multiple-slide transfer presses: a modern production system

Journal of Materials Processing Technology 72 (1997) 226–238

Multiple-slide transfer presses: A modern production system

Peter Bogon *Daimler-Benz AG, 71065 Sindelfingen, Germany

Received 15 July 1996

Abstract

The fundamental features and the range of use of multiple-slide transfer presses in the automobile industry are discussed. Inaddition to an economic comparison of conventional press lines with multiple-slide transfer presses, the different types of machineconcepts are illustrated. The classification of the different sizes and the types of transfer systems is given. Because themanufacturing accuracy of the final part depends on the quality of the drawing station, the different types of drawing facilitiesare explained. Another point is raised in the discussion of the utilisation ration time. Types of overload safety devices areintroduced and the machine behaviour in the case of overload is also discussed, concerning the influence of the spring constantof the machine on the force of impact. A comparison of the accuracy parameters of multiple-slide transfer presses withconventional stamping presses is made. © 1997 Elsevier Science S.A.

Keywords: Multiple-slide transfer presses; Automobile industry; Manufacturing process

1. Press line concepts

Mechanical presses interlinked in press lines havebeen used in the automobile industry since about 1940for enhancing productivity and thus for achieving eco-nomic production. The simplest form of panel handlingbetween individual machines is manual handling, Fig.1(a). Depending on the panel size, the stroke rateachieved with this method is not more than two tothree panels a minute. Progress, even if only slight, isachieved by using conveyor belts between individualmachines. In the next higher level of mechanisation,Fig. 1(b), the blank at the drawing press is placed intothe die automatically by a de-stacker and loader sys-tem, whilst the drawn part is removed from the die byan unloader and then thrown onto the conveyor belt.The part is then inserted into the next machine manu-ally. Fully automated manufacturing is only possible byusing loader and unloader feeders, as shown in Fig.1(c), or by the use of robots at all of the machines.

Press lines of types A and B operate in the single-stroke mode of operation and not in the continuous-stroke mode. Large investment in control system would

be necessary to achieve the necessary synchronisationof the individual machines of the press line. In otherwords, the clutch is engaged prior to each individualstroke, the elements of the clutch secondary side and ofthe link drive being accelerated and the clutch thenbeing disengaged when the upper dead centre (UDC) isreached. The rotational energy of the clutch secondaryside is partially braked. Apart from the noise problem,this causes intensive wear-and-tear of the clutch andbrake. At the same time the overall efficiency of thepresses is significantly impaired [1].

The provisional apex in the development of the fullyautomated press line, of press design and press-shoptechnology was achieved in 1973 [2] in the technologyof sheet metal forming with multiple-slide transferpresses, see Fig. 1(d). These machines are used exclu-sively in the fully-automated continuous-stroke modeof manufacturing. Fig. 2 shows an economic compari-son of conventional press lines with multiple-slidetransfer presses, according to which, a switch from thepress-line production system to the multiple-slide trans-fer press makes it possible to reduce investment cost by:20–40%; shop floor requirements by :50–70%; andelectrical power by 40 to 50%; with a parallel doublingof productivity [3]. The purchase price of such produc-tion systems, depending on the size and equipment of

* Corresponding author. Tel: +49 70 31906907; fax: +49 7031903606.

0924-0136/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved.

PII S 0 924 -0136 (97 )00173 -8

P. Bogon / Journal of Materials Processing Technology 72 (1997) 226–238P. Bogon / Journal of Materials Processing Technology 72 (1997) 226–238 227

Fig. 1. Comparison of different press line concepts.

Fig. 3. Characteristics of multiple slide transfer presses; threecolumns, two slides, five stages.

this concept that results in the absolute synchronisationof all the slides and which makes fully automated panelhandling possible as a result of an integrated transfermechanism. The systems used are either three-axistransfer systems with pickers or two-axis transfer sys-tems with vacuum cups.

The total nominal force of the machine is the sum ofthe individual nominal forces of the sub-machines. Thetechnical system for manufacturing with multiple-slidetransfer presses consists essentially of: a blank de-stacker and loader; a multiple-slide transfer design ofpress with a drawing device; moving bolsters with inte-grated scrap disposal; and a discharge belt for thedisposal of the parts.

It is also possible to position a blanking press insteadof the de-stacker and loader in front of the multiple-slide transfer press, which supplies the blanks directlyfrom the coil in the machine cycle ‘just in time’ forfurther processing. This, however, again necessitatesexact synchronisation of the two machines.

On the basis of a proposal made in Ref. [4], transferpresses can be divided on the basis of their totalnominal forces into five groups of mini, small, medium,large and super large (Table 1). The classes used formanufacturing panels are for the most part small tolarge, although machines of the super large class havealso been used increasingly in recent times for themanufacture of mono-piece side panels for cars [5].

Fig. 4 shows the specific press forces of multiple-slidetransfer presses. The respective nominal forces of themachines studied are related to the respective numberof stages and the individual bolster area. With a fewexceptions, the specific press forces are within the rangeof 1.0–1.5 MN m−2. Machines that are used for themanufacture of chassis parts for trucks (sheet thicknessof up to 8 mm), in contrast, have specific forces of2.5–3.3 MN m−2.

