Rockwell Automation Publication 1444-UM001D-EN-P - June 2018 159
Measurement Definition Chapter 4
Before we can configure the module for thrust measurement we must
understand the relationship between the position of the rotor, the thrust
bearings, and probe locations. The thrust bearing consists of two sets of thrust
pads, the active pads and the inactive pads. The rotor thrust collar runs against
the active pads during normal operation.
There is clearance between the two sets of pads. You must know this clearance
amount to configure the thrust setup. Under ambient conditions, with the
machine stopped, this clearance or “float” is referred to as the “Cold Float” and
can be anywhere between 0.15...0.5 mm (6mils to 20mils). When the machine
is running at normal temperature and normal load, this clearance or float
increases by potentially up to 50% and is referred to as the “Hot Float”.
Normal practice is to establish the Cold Float by “bumping” the rotor shaft
between the inactive and active pads and then measuring this distance. Use
bumping to push the rotor (it requires the coupling spacer to be removed and
can require jacking equipment) first against one set of pads and then against
the other. This process establishes the Cold Float. The Hot Float can only be
determined by reference to the machine OEM.
The two most common applications where thrust monitoring is encountered
are steam turbines and compressors.
IMPORTANT If you are updating from a Firmware Revision 1 system to Revision 2 system,
refer to Updating a System from Revision 1 AOP and Firmware to Revision 2
on page 163.
IMPORTANT The following information of setting normal thrust measurements on steam
turbines and compressors, monitoring thrust position with an HMI, and
setting alarm setpoints, assumes the use of standard negative-bias eddy-
current probes.
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