Accurately balancing wheels is critical; a balanced wheel produces superior roundness or, in the case of surface grinders, superior flatness. Only a small amount of wheel imbalance will damage the form and surface of a ground workpiece.

Traditionally, balancing was time-consuming and imprecise. A grinding wheel may be unbalanced because of the un-evenly distributed grains in the wheel. Additional imbalance and eccentricities may occur in the wheel mount, the arbor, and even coolant absorption in the pores of the wheel.

Simple balancing systems employing computer control minimize machine downtime. With them balancing time is typically cut from an hour or more for large wheels to about a minute or two, and balance improved by about a factor of five.

Schmitt Industries Inc. (Portland, Oreg.) developed its SBS Dynamic Balance System without using fluids or freon gas. This system has been in use successfully for over six years. A shop can install it without assistance. The quality of older grinders may be upgraded by retrofitting such a system.

Thus imbalance is stated in terms of a mass (in grams) times a distance (in centimeters).

To correct imbalances, the system uses two moveable weighted masses which can be positioned independently. The weights, located in the balance head, are driven by electrical motors through a precision gear train. A sensor monitors spindle movement or vibration. It sends a signal to the control unit, which filters the signal to the control unit, which filters the signal by rpm. The control unit then drives the two balance-head masses in the direction that reduces the signal amplitude. When the vibration is lowest, the balance cycle is complete.

Because the system operates by filtering out only the effect of the spindle rpm from vibration, other sources of vibration may not be distinguished from vibrations originating in the rotating wheel. Thus the system needs minimal external vibrations. Note that the balance system cannot compensate for vibrations caused by the grinding spindle itself or by components in the drive train of the spindle.

It will balance to a level of vibration specified by the user. The lower the balance limit is set, the more time the system takes to achieve balance. No system can balance the wheel to a value below the ambient or background level.

Navistar International Transportation (Melrose Park, Ill.) currently employs SBS balancers on Landis 5R crankshaft grinders for crank pin grinding. Manufacturing engineer Pat Tagney says the units were easy to install, and unlike water-based balancers, are easy to maintain. He reports that the units "balance a large wheel in two minutes and the wheels stay balanced when the machine is shut down. With the water-based balancers, the water runs out when you shut the machine down; so the wheel must be rebalanced at start-up." Navistar is putting the balancers on a line of new grinders. In short, he says, "they do what the company says they do, and that's a pleasure."