In addition to the nominal force of the individualslides and the total nominal force of the machine,further classification parameters are the following char-acteristics: number of slides, number of stages, size oftransfer pitch, type of transfer mechanism, nominalstroke rate and type of drawing mechanism.

the plant, ranges from US$ 15 m up to more thanUS$ 70 m. At present, there are a few hundred ma-chines of this type in use world-wide.

2. Fundamental design and classification

Contrary to conventional transfer presses, in whichseveral tool set-ups (in other words stages of the pro-cess) are arranged below a single slide, a multiple-slidetransfer press has at least two slides within a commonframe and in most cases several tools are arrangedbelow each of these slides, see Fig. 3. The individualsub-machines of a multiple-slide transfer press are alsoknown as modules.

The principal characteristic of all the designs ofmultiple-slide transfer presses, however, is that all theslides are driven by a common main drive shaft. It is

Fig. 2. Economic comparison of conventional press lines with multi-ple-slide transfer presses.

P. Bogon / Journal of Materials Processing Technology 72 (1997) 226–238P. Bogon / Journal of Materials Processing Technology 72 (1997) 226–238228

Table 1Classification of transfer presses, proposal according to [4]

Rating (min−1)Size of bolster (mm)Transfer pitch (mm)Class Tool stagesNominal force (MN)

1300×500 10–24Mini 5.0 500 510–222300×9005900Small 12.5–15.0

6 2500×1200Medium 22.0–25.0 1200 8–206 2500×1800Large 30.0–32.0 1800 8–16

4500×20006 6–142300Super large 62.0

For drawing difficult panels, in some cases the ma-chines are equipped with a separate drawing station, i.e.in this case, the drawing tool is positioned exclusively inthe first sub-machine. Pneumatic or hydraulic cushionsare used in most cases for drawing operations by meansof single-action presses. Double-action presses makeuse of mechanically-driven blank-holder slides and in-ner drawing slides. The inner slide is installed in thepress bed with a bottom drive derived from the mainshaft to avoid turning the panel.

In the case of machines from the small to the largeclassification, both the press bed as well as the crowncan be designed as a single welded construction,whereas in the case of super large machines, they haveto be made up of individual segments (modules) be-cause of the size. These segments can be fitted togetherby means of tie rod connections. The columns arelikewise designed as steel welded constructions with across bar. Bed, columns and crown are connected by tierods. A design in which machines can be made up ofindividual modules has also been the subject of apublication [6].

2.1. Machine concepts

Fig. 5 provides an impression of the variety of thedifferent arrangements of the tool stages that are possi-ble with regard to the individual slides.

The majority of machines feature two or three slidesand are equipped with four to eight working stages.

The number of stages is identical with the number ofslides, as is true also of the ordinary press line. Only inthe case of super large machines is each tool set-uppowered by its own slide.

The demand for minimising tool-change time is sa-tisfied by using moving bolsters and by automaticbolster and tool clamping. In this case, each sub-ma-chine is provided with two moving bolsters, each ofwhich is able to move in and out of the machine on theright and left transverse to the transfer direction, seeFig. 6. When production is in progress, therefore, it ispossible to prepare the next tool change on the movingbolsters that are not in use at that moment. Dependingon the strictness of safety rules and accident preventionlegislation in the individual industrial nation and thesize of the machine, the die-changing time varies frombetween 2 and 6–10 min.

As the columns of the machine are positioned be-tween the moving bolsters, this produces an idle stationat this point in each of the machines, in other words notool set-up is arranged at this point. Since the panelsare transferred through the system, they are placedonto idle stations and picked up again for the nexttransfer step and placed in the subsequent die. Thisprocedure is quite often the cause of interruptions anddisturbances of the production in the press. For thisreason, attention has been paid to achieve designs in

Fig. 5. Machine concepts: Arrangements of slides and stages.Fig. 4. Specific press forces of multiple slide transfer presses.


Fig. 6. Tool change with moving bolsters. Arrangement of movingbolsters: Example of a two-slide five-stage transfer press.

that the sizes of the stroke and the slide travel-crankangle diagram for the various slides of the machine maydiffer. Fig. 7 shows the slide speed as a function of theslide stroke for a link drive and for the usual crankdrive. The slide speed is standardised to the maximumslide speed of the crank drive: Positive speeds indicatethe downward movement of the slide whilst negativespeeds are associated with the return stroke. The dia-gram clearly shows that the link drive offers a speedthat is two times slower in the working range (25%before the lower dead centre (LDC) down to LDC)compared to the crank drive.

In order to minimise the shock forces for the transfermechanism that are produced when the clutch is en-gaged, the press does not operate when starting up atthe required number of production strokes when theclutch is engaged, but at a number of strokes between 4and 6 min−1, depending on the size of the machine, thedrive being accelerated to production stroke speed onlyafter this. Consequently, the main drive of the machineis a mixture of a direct and an accumulator drive, incontrast to single machines. At a low number ofstrokes, by far the greatest portion of the work duringthe forming process is rendered by the motor, with alarge proportion of the electric current being drawnfrom the mains. It is only when the press is operating ata greater number of strokes that the flywheel is suffi-ciently charged to provide an appreciable level of en-ergy and thus relieve the load on the motor.

