May 17, 2022
The following are general safety precautions when balancing rolls:
Hard bearing type balancers measure residual unbalance. This is the force of a measured quantity in a known direction attributed to uneven centrifugal force caused by unbalance. Vibration sensing devices on the balance machine measure and plot the force while the roll is rotating.
The general method used to balance a roll on a hard bearing balancer is as follows:
Drive belt pulley and support roller diameters can affect unbalance readings if sized incorrectly for the roll diameter being balanced. It is recommended that the frequency between the roll being balanced and the rotating components of the machine be greater than 60 RPM.
As the roll is rotated, a dot of light appears on the vectormeter showing the magnitude of the unbalance by the number of units it is away from the center and the phase by the angle from the phase mark.
Instrumentation and software can be supplied with the balance machines to calculate the amount and location of counterweight needed to reduce the amount of residual unbalance to acceptable limits. Once the amount and location of counterbalance is determined, weight is removed or installed to reduce the unbalance to specified amounts and a final check is made.
All rolls built and balanced by Valmet after 1986 are marked with a serial number, phase angle reference mark and rotation direction stamp. Any subsequent balancing should use the same phase angle and rotation. The importance of phase angle and rotation direction when check balancing rolls is to help determine what changes may have caused unbalance. The original documentation can be compared with current data.
Rolls with tapered bearing fits cannot be supported on the tapers with balance machine rollers. The straight fit above the taper can be used to balance the roll on if it is concentric with the bearing fit. Effort should be made to maintain the journal fits used for balancing rolls with tapered bearing fits. Journal surfaces which have been metal sprayed or sleeved should not be used on hard rollers.
Soft bearing type balancers use a stroboscope and vibration pickups to measure vibration amplitude and direction. The amount of unbalance is measured by movement on ladder bearings, usually in .001 inch. Old style machines measure dynamic runout rather than residual unbalance.
The end result is that trial weights are placed in the correct amounts and locations to reduce the amount of movement of the roll on the ladder bearings to acceptable dynamic runout levels, referred to as free bearing amplitude. Permanent weights are made and installed on or in the roll and a final vibration level check is made.
Rolls should be balanced whenever possible on journals to avoid variables introduced by bearing runout.
Many IRD-style balance machines are not equipped to run rolls on rollers and anti-friction bearings are used. Some rolls are designed in such a way that they cannot be supported on rollers.
For assistance with roll balancing, contact your Valmet representative.