LINK Systems System 1100 Tonnage Monitor - Manual (Page 45)

THEORY OF OPERATION
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3-8
Although stress and strain at a given location on a machine load
bearing structural member is proportional to the load force
transmitted to that location, certain conditions of eccentric
loading can induce signals that are not equal to the actual force
exerted by tooling.
Tonnage monitors on Gap, OBI, and other "C" frame power presses and
other machines calibrated with a load centered under pitmans (or
cylinder rod connections on hydraulic machines), will indicate a
tonnage greater than that actually exerted by dies when tooling is
moved forward of center under the ram or slide. When the same
force is applied forward of slide center, it acts through a longer
lever arm on the "C" frame, stretching (straining) the front of
the frame and compressing the rear of the frame more than when
centrally located. The larger strains act on the tonnage monitor
strain links to indicate tonnages that are larger than actually
exerted by tooling. It should be noted that the forward located
tooling produces the same stress and fatigue of the frame as a
centrally located tonnage seen by the frame. In addition, the
forward load tries to cock the slide and introduces lateral forces
in gibs in ways that accelerate wear.
On straight side presses and other machines of similar design,
eccentric loading that is not supported by connections to the ram
or slide can cock the slide or ram and introduce bending moments
into uprights through gibbing that causes strain links to be
stretched more than by vertical forces alone. Again, however, the
effect on the machine is equivalent to a central load of the
tonnage indicated by the tonnage monitor.
As long as bearing and gib clearances are within recommended
tolerances, the tooling forces will be accurately indicated by a
tonnage monitor on two point machines, if the load is applied
beneath the connections or near the straight line between
connections. On a four point machine, accurate tooling forces are
indicated as long as loads are applied in the rectangular area
within the four connections to the slide or ram.
3.5 Automatic Zeroing
Temperature changes cause expansion or contraction of machine
structural members, introducing strains in typical steel or cast
iron machine frames of 6 to 7 microinches/inch per degree
Fahrenheit. The strains induced by temperature must be