2.2. Drawing facilities

As the manufacturing accuracy of the final part isdependent, as a rule, on the quality and behaviour ofthe drawing station, the drawing facility used plays amajor role. In addition to the accuracy parameters ofthe slide, the kinematics of the slide and the quality ofthe transfer, the press operations involved with sheetmetal panels are basically a combination of stretchforming and deep drawing. The sheet metal flow iscontrolled in this case by a combination of several orall of the following points listed below: The geometricalshape of the blank; the lock/draw-beads; the quantity

which the moving bolsters first move into the machineand then transverse turn in the opposite direction of thetransfer direction. This has the advantage of eliminat-ing idle stations, although the drawback of this designis that it is no longer possible to correct the position ofthe panel in the idle stations as these have now beeneliminated, the correction now having to be done dur-ing the transfer of the panel. The design of the transfertherefore becomes more complex [7].

Different concepts are likewise pursued in the dis-charge of the panel scrap. A distinction is made be-tween central scrap discharge [4] in the middle of thestage and discharge of the panel scrap at all four sidesof the particular stage. Central scrap discharge is aclearly arranged design and keeps panel scrap reliablyaway from the interior of the machine and does notproduce any blocked discharge chutes, although it doesdemand a more sophisticated type of die construction.A drawback of this type of scrap discharge, however, isthat it is not possible to use a die cushion in theparticular stage. A further factor is that the slidingbolsters unavoidably have a larger deflection underload.

Side discharge of the panel scrap requires greaterinstallation space despite the smaller passages, althoughit does make it possible to use die cushions in all of thestages and results in a greater stiffness of the slidingbolsters.

Modern designs of hydraulically-operated overloadsafety devices are used to protect the machine and thedie from overload. The basic differences in the event ofan overload between conventional stamping presses andmultiple-slide transfer presses are discussed in Section4.2.

The number of strokes per minute is optimised byusing a link drive with a four or six link design with thefamiliar characteristics [8]. In this case, it is possible

Fig. 7. Kinematics of different types of drive systems: Phase curves oflink drive and of crank drive.


Fig. 8. Drawing apparatus for drawing presses.

tion with a separately-powered mechanical blank-holder and the die cushion with a conventional designof pneumatic or hydraulic drive that is used in a largenumber of versions. It is not intended to deal in thispaper with the familiar properties of such die cushions,not all of which are positive, such as the irregulardistribution of the forces in individual pins resultingfrom wear-and-tear and the elastic deformation of theentire drawing facility [10], but solely to deal with thetrend in favour of multiple-point die cushions whichhas become evident in recent years, e.g. Ref. [9].

Fig. 9(a) shows the basic design of a modern multi-ple-point die cushion. This design features several hy-draulic cylinders, each with a pressure pin which actsdirectly on the blank holder. All of the cylinders aremounted on a common base plate. A deflection of thisplate has no negative effect on the drawing process. Inthis type of solution, the force in the die cushion can beproduced passively by compressing the fluid or activelyby a pre-stressing pressure in the cylinder. If the passivesolution is adopted, the distance which should be al-lowed for creating the pressure is 10–20 mm, dependingon the design of the cylinder (cross-section and heightof the oil column).

In contrast to this, if the active solution is used, anadequately high holding force is already available at thestart of drawing, see Fig. 9(b). In addition, it is possibleto fix a different force for each individual cylinder. Thisforce can also be changed continuously or suddenlyalong the course of the punch travel (Fig. 9(c)).

In addition, reports exist of designs which have thedie-cushion plate travelling parallel with the slide andwhich therefore abandon the displacement principleand return to the clamping principle of the mechanicaldie holder and thus relieve the strain on the main drive,e.g. Ref. [11].

Using such drawing plans to suit particular manufac-turing processes necessitates sensitively translating thechanges in pressure of the individual die-cushion pinsinto appropriately local changes in force in the actioninterface between the lower and the upper blank-holderframe. To exploit optimally the features of the drawing

and type of lubricant; and the distribution of the sur-face pressure and normal compressive stresses.

A common feature of all these measures is that theyare intended to influence the friction characteristicsbetween blank and the blank holder [9]. In addition, itis possible to alter the friction characteristics by locallysetting the normal compressive stress between the blankholder and the blank and thus to better adapt thedrawing process.

In technical terms, these different friction characteris-tics are produced by the different spacings between theblank holders during spotting, this method beingknown as ‘hard–soft spotting’. A basic disadvantage ofthis method is the fact that the elastic characteristics ofthe machine and of the die also influence the result. Ifthe aim is persued of trying out tools and tool set-upsmore rapidly and efficiently in order to have them in aproduction state at an earlier date, it is then necessaryto avoid ‘hard–soft spotting’, or it must at least bereduced to a minimum. Open-loop and also closed-loopcontrol of the material flow with different surface pres-sures with progressive punch travel is still possible, ifthe blank holder is elastically deformed by specificallyintroducing forces locally into the blank holder in sucha way that the ‘hard–soft properties’ are now re-cre-ated.

The drawing facilities used in multiple-slide transferpresses thus result in new demands in respect of theopen-loop/closed-loop control of the pressure crankangle cycle in the die cushion and, at higher impactvelocities, the pre-acceleration of the entire blankholder. This is particularly true when seen from theaspect of the ever larger deep-drawn parts such assingle-piece car side panels, and double or four-foldparts.

