Force and work are two terms that canbe related only by means of a thirdvariable, distance. When force is

applied over a certain distance, work isperformed. This corresponds in theforce-distance graph to the area of therectangle below the force curve. Whenwork is performed, the distance overwhich it is performed determines themagnitude of the generated force.These principles apply to mechanical

presses, but rather than a raised weightmoving over a distance to performwork, energy is stored in the rotatingmass of the press flywheel. Since theenergy in the rotating flywheel is onlypartially used during a press stroke, theelectric motor driving the flywheel isnot overloaded and does not need tohave a large power capacity. In contin-uous operation, a flywheel slowdown of15 to 20 percent is estimated to be thegreatest speed drop permissible. How-ever, this gives no indication of theresulting forces and of the stress exert-ed on the press components.

Press LoadConsider a press with rated force

FN0 = 1000 kN = at 30 deg. before bot-tom dead center (BDC); usable energy/press stroke during continuous opera-

tion WN = 5600 Nm; continuousstroking rate n = 55/min.Assuming a slowdown of 20 percent

during continuous stroking, the usableenergy is 36 percent of the total energyavailable in the flywheel. The overallenergy 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 valueis based on the strength calculations ofthe frame and the moving elementslocated in the force flow—crankshaft,connecting rod and slide. Nominal loadrepresents the greatest permissible forcein operating the press, defined on thebasis of the permissible level of stress orby the deflection characteristics. In most

Fundamentals ofMechanical-Press Design

Ram-activesystem

Upper partof die

Blank

Blankholder

Drawingpunch

Pressurepin

Drawingcushion-passivesystem

Information for this articlecomes from the

MetalForming Handbook, bySchuler GmbH, published inJuly 1998 by Springer. The

hardcover book, 568 pages, isavailable on Amazon.com.

Double-action presses (above)equipped with mechanical orhydraulic drives find use fordeep drawing. On mechanicalpresses, a knuckle-joint sys-tem retains the blankholderand a multi-link drive executesthe drawing process. Blank-holder and drawing slideare driven from onemain shaft, so the twofunction in a fixedgeometrical relationship.Deep drawing also is increas-ingly being performed insingle-action presses (right)with drawing cushions.

12 METALFORMING / JUNE 2009 w w w. m e t a l f o r m i n g m a g a z i n e . c o m

cases, stress on the frame is kept low toachieve maximum possible rigidity. Thenominal load is specified at 30 deg.before BDC to indicate that from here toBDC, the drive components that trans-mit the power—driveshaft, clutch, etc.—also have been designed for the torquecorresponding to the nominal press force.Therefore, between 90 and 30 deg. beforeBDC, the movable parts must be sub-jected to smaller stresses to avoid over-loading. The force-vs.-crank angle curveindicates that the press in question maybe subjected to a nominal load of 1000kN between 30 deg. before BDC and atBDC,while at 90 deg. before BDC a slideloadof onlyFN0/2=500kN ispermissible.If a metalformer uses a press with the

parameters specified above to apply aconstant force of 1000 kN over a dis-tance h of 5.6 mm, the energy used dur-ing forming is W = F x h= 5600 Nm.The press then is being used to the lim-its of its rated force and energy. If thesame force were to act over a distance ofonly 3 mm, the energy expended is 3000Nm, and the force of the press then isfully used, while the available energy isnot completely used.The situation is much more unfa-

vorable if the rated flywheel energy of5600 Nm is applied over a working dis-tance of h = 3 mm. In this case, theeffective 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 thepress is being severely overloaded.Although the flywheel slowdown is with-in normal limits and gives no indicationof overloading, all elements subjected tothe press force may be damaged, such asthe press frame, slide and connectionrods. Serious overloading often occurswhen press forming is conducted usinghigh forces over small distances, such asduring blanking or coining. The dangeris that such overloading may go unde-tected, which requires the use of over-loading safety devices to protect the press.Another form of overloading results

from taking excessive energy from theflywheel,which can cause extremely highpress forces to develop if the displace-

ment during deformation is too small.However, if the energy is applied over alarge displacement, this type of overloadis much less dangerous. For example, ifthe press described above is brought toa stop during a working distance of h =100 mm, the entire flywheel energy ofW = 15,600 Nm is utilized. Assumingthat no peak loads have occurred, themean press force exerted is only F= w/h= 15,600 Nm/0.1 = 156 kN.The press is, therefore, not over-

loaded although the flywheel has beenbrought to a standstill. In this case, onlythe drivemotor suffers a large slowdownand it is necessary to use a press of alarger flywheel-energy capacity, althoughthe permissible press force of 1000 kNis more than adequate. Overloads ofthis type occur more frequently if defor-mation is done over a large distance—during deep drawing, for example.

