Integral to cost-efficient operation in most aggregate plants, the vibrating grizzly feeder (VGF) is often the vital link between the dump hopper or surge loading – and the smooth operation of the processing system.
Its functions are threefold – safely absorbing the impact of incoming material, scalping off the fines that do not require primary crushing and, most importantly, providing a smooth, steady, well-metered material feed.
If the selected feeder is a good fit for the application, its optimum performance is easily ensured via a simple seven-point maintenance plan. The first challenge though is to ensure the feeder correctly matches the application.
Proper feeder selection
Vibrating grizzly feeders are available in a variety of sizes (width and length) as well as service duty.
Firstly, width is selected based on balancing factors, such as the tonnes per hour of feed rate, size of material to be fed and the size of the crusher being fed.
Secondly, length should be matched to dump hopper design and an ability to handle the surge load.
Thirdly, service duty must be capable of safely handling the impact or load on the feeder due to dumping, and is often selected based on truck-dump versus loader-fed operation.
By properly matching the feeder (primarily its length and service duty) to the application, scheduled maintenance requirements will remain rather minimal.
Consider a stationary application, where quarry haul trucks dump large, metre-long stone slabs, perhaps nine metres or more, onto the pan of a feeder. Obviously, this significant impact demands a heavier-duty (HD) unit, with thicker steel in the side plate and pan. A HD design would also incorporate a tall side plate for rigidity from front to back, and deep structural steel crossmembers to support the pan. Heavy springs with plenty of travel would be used to absorb the load, without bottoming out. Often these HD designs incorporate two-stepped grizzly bar sections, which significantly improve the scalping ability of the feeder.
Alternatively, the VGF used in a loader-fed, road-ready portable operation would not require the same HD construction.
It is an application with less volume and smaller top-size feed, so far less impact is placed upon the feeder.
Furthermore, portable plant mobility requirements dictate a lighter and more shallow feeder design.
The standard duty, loader-fed design will typically have more shallow side plates, less support under the pan, shorter springs and a single set of grizzly bars.
Feeder length is also extremely important for minimising chronic maintenance problems. A longer feeder prevents a heavy load from being dumped directly onto the grizzly bars, where stone can wedge between the bars, prying them apart and causing damage and chronic maintenance downtime. Longer feeders also yield better scalping, by allowing material to stratify before hitting the bars. The proper feeder length should be determined in conjunction with the hopper design and anticipated surge capacity. The common error is to select a feeder that is too short.
By selecting a feeder with the proper service duty and length for the load, you have created an environment for optimum performance and low maintenance requirements. A regular maintenance plan should deliver long service life with minimum downtime.
A seven-point maintenance plan
Once the right feeder is installed, its scheduled maintenance is composed of a simple, seven-point plan that focuses on each key VGF component.
1. Properly lubricate the vibrator assembly. Typically, the vibrator assembly consists of two eccentric shafts, gears and bearings, all operating in a sealed housing with an oil bath. Maintenance boils down to following the manufacturer's recommendations for checking and maintaining oil levels, cleaning breathers and changing oil at proper intervals. A regular oil sampling program can be used to determine oil change intervals and as a predictive maintenance indicator.
2. Prevent live frame damage. The cracking of side plates can occur if the live body or frame contacts any stationary structure, hopper or chute. Maintain a minimum of 13mm in side-to-side clearance, 25-13mm in the up direction, 76mm in the front or back direction, and 152mm in the down direction. Get in the habit of inspecting the feeder at shutdown, examining the live frame for any signs of impact, and immediately creating clearance for any area that is making contact.
3. Monitor replaceable pan liners. Liners are proven to deliver a healthy payback in reduced maintenance costs. Feeder pans are typically lined with abrasion-resistant steel that are either welded or bolted in place. As the liner wears thin, it may buckle or warp, indicating it is time to replace the liner. Never allow the liner to wear through, exposing the pan to wear. Inspect the liners monthly, and replace them as needed. In extremely abrasive or sticky applications, specially formulated rubber liners, as well as rubber-capped grizzly bars may be used. These liners are heavier and thicker than steel liners, impacting the live weight and requiring more depth in the pan. Allowance for these types of liners should be incorporated into the feeder design, and discussed at the time the feeder is purchased.
4. Inspect the grizzly bars after each shift. Prevent reduced scalping capability and damage to the grizzly bar section, by inspecting the grizzly bars daily for any stone that is wedged between the bars. Clear out the bars after every shift. Also, grizzly bar caps should be repaired or completely replaced if sufficiently worn. Worn or bent bars are far more likely to have material hang up creating a daily problem. Lastly, while breaker hammers are commonly used at the primary, avoid breaking stone on the grizzly bar section as hammering can cause damage to the bars and the bar support.
5. Inspect springs weekly. To minimise spring breakage, remove any stone or dust build-up appearing around the base of the spring. Material can wear or scratch the spring, creating a stress point that may ultimately lead to cracking and failure. Importantly, springs should never bottom out. Replace weak or broken springs immediately.
6. Do not exceed maximum drive speed. Commonly used with a grizzly feeder, variable speed drives maximise production and handle wide variances of feed. It is strongly recommended that a limit be set on the drive so that it cannot exceed the feeders maximum rated speed. Maximum recommended speed is often determined by 'G' force, which is established by the relationship between speed and stroke (see Table 1, Determining the G Force). Most feeders are designed to withstand between 4 to 5 Gs, depending on the manufacturer and model. Excess speed can exceed the designed G force limit and quickly cause structural failure. Use extreme caution not to exceed the G force limit, if your common practice is to momentarily run the feeder at maximum speed to clear stone from the grizzly bars.
7. Practise sound operating tactics. It is best to empty out the feeder at the end of the day. However, during operation, it is a good practice to leave a little material in the pan between dumps. In this way, the next load of dumped material will land on stone – not the pan. This cushions the impact and reduces wear.
The combination of proper equipment selection and a well-trained crew can deliver outstanding performance and low operating costs. If errors are made, it is typically because the feeder is too short or too light duty for the job, causing lost production and costing thousands of dollars annually. Once the right feeder is installed, the trained eye and simple maintenance delivers consistent performance year in and year out.
Jim Schreiner is director of marketing for Telsmith. He has worked for 23 years in the aggregate equipment industry, 10 of which have been at Telsmith.