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Phase Angle Does Not Settle When Balancing

 

SYMPTOMS

When I use the instrument in Balance Mode the screen readout for phase angles seems to wander and does not lock in.

CAUSES (match the number of the cause with its solution below)

1. There are multiple items moving on any balance stand, not just the piece being balanced. There is the drive motor, pulleys and the belt just to name a few.

There are various vibrations being picked up by the shop surroundings.

2. The base or mounting of the machine or balance stand is not secure.
3. The piece being balanced is loose. This is a major cause of phase angle movements e.g. a cracked shaft, a loose fan or particulate located inside a roll.
4. The piece being rotated may be hitting the stand trunions causing an irregular bumping.
5. There are bad bearings in the balance stand trunions.
6. The accelerometers are located on a non-stable surface.
7. You are not allowing enough settling time for the rotating system to seek its own stability.

SOLUTIONS

1. Set the instrument to balance using the narrow filter bandwidth. It is not uncommon to read phase angles properly when starting the balance session and then as the unit imbalance lowers the phase readings start to wander. This is because initially the unit being balanced had enough imbalance to control the input to the vb instrument. As the actual imbalance is brought under control, imbalance frequencies of the balance machine and surrounding area equipment start to become a larger part of the total balance session. The narrow filter bandwidth will discard any stray imbalance readings and zero in on the RPM of the instrument only.
2. Secure and/or grout with machinery grout, the base of the balance stand. This machine must be a part of the floor area.
3. Before starting your balance session, hand rotate the unit to look and listen for any loose or broken parts.
4. Ensure that the unit being balanced has enough clearance to rotate without hitting any components of the balance machine.
5. Replace any balance trunion bearings that may be questionable. Given that the shaft rides on the outer surface of these bearings, it is essential that there be no nicks or flat spots on the outer races.
6. Do not place the accelerometer on a component that can have more than one plane of directional movement. The trunions can and do have a forward-and-back motion as well as a slight side-to-side motion. This wobbling will cause erratic readings. Locate the accelerometers on the most stable surface or edge you can find. On some balance stands, that may be located inside the left and right legs of the stand. A good rule is to locate the accelerometers in the same position as the OEM designed them to be.
7. The larger the unit, the more time is required for the entire system to stabilize. As the RPM is increased on any stand, inherent imbalances peak and diminish. These criticals have an oscillation to them that affects the stand when first starting up. Once at running speed, allow the balancing stand to settle down. The phase angles of the piece being balanced will stabilize quickly. Just give it time and do not hurry it up. Usually a maximum of 45 seconds is required on a larger piece to allow for this settling.