Ensure that the operations staff and assisting personnel are fully conversant with the plan for access and trial runs on the machinery. This can be achieved via a toolbox talk which is mandatory in most client sites.
Familiarise yourself with the location and operation of the emergency stop mechanism.
Prior to running the machine or taking measurements the engineer should ensure all machine guards are installed. If measurements must be taken with bare shaft exposed then all cables should be firmly secured using cable ties or similar to ensure that they cannot move when the machine is started. The cables should be long enough to allow the engineer to retire to a safe distance from the machine while it is started.
When the machine is stopped for fitting of balance weights then the engineer must ensure that all appropriate permitery is in place and the machine is electrically isolated (and mechanically isolated where appropriate) before any guards are removed or access to the rotor is gained.
Ensure all trial weights are firmly attached. If possible, positive centrifugal restraint should be used via existing bolts or overturned section of shrouding.
PREPARATION FOR IN-SITU MACHINE TRIM BALANCING
Before attempting to balance, analyse the machine response to be sure the problem is due to imbalance.
Ensure that the vibration characteristics throughout the machine are consistent with imbalance and the vibration levels are stable. Analyse the response of the complete machine and be sure that the component to be balanced is the unbalanced component within the machine. Do not automatically assume that all vibration problems can be solved by balancing.
Inspect the machine for obvious signs of damage such as cracks in the rotor or shaft. Make sure all mounting bolts are tight. Check the rotor for build up of dirt or other deposits. Check vertical and horizontal response amplitudes and ensure ratios are consistent for the type and installation of machine being considered. Consider the effect on other machine components, which might result from the balancing work which you are about to perform.
From your analysis data note which of the radial readings exhibits the higher 1 x rpm component, horizontal or vertical, and attach your vibration transducer in the direction of the higher measured amplitude.
Before recording your unbalance readings make sure the machine has had time to stabilise.
When the machine is rundown for the addition of the first trial weight observe the vibration response as the machine coasts down. If the machine exhibits a shaft critical speed, then this technique should not be used for the balancing of this rotor.
When the machine is stopped for adding trial weights, observe and record the presence of any background vibration from nearby machines. If these levels are high at the rotational speed of the machine you wish to balance, then this technique should not be adopted.
TRIAL WEIGHT SELECTION
Take care in selecting the size of a trial weight. If the trial weight is too small, no change in amplitude will be noted, and a balance run will have been wasted. On the other hand, a trial weight which is too large may damage the machine.
There is no substitute for experience in the selection of such weights and many factors require to be taken into account. However a common approach for selecting a trial weight is to use a weight which will produce an unbalance force at the support bearing equal to 10% of the rotor weight supported by the bearing.
The size of the trial weight can be calculated via the following abbreviated formula:
m = (44,700,000 x M) / ( r x N2 )
m = mass of trial weight (g)
M = mass of rotor (kg)
N = speed of rotor in (rpm)
r = radius of balance weight position (mm)
Hence for the calculation of a suitable trial weight for a two bearing machine running at 3600 rpm with a centrally mounted rotor weighing of 2039 kg, we would choose a trial weight which would produce a force equivalent to 10% of the rotor mass supported by each bearing.
Having inspected the rotor and found a suitable balance plane the engineer will measure the radius at which the weight will be placed. Let us assume we are balancing on a coupling hub with balance plane radius of 200 mm.
A suitable trial weight would therefore be
m = (44,700,000 x 2039) / (200 x 3600)2
m = 35 grams