ASM Metals HandBook Vol. 14 - Forming and Forging

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Fig. 12 Two common types of screw press drives. (a) Friction drive. (b) Direct electric drive

Advantages and Limitations

Screw presses are used for open- and closed-die forging. They usually have more energy available per stroke than

mechanical presses with similar tonnage ratings, permitting them to accomplish more work per stroke. When the energy

has been dissipated, the ram comes to a halt, even though the dies may not have closed. Stopping the ram permits multiple

blows to be made to the workpiece in the same die impression. Die height adjustment is not critical, and the press cannot

jam. Die stresses and the effects of temperature and height of the workpiece are minimized; this results in good die life.

Impact speed is much greater than with mechanical presses. Most screw presses, however, permit full-force operation

only near the center of the bed and ram bolsters.

Drive Systems

In the friction drive press (Fig. 12a), two large energy-storing driving disks are mounted on a horizontal shaft and

rotated continuously by an electric motor. For a downstroke, one of the driving disks is pressed against the flywheel by a

servomotor. The flywheel, which is connected to the screw either positively or by a friction-slip clutch, is accelerated by

this driving disk through friction. The flywheel energy and the ram speed continue to increase until the ram hits the

workpiece. Thus, the load necessary for forming is built up and transmitted through the slide, the screw, and the bed to the

press frame. The flywheel, the screw, and the slide stop when the entire energy in the flywheel is used in deforming the

workpiece and elastically deflecting the press. At this moment, the servomotor activates the horizontal shaft and presses

the upstroke-driving disk wheel against the flywheel. Thus, the flywheel and the screw are accelerated in the reverse

direction, and the slide is lifted to its top position.

In the direct-electric-drive press (Fig. 12b), a reversible electric motor is built directly on the screw and on the

frame, above the flywheel. The screw is threaded into the ram or slide and does not move vertically. To reverse the

direction of flywheel rotation, the electric motor is reversed after each downstroke and upstroke.

Other Drive Systems. In addition to direct friction and electric drives, several other types of mechanical, electric, and

hydraulic drives are commonly used in screw presses. A relatively new screw press drive is shown in Fig. 13. A flywheel

(1) supported on the press frame is driven by one or more electric motors and rotates at a constant speed. When the stroke

is initiated, a hydraulically-operated clutch (2) engages the rotating flywheel against the stationary screw (3). This feature

is similar to that used to initiate the stroke of an eccentric mechanical forging press. Upon engagement of the clutch, the

screw is accelerated rapidly and reaches the speed of the fly-wheel. As a result, the ram (4), which acts as a large nut,

moves downward. The downstroke charges a hydropneumatic lift cylinder system. The downstroke is terminated by

controlling the ram position through the use of a position switch or by controlling the maximum load on the ram by

disengaging the clutch and the flywheel from the screw when the preset forming load is reached. The ram is then lifted by

the lift-up cylinders (5), releasing the elastic energy stored in the press frame, the screw, and the lift-up cylinders. At the

end of the upstroke, the ram is stopped and held in position by a hydraulic brake.

Fig. 13 Screw press drive combining the characteristics of mechanical and screw presses. 1, flywheel; 2, air-

operated clutch; 3, screw; 4, ram; 5, lift-up cylinders

This press provides several distinct advantages:

• A high and nearly constant ram speed throughout the stroke

• Full press load at any position of the stroke

• High deformation energy

• Overload protection

• Short contact time between the workpiece and the tools

Limitations of this type of drive system include:

• Only two levels of energy are available, high and low

• Maintenance is increased on the clutch and hydraulic cylinders

• Force is controlled through slippage of the clutch, which can lead to unpredictable application of power

• The large amount of energy available can create material flow problems

Capacities and Speed

Screw presses are generally rated by the diameter of the screw. This diameter, however, is comparable to a listing to

nominal forces that can be produced by the press. The nominal force is the force that the press is capable of delivering to

deform the workpiece while maintaining maximum energy. The coining, or working, force is approximately double the

nominal force when forging occurs near the bottom of the stroke.

