Altan T. Metal Forming Handbook

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imum pneumatic pressure is generally 6bar, although high-pressure

networks or compressors producing a maximum of 16bar are increas-

ingly employed.

The engineering of these auxiliary units is determined by valid stan-

dards and legal requirements. In general, galvanized gas lines between

1'' and 6'' are used for installation purposes. Smaller lines are configured

using steel piping and cutting ring or beaded unions. The installation

required for a large-panel transfer press with a structural volume of

around 40 310 315m can comprise up 40t, including compressed air

tank, pipes and appliances.

3.2.10Hydraulic system

The use of hydraulics in mechanical press construction is relatively

minimal. Standard use of hydraulic engineering in this type of press is

restricted to the hydraulic overload safeguard, raising the moving bol-

ster and hydraulic die clamping systems. All these functions are gener-

ally supplied from a central hydraulic unit. In the case of transfer press-

es, these basic functions are complemented by other functions such as

engagement of the clutch between the press and transfer drive systems

and the comprehensive system of scrap ejection flaps. In addition, var-

ious clamping and locking devices around the press are hydraulically

driven.

The most important function in mechanical presses, engaging and

braking the slide drive system, is being increasingly done by hydraulic

means. The use of hydraulics permits higher switching frequencies in

single presses and is also more environmentally friendly in terms of

noise and air pollution.

The clutch and brake are supplied by a central hydraulic unit

equipped with a cooling system. A dual-channel safety control with

regulated damping facility and pressure filtration to 10 µm is required.

The increased use of deep drawing with single-acting presses places

particularly stringent demands on the blankholding function and thus

the draw cushion in the press bed (cf. Sect. 3.1.4), particularly in large-

panel presses. Multiple-point bed cushions with pressure control capa-

bility are then only possible using a complex servo hydraulic system.

Mechanical presses

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

This requires separate supply units with oil tank, pump outputs of

approx. 150 kW and oil flow rates of approx. 1,000 l/min for the draw

cushion. Control is performed using proportional valves complying to

DIN and ISO standards in the pressure range up to 300 bar.

3.2.11Lubrication

The complex lubrication systems used to supply the lubrication points

in mechanical presses are substantially simplified by using valve blocks

(cf. Fig. 3.3.4). The system most commonly used here consists of paral-

lel-switching progressive distributors. Allocation of the required quan-

tity of lubricant is aided by volume controllers, dividers and similar ele-

ments. The flow of oil is monitored at the distributor blocks. To ensure

optimum operating conditions, a control system regulates the temper-

ature of the lubricant within a relatively narrow bandwidth by using

auxiliary heating and cooling functions.

To guarantee maximum operating safety, complex production instal-

lations for metal forming are always fitted with twin supply unit ele-

ments such as pumps and filters. If one pump fails, there is no need to

interrupt production, as the second system cuts in automatically.

Maintenance or exchange of the defective system does not result in

production standstill, increasing uptime. The quality of the oil is safe-

guarded by supplementary filtration in the secondary circuit, increas-

ing its service life.

In smaller presses, and in particular by knuckle-joint presses with

bottom drive, central lubrication systems with multiple-circuit geared

pumps are used. Up to 20 main lubrication points are continuously

supplied directly by the gear stages with a constant supply of oil.

During the short swivel movements executed by the thrust elements

and joints, a constant flow of lubricant is available even under varying

resistance levels in the individual user and supply lines.

Fundamentals of press design

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

3Fundamentals of press design

3.3 Hydraulic presses

3.3.1 Drive system

Hydraulic presses operate on the physical principle that a hydrostatic

pressure is distributed evenly through a system of pipes, and that a pres-

sure p [N/m

], acting on a surface A [m

], produces a force F [N]:

The force acting on the slideof the press depends on the forming or

blanking process performed. For this reason, the pressure p acting on

the piston surface is also a function of the deformation process con-

ducted. Its measurement may be used directly to calculate the force act-

ing on the slide. The maximum slide force can be selected by limiting

the maximum hydraulic pressure through a relief valve at any position

of the slide.

The drive powerP [W] of a hydraulic press depends on the volumetric

flow of hydraulic fluid V[m

/s] and therefore on the speed of the slide

as well as on the hydraulic pressure p, the forces at the slide, and the

mechanical and electrical losses h

ges

In contrast to mechanical presses, there is generally no reserve of ener-

gy available in hydraulic presses. Thus, the entire power must be

applied when carrying out the forming operation. For this reason,

hydraulic presses require a significantly higher drive power than in

mechanical presses, with comparable force capacity (cf. Sect. 4.4.1).

FpA=⋅

ges

⋅

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

Fundamentals of press design

Mechanical presses operate according to the throughfeed principle,

and the output capacity is determined by the drive speed. In the case of

hydraulic presses, in contrast, the flow direction of the hydraulic medi-

um is reversed to close and open the press.