Fig. 8 provides an overview of the types of drawingfacilities available today. On the basis of this, it ispossible to make a rough distinction between the solu- Fig. 9. Basic design of a modern multiple-point die cushion.


Fig. 10. Ratio of area contact for blank holders of different stiffness.Fig. 12. Principle of three-axis transfer.

plan, it is essential to know the interactions between theindividual pins and the blank holder and to harmonisethe design of the stiffness of the blank holder to thesecircumstances.

Fig. 10 shows the change of the related contact arearatios in the action interface and thus the effectivenessof the pressure change in the individual pins of a15-point die cushion with blank holders of differentstiffnesses as a function of the force, e.g. for a singlepin. The left-hand half of the diagram shows the resultsfor a blank holder of low stiffness. The related contactarea ratio increase is nearly linear with the pin forceover a wide range of pin forces, from 0.1 to 0.3 MN. Inother words, it is possible here to effectively influencethe sheet metal flow by changing the contact area ratio.The opposite can be stated for the blank holder withgreat stiffness in the right-hand half of the diagram.Although the pin force was changed within the samerange, no significant change in the contact area ratiocan be ascertained. In this case, it is not possible toeffectively influence the sheet metal flow.

These few examples show the urgent necessity to alsoharmonise die design to the technical aspects of themachine as part of a global view of the manufacturingprocess.

2.3. Panel flow and transfer

As stated in Section 2, a production facility with amultiple-slide transfer press essentially consists of ade-stacker and loader, the actual machine and a panel-racking system. A typical die de-stacker and loader is

shown in a the schematic diagram in Fig. 11. This isrequired if the blanks are not produced by a separateblanking press upstream of the multiple-slide transferpress itself. The panel flow through the system is de-scribed here for such a case.

The first step is to place the blank stack onto thestacker and to move it manually or automatically,depending on the system concept, into the de-stackingstation into the working position. In the case of steelblanks, magnets are used to lift the top blank from offthe stack. The vacuum cup beam moves down, lifts offthe blank and moves up together with the blank that ithas picked off the stack. The thickness of the blank isdetermined using a thickness-measuring device. Thevacuum cup beam moves the blank onto a magnetictransfer belt which transfers it to the cleaning station. Ifa further measurement confirms the presence of a sec-ond blank, both blanks are then ejected down thedouble-blank chute. In the other case, the blank istransported through the cleaning and lubrication sta-tion and when it passes out of the lubrication unit, it isthen centred on the lowering belt and lowered to theheight of the transfer level of the clamps of the three-axis transfer system or to the level of the vacuum cupsin the case of the two-axis transfer system. It is at thispoint that the blank is transferred into die stage 1 andinserted. After the forming process, the deep-drawnpanel is moved by the next transfer step into stage 2and transferred on through the system with each fur-ther cycle step until it is removed at the end on thedischarge belt manually or by means of an automaticpanel racking system.

The integrated transfer system essentially consists oftwo transfer beams, see Fig. 3, which transport theaccessories of the parts, such as the pickers and thepanels, themselves. The transfer beams are moved bycomplex drive systems which operate in line with thespeed of the main shaft. In addition to this, there arealso electrically powered transfer systems that arelinked electronically to the main drive shaft. In the caseof two-axis transfer systems, the transfer beams moveonly in the y- and z-axes, whereas the beams of thethree-axis transfer system are moved in the x-, y- andz-directions.

Fig. 11. General view of blank feed. Separating, cleaning, lubricatingand feeding of blanks.


Transfer of the panels using a three-axis system isillustrated in Fig. 12. This transfer system can be usedin all those cases where the parts have sufficient inher-ent stability and do not exceed a particular size.

At the same time as the slide is raised, the transferbeams are closed (−x) and raised with the cross-barsattached to them (+z). During this operation, thepanel is fixed in position for transfer by means offingers and transferred to the next stage (+y). Thetransfer beams are then lowered (−z) and the panelinserted in position in the next stage. At the same timeas the die closes, the transfer beams open (+x) fol-lowed by return (−y) and the transfer beams areclosed (−x) before the next raising operation.

The two-axis transfer system, Fig. 13, is always usedfor large panels or for those which do not have suffi-cient inherent stability. In addition, manufacturing dou-ble or four-fold parts is only possible with such asystem if these have to be separated before the laststage. In this case, the panels are not fixed in positionby pickers or cases but by vacuum cups.

In this case, the transfer operation is sub-divided intothree sub-steps. First of all, the transfer bars move inparallel to the slide being raised (+z) and at the sametime, in a short step in the opposite direction of transfer(−y), into the opened die and then move down (−z).In a second step, the panel is picked up by the cross barfitted with vacuum cups, raised (+z) and transferred ina large step (+y) into the intermediate store, where itis placed down (−z). In a short third step, transferbars move up (+z) and in the opposite direction oftransfer (−y) into a waiting position ahead of the die,where they are lowered (−z). Depending on the designof the transfer, the actual vacuum cup bar may still beswivelled about the x-axis to make it possible to changethe position of the part.