Types of Drive SystemsEccentric or crank drive—For a long

time, eccentric or crank-drive systems,were the only drive mechanism usedin mechanical presses. The relativelyhigh impact speed on die closure andthe reduction of slide speed duringforming are drawbacks that often pre-clude the use of this type of press fordeep drawing at high stroke rates.However, in presses with capacities to

a nominal force of 5000 kN, such as uni-versal or blanking presses, a crank driveis still the most effective system. This isespecially true when using automatedsystems where the eccentric drive offers agood compromise between the requiredprocessing time and the time requiredfor part transport. Even with the latestcrossbar-transfer presses, eccentric-drive

systems used in subsequent processingstations—after drawing—satisfy therequirements for system simplification.

Linkage drive—Attempting to max-imize stroke rates using crank- or eccen-tric-drive systems requires increasing theslide speed. However, when deep draw-ing, ram speed typically must not exceed0.4 to 0.5 m/sec. during deformation. Alinkage-drive system can be designed formechanical presses so that slide speedduring drawing can be reduced by asmuch as half compared to an eccentricdrive. The slide in a double-acting deep-drawing press, for example, is actuatedusing a specially designed linkage drive.This kinematic characteristic offers idealconditions for deep drawing. The slidehits the blank softly, allowing it to buildup high press forces right from the startof the drawing process, forming thepart at a low, almost constant speed. Inaddition, this system ensures smoothtransitions between the various portionsof the slide motion. During deforma-tion, the drive links are stretched to analmost extended position; clutch torque,the gear load and the decelerating andaccelerating gear masses are between20 and 30 percent less thanwith a compa-rable eccentric press. In the case of single-acting machines, for instance, reducingimpact increases the service life of thedies, the draw cushion and the press itself.This offers a number of important

benefits in production. For the samenominal press force and slide stroke, alink-drive press can be loaded substan-tially earlier in the stroke, because thepress-force displacement curve is steeperwithin the deformation range. There-fore, a linkage press has a more favorable

Flywheel

Drive shaft

Slideadjustment

Motion curve,point B

Press slideEccentric drive system

Modified Knuckle JointModified knuckle-joint drivesystems can be either top- or

bottom-mounted. Particularly forsolid forming, the modified top-

drive system, shown here, is popu-lar. The fixed point of the modi-fied knuckle-joint is mounted inthe press crown. While the upper

joint pivots around this fixedpoint, the lower joint describes thecurved path, as-illustrated. Thisresults in a change of the stroke-

vs.-time characteristic of theslide, compared to the largely

symmetrical stroke-time curve ofan eccentric drive system.

w w w. m e t a l f o r m i n g m a g a z i n e . c o m METALFORMING / JUNE 2009 13

force-displacement curve than does aneccentrically driven press. Withoutincreasing the impact speed, it is possi-ble to achieve an appreciable increase inthe stroke rate and output. Due to theimproved drawing conditions, a higherdegree of product quality is achieved;even low-quality sheetmetal can be usedwith satisfactory results.In addition, this system reduces stress

on the die and the draw cushion andalso on the clutch and brake. And, noiselevels are reduced due to the lowerimpact speed of the slide and the quieterherringbone gears of the drive wheels.In the case of large-panel transfer

presses, the use of six- or eight-elementlinkage-drive systems is largely deter-mined by deformation conditions, partmovement and the overall structure ofthe presses. As a result, in the case oftransfer presses with tri-axis transfersystem, a six-element linkage-drive sys-tem often is used, representing the bestpossible compromise between optimumpress geometry and manufacturingcosts. In the case of crossbar transferpresses, either an eight-element linkagedrive or a combination of linkage andeccentric drives is used to optimize thedie-specific transfer movements.

