Cold and Hot Forging: Fundamentals and Applications / Edited by Taylan Altan, Gracious Ngaile, Gangshu Shen

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Principles of Forging Machines / 109

Fig. 10.2

Load versus displacement curves for various forming operations (energy ⳱ load ⳯ displacement ⳯ M, where M is a

factor characteristic of the speciﬁc forming operation). [Altan et al., 1973]

and die-chilling effects and, therefore, the type

of equipment used, determine the metal ﬂow be-

havior and the forging load and energy required

for the process. Surface tearing and cracking or

development of shear bands in the forged ma-

terial often can be explained by excessive chill-

ing of the surface layers of the forged part near

the die/material interface.

110 / Cold and Hot Forging: Fundamentals and Applications

Fig. 10.3

Load versus displacement curves obtained in

closed-die forging an axisymmetric steel part at

2012 ⬚F (1100 ⬚C) in three different machines with different initial

velocities (V

). [Altan et al., 1973]

10.4 Classiﬁcation and Characteristics

of Forming Machines

In metal forming processes, workpieces are

generally fully or nearly fully formed by using

two-piece tools. A metal forming machine tool

is used to bring the two pieces together to form

the workpiece. The machine also provides the

necessary forces, energy, and torque for the pro-

cess to be completed successfully, ensuring

guidance of the two tool halves.

Based on the type of relative movement be-

tween the tools or the tool parts, the metal form-

ing machine tools can be classiﬁed mainly into

two groups:

●

Machines with linear relative tool movement

●

Machines with nonlinear relative tool move-

ment

Machines in which the relative tool move-

ments cannot be classiﬁed into either of the two

groups are called special-purpose machines. The

machines belonging to this category are those

operated on working media and energy. The

various forming processes, discussed in Chapter

2, are associated with a large number of forming

machines. These include:

●

Rolling mills for plate, strip and shapes

●

Machines for proﬁle rolling from strip

●

Ring rolling machines

●

Thread rolling and surface rolling machines

●

Magnetic and explosive forming machines

●

Draw benches for tube and rod; wire and rod

drawing machines

●

Machines for pressing-type operations, i.e.,

presses

Among those listed above, “pressing”-type ma-

chines are most widely used and applied for a

variety of different purposes. These machines

can be classiﬁed into three types [Kienzle, 1965]

[Kienzle, 1953]:

●

Load-restricted machines (hydraulic presses)

●

Stroke-restricted machines (crank and eccen-

tric presses)

●

Energy-restricted machines (hammers and

screw presses)

Hydraulic presses are essentially load-re-

stricted machines; i.e., their capability for car-

rying out a forming operation is limited mainly

by the maximum load capacity. Mechanical (ec-

centric or crank) presses are stroke-restricted

machines, since the length of the press stroke

and the available load at various stroke positions

represent the capability of these machines. Ham-

mers are energy-restricted machines, since the

deformation results from dissipating the kinetic

energy of the hammer ram. The hammer frame

guides the ram, but is essentially not stressed

during forging. The screw presses are also en-

ergy-restricted machines but they are similar to

the hydraulic and mechanical presses since their

frames are subject to loading during forging

stroke. The speed range and the speed stroke

behavior of different forging machines vary con-

siderably according to machine design, as illus-

trated in Table 10.1.

The signiﬁcant characteristics of these ma-

chines comprise all machine design and perfor-

mance data, which are pertinent to the machine’s

economic use. These characteristics include:

●

Characteristics for load and energy

●

Time-related characteristics

●

Characteristics for accuracy

In addition to these characteristic parameters,

the geometric features of the machine such as

the stroke in a press or hammer and the dimen-

sions and features of the tool-mounting space

(shut height) are also important. Other important

values are the general machine data, space re-

quirements, weight, and the associated power re-

quirements.

Principles of Forging Machines / 111

Table 10.1 Speed-range and speed-stroke behavior of forging equipment

Speed range

Forging machine ft/s m/s Speed-stroke behavior

Hydraulic press 0.2–1.0(a) 0.06–0.30(a)

Mechanical press 0.2–5 0.06–1.5

Screw press

Gravity drop hammer

Power drop hammer

Counterblow hammer (total speed)

HERF machines

Low-speed Petroforge

2–4

12–16

10–30

15–30

20–80

8–20

0.6–1.2

3.6–4.8

3.0–9.0

4.5–9.0

6.0–24.0

2.4–6.0

Source: [Altan et al., 1973]

Apart from the features mentioned previously,

some of the basic requirements that are expected

of a good forging machine can be listed as:

●

High tool pressure, which requires the stock

to be tightly gripped and upsetting forces

completely absorbed.

