Counter Rotation and Synchronization.
The Cleveland Vibrator Company manufactures a complete line of vibratory equipment. The product line includes a variety of vibratory table configurations including flat deck, FA, vibratory grid tables, GT, and vibratory belt tables, BT, along with Electro-mechanical feeders, EMF and Electro-mechanical screeners, EMS, units. The majority of these units, perhaps 97% of what is built and sold, are of the general “Brute force” design type. This broad design category is based on the principles of counter rotation and synchronization when applied to two electromechanical rotary electric vibrators.
Counter Rotation – Brute force units require the use of two rotary electric vibrators. When mounted to a rigid structure the two vibrators will “sense” the presence of the other vibrator and make an effort to run together at the same speed. To produce linear force with two vibrators, which when operated alone produce a rotational or centrifugal force vector, the pair must run together and Counter Rotate. When mounted side by side and viewed from the end, one vibrator must run clockwise and the other vibrator must run counter-clockwise. In typical applications it isn’t important which vibrator runs in which direction, it’s just critical that the two run in different directions. When running properly and in opposite directions, the forces produced by the two vibrators cancel each other except for two times in a cycle with the eccentric weights of the vibrator both point in the same direction. With the Cleveland Vibrator line of three phase powered rotary electric vibrators, changing the direction of rotation on one of the vibrators is as simple as “flipping” two of the power legs of the vibrator. Refer to the attached diagram regarding counter rotation and force cancellation.
For the sake of completeness and since I don’t see that anyone else has touched on the topic in their Blogs, I’ll take a moment to discuss in very basic terms the design of the rotary electric vibrator as it is important to understand when we talk about Counter Rotation and Synchronization. Very broadly, the rotary electric vibrator is a through shaft motor with bearings on each end of the shaft. On each end of the shaft, outside of the bearings there are two eccentric weights that produce an “unbalanced” condition when the motor runs. This unbalanced condition is what produces the vibration used to move, screen or compact a variety of products. These eccentric weights are more or less “pie” shaped and are clamped onto the shaft with a bolt. The eccentricity or unbalanced condition of the vibrator can be varied from 0 – 100% based on the relationship of these weights.
To access the eccentric weights, you must remove the four bolts that hold the weight cover in place, and then remove the weight cover. To adjust the unbalanced condition, the outer weight, the one furthest from the center of the vibrator on each end of the vibrator, can be rotated to a new locating changing its relationship to the inner weight thereby increasing or decreasing the force output of the vibrator. As outlined in the rotary electric user’s manual, only the outer weight is adjusted, never change the location of the inner weight. It is critical to remember that if one outer weight is changed then ALL outer weights on both vibrators, assuming installation on a piece of equipment, must also be moved to the new location.
On each end of the motor shaft you’ll see a dial with increments between 0 and 100 percent. The outer eccentric weight has a small “punch mark” or “dot” on the face of the weight. This punch mark is aligned with the desired weight setting on the dial.
For example: if the goal is to reduce the force produced by a vibratory table, the customer might decide to “turn the weights back,” in other words, change the position of all outer weights to say 70%. All four outer weights are rotated so that their punch marks align with the 70% mark on the dial on the end of the shaft. By changing the relationship of the inner to outer weights, the unbalance of the vibrator is reduced and with it the force it can produce at any given speed.
On a piece of equipment it is critical to the successful operation and serviceable life of that unit that all out weights on each end of both vibrators are set in the same location. Again, refer to the rotary electric manual or call Cleveland Vibrator if there is any question on the positioning of the outer weights. It’s critical that all weights be set probably.
Synchronization – Once the outer weights are all set the same and the rotation has been checked to confirm counter rotation the unit can be run and the vibrators checked for synchronization. Synchronization simply means that the vibrators are running at the same speed in opposite directions and thereby producing linear vibration. Typical controls for equipment when supplied by Cleveland Vibrator are either a magnetic starter or a variable frequency drive (VFD), it’s important regardless of the means of starting the two vibrators, that they start together and be controlled by one starter device.
While doing a final check on the equipment prior to shipping to the end user, Cleveland Vibrator will periodically check for synchronization of the vibrators. To check the synchronization of the two vibrators as mounted to the equipment, one weight over is removed from each vibrator exposing the two eccentric weights on the same end of each vibrator. Once the covers are removed the unit is restarted and run. Use caution when operating the vibrators with the weight covers removed. While the unit is running a strobe tachometer is used to “stop” the motion of the rotating weights and check to see that the vibrators weights are a “mirror” of each other. To “stop” the motion of the vibrators, adjust the frequency of the strobe to match the operational speed of the vibrators. At the operational speed the motion will appear to “stop” and you can easily observe the relationship of one set of weights to the other. The procedure was recently videoed and you can see the results on our YouTube Channel.
To make the video an Extech Digital Stroboscope or “strobotach” was used. In the video, the initial frequency of the strobe was slightly off that of the vibrators. As the speed of the strobe approaches that of the vibrators, the two sets of weights appear to slow to a “stop”. As seen in the video, the orientation of one set of weights should be mirrored onto the other vibrator. When the weights appear to “stop” and the orientation is observed, if the position of the weights is not a mirror copy of each other, then the vibrators are not in synchronous operation and will not produce linear motion. This is not the desired condition for successful operation for a piece of vibratory equipment utilizing the Brute force design.
What does it mean if the vibrators are not running together in synchronous operation? This could possibly indicate a problem with one of the vibrators, perhaps an early indication of a pending bearing failure or lack of grease on those vibrators with grease fittings. Please refer to the operation manual for the recommended maintenance schedule for grease application, both quantity and type of lubrication. Some of the smaller vibrators are permanently lubricated and don’t require maintenance of this type. Another option would be to double check the tightness of the vibrator mounting bolts. As discussed in the maintenance manual the vibrator mounting bolts should be checked periodically for tightness. Loose bolts can prevent proper operation and synchronization of the two vibrators.
If there is any question regarding the proper operation of the equipment or vibrators it is best to contact Cleveland Vibrator as soon as possible. It’s always best to error on the side of caution and ask a few questions, rather than ignore a possible problem. Cleveland Vibrator has a very knowledge staff and they are happy to discuss your observations and assist in any way.