Tuning a bowl to its proper frequency involves the proper proportions between the mass and the springs. The natural frequency of the bowl should be as close as possible to the 60 cycle (rectified) or 120 cycle running frequency of the coil.
Sixty cycle base units normally use thinner springs than 120 cycle base units. You can sometimes tell if a bowl is rectified (60 cycle) by finding an extraneous bolt head on a controller indicating the addition of a diode.
Generally a 3/8″ thick spring = 2 1/2-1/4″ springs 1/4″ thick spring 2 1/2 = 1/8″ springs.
When springing a base unit, you should attempt to maintain equal springing in all spring packs. If this is impractical, the spring packs that have striker plates bolted to them should be spring heaviest, still attempting to hold as even and symmetrical springing as possible. If you add a spring to one side, the next spring should be placed exactly opposite. Also, keep the thickest springs nearest the cross arm.
Base units have fairly wide range between the lowest and highest points of the tuning and should be finally tuned with the bowl carrying a normal load of parts and running at the slowest possible control setting, while maintaining the specific rate.
Extremely heavy parts require a few more degrees of over-tuning than normal.
Tuning Check
Loosen top spring bolts on one sprint pack while the bowl is running.
Under-tuned
bolt loosened – parts slow down. If a slowdown of parts travel is noted, the unit is under-tuned. If more speed is needed, adding one or more springs, with proper spacers, or substituting another spring with greater thickness is required. Always recheck tuning after each change. If springs are added, be certain you have ample threads to withstand the torque necessary for tuning.
All bolts should be tight when the tuning is complete.
- 1/2″ bolts min 90 ft/lbs. torque
- 3/8″ bolts min. 110 ft/lbs. torque
- 3/4″ bolts min. 130 ft/lbs. torque
Over-tuned
Bolt loosened – parts speed up. If a bolt is loosened slightly and parts speed up, the unit is over-tuned. The degree of over-tuning must now be established. When this bolt (at the top of the spring pack) is loosened just slightly more and the parts slow down, the unit is not excessively over-tuned and can be correctly tuned by removing one or more springs.
Slight over-tuning is better than slight under-tuning.
Adjusting the coil air gap
- 60 cycle units-.045 to .090 gap
- 120 cycle units- .025 to .040 gap
- To adjust air gap, loosen set screws in the back part of the coil stand. Loosen the two bolts holding the coil stand to the unit. Slide the coil assembly forward or back to obtain the proper air gap. The gap must be sufficient to prevent the armature plate from striking the coil assembly.
- Tighten the coil stand and tighten screws.
- Recheck gap and alter tightening cover.
Note: A bowl that is sprung properly will run rate at about 60% to 80% of full power, will not be weight sensitive, and will pull anywhere from 2-6 amps.
The above would apply not only to new equipment, but older units as well. Most problems are caused by improper use, improper installation, or damage from handling in transit between the factory and the customer.