3. Utilization ratio time

In addition to the purchase price of the machine, it isthe possible, and much more the actual, utilizationperiod for production that is the critical element indetermining the economy of the production process. Itis necessary to provide at this stage a number ofdefinitions for the presentations which follow.

The number of panels to be manufactured for duringan order is the batch size bs. The term die change timetdc refers to the time that is required from the manufac-ture of the last part with die set A to the manufactureof the first part with die set B. The time for achievingthe first fault-free part is not included in the die-changetime. This time is termed the lead time tlead, whilst theproduction time tprod starts with the commencement ofthe manufacture of the first fault-free parts and endswith the forming of the last part of the batch size,reduced by the total of all of the interruption timestinterr. The occupancy time tO is consequently the totalof the die change time, the lead time, the interruptiontime and the production time. It is possible to combinethe die change, the lead time and the interruption timesto form the idle time tidle.

tidle= tdc+ tlead+ tinterr (1)

During the production time, the system operates atproduction strokes per minute nprod. This is the maxi-mum number of strokes with which a tool set-up can beoperated before frequent interruptions to productionoccur. The production strokes per minute are selectedso as to achieve a maximum in terms of productivity.This need not necessarily be when the machine isoperating at the highest possible number of strokes, asis illustrated below. The nominal strokes per minutennom, in contrast, is the number of strokes for which themachine is designed to operate in the continuous-strokemode of operation (main drive, transfer accelerationand die cushion).

The utilisation ratio time ht of multiple-slide transferpresses is defined here as the ratio of benefit to cost.Benefit in the case of the utilisation ratio time ht is thetime tprod, with which production is possible by meansof the machine. Cost here is the entire time required forworking through the batch size, in other words in thesimplest case the production time tprod plus the idle timetidle. Consequently, the utilisation ratio time ht can bestated as:

ht=tprod

tprod+ tidle

(2)

Hence from Eq. (2) with tprod=ntidle:

ht=n

1+n(3)

The generally applicable evaluation of this Eq. (3) ispresented in the left-half of Fig. 14.

To attain a utilisation ratio time of 85%, for example,the production time must be at least 5.6 times the idletime. The aim is to achieve pressing cycles of 1–4 daysin order to avoid having high levels of capital tied up inexcess stocks. In other words, with a daily output of,e.g. 1000 vehicles, the batch size is about 4000 parts. Afigure of 18 production strokes per minute thus resultsFig. 13. Principle of two-axis transfer with vacuum cups.


Fig. 14. The utilisation ratio as a function of the production time (onleft); and (on right) the idle time as a function of the productionstroke rate. The parameter is the batch size.

Fig. 16. Influence of short-time interruptions on productivity.

parameter number of short-time interruptions per hour.This shows clearly that productivity does not, as onemight expect, have its extreme level at the maximumnumber of production strokes per minute, but at lowernumbers of production strokes per minute dependingon the number of short-time interruptions. If, as wasstated above, about 13 min are available for each of thethree unproductive time portions of die change tdc, leadtime tlead and interruption time tinterr, then in order toachieve the desired utilisation ratio time of 85%, thenumber of short-time interruptions which may occurduring the entire processing of the total order is nomore than seven!

Fig. 17 shows the development of the average num-ber of production strokes per minute over a period of 6months after full production start-up for a parts varietyof two outer panels and two inner parts. The number ofproduction strokes per minute develops from an aver-age of 9.8 min−1 at the start of full production up to12.5 min−1 after completion of the optimisation phase.

4. Accuracy parameters and machine protection

4.1. Accuracy parameters

Although systematic studies into the field of accuracyparameters have existed since the mid 1970s (an exam-ple which can be mentioned here is that of the pressesfor cold extrusion [12]), no data is available even todayfor conventional stamping presses and multiple-slidetransfer presses. No series investigations aimed at deter-mining coefficients of vertical-stiffness and angular-stiffness coefficients of conventional drawing andstamping presses are known.

in a production time of about 220 min. In order tomaintain the utilisation ratio time of 85% in this case,the maximum permissible idle times which result arethus about 39 min, see right-hand half of Fig. 14. Inother words, 13 min each remain for die change timetdc, lead time tlead and interruption time tinterr. Theright-half of Fig. 14 also clearly shows that the idle timetidle which can be allowed becomes ever less with thedemand for a constantly high utilisation ratio time,decreasing batch size and increasing number of produc-tion strokes per minute.

The desired trouble-free operation of the plant isrepeatedly negatively affected in practice by all of thepossible interruptions to operations. Not only faults inthe electrical system, in the transfer system or in theperipherals of the machine, but also problems in thelogistics, are mentioned here as examples of reasons forinterruption to production. If all of the interruptionsare recorded in terms of their duration and frequency,what results is the typical pattern, shown in Fig. 15.

Here, the number of interruptions has been plottedagainst the log of the duration of the interruptions. Thetypical average duration of an interruption is of theorder of about 105 s; in this example, it representsabout 2/3 of all such incidents. This typical interruptiontime is designated in the sections which follow asshort-time interruption.

Fig. 16 shows the productivity per hour as a functionof the production strokes per minute, linked to the

Fig. 15. Interruption duration pattern of a multiple-slide transferpress. Example: driver door panel.

Fig. 17. Development of the number of production strokes perminute of a typical tool set after full production start-up.