●

Sufﬁcient tool length to permit rigid bar re-

ception apart from ﬁlling up the impression.

●

The gripping tools must not open during the

upsetting process.

●

The device for moving the tools must be se-

cured against overloading.

●

The heading slide must be provided with

long and accurate guides.

●

The whole machine must be elastically se-

cured against overloading.

●

Design of a crankshaft of special rigidity.

●

Readily interchangeable gripping and head-

ing tools.

●

The driving motor and the machine must be

connected through a security coupling.

●

The machine must have central lubrication.

10.5 Characteristic Data for

Load and Energy

Available energy, E

(in ft-lb or m-kg), is the

energy supplied by the machine to carry out the

deformation during an entire stroke. Available

energy, E

, does not include either E

, the en-

ergy necessary to overcome the friction in the

bearings and slides, or E

, the energy lost be-

cause of elastic deﬂections in the frame and driv-

ing system.

Available load, L

(in tons), is the load avail-

able at the slide to carry out the deformation

process. This load can be essentially constant as

in hydraulic presses, but it may vary with the

slide position in respect to “bottom dead center”

(BDC) as in mechanical presses.

Efﬁciency factor, g, is determined by divid-

ing the energy available for deformation, E

,by

the total energy, E

, supplied to the machine;

i.e., g ⳱ E

. The total energy, E

, also in-

cludes in general: (a) the losses in the electric

motor, E

, (b) the friction losses in the gibs and

in the driving system, E

, and (c) the losses due

to total elastic deﬂection of the machine, E

The following two conditions must be satis-

ﬁed to complete a forming operation: ﬁrst, at any

time during the forming operation,

L ⱖ L (Eq 10.1)

where L

is the available machine load and L

is the load required by the process; and second,

for an entire stroke,

E ⱖ E (Eq 10.2)

where E

is the available machine energy and

is the energy required by the process.

If the condition expressed by the former in-

equality above (Eq 10.1) is not fulﬁlled in a

hydraulic press, the press will stall without ac-

complishing the required deformation. In a me-

chanical press, the friction clutch would slip and

the press run would stop before reaching the bot-

tom dead center position. If the condition ex-

pressed by the latter inequality (Eq 10.2) is not

satisﬁed, either the ﬂywheel will slow down to

unacceptable speeds in a mechanical press or the

part will not be formed completely in one blow

in a screw press or hammer.

112 / Cold and Hot Forging: Fundamentals and Applications

10.6 Time-Dependent

Characteristic Data

Number of strokes per minute, n, is the

most important characteristic of any machine,

because it determines the production rate. When

a part is forged with multiple and successive

blows (in hammers, open-die hydraulic presses,

and screw presses), the number of strokes per

minute of the machine greatly inﬂuences the

ability to forge a part without reheating.

Contact time under pressure, t

, is the time

during which the part remains in the die under

the deformation load. This value is especially

important in hot forming. The heat transfer be-

tween the hotter formed part and the cooler dies

is most signiﬁcant under pressure. Extensive

studies conducted on workpiece and die tem-

peratures in hot forming clearly showed that the

heat transfer coefﬁcient is much larger under

forming pressure than under free contact con-

ditions. With increasing contact time under pres-

sure, die wear increases. In addition, cooling of

the workpiece results in higher forming load re-

quirements.

Velocity under pressure, V

, is the velocity

of the slide under load. This is an important vari-

able because it determines (a) the contact time

under pressure and (b) the rate of deformation

or the strain rate. The strain rate inﬂuences the

ﬂow stress of the formed material and conse-

quently affects the load and energy required in

hot forming.

10.7 Characteristic Data for Accuracy

Under unloaded conditions, the stationary

surfaces and their relative positions are estab-

lished by (a) clearances in the gibs, (b) parallel-

ism of upper and lower beds, (c) ﬂatness of up-

per and lower beds, (d) perpendicularity of slide

motion with respect to lower bed, and (e) con-

centricity of tool holders. The machine charac-

teristics inﬂuence the tolerances in formed parts.

For instance, in backward extrusion a slight non-

parallelism of the beds, or a slight deviation of

the slide motion from ideal perpendicularity,

would result in excessive bending stresses on the

punch and in nonuniform dimensions in ex-

truded products.

Under loaded conditions, the tilting of the

ram and the ram and frame deﬂections, particu-

larly under off-center loading, might result in

excessive wear of the gibs, in thickness devia-

tions in the formed part and in excessive tool

wear. In multiple-operation processes, the tilting

and deﬂections across the ram might determine

the feasibility or the economics of forging a

given part. In order to reduce off-center loading

and ram tilting, the center of loading of a part,

i.e., the point where the resultant total forming

load vector is applied, should be placed under

the center of loading of the forming machine.

