

Fig. 3.2.9 Drive system of a transfer press with crossbar transfer

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

3.2.7Press crown assembly

The connecting rod, links, gears and main drive system determine the

capacity and the size of the press crown. The welded components weigh

up to 150t, as do the mounted gear drive elements contained in the

crown of a large-panel press.

The dimensional limits of such large-scale parts are around 6,000 3

3,600 312,000mm, whereby width, height and weight depend on the

possibilities offered for road transport on low-bed trucks – which gen-

erally require special authorization. Waterways represent a less difficult

method of transporting this type of components.

Stress-relief annealing of large welded assemblies and also machining

on drilling centers is restricted to a maximum length of 12m. The entire

problem of transport logistics plays a major role in large-panel press

design and must be taken into consideration right from the planning

stage.

3.2.8Slide and blank holder

The rotary motion generated by the press drive is transmitted to the

linear motion of the slide and blank holder using a suitable speed and

force progression. The most important functional units, particularly in

large-panel presses, are the pressure pointscontaining slide adjustment

and an overload safety device.

Slide adjustment

In individual presses, pressure points with a slide adjustment of up to

600mm are used in the main spindle in order to adjust the press shut

height to various die heights (Fig.3.2.10). In universal and transfer

presses, slide adjustment distances of 150mm are sufficient. In this

case, pressure points with a low overall height are useful. Using brake

motors and worm gears, adjusting speeds of 60mm/min are obtained.

With multiple-stage presses used for solid forming, to achieve high stiff-

ness, wedge adjustments are frequently used in the die set or at the slide

clamping plate. In this case, a slide adjustment is no longer necessary.

Fundamentals of press design

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

Overload safety devices

The nominal force of the press is limited and safeguarded by the

hydraulic overload safety device in the slide. If the nominal press force

is exceeded, the pressure exerted on a hydraulic cushion integrated in

the pressure point is quickly relieved, allowing an overload displace-

ment.

For blank holders used in double-acting mechanical presses, the over-

load safety function is coupled to the adjustment function of the blank

holder force. The overload safety function is activated only when the

deflection of the four blank holder pressure points has exceeded a per-

missible magnitude of approx. 3mm. The safety function is actuated by

an air-hydraulic pressure scale. By changing the compressed air setting,

the blank holder force can be adjusted to the requirements of the die at

the four pressure points within a range of 40 to 100%.

Slide counterbalance

The slide weight is compensated by the slide counterbalance system.

Thus, the weight of all moving parts such as joints, connecting rods,

the slide and upper die, is balanced out by pneumatic cylinders

(Fig. 3.2.11). As a result, the drive system is largely free of gravity forces.

There are no stresses acting on the slide adjustment and there is an addi-

Mechanical presses

slide

adjustment

device

connecting rod

wrist pin

adjustment nut

overload bolt

oil cushion

pressure pad

adjustment spindle

adjustment device

connecting rod

wrist pin

adjustment nut

adjustment pad

overload bolt

oil cushion

Fig. 3.2.10 Spindle (left) and pressure point (right)

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

tional safeguard against downward travel of the slide caused by gravity.

Pneumatic weight compensation guarantees low-noise, vibration-free

operation, smooth loading of the motor and short braking distances.

For counterbalancing weights up to 200t, at 10bar air pressure,

cylinder sizes up to a 900 mm diameter are required. Long slide strokes

of up to 1,300 mm and permissible pressure spikes of up to 25% require

substantial surge tanks. The arrangement of cylinders and tanks de-

pends on the type of press and safety issues. In the case of counterbal-

ance cylinders which are not externally enclosed, double piston rods

are often required to safeguard against the risk of breakage (Fig. 3.2.11).

Conversely, if the system is integrated in the press crown, this addi-

tional safety feature is not necessary.

With every die change, the weight compensation system must be

automatically adjusted to the new die weights for this purpose.

Individual presses are fitted with special weighing and control func-

tions. Program-controlled automatic systems used in transfer presses

automatically set the program inherent pressure value when changing

dies in order to reduce set-up times to a minimum.

Fundamentals of press design

slide

breakage sensor

tubular rod

safety rod

piston

cylinder

compressed

air tank

Fig. 3.2.11

Compensation of slide speed with

double piston rod

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

Slide cushion

Some users of presses prefer to use slide cushions as an ejector for the

top die. In these cases, pneumatic bellow cylinders are used to supply

the required ejection forces (Fig.3.2.12). Here, ejection strokes can be

anywhere between 100 and 200 mm, with pressure increases of up to

30%. The bellow cylinders act on an ejector plate which translate the

force to the pressure pins of the cushion and of the die.

During the upstroke of the slide, the ejector plate and the pins are

returned by force from the bellow cylinders to their lowest position.

Damping elements are used to reduce the noise resulting from the hard

impact.

Stroke adjustment

Today, the stroke of small mechanical presses can be adjusted by motor

power continuouslyaccording to requirements of the formed parts with

the aid of two interactive eccentrics (Fig.3.2.13). The stroke can, for

example, be set in such a way that the required part height is reached

during the deep drawing process and the part can be removed from the

die without difficulties on completion of the deep-drawing operation.

The elastic clamp connection between the eccentric shaft and the

eccentric bushing is hydraulically released. This involves expanding the

Mechanical presses

bellows

cylinder

dampingrail

pressurepin,

slidecushion

pressurepins,

die

ejectorplate,

die

separate

slideplate

pressure

plate

bottom

stop

slide

upperdie

Fig. 3.2.12 Slide cushion with bellow cylinders

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

bushing against the clamp so that it can be rotated using a gliding

motion in any direction on an oil cushion. Special seals are used to pre-

vent oil leakage. The position of the eccentrics in relation to each other

is modified from the control desk in accordance with the specific

requirements of the die until the sum of the eccentricities reaches the

required stroke. The positive connection between the eccentric shells is

created as a result of pressure relief coupled with run-away of the oil

through drainage grooves. For each die, it is possible to store the

required stroke in the die data record. This information can be accessed

again and used for automatic adjustment of the slide stroke.

3.2.9Pneumatic system

The major pneumatic functions of mechanical presses are the clutch,

brake, flywheel brake, slide counterbalance, slide cushion and bed

cushion. In the case of transfer presses, pneumatic functions also

include the application of air pressure to the transfer cam followers and

transport by suction cups. Large volumes of air require compressed air

tanks with a monitoring safety function and drainage system. The max-

Fundamentals of press design

eccentric

eccentricbushing

innertaperedring

outertaperedring

clampingscrew

centralborefor

pressuremedium

connectingrod

groovesfor

pressuremedium

Fig. 3.2.13 Continuous stroke adjustment in an eccentric press

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