6 the machine rating de-rate table screen, Figure 19: example tonnage de-rate curve – LINK Systems 5100-8 Tonnage & Analog Signal Monitor User Manual

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4.6 The Machine Rating De-rate Table Screen

Press the SELECT key to the right of the “Tonnage Monitor Calibration” in the Tonnage Monitor
Configuration screen as shown in Figure 16 to display the screen shown in Figure 20.

A mechanical power press is typically specified by its manufacturer with a tonnage capacity rating and a
height off of the bottom of the stroke at which this rating applies. One reason for this is that the
mechanical advantage created in the translation of the rotary motion of the crankshaft to the linear
motion of the slide changes depending upon crankshaft angle. The constant torque of the clutch
develops more downward force as the crankshaft angle travels from 90 degrees (mid stroke) to 180
degrees (bottom).

If torque were the only limiting factor, the press could deliver infinite tonnage at the bottom of the
stroke. However, the elastic limits of the press frame place an additional limitation on tonnage near the
bottom. Below the point where the machine is rated, a Machine Rating limit of no more than 125% of
rated capacity is placed on each strain gage mounted to the machine frame in order to stop the machine
before permanent damage is done to the structural members (see section 4.4.3 on page 45 for how this
limit is set).

Above the point where the press is rated, an additional limit can be set so that the torque available from
the clutch is not exceeded. This torque is delivered to the entire machine frame and is measured by
examining only the total tonnage (combined tonnage on all frame members). Since the limit is placed
on the tonnage (and not directly measuring torque), it must decrease as crankshaft angle moves from 180
to 90 degrees (de-rates the machine rating). Tonnage curves are available from the press manufacturer
that describes the amount of total tonnage that can be developed at different points in the stroke.

If desired, the total tonnage
can be de-rated by
examining the press
manufacturers' tonnage
rating curve for a particular
type machine and entering
this information into the
tonnage monitor. The entry
process requires that the
machine curve be divided
into 16 discrete regions
with a single tonnage limit
that applies for each region.
Height off the bottom of
the stroke must be
converted to crankshaft
angle in order to position
each region. Figure 19
shows an example tonnage
de-rate curve for a 12 inch stroke press with a 30 inch connecting rod. Rated tonnage is specified at 0.25
inches off the bottom. In the example, the limits are placed approximately 5% beyond the rating.

150%

125%

100%

75%

50%

25%

30%

35%

6.0

(95°)

5.0

(105°)

4.0

(115°)

3.0

(125°)

2.0

(135°)

1.0

(149°)

0.5

(158°)

0.0

(180°)

0.25

(164°)

Height Off Bottom in Inches

(Crankshaft Angle in Degrees)

achi

Piecewise Approximation For
Total Machine Rating Alarm

Press Rating As Supplied By Manufacturer