In the world of aggregate crushing, the word “feeder” can apply to many different devices. There are roller belt feeders, apron feeders, electromagnetic feeders, pan feeders and others. This article focuses on a feeder type that plays a critical role in the efficient operation of primary crushers – the vibrating grizzly feeder (VGF).
{{image2-a:r-w:325}}The purpose of a VGF is two-fold: to absorb the shock of large dump loads and to maximise the primary crusher efficiency. It makes the primary crusher more efficient by supplying a constant, metered rate of feed to the crusher and by pulling fines from the feed material to keep it out of the crushing chamber. VGFs handle a variety of material types: from large shot rock to sand and gravel, and from high clay aggregate to recycled concrete. The VGF also helps to clean and separate sticky or conglomerate-type materials prior to entering the crushing chamber. The VGF takes a beating in most primary applications, allowing the rest of the circuit downstream to operate at an optimum level. For this reason, the VGF should be a point of focus for regular inspections and preventative maintenance.
Like all discussions on aggregate processing equipment, the starting point is to talk about safety. Operators should follow the original equipment manufacturer (OEM) guidelines and the company’s lock-out/tag-out procedures prior to starting maintenance or repair work on a VGF. They should always use the correct lifting devices that have been inspected and calibrated for lifting requirements. Since most VGFs operate on the basis of unbalanced weight rotation, all the stored energy has to be relieved prior to working on the machine. The catchphrase should be: “Work smart and stay healthy!”
Right application settings
VGFs use medium speed rotating eccentric shafts to “throw” material down the feeder and into the crushing chamber. Even though most feeders operate at lower rotations per minute (rpm) than typical material sizing devices (screens) – typically in the 500 to 800 rpm range – the bearings on the feeder shafts are still subject to the stresses of friction at a medium speed over long time periods. Friction leads to high heat generation, the enemy of bearings in any situation. This is one of the many forces acting to wear down vibratory equipment. Maintenance personnel are charged not only with keeping equipment in good shape and running smoothly, but also attaining the most uptime from every piece of equipment and making it last as many years as possible. There are several factors involved with operating and maintaining {{image3-a:r-w:200}}a VGF to ensure optimum daily operation, as well as maximum longevity. These include the following:
- Stroke angle of the feeder.
- The rpm of the driven shaft.
- Maintenance schedules.
Let’s look at these factors in closer detail. Most VGFs in the quarrying industry have adjustable throw capability, which allows the angle of eccentricity to be changed. A more vertical throw will concentrate the agitation toward the intake end of the feeder. This can assist in loosening sticky material and allow more fines to work down to the bottom of the material bed for removal at the grizzly. Applications with a high percentage of fines or sticky material may benefit from this setting by retarding material flow and keeping it on the feeder longer.
In applications where material is at the top end of allowable feed size to a jaw crusher, the more vertical VGF stroke angle will feed the material at a slower feed rate and allow the crusher to more efficiently process the aggregate without overfilling the crusher feed hopper. Varying the feeder rpm, preferably with a variable frequency drive controller on the feeder drive, can also control the VGF feed rate. The variable speed controller should be tuned to the OEM minimum and maximum allowable speeds.
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A lower VGF stroke angle, on the other hand, will concentrate the agitation towards the discharge or grizzly end of the feeder. Imparting a more lively action to the grizzly can reduce the chances of material wedging between the grizzly bars, maintain a thinner bed depth of material and move it more quickly down the feeder and into the crusher. Again, in combination with a variable frequency controller, the operator can tailor a horizontal stroke angle to move material more quickly into a crusher to keep it properly fed.
By ensuring the feeder is set to the correct operating angle, both feeder and crusher efficiencies will improve. Operators should always refer to the OEM manual for the correct stroke angle for the application and keep a magnetic stroke gauge handy to accurately check the feeder’s performance.
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Anyone connected with the maintenance or repair of aggregate equipment knows the old adage “speed kills” is applicable to all vibratory equipment, including vibrating grizzly feeders. Simply running a VGF faster will not always equate to more material into the crusher, but it will lead to more frequent bearing changes and possible costly failures. The faster a bearing turns, the sooner it will wear out or fail, so it’s important to understand the “sweet spot” of speed versus production. Operators should consult the OEM guidelines for the rpm range for the feeder and the nominal operating temperatures of the feeder bearings, then set up the feeder drive according to those specifications, and keep a good log of the feeder bearing operating temperatures as compared to the OEM temperature limits. This will allow operators to recognise a bearing problem early on and have time to address it before a major failure. Maintaining the correct feeder rpms is sound advice to reduce labour costs and downtime — as well as saving the new equipment warranty!
Preventative routines
{{image6-a:r-w:200}}All the above information goes out the window if the VGF is not being regularly and correctly maintained. Most OEMs will require the completion of a delivery inspection form upon receipt of a new feeder at the dealer/customer. This is a critical document that will be required by a transport company should alleged damage occur in transit. Both the operator and the OEM benefit from the time taken to accurately complete and submit this form. Some OEMs require this document to be completed and submitted to initiate the new machine warranty period. Along with this form, some manufacturers will also specify a new equipment start-up procedure to help ensure the new machine is started up correctly. Again, following and completing these documents is the best way to protect the operator, dealer and OEM when processing new equipment.
Once a new feeder has been properly processed and all forms completed and submitted, a regular maintenance schedule needs to be in place. Most OEMs will outline required maintenance items in the VGF operations and maintenance manuals.
These regular maintenance checks are typically broken down into daily (before, during and after operation), weekly and special instructions for extended shutdown and storage. See the boxout (above left) for some examples of these checks.
When topping off or changing oil in any vibratory unit, it’s important to understand the lubricants that are required. All manufacturers provide recommended lubrication for their equipment in the operations and maintenance manuals, so be sure to keep that manual nearby. It’s important to follow lubrication recommendations for brands, number designation, temperature, viscosity and synthetic or natural-type oils.
As long as an operator keeps the VGF running at peak efficiency, it will serve the rest of the crushing circuit well and provide years of dependable operation.
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DAILY MAINTENANCE CHECKS |
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Before start up:
During operation:
After operation:
Weekly maintenance checks:
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Dave Anderson is an applications specialist with Terex Minerals Processing Systems.
This article first appeared in the May 2015 issue of Aggregates Manager US and reappears in Quarry with kind permission.