Tsch?tsch H., Koth A. Metal Forming Practise: Processes - Machines - Tools
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Part II: Presses
21 Types of press
According to their characteristics, presses can be classified into those controlled mainly by
work (energy), the ram path or force.
21.1 Presses controlled by work
Hammers and screw presses are machines whose main characteristic is their work capacity
(energy). A hammer’s work capacity is determined from the ram mass and drop height. With
screw presses, the work capacity is stored in the rotating masses (mainly in the flywheel),
therefore it depends upon the angular velocity and the mass moment of inertia. The two kinds
of machine are similar in that the work capacity can be influenced or adjusted.
The force, on the other hand, can not be directly adjusted, but depends upon the kind of work-
piece and the deformation distance.
Figure 21.1 The principle of presses controlled by work, a) drop hammer, b) screw press
21.2 Presses controlled by the ram path
These include crank presses and knuckle-joint presses. With these presses, the deformation
is complete when the ram has reached its lowest position (bottom dead centre í BDC). This
means their characteristic value is the ram path limit, which comes from the crank radius r
in crank presses and from the leverage with knuckle-joint presses (Figure 21.2).
With crank presses the nominal force of the press is available at a crank angle of 30° before
BDC up to BDC, whereas with a knuckle-joint press, the nominal press force (depending on
the leverage) is only available within a range of 3 to 4 mm before BDC.
21.4 Question on Chapter 21 263
Figure 21.2 Drive layout of presses controlled by the ram path, a) crank press, b) knuckle-joint press
21.3 Presses controlled by force
The operation of hydraulic presses depends upon force, as the only thing which can be ad-
justed about them is the force (via the operating pressure).
As the deformation forces fluctuate within certain limits (differences in material strength,
blank tolerances, lubrication and condition of the tooling), a dimensionally accurate part can
only be produced on a hydraulic press if the deformation distance is limited. This can be car-
ried out in the press itself using positive stops, or in the die.
This kind of ram path limit is also needed for presses controlled by work.
21.4 Question on Chapter 21
How are presses classified?
22 Hammers
22.1 Columns and frames
Figure 22.1 shows the most important types of hammer column.
Figure 22.1 Hammer frame designs. I) Single-column frame, 1 anvil, 2 column, 3 ram guide, 4 ram, 5 air
cylinder; II) structural designs of two-column frames, a) anvil 1, side column 2 and head 3,
separate, b) side columns and head in one piece, c) anvil, side columns and head in one piece
22.2 Types of hammer
Hammers are classified according to the drive design into:
drop hammers
double-acting hammers
counterblow hammers.
With drop hammers, the ram drops freely. The impact energy comes from the mass of the ram
and the drop height. Belts, chains or with hydraulic drives the piston rod are used to lift the
ram.
At present, hydraulic lifting devices are mainly used for reasons of cost-efficiency.
W = m · g · H
With double-acting hammers a pressure medium (air or compressed oil) is used as well as the
drop energy to further accelerate the ram. This means that higher ram impact velocities can be
achieved, thus raising the impact energy.
W =
2
2
m
X
Table 22.1
Types of hammer
Hammers with anvil block
Hammers without anvil block
Drop hammers
Counterblow hammers
Coupling with
belt
with chain
with hydraulics
with bands
with hydraulics
Elements and parameters
Double-acting ham-
mers
Lifting mechanism
Belt
Chain
Piston rod
Piston rod
Differential piston
Pressure mechanism
–
–
–
Piston
Piston
Energy for working mo-
vement
Free fall
Air Compressed oil
Air Compressed oil
Compressed oil
Max. drop height in m
2
1.3
1.3
1.3
–
–
Ram acceleration
a (m/s)
a g
a ! g
a ! g
Impact velocity
X
A
(m/s)
approx. 5
6
6 – 8
8 – 14
Work capacity
W = m
·
X
2
/2
m · g · h
m · g · H+
P · 10
–1
· A · H
12
2
()
2
mm
X
Max. work capacity
W (kN
m)
80
100
160
200
1000
1000
266 22 Hammers
With double-acting hammers, in order to have the same available impact energy as with a drop
hammer, the following is needed:
smaller stroke heights
smaller ram masses.
This results in a higher number of blows per time unit and more economical processing during
the forging operation.
From the point of view of the drive, counterblow hammers are double-acting hammers with no
extra drop energy. Only the upper ram is driven. The lower ram is driven indirectly by means
of a mechanical coupling using strip steel or a hydraulic coupling.
At present only hammers with hydraulic couplings are built. Table 22.1 (previous page) shows
how hammers are classified.
22.3 Constructional design
22.3.1 Drop hammers (Figure 22.2)
F
WmgH
K
(potential energy)
W in Nm impact energy
W
e
in Nm effective work
m in kg ram mass
H in m drop height
X
in m/s ram impact velocity
g in m/s
2
drop acceleration
K
F
– drop efficiency
K
i
– impact efficiency
(
K
i
= 0.5 – 0.8).