Friction screw presses have screw diameters ranging from 100 to 635 mm (4 to 25 in.). These sizes translate to nominal

forces of 1.4 to 35.6 MN (160 to 4000 tonf). Direct-electric-drive screw presses have been built with 600 mm (24 in.)

diam screws, or 37.3 MN (4190 tonf) of nominal force capacity.

Hydraulically driven screw presses with hard-on-hand blow capacities up to 310 MN (35,000 tonf) have been built.

Press speed, in terms of the number of strokes per minute, depends largely on the energy required by the specific forming

process and on the capacity of the drive mechanism to accelerate the screw and the flywheel. In general, however, the

production rate of a screw press is lower than that of a mechanical press, especially in automated high-volume operations.

Small screw presses operate at speeds of up to 40 to 50 strokes per minute, while larger presses operate at about 12 to 16

strokes per minute.

Hammers and Presses for Forging

Revised by Taylan Altan, The Ohio State University

Multiple-Ram Presses

Hollow, flashless forgings that are suitable for use in the manufacture of valve bodies, hydraulic cylinders, seamless

tubes, and a variety of pressure vessels can be produced in a hydraulic press with multiple rams. The rams converge on

the workpiece in vertical and horizontal planes, alternately or in combination, and fill the die by displacement of metal

outward from a central cavity developed by one or more of the punches. Figure 14 illustrates the multiple-ram principle,

with central displacement of metal proceeding from the vertical and horizontal planes.

Fig. 14 Examples of multiple-ram forgings. Displac

ement of metal can take place from vertical, horizontal, and

combined vertical and horizontal planes. Dimensions given in inches

Piercing holes in a forging at an angle to the normal direction of forging force can result in considerable material savings,

as well as savings in the machining time required to generate such holes.

In addition to having the forging versatility provided by multiple rams, these presses can be used for forward or reverse

extrusion. Elimination of flash at the parting line is a major factor in decreasing stress-corrosion cracking in forging alloys

susceptible to this type of failure, and the multidirectional hot working that is characteristic of processing in these presses

decreases the adverse directional effects on mechanical properties.

Hammers and Presses for Forging

Revised by Taylan Altan, The Ohio State University

Safety

A primary consideration in forging is the safety of the operator. Therefore, each operator must be properly trained before

being allowed to operate any forging equipment. Protective equipment must be distributed and used by the operator to

protect against injuries to the head, eyes, ears, feet, and body. This equipment is described in ANSI standard B24.1.

The forging machines should be equipped with the necessary controls to prevent accidental operation. This can be

achieved through dual pushbutton controls and/or point-of-operation devices. Guards should be installed on all exterior

moving parts to prevent accidental insertion of the hands or other extremities. Guards should also be installed to protect

against flying scale or falling objects during the forging operation.

All forging equipment must be properly maintained according to manufacturer's recommendations. During machine repair

or die changing, the power to the machine should be locked out to prevent accidental operation; the ram should be

blocked with blocks, wedges, or tubing capable of supporting the load. The strength and dimensions of the blocking

material are given in ANSI B24.1. More information on safety is available in the publications cited in the Selected

References at the end of this article.

Hammers and Presses for Forging

Revised by Taylan Altan, The Ohio State University

Selected References

Forging Equipment

•

T. Altan, "Characteristics and Applications of Various Types of Forging Equipment," SME

Technical Paper MFR72-02, Society of Manufacturing Engineers, 1972

•

T. Altan et al., Ed., Forging Equipment, Materials, and Practices, Battelle-

Columbus Laboratories,

Metalworking Division, 1973

•

T.G. Byrer, Ed., Forging Handbook,

Forging Industry Association and American Society for

Metals, 1984

•

K. Lange, Ed., Machine Tools for Metal Forming, and Forging, in Handbook of Metal Forming,

McGraw-Hill, 1985

•

A.M. Sabroff, F.W. Boulger, and H.J. Henning, Forging Materials and Practices, Reinhold, 1968

•

C. Wick, J.T. Benedict, and R.F. Veilleux, Ed., Hot Forging, in

Tool and Manufacturing Engineers'

Handbook, Vol 2, 4th ed., Forming, Society of Manufacturing Engineers, 1984

Safety

•

C.R. Anderson, OSHA and Accident Control Through Training, Industrial Press, 1975