In presses (mechanical, hydraulic, or screw),

where the press frame and the drive mechanism

are subject to loading, the stiffness, C, of the

press is also a signiﬁcant characteristic. The

stiffness is the ratio of the load, L

, to the total

elastic deﬂection, d, between the upper and

lower beds of the press, i.e.:

C ⳱ L /d (Eq 10.3)

In mechanical presses, the total elastic deﬂec-

tion, d, includes the deﬂection of the press frame

(⬃25 to 35% of the total) and the deﬂection of

the drive mechanism (⬃65 to 75% of the total).

The main inﬂuences of stiffness, C, on the form-

ing process can be summarized as follows:

●

Under identical forming load, L

, the de-

ﬂection energy, E

, i.e., the elastic energy

stored in the press during buildup, is smaller

for a stiffer press (larger C). The deﬂection

energy is given by:

E ⳱ dL /2 ⳱ L /2C (Eq 10.4)

dM M

●

The higher the stiffness, the lower the de-

ﬂection of the press. Consequently, the var-

iations in part thickness due to volume or

temperature changes in the stock are also

smaller in a stiffer press.

●

Stiffness inﬂuences the velocity versus time

curve under load. Since a less stiff machine

takes more time to build up and remove pres-

sure, the contact time under pressure, t

,is

longer. This fact contributes to the reduction

of tool life in hot forming.

Using larger components in press design in-

creases the stiffness of a press. Therefore,

greater press stiffness is directly associated with

increased costs, and it should not be speciﬁed

unless it can be justiﬁed by expected gains in

part tolerances or tool life.

Principles of Forging Machines / 113

REFERENCES

[Altan et al., 1973]: Altan, T., et al., Forging

Equipment, Materials and Practices, Metal

and Ceramics Information Center, HB03,

1973, p 4–7.

[Kienzle, 1965]: Kienzle, O., Characteristics of

Data in Machine Tools for Closed Die Forg-

ing, (in German), Werkstatttechnik, Vol 55,

1965, p 509.

[Kienzle, 1953]: Kienzle, O., The Characteristic

Data on Presses and Hammers (in German),

Werkst. Maschin., Vol 43, 1953, p 1.

SELECTED REFERENCES

[Altan et al., 1983]: Altan, T., Oh, S.I., Gegel,

H., Metal Forming: Fundamentals and Appli-

cations, ASM International, 1983.

[FIA 1997]: Forging Industry Association,

Product Design Guide for Forging, 1997.

[Geleji et al., 1967]: Geleji, A., et al., Fo rge

Equipment, Rolling Mills and Accessories (in

English), Akademiai Kiado, Budapest, 1967,

p 168.

[Lange, 1972]: Lange, K., Ed., Study Book of

Forming Technology, (in German), Vol 1,

Fundamentals, Springer-Verlag, 1972.

CHAPTER 11

Presses and Hammers for

Cold and Hot Forging

Manas Shirgaokar

11.1 Introduction

The continuous development of forging tech-

nology requires a sound and fundamental un-

derstanding of equipment capabilities and char-

acteristics. The equipment, i.e., presses and

hammers used in forging, inﬂuences the forging

process, since it affects the deformation rate and

temperature conditions, and it determines the

rate of production. The requirements of a given

forging process must be compatible with the

load, energy, time, and accuracy characteristics

of a given forging machine.

There are basically three types of presses: hy-

draulic, mechanical, and screw presses. These

machines are used for hot and cold forging, cold

extrusion trimming, and coining.

Developments in the forging industry are

greatly inﬂuenced by the worldwide require-

ments for manufacturing ever-larger and more

complex components for more difﬁcult-to-forge

materials. The present and future needs of the

aerospace industry, the increase in demand for

stationary power systems, jet engines, and air-

crafts components, and the ever-increasing for-

eign technological competition require continu-

ous upgrading of today’s technology. Thus, the

more efﬁcient use of existing forging equipment

and the installation of more sophisticated ma-

chinery have become unavoidable necessities.

Development in all areas of forging has the ob-

jectives of (a) increasing the production rate, (b)

improving forging tolerances, (c) reducing costs

by minimizing scrap losses, by reducing pre-

forming steps, and by increasing tool life, and

(d) expanding capacity to forge larger and more

intricate parts.