Fig. 18. Coefficients of total vertical stiffness of different types ofpresses. Fig. 20. Coefficients of angular stiffness of double-action presses.

A comparison of the coefficients of total verticalstiffness qz determined from measurements betweendeep-drawing presses (double-action presses with me-chanically-driven blank-holder slides) and transferpresses (single-action presses, possibly with bolster diecushions) of various press lines and of different designswith the stiffness coefficients of individual sub-machinesof multiple-slide transfer presses in a major pressingplant, is shown in Fig. 18. Only press lines that aredesigned as an integrated facility are used in the com-parison to ensure that the comparison with multiple-slide transfer presses is valid.

Double-action presses are clearly somewhat less stiffbasically than the corresponding transfer presses of thesame press line, although the coefficients of total verti-cal stiffness for both groups are between 15 and 20. Thesituation in the case of multiple-slide transfer presses isdifferent, as shown in Fig. 19. In this case, most of thecoefficients are between 30 and 40, in other words twiceas stiff.

The angular stiffness coefficients px,y about the x-and y-axes are presented in Figs. 20 and 21 for thesame machines.

The results spread across a large range between oneand eight, a further striking feature being that thefigures at right angles to the transfer direction of themachine are generally two to three times greater thanthose in the transfer direction. This cannot be explainednot only by the positioning of the guides, but, first andforemost, by the positioning of the columns and thusby the different moments of resistance to bending in thex- and y-direction.

Finite-element method (FEM) can be used today topre-determine the accuracy parameters of multiple-slide

transfer presses as early as the design phase. Calculat-ing the elastic properties of the entire machine fordifferent design loads, for example the pass of an idlestation after discharging double panels, is state-of-the-art today [13].

4.2. O6erload protection

The slides of modern presses (two- and four-pointdrive) are basically equipped with hydraulic overloadsafety devices to protect the machine and the die fromexcessive loads.

The designs that are commonly found today can besub-divided in accordance with various criteria. Thefollowing properties are mentioned here by way ofexample:

(i) Overload safety device with set-point character.The overload force cannot be varied, 110% of therelevant slide nominal force being generally set as asafeguard.

(ii) Variable pre-stress pressure. The response forceof the overload safety device can be set by altering thepre-stress pressure.

(iii) Different pre-stress pressures for the front andrear side of the slide, looking in the direction of trans-fer. This enables individual stages to be safeguardeddifferently. In the event of an overload (eol), the trig-gering of one pressure point also triggers all of theother pressure points.

(iv) Soft and hard versions of pressure points. In thecase of the soft version, the pre-stress pressure in the oilchamber is relatively low and the pressure increases upto the nominal load are relatively high. In the other

Fig. 21. Coefficients of angular stiffness of single-action presses.

Fig. 19. (a) Coefficients of total vertical stiffness of individual sub-machines of multiple-slide transfer presses. (b) Individual sub-ma-chines of different multiple slide transfer presses.


Fig. 22. Design of a hydraulic overload safety device.

This force of impact can be determined in a simplifiedway and as a model from the kinetic energy Ekin of theslide and of the system spring-constant of the machineand the dies. The kinetic energy of the solid is:

Ekin=12mtot6

2eol with mtot=mslide+mupperdie (4)

and spring work Wspring which the machine/tool systemabsorbs during the die-to-die impact blow is:

Wspring=12

F2

csys

(5)

Ekin=Wspring

Fimprel=6eolcsysmtot (6)

Fimprel=

FimpMSTP

FimpDAP

(7)

Fimprel=6eolMSTP

6eolDAP

'csysMSTP

csysDAP

mtotMSTP

mtotDAP

(8)

Velocity 6eol in this case is the momentary velocity ofthe slide when the overload case occurs. This velocity isessentially determined from the elasto-kinematics of thepress drive-die system and is smaller than the velocityof the unloaded drive system.

Quantity csys is the spring constant of the systemfrom the die and the press elements subjected to theeffect of the force of the machine such as the press bedand the slide. The static overall spring constant can beused approximately for these still-elastically-deformedcomponents of the machine. Typical characteristic datafor a multiple-slide transfer press (MSTP) and for adouble-action press (DAP) are presented in Fig. 23.

For the comparative studies that to be undertakenhere, it is assumed that the number of strokes perminute is approximately in the ratio 1:1.5 (DAP:MSTP)and that the machines are operated with the same slidekinematics.

In absolute terms, the multiple-slide transfer pressunder study is 3.3 times stiffer than the double-actionpress. The situation is even more pronounced with theratio of slide masses, these being 4.25–6.25 times heav-ier for the multiple-slide transfer press, depending onwhether the slide is designed for two or three toolstations.

The analysis of Eq. (8) is presented in the left half ofFig. 24. Here, the manifold of the force of impact of aslide of a multiple-slide transfer press is juxtaposed withthe force of impact of the double-action press de-scribed, with a variation of the spring constant of thetool and the ratio of masses of the slides of the twopress types. This reveals clearly that the force of impactproduced by the slide of a multiple-slide transfer pressis basically three to four times greater than that of acomparable conventional double-action press. This isessentially explained, on the one hand, by the signifi-cantly greater stiffness in the z-direction of the multi-

case, the pre-stress force is high and the pressure in-crease is only slight.

