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

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24.4 Eccentric and crank press drives 301

Figure 24.8

Crank press.

1 flywheel, 2 crankshaft,

3 protective cover, 4 ram,

5 press column, 6 bottom

platen

Figure 24.7

Drive of a gap-frame

eccentric press with transmission

gear. 1 press frame, 2 valve, 3 and

5 clutch and brake, 4 flywheel,

6 transmission gear, 7 electric shaft,

8 eccentric bush for stoke length

adjustment, 9 round-headed bolt to

adjust stroke position, 10 load-

limiting device, 11 ram, 12 cover

(Illustration from Weingarten

works, Weingarten, Germany)

Figure 24.6

Drive of a gap-frame ec-

centric press. 1 press frame, 2 valve,

3 and 5 clutch and brake, 4 flywheel,

6 eccentric shaft, 7 stroke adjustment

(stroke length adjustment), 8 round-

headed bolt to adjust stroke position,

9 load-limiting device, 10 ram,

11 cover to clamp tooling in place

(Illustration from Weingarten works,

Weingarten, Germany)

302 24 Eccentric and crank presses

24.4.1 Clutches

Clutches are very important to keep the press and operator safe. When the press is deactivated,

the clutch must disconnect the connection between the flywheel and the eccentric shaft or

crankshaft. In order for this shaft to stop immediately, the clutch is coupled to a brake.

Clutches are categorised either as (form-

closed, positive) driving clutches or

(force-closed, non-positive) friction

clutches.

1. Driving clutches

These clutches are named after the type

of driver:

– bolt clutch

– rolling key clutch

– tangential key clutch.

The clutch used the most in older ma-

chines was the rolling key clutch. As all

positive clutches run very roughly and

have long operating times, today non-

positive friction clutches are mainly

used.

2. Friction clutches

All clutches of this kind (Figure 24.10)

run quietly and stand out for their short

operating times. The contact pressure

force is supplied either pneumatically or

hydraulically.

These clutches are safe against overload

as they transmit a very precise, adjust-

able limiting moment M.

sin



F ram force, M moment set on the clutch,

r crank radius,

crank angle

Figure 24.9 Rolling key clutch (clutch disen-

gaged), a) pawl, b) threaded bush, c) fastening bolt,

d) supporting lever, e) locking lever, f) 1 latch, g)

feather key, h) locking ring, I) 2 latch, k) tension

spring, 1) nose of rolling key, m) cam, n) release

lever, o) locking bolt, p) shift linkage

Figure 24.10 Single-disc functional clutch with

brake, a) brake open, b) brake closed, c) clutch en-

gaged, d) clutch disengaged

24.4 Eccentric and crank press drives 303

Figure 24.11 shows a modern disc clutch/brake

system.

This drive construction has an integrated safety

lock to halt the press. The two independently-

operating brakes ensure that if one brake fails,

the second dynamically halts the press cycle.

The particular advantages of this kind of con-

struction are:

– lower ram deceleration path

– fast single-stroke operating cycles.

Figure 24.11

Pneumatic single-disc clutch/brake system.

1 flywheel, 2 clutch, 3 brake, 4 pneumatic piston

24.4.2 Load-limiting safety elements on the press frame

Apart from the clutch, which as well as activating and deactivating is mainly intended to pro-

tect the drive elements against overload, other elements are necessary to protect the press

frame against overload. These load-limiting elements must fulfil the following requirements:

– fast response on overload,

– precise load limit setting must be possible,

– load limit must be independent of the frequency of the load (must not be prone to failure

when subjected to continuous load),

– it must be possible to quickly replace the fail-safe element.

Different types of load-limiting device

1. Mechanical safety devices

These elements are built into the ram unit below

the ball socket in the ram (Figure 24.12). For

example:

– shear plates

– steel shear plates

– spring releases.

1.1 Safety device with shear plate

Here, the shearing force at which the safety de-

vice reacts (Figure 24.13) can be calculated from

the shearing area and the shear strength of the

plate material:

Figure 24.2 Shearing member load-limiting device in

press ram, 1 connecting rod, 2 round-headed bolt, 3 ball

socket, 4 shear ring, 5 shearing member made of grey

cast iron, shearing member holder, 7 press ram

304 24 Eccentric and crank presses

Ad s





F in N shearing force

s in mm thickness of the shearing

plate at the shear-off point

in N/mm

shear strength

A in mm

shearing area.