The purchase of new forging equipment re-

quires a thorough understanding of the effect of

equipment characteristics on the forging opera-

tions, load and energy requirements of the spe-

ciﬁc forging operation, and the capabilities and

characteristics of the speciﬁc forging machine to

be used for that operation. Increased knowledge

on forging equipment would also speciﬁcally

contribute to:

●

More efﬁcient and economical use of exist-

ing equipment

●

More exact deﬁnition of the existing maxi-

mum plant capacity

●

Better communication between the equip-

ment user and the equipment builder

●

Development of more reﬁned processes such

as precision forging of gears and of turbine

and compressor blades

11.2 Hydraulic Presses

The operation of hydraulic presses is rela-

tively simple and is based on the motion of a

hydraulic piston guided in a cylinder [Geleji

1967], [Mueller 1969], [Peters 1969]. Hydraulic

presses are essentially load-restricted machines;

i.e., their capability for carrying out a forming

operation is limited mainly by the maximum

available load. The following important features

are offered by hydraulic presses:

●

In direct-driven hydraulic presses, the max-

imum press load is available at any point

Cold and Hot Forging Fundamentals and Applications

Taylan Altan, Gracious Ngaile, Gangshu Shen, editors, p115-139

DOI:10.1361/chff2005p115

www.asminternational.org

116 / Cold and Hot Forging: Fundamentals and Applications

Fig. 11.1

Schematic illustration of drives for hydraulic presses. (a) Accumulator drive. (b) Direct drive. [Riemenschneider et al.,

1959]

during the entire ram stroke. In accumulator-

driven presses, the available load decreases

slightly depending on the length of the stroke

and the load-displacement characteristics of

the forming process.

●

Since the maximum load is available during

the entire stroke, relatively large energies are

available for deformation. This is why the

hydraulic press is ideally suited for extru-

sion-type forming operations requiring a

nearly constant load over a long stroke.

●

Within the capacity of a hydraulic press, the

maximum load can be limited to protect the

tooling. It is not possible to exceed the set

load, because a pressure-release valve limits

the ﬂuid pressure acting on the ram.

●

Within the limits of the machine, the ram

speed can be varied continuously at will dur-

ing an entire stroke cycle. Adequate control

systems can regulate the ram speed with re-

spect to forming pressure or product tem-

perature. This control feature can offer a con-

siderable advantage in optimizing forming

processes.

11.2.1 Drive Systems for

Hydraulic Presses

The operational characteristics of a hydraulic

press are essentially determined by the type and

design of its hydraulic drive system. As shown

in Fig. 11.1, two types of hydraulic drive sys-

tems give different time-dependent characteris-

tic data [Hutson 1968] [Riemenschneider et al.,

1959].

Direct-driven presses usually employ hy-

draulic oil or water emulsion as the working me-

dium. In earlier vertical press designs, at the start

of the downstroke the upper ram falls under

gravity and oil is drawn from the reservoir into

the ram cylinder through the suction of this fall.

When the ram contacts the workpiece, the valve

between the ram cylinder and the reservoir is

closed and the pump builds up pressure in the

ram cylinder. This mode of operation results in

relatively long dwell times prior to the start of

deformation. As illustrated in Fig. 11.1(b), dur-

ing the downstroke in modern direct-driven

presses a residual pressure is maintained in the

return cylinders or in the return line by means

of a pressure relief valve. Thus, the upper ram

is forced down against pressure and the dwell

inherent in the free fall is eliminated. When the

pressure stroke is completed, i.e., when the up-

per ram reaches a predetermined position, or

when the pressure reaches a certain value, the

oil pressure on the ram cylinder is released and

diverted to lift the ram.

Accumulator-driven presses usually em-

ploy a water-oil emulsion as the working me-

Presses and Hammers for Cold and Hot Forging / 117

Fig. 11.2

Schematic illustration of two types of hydraulic press drives. (a) Push-down drive: 1, stationary cylinder cross head; 2,

moving piston-ram assembly; 3, stationary press bed with return cylinders. (b) Pull-down drive: 1, movable cylinder-

frame assembly; 2, press bed with return cylinders; 3, moving cross-head. [Kirschbaum, 1968]

dium and use nitrogen, steam, or air-loaded ac-

cumulators to keep the medium under pressure

(Fig. 11.1a). The sequence of operations is es-

sentially similar to that for the direct-driven

press except that the pressure is built up by

means of the pressurized water-oil emulsion in

the accumulators. Consequently, the rate of pen-

etration, i.e., the ram speed under load, is not

directly dependent on the pump characteristics

and can vary depending on the pressure in the

accumulator, the compressibility of the pressure

medium, and the resistance of the workpiece to

deformation. Toward the end of the forming

stroke, as deformation progresses, the working

medium expands, the force required to form the

material increases, and the speed of penetration

and the load available at the ram decrease.

