Vibratory equipment utilizing reaction springs (natural frequency design) depends on a specifically calculated operating speed (eccentric shaft RPM or vibratory motor RPM) and natural frequency to function properly. A change in the operating RPM affects the operating stroke dramatically. As compared to other types of equipment, a 10 RPM variation on natural frequency vibratory equipment can be too much for proper operation.
If no other changes have been made, reducing the operating RPM decreases the stroke, while increasing the RPM increases the stroke. Some RPM changes can be accommodated by re-tuning the machine (adding or removing springs). However, always consult ACTION before making a speed change to be certain it does not exceed design parameters.
One often overlooked cause of RPM loss is wear on the drive belts and sheaves. Over time in a harsh, dirty environment, the V-grooves on the sheaves can wear causing a gradual reduction in the pitch diameter. This (or belt wear) allows the belts to drop in the grooves resulting in a lower operating RPM and lower stroke. Check the RPM with a tachometer or strobe light. If low, replace the belts and sheaves, especially if you are troubleshooting an older machine.
RPM variation from design can also be caused by component replacement such as a mismatch between belt cross-section and sheave V-groove cross-section or installing a sheave with a different pitch diameter. For example, BX belts will run differently than B section belts, or variations between belt manufacturers are possible.
Always verify the part number and pitch diameter is correct as shown on the General Arrangement drawing machinery list if replacing sheaves. Sheaves one incremental size apart may look alike but result in a very different operating RPM.
A VFD is not recommended, but if one is used, consult ACTION before making frequency adjustments
