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4.8 TRIAL RUN
Known unbalance is added on the rotor to compute the relation between measured vibration and rotor unbalance.
The number of trial runs is equal to the number of balancing planes.
4.8.1 Trial run definition
You must indicate the value of the weight and its angular position in each of the balancing planes. Carefully
observe the angle convention (direction of rotation = positive direction of angles). The angle unit defined in the
setup is recalled on the left hand side of the input field. Generally, the first trial run is performed one with the weight
in plane 1, the second one with the weight in plane 2 and so on until all the runs are completed.
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See the rotor from the other side (for 2-plane balancing only)
Trial weight estimation:
This function estimates a trial weight value to avoid adding a too high unbalance which could damage the
machine.
The result is only indicative. It is not required to add a weight exactly identical to the calculated value.
The trial weight must at least change the vibration amplitude by 20% or its phase by 20 degrees. If the
weight is not sufficient, a warning indicates it after the trial run measurement. In this case, it is
recommended to increase the weight value and to repeat the measurement.
Principle:
A usually adopted method involves the calculation of a weight m, which generates a force equivalent to one
tenth of the weight of the rotor to avoid adding too high an unbalance, which could damage the machine.
m
N
x
r
x
M
x
x
x
V
1 1
10
981 3600
4
1
2
2
,
With:
r radius at which the trial weight m is located
M: weight of the rotor in kg
V rotation speed in CPM
N number of planes
m trial weight in kg
If the sensor position has been defined in the setup and if the dephasing due the bearing structure can be
evaluated, this tool also gives an estimation of the angular position in opposition with the unbalance to
avoid increasing the vibration.
Note:
the origin of angular position on the rotor must in front of the triggering position (the reflective
adhesive tape for instance)
for a measurement with accelerometer or velocimeter, there is an additional error for low-speed
machine (< 300 CPM) due to the phase shift between signal input and tachometer input.
Equivalent weight:
Tool to compute the weight equivalent to 3 weights W1, W2 and W3 positioned on the selected plane
Go to the steps browser to reach directly any other accessible step (see § 4.11)
Exit: return to balancing test list (see § 4.3)
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