In both direct and accumulator drives, as the

pressure builds up and the working medium is

compressed, a certain slowdown in penetration

rate occurs. This slowdown is larger in direct oil-

driven presses, mainly because oil is more com-

pressible than a water emulsion.

The approach and initial deformation speeds

are higher in accumulator-driven presses. This

improves the hot forming conditions by reduc-

ing the contact times, but wear in hydraulic ele-

ments of the system also increases. Sealing

problems are somewhat less severe in direct-oil

drives, and control and accuracy are in general

about the same for both types of drives.

From a practical point of view, in a new in-

stallation, the choice between direct or accu-

mulator drive is decided by the economics of

operation. Usually, the accumulator drive is

more economical if several presses can use one

accumulator system, or if very large press ca-

pacities (10,000 to 50,000 tons) are considered.

The frame of a hydraulic press must carry the

full forming load exerted by the hydraulic cyl-

inder on the press bed. The load-carrying capa-

bility of the frame is achieved by using various

designs such as cast (or welded) structures pre-

stressed by forged tie rods or laminated plates

assembled through large transverse pins.

As can be seen in Fig. 11.2, the two principal

types of press construction are designated as

“pull-down” and “push-down” designs [Kirsch-

baum, 1968]. The conventional push-down de-

sign is often selected for four-column presses of

118 / Cold and Hot Forging: Fundamentals and Applications

Fig. 11.3

Load and displacement versus time curves obtained on a 2500 ton hydraulic press in upsetting with direct drive. 1, start

of deformation; 2, initial dwell; 3, end of deformations; 4, dwell before pressure release; 5, ram lift. [Altan et al., 1973)

all sizes. The cylinder cross head and base platen

are rigidly connected by four columns that take

up the press load and simultaneously guide the

moving piston-ram assembly. Considerable elas-

tic deﬂections are exhibited under off-center

loading. This type of press requires a relatively

tall shop building. In the pull-down design, the

base platen rests on a foundation. The cylinder

cross head, located below ﬂoor level, is rigidly

connected to the press columns. This assembly

is movable and is guided in the bed platen. The

center of gravity of the press is low, at approx-

imately ﬂoor level, and the overall static and dy-

namic stiffness of the press is increased accord-

ingly. Pull-down presses are particularly suitable

for installation in low buildings. Most of the hy-

draulic and auxiliary equipment may then be ac-

commodated beneath ﬂoor level. This arrange-

ment is particularly favorable for direct-oil

drives since it minimizes ﬁre hazard and reduces

the length of piping between the pumping sys-

tem and the press cylinder.

11.2.2 Characteristics of

Hydraulic Presses

In direct-driven hydraulic presses, the maxi-

mum press load is established by the pressure

capability of the pumping system and is avail-

able throughout the entire press stroke. Thus,

hydraulic presses are ideally suited for extru-

sion-type operations requiring very large

amounts of energy. With adequate dimensioning

of the pressure system, an accumulator-driven

press exhibits only a slight reduction in available

press load as the forming operation proceeds.

In comparison with direct drive, the accu-

mulator drive usually offers higher approach and

penetration speeds and a short dwell time prior

to forging. However, the dwell at the end of pro-

cessing and prior to unloading is larger in ac-

cumulator drives. This is illustrated in Fig. 11.3

and 11.4, where the load and displacement var-

iations are given for a forming process using a

2500 ton hydraulic press equipped with either

accumulator or direct-drive systems.

Parallelism of the Slide. The capacity of the

press frame to absorb eccentric loads plays a

major role in forming a part with good dimen-

sional accuracy. Eccentric forces occur during

the forming process when the load of the re-

sulting die force is not exerted centrally on the

slide, causing it to tilt (Fig. 11.5). The standard

press is able to absorb a maximum slide tilt of

0.8 mm/m. If a higher offcenter loading capa-

Presses and Hammers for Cold and Hot Forging / 119

Fig. 11.5 Control system for maintaining slide parallelism. [Schuler Handbook, 1998]

Fig. 11.4

Load and displacement versus time curves obtained on a 2500 ton hydraulic press in upsetting with accumulator drive.

1, start forming; 2, initial dwell; 3, end of forming; 4, dwell before pressure release; 5, ram lift. [Altan et al., 1973]

bility 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.

Often it is necessary to use hydraulic paral-

lelism control systems, using electronic control

technology (Fig. 11.5), for example, in the case

of hydraulic transfer presses. The parallelism