Knuckle-joint drive—This designprinciple is applied primarily for coin-ing. The knuckle-joint drive systemconsists of an eccentric or crank mech-anism driving a knuckle-joint. The fixedjoint and bed plate form a compactunit. The lower joint moves the pressframe and acts as a slide to move theattached top die up and down. Due tothe optimum force flow and the favor-able configuration possibilities offeredby the force-transmitting elements, ahighly 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 thesame drive moment it is possible toachieve three to four times more forcethan with an eccentric press. Further,the slide speed in the region 30 to 40deg. above BDC is appreciably lower.Both of these design features representa 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- orbottom-mounted. Particularly for solidforming, the modified top-drive sys-tem is popular. The fixed point of themodified knuckle-joint is mounted inthe press crown.While the upper jointpivots around this fixed point, the lowerjoint describes a curved path. Thisresults 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 modifyingthe arrangement of the joints, or byintegrating an additional joint.As a rule, it is desirable to reduce the

slide velocity during deformation,reducing impact and the pressing speedof the slide. Using this principle, theslide displacement available for defor-mation can be increased by three orfour times than when using eccentricpresses with a comparable drive torque.

Blankholder drive—Sometimesused in double-acting deep-draw presses,this drive is a special case. The requiredstandstill of the blankholder duringdeep drawing is achieved in this type ofmachine by superimposing a doubleknuckle-joint system with an eccentricor linkage drive of the slide. The stand-still of the blankholder represents acrank angle of between 90 and 130 deg.,with a maximum residual movement of0.5 mm. This residual movement doesnot, however, influence the deep-draw-ing process because the blankholderoverload safeguard acts as a storage sys-tem and the elastic deflection of theentire press—a few millimeters—hasan overriding effect. The slide drive,the blankholder drive and wheel gearare integrated in the press crown.

Drive Motor and FlywheelIn larger-scale presses, the main drive

system usually is powered byDCmotors,primarily to provide a large strokingrate. Frequency-controlled three-phasemotors offer an alternative, particularly

Mechanical-Press Design

14 METALFORMING / JUNE 2009 w w w. m e t a l f o r m i n g m a g a z i n e . c o m

where a high-protection class rating iscalled for. The output of a press, deter-mined by force, slide displacement andspeed, including lost energy, is provid-ed by the main motor. Periodicallyoccurring load peaks are compensatedby the flywheel, which stores energy.The highly elastic DC drive system cancompensate for a drop of the flywheelspeed of as much as 20 percent in everypress stroke and can replace the con-sumed energy by acceleration of theflywheel prior to the subsequent stroke.Useful energy also should be avail-

able during setup with a reduced strokerate, 5 strokes/min. for example. In thiscase, a speed drop of 50 percent isadmissible. An important criterion inconfiguring the energy balance is toensure a short run-up time. Running upa large-panel press from its minimum tomaximum production stroke rate gen-erally takes less than 1 min. under load.Due to the required useful energy

and the permissible flywheel slowdownof 20 percent, the flywheel generally isdesigned to operate under themost ener-gy-consuming condition—in the lowerstroking range of the press. Although ahigh flywheel speed is beneficial, this islimited by the admissible speed of theclutch, brake and flywheel itself.

Clutch and BrakeMechanical presses use a clutch to

transmit motor and flywheel torque tothe gear shaft. After clutch release, abrake decelerates the slide, the top dieand the gear. Particularly when workingin single-stroke mode, the masses intranslational or rotational motion mustbe brought to a standstill after everystroke within an extremely short time:200 to 300 msec. for large-panel press-es, 100 to 150 msec. in universal press-es. Conversely, after engaging the clutch,the same masses must be acceleratedfrom zero to operating speed.For safety reasons, braking is generat-

edmechanically by spring power.Clutchtorque is calculated from the nominalpress force and the required working dis-tance, generally 13 to 25mmabove BDC.Pneumatic single-disk clutch and

brake combinations with minimum

w w w. m e t a l f o r m i n g m a g a z i n e . c o m METALFORMING / JUNE 2009 15

rotating masses have been used fordecades. Pneumatic control systemswith safety valves and damping devicesare reasonable in cost and generallycomply with requirements. One of theproblems of pneumatic systems, how-ever, is the limited switching frequencyof single-stroke presses and the envi-ronmental damage caused by wear tothe clutch and brake. In order to elim-inate these drawbacks, hydraulic sys-tems have become more popular.Withthe aid of cooling systems, these unitspermit far higher switching loads pertime and are practically immune towear. In single-stroke presses, separateunits that permit switching frequen-cies to 30/min. have been used to permitmore effective control and to avoidovershooting and undershooting by theclutch and brake. Large-panel transferpresses use clutch-brake combinationsbased on sintered disk sets with as manyas 20 friction surfaces, and transmissiontorque levels to 500,000 Nm.The compact drive system used in