One disadvantage of shear plate safety devices is

their susceptibility to fatigue fracture. An ad-

vantage is that the shear plates can be quickly

replaced.

Figure 24.13 Shear plate safety device,

a) ram, b) ball socket, c) shear

plate

1.2 Spring releases

A pivotable lever (Figure 24.14) presses against a dog

which is mounted on a bolt pre-tensioned by disc springs.

When there is an overload the dog moves out of the hori-

zontal position and the pivoted lever is released.

2. Hydropneumatic and hydraulic safety devices

2.1 Pneumatic safety devices in the ram

In this design (Figure 24.15) a piston is mounted in the

ram, the interior of which is a cylinder; the lower end of

this piston is connected to the die bolster plate. The piston

is actuated with compressed air, so the safety force can be

calculated from the pressure times the area. When there is

an overload the piston is pushed out of the zero position.

In doing so it deactivates the machine by means of an

electronic limit switch.

2.2 Hydraulic safety device in the bottom platen

Here, with a similar principle to the pneumatic safety

device, a hydraulic piston is built into the bottom platen.

Figure 24.14

Release lock.

a) ball socket

b) pivoted lever

c) dog, d) disc springs

Figure 24.15

The principle of a pneumatic press

safety device, a) limit switch, b) air

inlet, c) ram, d) bolster plate

24.4 Eccentric and crank press drives 305

Figure 24.16 Drive shafts a) crankshaft, b) eccen-

tric shaft

24.4.3 Stroke setting

1. Length of the stroke

– Presses with a fixed stroke

For presses with a fixed, unadjustable stroke,

the stroke length results from:

x 

H in mm stroke

x in mm eccentricity between the pin

and the shaft in eccentric

presses; x = r in mm, crank

radius in crank presses

í Presses with an adjustable stroke

Presses with an adjustable stroke have an

adjustable eccentric bush (Figure 24.17) on

the crank web (crank presses) or on the ec-

centric pin (eccentric presses). With this kind

of machine the stroke results from the sum of

the two eccentricities.

max

min

2( )





y in mm eccentricity of the bush bo-

x in mm eccentricity of the eccentric

pin.

Figure 24.17

Layout of eccentric bush and eccentric pin on

presses with adjustable stroke. a) eccentric shaft,

b) eccentric pin, c) eccentric bush, y eccentricity

of the eccentric bush, x eccentricity between the

eccentric pin and the shaft

2. Stroke position

In order to set up the tooling precisely it must be possible to adjust the height of the ram. As

the tools have different overall heights, the stroke position must be adjusted according to the

overall height of the tooling. This adjustment is especially important with embossing work as

the embossing force is strongly influenced by height differences and the machine may be over-

loaded. With smaller machines the stroke position is adjusted using round-headed bolts (Figure

24.12).

With larger machines the ball socket in the ram is adjusted.

306 24 Eccentric and crank presses

24.5 Calculating the parameters

24.5.1 Clutch moment

def M

9554 P



def

(360 )

q q

in deg. crank angle

M in Nm moment

n in min

–1

strokes per minute

P in kW motor power

def

in kW short-term capacity available for forming between

and BDC

in deg. angle at which the energy is released until BDC

– press efficiency.

Because the energy stored in the flywheel is released between

and the BDC (bottom dead

centre), in this range a short-term power of P

def

is available.

24.5.2 Tangential force

– Dynamic tangential force

The dynamic tangential force results from:

def d c

def

PM Tr



 





in N dynamic tangential force

T in N static tangential force

in m effective radius on the clutch.

í Static tangential force

The static tangential force results from:

30P



24.5 Calculating the parameters 307

With a known crank radius, the following then results from the constant moment:

T =

r in m crank radius.

24.5.3 Permissible press force

For the permissible force in a crank press a difference is made between

nominal press force,

permissible press force from the drive moment,

permissible press force from the work capacity.

From the point of view of press force, the press can only be used for a certain forming opera-

tion if the force required for the deformation is smaller than, or at most equal to, the permissi-

ble forces mentioned above.