•

"Concepts and Techniques of Machine Safeg

uarding," OSHA 3067, Occupational Safety and

Health Administration, 1981

•

Guidelines to Safety and Health in the Metal Forming Plant,

American Metal Stamping

Association, 1982

•

Power Press Safety Manual, 3rd ed., National Safety Council, 1979

•

C. Wick, J.T. Benedict, and R.F. Veilleux, Ed., Safety in Forming, in

Tool and Manufacturing

Engineers' Handbook, Vol 2, 4th ed., Society of Manufacturing Engineers, 1984

Selection of Forging Equipment

Taylan Altan, The Ohio State University

Introduction

FORGING EQUIPMENT influences the forging process because it affects deformation rate, forging temperature, and rate

of production. The forging engineer must have sound knowledge of the different forging machines in order to:

• Use existing machinery more efficiently

• Define the existing plant capacity accurately

• Communicate better with, and at times request improved performance from, the machine builder.

• Develop, if necessary, in-

house proprietary machines and processes not available in the machine tool

market

• Utilize them in the most cost-effective manner

This article will detail the significant factors in the selection of forging equipment for a particular process. The article

"Hammers and Presses for Forging" in this Volume contains information on the principles of operation and the capacities

of various types of forging machines.

Selection of Forging Equipment

Taylan Altan, The Ohio State University

Process Requirements and Forging Machines

Figure 1 illustrates the interaction between the principal machine and process variable for hot forging conducted in

presses. As shown at the left in Fig. 1, flow stress σ, interface friction conditions, and part geometry (dimensions and

shape) determine the load L

at each position of the stroke and the energy E

required by the forming process. The flow

stress

increases with increasing deformation rate

•

and with decreasing work metal temperature, θ. The magnitudes of

these variations depend on the specific work material (see the Sections on forging of specific metals and alloys in this

Volume). The frictional conditions deteriorate with increasing die chilling.

Fig. 1 Relationships between process and machine variables in hot-forging processes conducted in presses

As indicated by the lines connected to the "Work metal temperature" block in Fig. 1, for a given initial stock temperature,

the temperature variations in the part are largely influenced by the surface area of contact between the dies and the part,

the part thickness or volume, the die temperature, the amount of heat generated by deformation and friction, and the

contact time under pressure t

The velocity of the slide under pressure V

determines mainly t

and the deformation rate . The number of strokes per

minute under no-load conditions n

, the machine energy E

, and the deformation energy E

required by the process

influence the slide velocity under load V

and the number of strokes under load n

; n

determines the maximum number of

parts formed per minute (the production rate) if the feed and unloading of the machine can be carried out at that speed.

The relationships illustrated in Fig. 1 apply directly to hot forging in hydraulic, mechanical, and screw presses.

For a given material, a specific forging operation, such as closed-die forging with flash, forward or backward extrusion,

upset forging, or bending, requires a certain variation of the load over the slide displacement (or stroke). This is illustrated

qualitatively in Fig. 2, which shows load versus displacement curves characteristic of various forming operations. For a

given part geometry, the absolute load values will vary with the flow stress of the material and with frictional conditions.

In forming, the equipment must supply the maximum load as well as the energy required by the process.

Fig. 2 Load versus

displacement curves for various forming operations. Energy developed in the process = load

× displacement × m, where m is a factor characteristic of the specific forming operation. (a) Closed-

die forging

with flash. (b) Upset forging without flash. (c) For

ward and backward extrusion. (d) Bending. (e) Blanking. (f)

Coining. Source: Ref 1, 2

References cited in this section

J. Foucher, "Influence of Dynamic Forces Upon Open Back Presses," Doctoral dissertation, Technical

University, 1959 (in German)

T. Altan, Important Factors in Selection and Use of Equipment for Metal-working, in

Proceedings of the

Second Inter-American Conference on Materials Technology (Mexico City), Aug 1970

Selection of Forging Equipment

Taylan Altan, The Ohio State University

Classification and Characterization of Forging Machines

Forging machines can be classified into three types:

• Force-restricted machines (hydraulic presses)

• Stroke-restricted machines (mechanical presses)

• Energy-restricted machines (hammers and screw presses)

The significant characteristics of these machines constitute all machine design and performance data that are pertinent to

the economical use of the machine, including characteristics of load and energy, time-related characteristics, and

characteristics of accuracy. More information on these machines is available in the article "Hammers and Presses for

Forging" in this Volume.

