September 1, 2012
Follow these basic guidelines during routine maintenance inspections to help minimize feeder downtime.
By Carla Phelps
With proper maintenance and regular inspection, vibratory feeders can operate for long periods of time with minimal maintenance and repair. The feeder’s consistent operation ensures that the rest of the equipment in the process operates at desired capacity. Downtime of a feeder can be detrimental to the plant’s successful operation.
There are a number of issues that can cause a reduction in feeder output. Many are issues that occur gradually and are not noticed, as there are often multiple feeders in the process and gradual compensation has been taking place.
Feeders are designed to operate within a specific range of stroke (also termed amplitude or displacement) that is the distance the feeder deck moves during operation. All feeders are provided with some type of stroke- indicating sticker or plate. It is very important that the maximum stroke of the feeder is known and that plant personnel understand how to read the stroke plate. With the feeder control turned to maximum, the feeder should operate at the maximum stroke. If the feeder does not have a variable speed control, it should be at or just below the maximum stroke. A feeder should never operate above the maximum stroke. If operated in an over- stroking condition, it will cause damage to the feeder.
Never add or take away material from the feeder. It is designed with a certain weight, and altering the weight or balance of the feeder will cause damage to the feeder and alter the feeding characteristics.
Worn wear liners should be replaced with the same weight material as the original liners. Always remove the worn liners; never install new liners over the worn liners. The liners should be installed in the same manner as the original ones. Bolt-in liners need to have the same bolt pattern as the original liners. This will ensure the liners will not diaphragm or have material build up under them. If the liner wear is excessive, consider having the liners fabricated from a more abrasion-resistant material, keeping the thickness the same. If you convert to a poly-type liner, you may have to increase the thickness or add a backing plate to maintain the original weight.
The importance of the infeed hopper cannot be overstated. Most manufacturers understand the importance of the hopper and will help you determine the proper design for your application. Here are some guidelines that should not only protect the feeder from overburden, but also help avoid hopper plugging and rat-holing of material above. The hopper opening above the feeder deck is referred to by two dimensions; the gate height (the parallel distance up from the inside bottom of the deck) and the throat (the distance from the inlet bottom of the deck horizontally to the hopper opening). The gate height should be twice the throat dimension. Generally, the hopper opening should not be more than 1/3 the deck length. This ratio will allow the feeder to take away all the material that is delivered by the hopper, developing the depth of material on the deck. If this ratio is not maintained, there may be more material trying to exit the hopper than the feeder can deliver. This can cause hopper blockage, spillage at the back of the feeder, and increased liner wear. The feeder will try to feed material back up into the hopper. The walls of the hopper should be sloped to approximately 60 degrees in the rear and 55 degrees in the front. This, along with the proper gate and throat dimensions, will ensure that all the material in the hopper is active, minimizing blockage problems.
Worn wear liners should be replaced with the same weight material as the original liners. Always remove the worn liners; never install new liners over the worn liners.
The side skirts must be tapered on the bottom to prevent material from being wedged under them. The dimension from the inside of the deck to the bottom of the side skirts should be at least 1inch greater at the discharge end than at the inlet end. The clearance between the side skirts and the side walls of the deck should be 1/2 inch minimum; however, as this dimension increases, the volume of material decreases. For example, if there is a 3-inch clearance on each side, you will have the effective capacity of a feeder that is 6 inches narrower.
If these hopper and side skirt rules are followed, the feeder should be able to deliver the maximum design capacity.
There are a number of simple visual checks that can be done when in the area of the feeder. The timing of the inspections is generally determined by the application.
• Check for proper clearance. When the hopper above is emptied, a suspended feeder should be able to swing freely for a minimum of 1 inch front to back and 1/2 inch side to side without contacting any solid structure.
• Look under the feeder deck for missing liner bolts. If there are bolts missing, it is usually an indication that the liners are worn below the bolt heads and likely need to be replaced.
• Inspect suspension springs. If collapsed, they must be replaced. A good rule of thumb is that, if the space between the coils is less than 1/32 of an inch, the coil spring should be replaced.
• Look over the cables during operation to see if they are stable. If any of the cables are whipping, it may be an indication that the loading is not distributed properly. All suspension points must share the loading equally. If one of the suspension points has become loose or experiences a spring failure, it will overload the remaining springs. If turnbuckles have been used, ensure that they are locked in place by pinning, double-nutting, or tack welding. Feeder vibration can loosen the turnbuckles if they are not properly locked.
• Study the condition of the safety cables on suspended units. Ensure that they are not in contact with the feeder when the feeder is loaded with material. A good way to prevent the safety cable from interfering with material on a belt is to run the cable through a flat piece of metal conduit that is slightly wider than the feeder pan to take the sag out of the cable.
• Inspect the inside of the feeder deck for signs of material buildup. This material will affect the operation of the feeder and should be removed to prevent damage to the feeder. If inspected on a regular basis, the operators will soon become aware of the changing moisture, temperature, or material sizes that will likely cause the buildup. If sticking is a constant problem, contact the manufacturer to discuss alternate liner materials.
• Pay attention to the sound of the feeder. Normally, there is noise from the material being fed, but when the feeder is running empty, you should be able to carry on a conversation without a problem. If there is excessive noise from the feeder, there is a potential problem. There may be a loose liner or connection springs. On an electromagnetic feeder, there may be a striking condition in the drive. Check the air gap of the feeder and make sure it is to the manufacturer’s specifications. Do not over compensate this air gap setting; if it is set too wide, the feeder will draw excessive amperage and cause fuse failures.
Following these basic guidelines during routine maintenance walk-through inspections can minimize feeder downtime and help find any issues before they become problems. When found early, issues can be resolved and corrected during scheduled shut downs. AM
Carla Phelps is product manager, vibratory feeders, for Jeffrey Rader Corp. She can be contacted at firstname.lastname@example.org.