2
2
m
W
X
(kinetic energy)
ei
WW
K
2
g
H
X
Figure 22.2 Electro/oil-hydraulic drop hammer
1 hydraulic drive, 2 con rod guide, 3 ram lock, 4 ram, 5
guides, 6 columns, 7 anvil bolster, 8 anvil block (Illus-
tration from LASCO Umformtechnik works, Coburg,
Germany)
22.3 Constructional design 267
The hydraulic drop hammer shown in Figure 22.2 has a three-part frame. The materials used
are:
a) Anvil and side columns: grey cast iron with added steel.
This special cast iron is a better damper and has a more ho-
mogeneous structure than cast steel.
b) Ram: high-alloy, hardened and tempered cast electric steel or
quenched and tempered steel.
c) Guides: hardened, ground steel prism guides.
For hammers with an anvil block, the impact effect depends upon the ratio of
impact energy
mass of the anvil
.
Modern hammers have values of Q = 1.0 – 1.2.
a
(Nm)
m(kg)
W
Q
The impact efficiency of a hammer is determined by this value. In the past, the following ratio
was used to measure the impact effect:
anvil mass (kg) 10 20
to
ram mass (kg) 1 1
However, as the anvil mass remains relatively small with small ram masses, this mass ratio can
not be used to measure the impact effect.
22.3.2 Double-acting hammers
With double-acting hammers the work capacity of the falling ram is raised further by a propel-
lant (air or compressed oil).
W = m · g · H + p · 10
–1
· A · H
1
(10)WHmgp A
W in Nm impact energy
m in kg ram mass
g in m/s
2
drop acceleration
H in mm drop height
p in bar operating pressure
A in cm
2
piston area
10
–1
conversion from bar to N/cm
2
268 22 Hammers
With air-powered hammers, pressures are used of p = 6 – 7 bar.
Double-acting hammers are built as both single- and double-column hammers. The advantage
of double-column hammers is the better ram guide and the more rigid self-contained column.
The hydraulic double-acting hammer shown in Figure 22.3, with fully electronic controls, is a
development of this kind of hammer. The hydraulic pressure from above means that the ram
reaches its impact velocity of around 5 m/s in the shortest distance possible. Thus, the impact
frequency is considerably higher than with a drop hammer.
The result is short tooling contact times, lengthening tooling lives. These LASCO hammers are
built with an impact energy of 6.3 - 400 kJ (63,000 – 400,000 Nm). Up to about 150 kJ
(150,000 Nm) work capacity, the hammer is built with what is known as a U frame. When the
work capacity is higher, a segmented frame construction is used because of the higher anvil
weight and the transport problems this involves.
Figure 22.3
HO-type hydraulic double-acting hammer
(Ilustration from LASCO Umformtechnik works,
Coburg, Germany)
22.3 Constructional design 269
The U-frame shape is ideal, with optimum mass distribution. It stands out for its high rigidity.
The stress-optical characteristics of the most crucial cross-sectional transitions are optimised.
The frame material is specially heat-treated alloyed cast steel. The weight of the frame and the
base area are set up to accommodate spring dampers (Figure 22.3). A well-dimensioned bolster
to hold the die, made of quenched and tempered steel, is wedged in the U frame.
The ram, made of forged quenched and tempered steel, is a block ram. The guides are in an
X arrangement. The ram is guided precisely, with as little play as possible, in adjustable
guide bars made of hardened steel mounted in the U frame.
Figure 22.4
Double-acting hammer hydraulic drive
(Illustration from LASCO Umformtechnik
works, Coburg, Germany)
The drive system (Figure 22.4) is fully enclosed and cast in a modular design. It is installed in
the head of the hammer which also serves as an oil chamber. In the forged control block, whe-
re the main control elements are combined in an integral hydraulic lift system, there are as few
failure-prone pipes as possible.
At the heart of the hydraulic unit are long-lasting axial piston pumps, driven by special rotary
current motors via elastic couplings, in connection with hydraulic accumulators.
Figure 22.5 again compares various structural designs of double-acting hammers.
22 Hammers 270
Figure 22.5
Structural designs of double-acting ham-
mers, a) double-column double-acting
hammer with pneumatic or hydraulic drive
(Banning principle), 1 anvil bolster, 2 ram
guide, 3 hammer body, 4 ram, 5 propellant
feed, 6 pilot valve, 7 propellant outlet, 8
cylinder liner
b) Single-column air drop hammer (Bêché & Grohs principle).
a extra ram guide, b anvil, c dies
c) Hydraulic U-frame design double-
acting hammer (LASCO Umform-
technik)