Minster IOI, Service Manual

Page: 72 / 92

out by setting the slide down “a little more” because
coining loads may be developed which reach danger-
ous proportions; frame parts could be damaged.
To summarize, remember that it is possible to pressure
overload a press near bottom of stroke because of the
increased mechanical advantage of certain drive parts
at that point. And, it is possible to induce excessive
stress into both frame and drive members by improper
die setting and loading. Avoid situations of this kind by
installing only dies which have capacity requirements
falling within maximum tonnage rating of the press, by
setting shutheight correctly, and by making certain that
the dies are physically large enough to prevent con-
centrated loading.
TONNAGE/STROKE RELATIONSHIP
Mechanical power presses use a crankshaft (or eccen-
tric shaft) to convert rotary motion from the flywheel and
gears to sliding straight line motion of the slide. The
geometry of this combination of parts will result in a
changing mechanical advantage between the drive
and slide. For example, mechanical advantage of
the crank arm and connection assembly will vary
from zero, at a point near midstroke, to infinity at
the bottom of the stroke.
This changing lever concept is illustrated in Figure 1I,
views A, B and C. Note that in View “A”, the crank is
positioned near the midstroke where the mechanical
advantage is about 1 to 1 (or no mechanical advantage
at all). Assuming that a torque input of 600 inch tons is
applied to the shaft, which has a crank throw of 6 inch-
es, a force of 100 tons would be delivered by the slide
at that point.
To illustrate how the tonnage will change with regard to
stroke position, select another point where the effective
moment arm is about half as long as it was at midstroke.
The slide will be nearer bottom of stroke and the effec-
tive moment arm will be one-half of distance “L”, or 3
inches. Substitute the value of 3 inches into the calcu-
lation and note that the force delivered by the slide will
now be 200 tons, an increase of two times the amount
at midstroke. See View “B”.
Finally, it can be shown in view “C” that on dead bottom
of stroke the effective moment arm would be zero, and
zero divided into any number other zero would produce
a quotient of infinity. Therefore, the drive could theoret-
ically deliver infinite tonnage at bottom of stroke. From
a practical standpoint, however, the elasticity of frame
parts would allow them to deflect under excessive
loads. And, if the overload should exceed elastic limits
of the frame components, they may be permanently
damaged or broken. Press users should make every
effort to keep the applied load within the design limits of
the press.
H - 3
Figure 1I. Changing lever concept showing tonnage/stroke relationship.
View “A” View “B” View “C”
Torque = 600
in. tons
Torque = 600
in. tons
Torque = 600
in. tons
Slide
Crankshaft
Connection
Force (tons) =
Torque (inch tons)
Lever Arm (inches)
F =
T
L
F =
600
6
= 100 tons F = 100 tons
L = 6”
F
F =
T
L
F =
600
3
= 200 tons F = 200 tons
F
F =
T
L
F =
600
0
=
∞
F =
∞
F
L
2
= 3”
L = 0”