The cycle timeis made up of a number of variable components,

whereby there is an optimum press setting value for each die set. The

total stroke, the drawing stroke, the forming force and blank holder

force are optimized in accordance with the dies. Each of these setting

values has an effect on the press cycle time, and thus on the press out-

put. The drive power is calculated on the basis of the ram displacement

versus time curve (Fig. 3.3.1)that is related to time constants of the

press.

Fig. 3.3.1 Displacement-time diagram of a hydraulic press

2 3 4 5

stroke

[mm]

parttransporttime

transporttimeT

presscycle

acceleration

loweringinrapidtraverse

500mm/s

braking

drawing1

drawing2

braking

beforeTDC

embossing

returnstroke

500mm/s

acceleration

time[s]

0,1 0,2 0,7 0,2

TDC

BDC

prorated

presstimeT

cycletimeT=0,2s+T+T

pressureholdingtime

andreversal

pressurerelief

0,1

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

Hydraulic presses

Design of the drive system

The piston force of the slide cylinder, fastened to the press crown, acts

directly on the slide (Fig. 3.3.2and cf. Fig.3.1.10). The nominal press

force is the sum of the active surfaces of all pistons multiplied by the

hydraulic pressure. The movement sequence during the press cycle is

executed by the slide cylinder with the slide control. The number of

active hydraulic cylinders does not affect the press function. Whether

the slide is driven at one, two or four points is determined on the basis

of static calculations (cf. Fig. 3.1.3).

The function of the hydraulic system

Hydraulic systems convert electrical energy into hydraulic energy,

which in turn is transformed into mechanical energy with the aid of

the individual hydraulic cylinders inside the press. The hydraulic fluid

in the cylinders actuates the press slide as follows:

Fig. 3.3.2 Functional principle of a hydraulic cylinder driving the slide of a hydraulic press

high-speedclosure

forming returnstroke

250bar

slide

control

fillingvalve

piston

die

part

slide

press

bed

pressurepipe

lowering

module

return

pipe

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

Fundamentals of press design

–Forces are generated through the action of the hydraulic fluid.

–Displacements are achieved by the supply of hydraulic fluid.

–Press (slide) speed is proportional to volumetric flow rates.

–Forward and reverse motions are controlled by the direction of

fluid flow.

Design of hydraulic systems in presses

The hydraulic system is subdivided into several functional assemblies

(Fig. 3.3.3).The hydraulic fluid is stored in the oil tank. All suction pipes

to the pumps or cylinders are supplied with fluid from the reservoir. The

fluid tank is suspended at the press crown to ensure that the cylinders are

filled when the press closes at high speed. The hydraulic drive system,com-

prised of pumps and motors, forms a structural unit together with the

clutch and connecting flange. The pumps used are either immersion

Fig. 3.3.3 Arrangement of a press hydraulic system

oiltank

slide

cylinder

secondary

systems

valvecontrol

slide

control

heating

cooling

filtration

pressure

andpump

control

pumpdrive

primarycircuit

secondarycircuit

oilmaintenance

pressure

accumulator

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

pumps suspended in the oil tank, or they are mounted on supports in the

press foundation. The motors drive the pumps via an elastic clutch. The

pumps are supplied by suction pipes, and the hydraulic fluid is conveyed

to the valve control by means of high-pressure pipes. The valve control

systemguides the hydraulic fluid to the appropriate units. Depending on

the function in question, the hydraulic cylinders are extended or retract-

ed, moved actively or passively in the press. The return flow is guided

back to the oil tank. The consumers, cylinders or hydraulic motors, move

the slide or cushions, or adjust spindles. They generate forces, lift loads or

clamp the dies. The service unitoperates in a bypass circuit away from the

main hydraulic flow. In this unit, the fluid is recirculated by means of fil-

ters and coolers in a separate circuit, purifying and cooling it to ensure

that the press remains ready for production at all times.

Pressure accumulator drive system

If the pump conveys the fluid to a pressure accumulator, it is possible to

smooth out the energy delivery from the main electrical net. Large

quantities of hydraulic oil can be drawn from the pressure accumulator

for a short time, which has to be replenished by the pump before the

next press cycle. If the extraction of hydraulic oil is followed by a long

idle period, the accumulator can be supplied by a small pump – for

example when loading and unloading the die after retraction of the

slide. However, for processes requiring large quantities of hydraulic fluid

to be drawn in quick succession, a higher delivery power must be avail-

able – for example a drawing operation followed by the opening of the

press. For the second example, a pressure accumulator-type drive sys-

tem is not suitable, since not only a large accumulator system but also

a large pump is required. Draw presses involve short cycle and transport

times which often cannot be achieved using an accumulator drive sys-

tem. Table 3.3.1 demonstrates additional differences between pump

and accumulator drive systems.

