Chuck Yung
EASA Technical Support Specialist
One of the unique motor applications we’re often called upon to service is the “shaker screen duty”or vibrator motor. See Figure 1. These are mechanically robust electric motors, fitted with large eccentric weights, designed to deliberately vibrate – a lot. The unusual application calls for some special considerations when repairing these motors. This article is intended to consolidate those tips in one place.
When dismantling the motor, the first step is to document the position of the eccentric weights on both ends, relative to each other, so the performance characteristics remain unchanged. Note that many of these are fitted with two weights on each end and that only one of the weights is keyed. The second weight can be shifted relative to the first to allow adjustment of the unbalance to suit the application. In some applications, for example, when shaking a product through a hopper, the weights might be adjusted to different settings to move materials of different density. See Figure 2.
Shift weights to opposing sides
While there are designs where the weights on both ends are offset in the same direction, it is sometimes more effective to shift the weights to opposing sides. The resulting structure movement is more conducive to energy transfer when the ends of the motor undulate. Installing the eccentric weights both on the same side (e.g., both ends of the motor have the eccentric weights in the 6:00 position at the same time) produces a more violent movement that is more likely to break welds and do structural damage.
It is critical that the eccentric adjustment of the counterweights on both ends be identical. Setting the offset on opposite ends to different force will result in transverse forces that are very likely to result in the destruction of the mounting supports, or the shaker screen duty motor itself.
All that shaking requires a massive shaft and oversized bearings as well as special bearing fits for the housing and bearing journal. In layman’s terms, the shaft fit is a slip fit and the housing fit is tight. As you might imagine, so much radial load requires special bearings. Aside from the relatively large bearing for the kW/hp rating, the internal clearance of the bearing is always greater than C3. Abearing with C4 internal clearance is most common for vibrator motors.
Repair methods
Not all journal repair methods are appropriate for the extreme duty of the application. Plating is the preferred option to restore an undersized bearing journal. Metalizing should be avoided; it may fracture under the pounding forces. Welding should not be used, as the shaft is likely to break. A severely damaged shaft should be replaced, and the replacement shaft should match original material.
The bearing fits are typically a looser than usual shaft fit, with a tight housing fit. Some older bearing fit tables refer to these as “rotating housing, stationary shaft” tolerances. Bearing manufacturers differ slightly in their tolerance recommendations; see Table 1 for an abbreviated comparison.
NJ roller bearings
Some shaker screen duty motors are fitted with NJ roller bearings, which have a locating shoulder on only one side of the inner race. Pay close attention to the bearing numbers. The inner race is specially ground or “crowned” to provide the necessary internal clearance for a shaker screen duty motor. The bearing numbers may be hand engraved on the bearing.
Do not substitute a standard C3 ball, or conventional NJ roller bearing, in a shaker screen duty motor. The windings will fail and the appearance can mislead one to interpret the failure as an overload condition.
When rewinding vibrator motors, the winder should be aware that the magnetic flux densities are often lower than usual. It is not desirable for the rotor to accelerate too quickly; that can cause structural damage to the installation. It takes time to establish the required movement of the hopper or structure, and too rapid acceleration might break welds near the motor mounting. The motor frame could even be damaged over the course of numerous starts.
Use extreme-pressure grease
Extreme-pressure grease is required to accommodate the large radial load on the bearings. Standard ball bearing grease can break down under the operating conditions.
In a good design, the end brackets and retaining bolts are also more substantial than those of similar kW/hp ratings. Bolt torque is of increased importance as is the importance of lock washers, thread-locker compounds when recommended, and jam nuts on designs where the bolt is not threaded into a tapped frame. Through-bolts are less suitable than designs with a substantial frame tapped for thread engagement of (at least) twice the bolt diameter.
Caution your customer that the base bolts must be of the correct size; the use of under-sized base bolts can result in catastrophic damage to the motor and driven equipment as well as increased risk of injury to personnel. Washers, when used, MUST be hardened washers to prevent distortion which would result in the loosening of the fasteners.
Lugs should be crimped and soldered for the added security. The lead opening/terminal box must be packed with material to prevent chafing of the leads against other surfaces. Suitable materials include vermiculite, duct seal or pump packing. Never use silicone sealant, which makes it difficult to access the leads when the motor is next removed from service.
Final test run
The final test run should be done without the counterweights, for practical reasons as well as safety. Bearings and no-load current can be monitored and counterweights should be installed prior to shipping. If you absolutely have to run a shaker screen duty motor with the counterweights, the motor must be securely bolted to a massive foundation or to a resilient structure built to withstand the forces.
Shaker screen duty motors are expensive to replace and often have long lead times. So, the repair option is attractive to end users. Use the tips in this article and work with your bearing supplier to ensure customer satisfaction with your repairs.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100- Section 2: Mechanical repair
- Section 4: Testing
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