First of all, it is appropriate to discuss the basicbehaviour of the slide and die system in the event of anoverload.

All centres of pressure of hydraulic overload safetydevices are so designed that a pre-stressed oil cushion isarranged in an oil chamber between the slide and theconnecting rod, see Fig. 22(a). If an overload occurs,the pressure in this oil cushion is suddenly released,which ensures that essential parts of the machine suchas the crown, the main drive and the columns areimmediately protected from excessive loads; but, incontrast, not the dies. The slide is completely discon-nected from the drive system in the event of an over-load, in other words the system (sys) is converted froma travel-bound system into an energy-bound system, seeFig. 22(b) and (c).

Consequently, the slide behaves like the ram of ahammer or a percussion press during an elastic die-to-die impact blow. At the moment the slide is discon-nected from the drive system, about 110% of thenominal force of the press is already active in theinterface between the upper and the lower die. Thework that is required to elastically deform the partslocated in the force-flow of the machine so that thenominal force of the press can be made available,comes from the motor-flywheel combination. To thisforce is added, in the event of an overload, the portionwhich originates from the die-to-die impact blow of theslide on the tool and the press bed and the foundation.

Fig. 23. Impact forces in the case of an overload: Comparison of aconventional double-action presses with a multiple-slide transferpresses.


Fig. 24. Related force of impact of a multiple-slide transfer presses:overload safety device activated.

Fig. 26. Vertical displacement of a single-action press.

force of impact and the increase in the response force ofthe overload safety device are plotted as a function ofthe stroke rate and the impact travel before UDC. Inboth cases, the force of impact increases with an in-creasing impact velocity.

The basic design of the centres of pressure for thedifferent versions is identical in principle, only thedown-stream valve engineering and the level of thepre-stress pressure differing Fig. 22(a) shows the basicdesign of centres of pressure. The pressure chamberbelow the connecting rod is designed as a plunger. Thepiston is clamped elastically to the clamp ring by thehydraulic pre-stress pressure. If an external force actson the piston, the clamping force is reduced as the forceincreases because of the compressibility of the oil, untilthe piston detaches from the clamp ring and immersesitself into the oil chamber by the amount of its displace-ment as the external load rises further. This now resultsin a change in the stiffness of the centre of pressure andconsequently also in that of the entire machine and thesub-machine: this is presented in Fig. 26.

The top curve shows the experimentally-determinedstatic deflection in the z-direction as a function of thestatic force acting at the slide. The deflection curveconsists of two linear sub-areas with different slopes,the point at which the piston detaches from the clampring being located at the point of intersection of the twostraight sections. The bottom curve indicates the rela-tive travel between the slide and the piston as a func-tion of the static load, the break points in these curvescoinciding. The middle curve shows that this behaviouralso exists in the case of dynamic forces. In this case,the machine under test was subjected to an increasingmaximum force from operation to operation by con-ducting a variety of coining operations. The maximumdynamic deflection for each individual coining opera-tion was determined from comparing the ram travelduring load stroke and idle stroke [14]. Here, too, thebreak point in the displacement curve can be seen.

As the extent of the elastic clamping between thepiston and the clamp ring is determined by the level ofthe pre-stress pressure of the overload system, the forceup to the moment of release must also increase parallelto the rising pre-stress pressure, this behaviour beingshown in the left hand of Fig. 27. In this case, the

ple-slide transfer press in comparison to the usual dou-ble-action press and, on the other hand, by the signifi-cantly greater mass of the slide of the multiple-slidetransfer press. It is necessary in this case to dissipate ahigher kinetic energy into a stiffer system. In the eventof an overload, this results inevitably in a significantlygreater force of impact and thus in a significantlygreater load of the dies.

Depending on the impact velocity, this results in aforce of impact of 8–10% of the nominal force for the8.0 MN double-action press described in Fig. 22. Forthe multiple-slide transfer press, this quantity is in-creased accordingly to 30–40% plus 10% from theusual statistical over-shoot.

Therefore the shift from the manufacturing system ofthe single machine to the manufacturing system of themultiple-slide transfer press means that the die is sub-jected to a significantly greater force of impact in theevent of an overload, this being true no matter whattype of overload safety device is used. This is all themore the case if the force of impact has to be absorbedby only one die. For reasons related to the system, it isconsiderably more difficult to pursue active tool protec-tion with a multiple-slide transfer press.

The time patterns of the hydraulic pressure measuredin the overload safety device and of the force acting inthe die in the event of an overload are shown in theright half of Fig. 24. Indeed the force in the dieincreases clearly although the hydraulic pressure col-lapses in the overload safety device.

The experimental verification of Eq. (6) derivedabove is presented in Fig. 25. Here, the increase in the

Fig. 25. Influence of number of strokes and impact velocity: (1) Max.force in die; (2) Response force of the centre of pressure.


Fig. 27. Release force of the overload system and coefficients of totalvertical stiffness as functions of the pre-stress pressure.

damage to the blades. For this reason also, it is neces-sary to work with a slight clearance of the guides at theslide.

