Tsch?tsch H., Koth A. Metal Forming Practise: Processes - Machines

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23.2 Functions of the individual styles of construction 281

23.2.5 Screw presses with direct electric motor drive

The percussion press (Vincent press) shown in

Figure 23.8 is powered in a non-contact man-

ner by a reversible motor (electric motor which

can be run in two rotation directions). In this

press, the reversible motor rotor is positioned

directly on the threaded spindle and the stator

is on the press frame. The moment required to

accelerate the flywheel is generated without

contact by the magnetic field between the rotor

and the stator. The direct electric drive means

there are no parts which are subject to wear in

the drive system. As there is no slip or other

mechanical loss apart from electrical slip and

losses in the linear (thread) drive itself, the

mechanical efficiency of this press is very high

(

= 0.7 – 0.8).

The work capacity can be precisely controlled

y the speed of the rotor. Up to a nominal press

force of 3150 kN, the press frame is built as a

one-piece cast-plate construction made of fine-

grained steel. For larger presses with a nominal

ress force of up to 23000 kN, a split press

frame is used, held together by two hydrauli-

cally-retracted pre-stressed steel tie rods.

Figure 23.8

Screw press with direct

electric motor drive.

1 rotor = flywheel, 2 ventilation wheel, 3 stator,

4 tie rod, 5 spindle, 6 column, 7 ram, 8 ram

guide. (Illustration: Müller-Weingarten works,

Weingarten, Germany)

For screw presses with a direct electric motor drive (Figure 23.9), small presses up to 6300 kN

press force, mainly used in forging, are powered with a reversible three-phase asynchronous

motor.

Because of their poor energy balance, for larger presses from 10,000 kN press force

direct electric drives with frequency converters

are mainly used.

282 23 Screw presses

Figure 23.9 SPR/SPP screw press with direct electric drive

(Illustration from LASCO Umformtechnik works, Coburg, Germany)

The frequency converter raises the electrical efficiency of the screw press by a factor of

three. As the drive accelerates, when the moment is constant the stress and frequency rise at

the same proportion until the pre-selected rotation speed is reached. If the rotation speed

and thus the energy stored in the flywheel is higher than that required for forming, then the

generator is used as a brake. The generator effect then means that the excess energy is fed

back into the network as electric energy.

On the return stroke this process is then repeated in the opposite direction. The upward accel-

eration and the generator braking are co-ordinated so that the mechanical brake only locks in at

the upper dead centre, meaning that there is no braking heat or wear.

The following energy diagrams, Figures 23.10a and b, show the energy balance of both types

of drive. Comparing the energy balances, it can be seen that when the drive is equipped with a

frequency converter, around 80% of the energy required for the return stroke can be reclaimed

in the form of electricity, in particular for the return stroke.

The energy for mechanical braking approaches zero, i.e. in practice, the mechanical brake is

only needed as a retainer.

23.2 Functions of the individual styles of construction 283

Figure 23.10a Energy path for a screw press with resistance motor

Figure 23.10b Energy path for a screw press with frequency-regulated drive

284 23 Screw presses

The energy from the electricity network (useful energy) is equal to the gross energy of the

press plus electric and mechanical losses.

With a frequency-regulated drive the total energy is reduced by the amount of energy which

flows back into the network due to the generator effect during braking. With the resistance

rotor this energy is lost as heat.

The advantages of the frequency-regulated drive with reactive current compensation are:

– lower load on electricity network

– lower energy costs

– low wear in clutch and brake

– lower maintenance costs

– shorter cycle times with reduced impact energy

– higher efficiency.

Table 23.1 below shows the energy balance in figures, taking as an example a screw press with

10,000 kN nominal press force and 130 kWs gross energy.

Table 23.1 Energy balance

Resistance rotor [kWs] Frequency-regulated

drive [kWs]

Total energy loss

480.0 161.0

Mechanical work

130.0 130.0

Energy from the network

610.0 291.0

Generator gain

0.0 49.1

Energy from the network, taking into ac-

count the generator gain

610.0 241.9

Constructional design

The press frame is a multi-part cast construction whose elements (table, side columns and

head) are joined to the press frame by four pre-stressed tie rods.

The bronze spindle nut is held in a steel casing, thus lengthening the guide. The threaded spin-

dle is forged and made of high-alloy CrNiMo steel. The shape of the thread is optimised to

have an extremely high fatigue strength. The cast steel flywheel is made up of several parts

with an independent load-limiting device. The ram is also made of cast steel. The extremely

long, heat-neutral guide system, mounted below 45q, has nitrated guide bars. Plastic-coated

support blocks, which can be adjusted from any direction, allow a very small guide clearance.

By mounting the press on spring dampers to insulate it against vibration and structure-borne

sound, press-induced vibrations can be almost completely avoided.

23.2 Functions of the individual styles of construction 285

Controls

LASCO screw presses come with an electronic stored-programme control system which en-

ables the process to be controlled fully automatically. All process data are shown on the

screen.

Figure 23.11 shows a complete screw press with automatic feed system and a two-jaw robot.

Among other things, this machine is used to manufacture automobile steering shafts.

