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Pressductor PillowBlock Load Cells, Horizontal Measuring PFTL 101, User Manual
6 Fault Tracing
6.4
6.4.1
6.4.2
6.4.3
6.4.4
6.4.5
3BSE009965R0401 en Rev E
Fault Tracing in the Mechanical Installation
There are a number of parts in the mechanical arrangement that can cause faults. The extent to
which these faults are repeatable differs. Possible causes fall into the following groups.
•
Defective mounting surface or adapter plates.
•
Force shunting.
•
Insufficient mounting of load cell and adapter plates.
•
Rolls and bearings.
•
Driven roll.
Defective Mounting Surface, Support or Adapter Plates
An unmachined or poorly machined mounting surface, which is uneven, may cause bending or
twisting of the load cell. This may result in instability of the zero point.
Force Shunting
Force shunting means that some of the force is diverted past the load cell. This may be caused by
some kind of obstruction to the force through the load cell. The connecting cables, for example,
have been incorrectly installed and are preventing movement. Another possible cause is that the
roll is not free to move in the direction of measurement, possibly because something is mounted
too close to a bearing housing, or because an object has worked loose and become trapped
between the bearing housing and adjacent parts.
Force shunting causes the web/strip tension indication to be lower than the actual web/strip ten-
sion.
Fastening of Load cell and Adapter Plates
Screw joints that have not been properly tightened or have lost their pre-tightening force, cause
sliding at the mating surfaces. Fastening of the load cell is especially critical. If a load cell is not
properly secured, the zero point will be unstable. Sliding between other surfaces may cause the
same symptoms.
Rolls and Bearings
An incorrectly designed bearing arrangement may give rise to high axial forces. The roll should be
fixed at one end and free at the other.
If both ends are fixed, there will be a high axial (thrust) force due to expansion of the shaft with
rising temperature.
Even a correctly designed bearing arrangement may deteriorate with time; bearings become worn,
and so on. This may give similar symptoms, such as slow zero point drift between cold and hot
machine, or sudden jumps in the signal.
Driven Roll
A source of error that is seldom suspected is the roll itself. The effect is especially critical when
measuring forces on the load cell are relatively low. Long drive shafts with their associated universal
joints may cause unstable signals if they are not properly maintained. It is important to lubricate
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