5. Outlook

In view of the high levels of capital tied up when anorder is placed for a multiple-slide transfer press, it isimportant to reduce significantly the turnaround timefrom ordering the press until delivery to the customer.This is attainable only by intensified use of CAD in thedesign phase, by a standardised design and by newproduction technologies for manufacturing the majormachine parts, the key phrase being ‘variant designsand semi-finished products for press assemblies’.

An optimal design matched to the expected maxi-mum blanking forces is essential in order to reducemachine costs. The design of the dies must also bematched to this. The individual blanking operationsmust not be performed simultaneously in the die: Thedesign must allow for staggering these operations,which enables the nominal force required for the indi-vidual blanking stages to be reduced significantly. Toenable multiple-point die cushions to be used success-fully, it is necessary to match the stiffness of the blankholder to them, the key phrase being ‘global study ofthe machine, the die and the manufacturing process’.

The costly mechanical coupling of the main driveshaft and the transfer mechanism has to be replaced bya reliable electronic coupling. Transfer systems that aremonitored in this way also makes it possible to uselow-cost hydraulically-powered multiple-slide transferpresses of the super large type, the key phrase being‘hydraulic multiple-slide transfer press’.

References

[1] P. Bogon, A. Jorg, H.W. Wagener, Energy flow and efficiency ofeccentric press system, Stahl und Eisen 106 (12) (1986) S687–S693.

[2] H.W. Wagener, New Trends in Press Design for Sheet MetalForming. Seminar on Sheet Metal Forming Technologies, 1994,Bombay/India.

[3] G. Spur, Th. Stoferl (Eds.), Handbook of Working Technology,vol. 2/3, Metal Forming and Cutting, Carl Hanser VerlagMunchen, Wien, 1985.

[4] D. Niemitz, S. Guse, D. Drazic, Demand at Multiple SlideTransfer Presses. VDI-Bericht Nr. 946, VDI-Verlag Dusseldorf,1992, S231–S253.

[5] E. Harsch, Press Technology in New Dimensions. Address onthe occasion of the official start-up of the Large-Panel TransferPress S7300 at Mercedes-Benz, Sindelfingen, 7 July 1995, Muller-Weingarten AG.

[6] W. Hartung, New possibilities for higher working accuracy ontransfer presses, Werkstatt und Betrieb 125 (1992) 5.

[7] Present Technology for Presses and Automated Stamping Shops,IHI, Tokyo, 1992.

pre-stress pressure was varied in the range between 55and 130 bar. At the same time, the release force of theoverload system increased as a result from 3.0 to 5.0MN with a nominal force of the machine of 8.0 MN.

The right-hand diagram of Fig. 27 shows that aconsiderable reduction in the stiffness of the total sys-tem takes place when the piston is immersed. Thecoefficients of total vertical stiffness are presented herein the z-direction as a function of the pre-stress pres-sure before and after the immersion of the piston. forthe range prior to and following detachment. As thestiffness of the centre of pressure also increases as thepre-stress pressure becomes greater, the coefficient oftotal vertical stiffness consequently becomes greater inrange 1. After the piston has detached, however, thereis only then a slight change in the stiffness (the com-pression module of the oil is a function of the pressureitself); consequently, the coefficients of total verticalstiffness in range 2 are practically constant.

In the case of an overload, a considerable tiltingmoment is exerted on the slide guides if all of thecentres of pressure do not detach simultaneously, as isverified by Fig. 28. The overload case was produced inthis instance by having the slide impact on a devicewith four load cells. The height of the individual loadcells is harmonised in such a way as to achieve the sameforces when the load is exerted. The spring constant ofthe device is of the same magnitude as that of a die.Although the load was applied to the centre of the slidewith this device, a greater supporting force results onthe left, looking in the direction of transfer, then on theright. This produces a high tilting moment about they-axis. In the case of cutting tools, this can cause

Fig. 28. Tilting moment in the event of an overload.


[8] E. Doege (Ed.), Drawing Technique on Multiple Slide TransferPresses. HFF-Bericht Nr. 11, 1987.

[9] K. Siegert (Ed.), Drawing Facilities of Single Action Presses forSheet Metal Forming, Universitat Stuttgart, DGM-Information-sgesellschaft, Oberursel, 1991.

[10] K.-J. Pahl, Elastic Interactions in the Drawing Apparatus ofSingle Action Presses. Dr.-Ing. Dissertation Universitat Kassel(1993). VDI-Fortschritt-Berichte Reihe 2 Nr. 307, Dusseldorf,VDI-Verlag, 1994.

[11] R. Zeidler, W. Petter, G. Geist, Double action presses withoutlimitation replaceable, Bander Bleche Rohre 12 (1991) S26–S28.

[12] M. Hanisch, The Behaviour of Mechanical Presses in ClosedDesign at Centric and Eccentric Load. Dr.-Ing. Dissertation,Tech. Universitat Hannover, 1978.

[13] U. Hoyer, U. Bottcher, H.J. Richter, FEM in Press Manufactur-ing 12. Reutlinger Seminar, Finite Elements in Praxis, 1994.

[14] P. Bogon, Parameters of Influence on the Dynamic Deflection ofEccentric Presses. Dr.-Ing. Dissertation, Universitat Kassel. Forts-chritt-Berichte Reihe 2 Nr. 215, Dusseldorf, VDI-Verlag, 1991.

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multiple-slide transfer presses: a modern production system

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