Figure 23.11

SPR 800 fully-automatic screw press

(Photograph from LASCO Umformtechnik

works, Coburg, Germany)

23 Screw presses 286

Table 23.2

Types of screw press

Screw presses with spindle moved axially by friction

Screw presses with fixed spindles (Vincent presses)

Three-disc

screw press

Four-disc screw

press

Hydraulically-

driven screw

press

Three-disc

screw press

Conical disc

screw press

Screw press with

direct electro-

magnetic drive

Wedge screw press

Energy

transmission

Friction-driven

Pinion

Friction-driven

Direct

Wedge

Max. work

capacity

(kNm)

800

7 500

800

7 000

800

Nominal

press force

(max.) (kN)

31 500

140 000

20 000

125 000

31 500

Recoil impact

force (max.)

(kN)

63 000

no longer built

300 000

40 000

no longer built

250 000

63 000

Impact velocity

v in m/s 0.7 to 1 m/s

Work capacity (kNm)

2302

InI

S

§·



¨¸

©¹

Required rotation speed

for W

in min

–1

)

182 W



= work capacity required for forming

23.3 Calculating the parameters for screw presses 287

23.3 Calculating the parameters for screw presses

23.3.1 Work capacity

S

§·

 

¨¸



©¹

W in Nm work capacity (stored in the flywheel)

in min

–1

flywheel rotation speed

in kg m

mass moment of inertia

30 in s/min conversion figure for the rotation speed from minutes to seconds

in s

–1

angular velocity.

23.3.2 Mass moment of inertia

With the most common flywheel forms (Figure 23.12), the mass moment of inertia is made

up of several elements.

tot web

crown hub

dd d d

II I I 

Figure 23.12

Flywheel made up of

three hollow cylinders

The mass moment of inertia for a solid cylinder can be determined as follows:

r 

in kg m

mass moment of inertia

r in m radius of the solid cylinder

m in kg mass of the cylinder

g in kg/m

density

h in m height / length of the cylinder.

The mass m is:

mr h



For cast iron, which is the material mainly used for flywheels, the expression ȡ · S/2 can be

condensed into a constant, z.

z =

7250

kg/m

S S

11382 kg/mz

With this constant, z, it is easy to determine the mass moments of inertia. For the most com-

mon forms, this results in:

288 23 Screw presses

Table 23.3 Formulae for calculating mass moments of inertia.

Solid cylinder

= z · h · r

Hollow cylinder

= z · h · (R

– r

)

Obtuse cone

= z · h ·

5( )



h, r, R in m; z = 11 382 kg/m

for cast iron, e.g. GG 18

23.3.3 Required flywheel rotation speed

The operator of a screw press knows which workpieces the press is to be used for and thus

the work capacity required for forming, as well as the press force. Depending on the press

force, the nominal press force of the machine is selected and the spindle or flywheel speed

is set according to the work capacity required.

The calculation is as follows:

182 W



in min

–1

required flywheel speed

in N m work capacity required for forming

in kg m

mass moment of inertia of the flywheel

182 in s

/min

constants integrating all individual factors from Eq. 1.

This speed must be calculated for all screw presses of the Vincent construction type. For

friction screw presses as in Figure 23.13, the driving disc radius r

is sought. Up until this

point, r

, the friction must be kept up between the flywheel and the driving disc so that the

energy required for forming is stored in the flywheel.

23.3 Calculating the parameters for screw presses 289

23.3.4 Required driving disc radius

This results from the assumption (Figure 23.13) that the peripheral speeds of the driving disc

and the flywheel are equal at a driving disc radius of r

. Thus:

· S · n

= 2 · r

· S · n



If n

í from Equation 2 í is inserted into Equation 3, the driving disc radius r

can be calcu-

lated up to which friction must be maintained (Eq. 4) in order to save the work capacity W

required for forming in the flywheel.

dd d

182



in mm required driving disc radius

in mm flywheel diameter

in mm

–1

driving disc rotation speed

in min

–1

flywheel rotation speed

in kg/m

mass moment of inertia of the flywheel

in Nm work capacity required for forming

Figure 23.13 Layout of a friction screw press.

initial radius, r

radius at which there is no more friction, d

flywheel diameter, d

driving disc di-

ameter, n

flywheel speed, n

driving disc rotation speed: 1 driving disc, 2 flywheel, 3 spindle, 4 frame,

5 ram

290 23 Screw presses

23.3.5 Ram impact velocity

The ram impact velocity can be determined from the spindle pitch and rotation speed.

Spindle pitch:

tanhd

S

h in m spindle pitch

in m pitch diameter of the thread

in deg. pitch angle.

Impact velocity:

60 s/ min



in m/s ram impact velocity

in min

–1

spindle rotation speed.

With screw presses, impact velocities are between 0.7 and 1.0 m/s, much lower than with

hammers (5 – 14 m/s).

If the calculation of the required radius r as well as that of the work capacity and the values

this produces for the rotation speed n take into account the press efficiency

then:

dM M

dd d M

Eq.1a

2302

182

Eq.2a

182

Eq.3a

WnI



§·

 

¨¸

©¹





in Nm available effective work

press efficiency (0.7 í 0.9)

in min required effective rotation speed

in m required effective diameter

in Nm work capacity required for forming

Tsch?tsch H., Koth A. Metal Forming Practise: Processes - Machines - Tools

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