smaller mechanical presses consists of aflywheel, clutch, brake and planetarygear in a complete unit. Such a systemachieves a particularly high level of effi-ciency with an extremely low momentof inertia and small braking angle, cou-pled with a long service life and low

maintenance costs. The electric motordrives the eccentric shaft via a flywheel,a clutch-brake combination—eitherhydraulically or pneumatically actuated,and a high-performance planetary gear.This setup eliminates the need for thereducing gear generally required foruniversal presses. Compact drive sys-tems using a hydraulic clutch-brakecombination are particularly environ-mentally friendly, as they produce noabraded particles and run at anextremely low noise level.

Longitudinal andTransverse Shaft DriveThe design of a mechanical press is

determined mainly by the arrangementof the drive shafts and axles, and bythe number of pressure points of theconnecting rod on the slide. The termlongitudinal or transverse shaft drive isdefined by the arrangement of the driveshafts relative to the front of the press,where the press operator stands.A longitudinal-shaft drive is used

mainly in single-acting one- and two-point presses with eccentric drive sys-tems. These generally are blanking anduniversal presses in the lower force rangeto 8000 kN. This classical drive arrange-ment also is used in long and narrowtransfer presses where two to six pressure

points or connecting rods are located inthe longitudinal direction. The con-necting rods are driven by a continuousdrive shaft. Transfer presses are designedwith nominal pressing forces between1000 and 35,000 kN, and have as manyas four pairs of uprights and three slides.The connecting rods are arranged

in the area of the uprights and tie rodsto minimize bending stress on the presscrown. This leaves the top of the work-stations in the slide free for ejectionfunctions. For simpler production andassembly in larger-scale transfer press-es, the longitudinal shaft drive also isreplaced by a transverse-shaft drive sys-tem. Knuckle-joint presses with single-point drive, covering a force range of1500 to 16,000 kN, represent anotherspecial construction.The transverse-shaft press is designed

to have one-, two- or four-point con-nections and pressing forces of 1600 to20,000 kN. This design permits a greaterbed depth, a longer stroke and the use ofa joint system. Double-acting deep-drawing presses are only available inthis configuration.

Slide Adjustment, OverloadProtection and CounterbalanceIn presses, pressure points with a

slide adjustment to 600 mm are used inthe main spindle to allow adjustment ofshut height. In universal and transferpresses, slide-adjustment distances of150 mm are sufficient; and, pressurepoints with a low overall height proveuseful. Using brake motors and wormgears, stampers can attain speeds of 60mm/min. With multiple-stage presses,used for solid forming, wedge adjust-ments are frequently used in the dieset or at the slide clamping plate, to

Compressedair tank

Compensatingcylinder

Piston

Piston rod

Safety rod

Tubular rod

Deformationelement

Breakagesensor

Press slide weight is compensated for bya slide counterbalance system. Pneumaticcylinders balance the weight of all mov-ing parts—joints, connecting rods, slideand upper die. As a result, the drive sys-tem is largely free of gravity forces—nostresses act on the slide adjustment, andthere is an additional safeguard againstdownward travel of the slide caused bygravity. Pneumatic weight compensationguarantees low-noise vibration-free oper-ation, smooth loading of the motor andshort braking distances.

Slide Counterbalance

16 METALFORMING / JUNE 2009 w w w. m e t a l f o r m i n g m a g a z i n e . c o m

achieve high stiffness. Here, slide adjust-ment is unnecessary.Nominal press force is limited and

safeguarded by a hydraulic overload safe-ty device in the slide. If this press force isexceeded, the pressure exerted on ahydraulic cushion integrated into the pres-sure point is quickly relieved, allowing anoverload displacement. For blankholdersused in double-actingmechanical presses,the overload safety function is coupled tothe adjustment function of the blankhold-er force. The overload safety function isactivated only when the deflection of thefour blankholder pressure points exceedsa permissiblemagnitude of approximate-ly 3mm.The safety function is actuated byan air-hydraulic pressure scale. By chang-ing the compressed-air setting,blankhold-er force can be adjusted to the require-ments of the die at the four pressurepoints,within a range of 40 to 100percent.Slide weight is compensated for by a

