An important requirement of all rotating machinery parts is that the rotation axis coincide with one of the principal axis of inertia of the body. After a roll is manufactured, it must be balanced to satisfy this requirement, especially for high speed machines. The condition of unbalance of a rotating body may be classified as static or dynamic unbalance.
An idler roll is statically balanced if the roll doesn't rotate to a "heavy side" when free to turn on its bearings. Thus, the roll's center of gravity is on the axis of rotation. Static unbalance creates a centrifugal force when rotating which causes deflection of the roll. Idler rolls are sometimes static balanced only when the roll operates at slow speeds.
Static (or force) Unbalance
An idler roll may be in perfect static balance and not be in a balanced state when rotating at high speeds. A dynamic unbalance is a “couple” or twisting force in two separate planes, 180 degrees opposite each other. Because these forces are in separate planes, they cause a rocking motion from end to end. A roll that is unbalanced will cause 1) machinery vibrations, 2) web flutter, and 3) a decrease in bearing life due to unnecessary forces.
To dynamically balance a roll, it must first be statically balanced. Then, the roll must be rotated to its operating speed and have the dynamic unbalance eliminated by adding or subtracting weight. The determination of the magnitude and angular position of the unbalance is the task of the balancing machine and its operator. Balancing machines are provided with elastically supported bearings in which the idler roll may spin. Because of the unbalance, the bearings will oscillate laterally, and the amplitude and phase of the roll are indicated by electrical pickups and a strobe light.
Dynamic (or couple) Unbalance
The allowable unbalance of a roll is routinely expressed in ounce-inch units. For example, if a roll is to be balanced to 3 oz-in, this is 1 ounce unbalance on a 3” radius, or 3 ounces on a 1” radius. The amount of unbalance allowed depends on the quality grade specified, the roll weight, and the balancing speed. Epoch recommends ISO grade G6.3 for most idler rolls. For even less unbalance, specify an ISO grade G2.5 for an additional fee of $50.00 per roll.
Occasionally in a fast turning roll with a high length / diameter ratio, there occurs a phenomenon called “whip” in which the center of the roll deflects significantly in the direction of the unbalance of the roll center. This deflection increases the unbalance of the roll the balancing machine is reading. To correct this unbalance, weights must be placed in the center of the roll where the “whip” is occurring. This is a very laborious task and increases the price of balancing the roll. For additional information on balancing, contact one of Epoch’s engineers.
Mar 14, 2018
Aug 10, 2016
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