3.3.2Hydraulic oil

Hydraulic systems in presses are operated using standard hydraulic oil

(HLP) according to DIN 51524 Part 2. Hydraulic oils are required to

comply with strict standards:

Hydraulic presses

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

Fundamentals of press design

– lubrication and wear protection,

– constant viscosity level at temperatures between 20 and 60 °C,

– resistance to temperature,

– low compressibility,

– low foaming characteristics,

– low absorption of air,

– good rust protection,

– good filtration properties,

– low cost.

All hydraulic units are designed for use with hydraulic oils conforming

to these standards. Characteristics, life expectancy and pressure ranges,

as well as the choice of seals and gaskets correspond to the oil type used.

Due to the compressibility of hydraulic oil, the volume of hydraulic fluid

changes under pressure. As pressure is increased, the fluid column is

compressed. This property affects the control and regulating functions

in the press: the higher the pressure, the longer the response time.

When calculating cycle times or determining the use of different drive

systems, the compressibility factor must be taken into account, as this

has a pronounced effect, particularly where short forming strokes are

involved, for example when blanking or coining. In the case of mineral

oils, a compressibility factor of 0.7 to 0.8% per 100bar oil pressure can

be expected. This property of hydraulic fluids also affects the release of

Table 3.3.1: Comparison of hydraulic drive systems

Pump drive system

The installed pump capacity determines the

achievable speeds with the motor output.

Requirement-oriented hydraulic oil delivery

results in peak loads on the electrical mains.

Work capacity is unlimited, as the pressing

force is available over the entire stroke.

The system pressure corresponds directly to

the power requirement of the die.

The pressing force can be simply limited by

means of pressure reducing valves.

Pressure accumulator drive system

Speeds are determined by valve cross-sections

and the pressure head.

Continuous hydraulic oil delivery exerts an even

degree of stress on the mains.

Work capacity is limited by the available useful

volume of the accumulator.

Pump pressure corresponds to the accumulator

boost pressure. In case of a lower energy

requirement, excess energy is converted into

heat.

It is not possible to regulate the pressure of

an accumulator, which always operates at full

capacity.

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

Hydraulic presses

pressure in the press cylinder. If the area under pressure is opened too

rapidly, this results in potentially damaging pressure relief spikes.

Flame-retardant hydraulic fluids should only be used where this is

absolutely mandatory and the appliance manufacturer has issued the

relevant approval. These call for the use of special gaskets, in some cases

lower operating pressure and also special measures during start up.

Between the pump drive system and the cylinders, the press func-

tions are controlled by valves. Directional, pressure and non-return

valves act in conjunction with proportional and servo valves.

Depending on the system and position – in the primary or secondary

circuit – valves of various dimensions are used. The execution of all the

necessary press functions calls for a large number of hydraulic connec-

tions which are grouped together in a block configuration (Fig. 3.3.4).

Block hydraulic systemshave gained in popularity, because:

–external connections only need to be directed from the pump and to

the cylinder,

–the valves are surface-mounted and easy to exchange,

Fig. 3.3.4Block hydraulic system

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

–their nature inevitably results in a standardization process: the valve

blocks do not need to be redesigned for each different press,

–all the valves are brought together to create a functional assembly on

a single block,

–integrated valves are sensibly combined with space-saving, low-cost

auxiliary valves,

–troubleshooting is simplified by the presence of drilled-in functional

and test connections.

3.3.3Parallelism of the slide

The capacity of the press frame to absorb eccentric loads plays a major

role. Eccentric forces occur during the forming process when the load of

the resulting die force is not exerted centrally on the slide, causing it to

tilt (Fig. 3.3.5). The standard press is able to absorb a maximum eccen-

tric moment of load of

via the slide gibs. This results in a maximum slide tilt of 0.8 mm/m. If a

higher off center loading capability is desired, then the press design

must be more rigid. In this case, the slide gibs will have greater stability,

the press frame will be more rigid, and the slide will be higher.

If it is not economically possible to achieve the allowable amount of

slide tilt within the required limits by increasing press rigidity, it is nec-

essary to use hydraulic parallelism control systems, using electronic

control technology (Fig. 3.3.5), for example in the case of hydraulic

transfer presses (cf.Fig. 4.4.14). The parallelism controlsystemsact in the

die mounting area to counter slide tilt. Position measurement sensors

monitor the position of the slide and activate the parallelism control

system (Fig. 3.3.5). The parallelism controlling cylinders act on the cor-

ners of the slide plate and they are pushed during the forming process

against a centrally applied pressure. If the electronic parallelism moni-

tor sensor detects a position error, the pressure on the leading side is

increased by means of servo valves, and at the same time reduced on

the opposite side to the same degree. The sum of exerted parallelism

Fundamentals of press design

M mF kNm

kNm

=⋅

[]

0075

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