slide counterbalance system. Pneumaticcylinders balance the weight of all mov-ing parts—joints, connecting rods, slide

and upper die. As a result, the drive sys-tem is largely free of gravity forces—nostresses act on the slide adjustment, andthere is an additional safeguard againstdownward travel of the slide caused bygravity. Pneumatic weight compensa-tion guarantees low-noise vibration-freeoperation, smooth loading of the motorand short braking distances.To counterbalance weights to 200

tons at 10-bar air pressure, cylinder sizemust be as great as 900-mm dia. Longslide strokes, to 1300 mm, and permis-sible pressure spikes to 25 percentrequire substantial surge tanks. Thearrangement of cylinders and tanksdepends on the type of press and safe-ty issues. In the case of counterbalancecylinders not externally enclosed, dou-ble piston rods often are required tosafeguard against the risk of breakage.Conversely, integrating the system intothe press crown makes this additionalsafety feature unnecessary.With every die change, the weight-

compensation system must be auto-

matically adjusted to the new dieweights. Program-controlled automat-ic systems used in transfer presses auto-matically set the program inherent pres-sure value when changing dies, tominimize setup time.

Pneumatic & Hydraulic SystemsThe major pneumatic functions of

mechanical presses are the clutch, brake,flywheel brake, slide counterbalance,slide cushion and bed cushion. Withtransfer presses, pneumatic functionsalso include the application of air pres-sure to the transfer cam followers andworkpiece transport by suction cups.Large volumes of air require compressedair tanks with a monitoring safety func-tion and drainage system. The maxi-mum pneumatic pressure is generally6 bar (90 psi), although high-pressurenetworks or compressors producing amaximum of 16 bar (230 psi) increas-ingly find use.The use of hydraulics in mechanical-

press construction is relatively minimal,

Mechanical-Press Design

18 METALFORMING

typically restricted to the hydraulicoverload safeguard, raising the movingbolster and hydraulic die-clamping sys-tems.All of these functions generally aresupplied from a central hydraulic unit.With transfer presses, additional

hydraulic functions include engage-ment of the clutch between the pressand transfer-drive systems, and thecomprehensive system of scrap-ejec-tion flaps. In addition, hydraulics maydrive various clamping and lockingdevices around the press. The mostimportant function in mechanicalpresses—engaging and braking the slidedrive system—is increasingly beingaccomplished via hydraulics. The use ofhydraulics permits higher switching fre-quencies in single presses, and is moreenvironmentally friendly in terms ofnoise and air pollution. The clutch andbrake are supplied by a central hydraulicunit equipped with a cooling system.Also required: A dual-channel safetycontrol with regulated damping facili-ty and pressure filtration to 10 µm.The increased use of deep drawing

with single-acting presses places strin-gent demands on the blankholdingfunction and, therefore, the draw cush-ion in the press bed, particularly withlarge panel presses. Multiple-point bedcushions with pressure control capabil-ity then are only possible using a complexservo hydraulic system. This requiresseparate supply units with oil tank,pump outputs of approximately 150kW and oil-flow rates of approximate-ly 1000 l/min. for the draw cushion.

Keeping PressesWell LubricatedThe complex lubrication systems

used to supply the lubrication points ofmechanical presses can be simplifiedby using valve blocks. The typical lubri-cation system comprises parallel-switch-ing progressive distributors. To allocatethe required quantity of lubricant, thesystem includes volume controllers,dividers and similar elements. The flowof oil is monitored at the distributorblocks and, to ensure optimum operat-ing conditions, a control system regu-lates lubricant temperature within anarrow bandwidth, by using auxiliary

heating and cooling functions.To maximize operating safety, com-

plex production installations for met-alforming always include twin supply-unit elements, such as pumps and filters,so that if one element fails there is no needto interrupt production, as the secondsystem cuts in automatically. Also,maintaining or exchanging the defectivesystem does not interrupt production.In smaller presses, and in particular

bottom-drive knuckle-joint presses,central lubrication systems with multi-ple-circuit geared pumps find use. Thesesystems continuously supply lubricantto as many as 20 main lubricationpoints. During the short swivel move-ments executed by the thrust elementsand joints, a constant flow of lubricantis available even under varying resist-ance levels in the individual user andsupply lines. MF

w w w. m e t a l f o r m i n g m a g a z i n e . c o m METALFORMING / JUNE 2009 19