Nominal press force (Figure 24.18)

When the work capacity is provided, and assuming that the tangential force T on the drive is

always available, it can be said, on condition that r/l is very small, that:

= sin

T in kN tangential force

F in kN press force

in deg. crank angle

r in m crank radius

l in m length of the connecting rod.

M = F · a = F · r · sin

Figure 24.18 Crank press drive layout.

For a normal crank press, a value of 0.5 is fixed for the T/F ratio. This is equal to a crank angle

of 30°. Thus:

sin30

F in kN nominal press force.

In other words, the nominal press force is available from a crank angle of 30° before BDC.

The press force is smaller when

> 30°. It reaches a minimum at

= 90° (sin 90° = 1).

308 24 Eccentric and crank presses

The maximum is at the BDC at

= 0° (sin 0° = 0), i.e. at the BDC the force approaches infin-

ity. The permissible press frame load (press frame strength) is set to the nominal press force.

For this reason the nominal press force must not be exceeded.

Permissible press force from the drive moment

This can be determined from the values which are always known about a press, using the fol-

lowing equation:

nmax

adj

Hhh





in kN permissible press force from the drive moment

in kN nominal press force

max

in mm max. stroke (H =2· r)

adj

in mm set stroke

h in mm ram displacement.

This force F

is the limiting force from

= 30 – 90°.

Permissible press force from the work capacity

This limiting force results from the press work capacity (or its centrifugal masses).

If it is exceeded then the press stops. As the limiting force F

Wct

resulting from the work capac-

ity is always lower than F

, it can not cause the machine to break if exceeded.

Wct

in N permissible press force from the continuous work capacity

in Nm continuous work capacity

h in m ram displacement.

The following table shows once more which forces at which crank angle limit the operation of

a crank press.

Table 24.4 Limits on the press force of a crank press

Crank angle

Force is limited by: Force

0° – 30° before BDC the strength of the press frame F

the drive moment F

30° – 90° before BDC

the work capacity F

Wct

24.5 Calculating the parameters 309

Figure 24.19

Characteristic curves of a crank press

If the three limiting forces are put together in a diagram (Figure 24.19) then it can be seen that

until

= 30° the force of a crank press is limited by F

and when

is higher than 30° by the

drive moment or the work capacity of the press.

The point where all three limiting forces reach a maximum is the peak efficiency point.

The ideal press load would be if the press was run in the peak efficiency range.

However, depending upon the operating process, this is only possible to a limited extent. Apart

from checking the three permissible forces í compared with the maximum force occurring with a

particular operating process í the work capacity of the process and the operating process must

also be checked. This kind of press can only be cleared for use for a certain operation if all forces

and the work capacity are within the permissible range.

24.5.4 Ram displacement

The ram displacement can be determined from the crank radius and the crank angle.

(1 cos )hr



h in m ram displacement

r in m crank radius

in deg. crank angle

H in m press stroke (H = 2r)

For

= 30° h is:

h |

310 24 Eccentric and crank presses

24.5.5 Work capacity

1. Continuous work capacity

This means the work capacity of a press in continuous mode, i.e. if the press runs without

interruption.

The continuous mode requires an automatic material and workpiece feed system.



in Nm work capacity in continuous mode

in Nm work capacity in single-stroke mode

H in m press stroke (H = 2r)

r in m crank radius

in N nominal press force.

2. Work capacity in single-stroke mode

si ct

2WW 

Single-stroke mode is when the press stops after every stroke and needs to be initiated again

(clutch engaged). When workpieces need to be inserted by hand, presses are run in single-

stroke mode. As the flywheel speed, which drops on the working stroke, can be fully recovered

during the insertion period (unlike in automatic mode), the work capacity in single-stroke

mode is around double that of continuous mode.

24.6 Example

The following must be found for an extrusion:

max. press force F = 1200 kN with a ram displacement of h = 20 mm.

A crank press is available with a

nominal press force of F

= 2000 kN and a

stroke of H = H

adj

= 200 mm (not adjustable).

Can the press be used for this operation?

Solution:

1. Determining the extrusion work

= F · h · x = 1200 kN · 20 mm · 1 = 24 000 kNmm

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

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