Hydraulic Presses

The operation of hydraulic presses is relatively simple and is based on the motion of a hydraulic piston guided in a

cylinder. Hydraulic presses are essentially force-restricted machines; that is, their capability for carrying out a forming

operation is limited mainly by the maximum available force.

The operational characteristics of a hydraulic press are essentially determined by the type and design of its hydraulic drive

system. The two types of hydraulic drive systems--direct drive and accumulator drive (see Fig. 11 in the article "Hammers

and Presses for Forging" in this Volume)--provide different time-dependent characteristic data.

In both direct and accumulator drives, a slowdown in penetration rate occurs as the pressure builds and the working

medium is compressed. This slowdown is larger in direct oil-driven presses, mainly because oil is more compressible than

a water emulsion.

Approach and initial deformation speeds are higher in accumulator-drive presses. This improves hot-forging conditions

by reducing die contact times, but wear in the hydraulic elements of the system also increases. Wear is a function of fluid

cleanliness; no dirt equals no wear. Sealing problems are somewhat less severe in direct drives, and control and accuracy

in manual operation are generally about the same for both types of drives.

From a practical point of view, in a new installation, the choice between direct and accumulator drive is based on the

capital cost and the economics of operation. The accumulator drive is usually more economical if one accumulator system

can be used by several presses or if very large press capacities (89 to 445 MN, or 10,000 to 50,000 tonf) are considered.

In direct-drive hydraulic presses, the maximum press load is established by the pressure capability of the pumping system

and is available throughout the entire press stroke. Therefore, hydraulic presses are ideally suited to extrusion-type

operations requiring very large amounts of energy. With adequate dimensioning of the pressure system, an accumulator-

drive press exhibits only a slight reduction in available press load as the forming operation proceeds.

In comparison with direct drive, the accumulator drive usually offers higher approach and penetration speeds and a

shorter dwell time before forging. However, the dwell at the end of processing and prior to unloading is longer in

accumulator drives. This is shown in Fig. 3, in which the load and displacement variations are given for a forming process

using a 22 MN (2500 tonf) hydraulic press equipped with either direct-(Fig. 3a) or accumulator-drive (Fig. 3b) systems.

Fig. 3

Load versus time and displacement versus time curves obtained on 22 MN (2500 tonf) hydraulic presses

with (a) direct-drive and (b) accumulator-drive systems. 1, start of deformat

ion; 2, initial dwell; 3, end of

deformation; 4, dwell before pressure release; 5, ram lift. Source: Ref 3

Mechanical Presses

The drive system used in most mechanical presses is based on a slider-crank mechanism that translates rotary motion into

reciprocating linear motion. The eccentric shaft is connected, through a clutch and brake system, directly to the flywheel

(see Fig. 7 in the article "Hammers and Presses for Forging" in this Volume). In designs for larger capacities, the flywheel

is located on the pinion shaft, which drives the eccentric shaft.

Kinematics of the Slider-Crank Mechanism. The slider-crank mechanism is illustrated in Fig. 4(a). The following

valid relationships can be derived from the geometry illustrated.

Fig. 4 Load, displacement, velocity, and torque in a simple slider-crank mechanism. (a) Schematic of slider-

crank mechanism. (b) Displacement (solid curve) and velocity (dashed curve). (c) Clutch torque M

and machine

load L

. Source: Ref 3

The distance w of the slide from the lowest possible ram position (bottom dead center, BDC; the highest possible position

is top dead center, TDC) can be expressed in terms of r, l, S, and α, where (from Fig. 4) r is the radius of the crank or one-

half of the total stroke S, l is the length of the pitman arm, and α is the crank angle before bottom dead center.

Because the ratio of r/l is usually small, a close approximation is:

(Eq 1)

Equation 1 gives the location of the slide at a crank angle α before bottom dead center. This curve is plotted in Fig. 4(b)

along with the slide velocity V, which is given by the close approximation:

(Eq 2)

where n is the number of strokes per minute.