Feed to the crusher is very important to optimize aggregate production and parts wear.
Feed conveyors or chutes should be mounted or adjusted so that the material is falling straight down the center of the feed tube. If the feed is entering the crusher off to one side of the feed tube, the feed tube will wear uneven, one or two impellers will wear our faster than the others and the anvils will wear out fast on one side of the crusher (usually 180 degrees from the side the material is entering the feed tube).
If the feed is centered, the wear parts will wear out evenly, there will be less recirculating load, and the amp draw on the motor will be more constant. This will also improve fracture.
The velocity of the material entering the crusher is also important. If the material is falling into the crusher from a high distance or the feed conveyor is running at a high rate of travel, the material can bounce up off the feed disc and be paddled by the impeller. This causes high recirculating loads, gap graded products, high lid liner wear, wear parts breakage and larger size discharge material. This also affects fracture.
Feed Tube Adjustment
The feed tube is generally adjusted to be level with the impellers or slightly higher, in high tonnage application, close to the max throughput. The tube may need to be raised 1″ above the impellers, if the feed starts to back up the tube. The feed tube should be checked every 3-5 impeller changes. If the feed tube wears above the impellers, the re-circulation load will increase and the lid liner wear will increase.
In addition, you will feel and hear rocks hitting the lid and the top leading edge of the impeller will wear off in a rounded pattern. This indicates material going over the top of the impellers.
The VSI crusher is not as sensitive to feed gradations as other crushers, however it does affect the horsepower and causes high wear.
In most cases this type of crusher will produce an even product from a gap graded feed stream and will produce approximately the same output gradation at high and low tonnages. A rock on rock style VSI relies more on feed gradations and particle density or higher input tonnage, to produce an even grated product.
Rate of feed is not important for the output gradation but is important for wear parts utilization and ultimately resulting in the cost per ton of operation. If the crusher is being operated at a low rate of feed (below 50% of max throughput) the impellers and anvils will generally wear as fast as at a high rate of feed except that they will have a thinner wear pattern. However, the total aggregate tons produced per change out will be less than at high rates resulting in higher cost per ton.
This is usually a bigger issue in medium to high abrasion materials having an impeller wear life of 100 hours or less.
Speed and Wear
Speed and wear are directly related. The result of faster RPM is better fracture on single pass products. However, impeller/anvil clearance, number of impellers, and number of anvil faces also affects fracture. In several wear applications your crusher may be re-configured to maximize wear life and maintain required production.
This approach can also be used to reduce horsepower draw and increase throughput capacity.
More Fines Vs. Less Fines
Generally speaking max fines production is produced with more impellers and more anvils. In very high tonnage applications a reduction of on impeller will result in higher production due to more flow through the table.
Fewer fines are produced through various combinations of lower RPM, 3-4 impellers and/or curved or rounded shaped impellers. When trying to maximize certain products such as 1/2″ – #4 chips while producing a minimum amount of fine, a higher speed like 100 RPM, a 3 shoe table, with curved impellers will maximize the chip production and produce less than 15% – #4.
Where only a courser product is required a slower RPM or curved impellers will be adequate. Any time a speed change or specific product is required consult the factory for advice.
Moisture and Dust Control
Moisture in the feed increases density and will result in a faster wear and higher horsepower draw. In low or medium abrasion materials most people put a hose in the top of the feed tube to control dust. This is very effective because the fan type characteristics of the impeller table sprays the water out radially and mixes it with the material easily.
Since the impeller table is similar to a fan, the crusher draws in and blows out a lot of air especially on startup and shut down, when the crusher is empty. If the crusher is not being operated at maximum production it will draw air and create more dust. The feed opening can be necked down to match the feed rate, which will minimize air draw, slow feed down and center the feed, resulting in even wear and steady amp draw. This can be achieved by simply cutting a hole in a piece of steel plate and putting it over the top of a feed tube.
The discharge conveyor should be covered and sealed at least 15′ from the crusher which will allow the dust to settle out of the air discharging from the crusher. An air re-circulation system or a bag house can also be installed to aid in controlling dust.
Tramp iron can seriously damage the VSI crusher due to its chrome iron wear parts, which will break when hit by metallic pieces. A magnet or metal detector is recommended to be installed on the feed conveyor. In most cases a magnet will pay for itself on the first time in detects a large piece of metal. Another option is to feed the crusher from a second deck of a screen, which will reduce the size of tramp iron that can get into the crusher. Any damage caused by tramp iron is not covered under warranty.
Feed Size Vs. RPM
The VSI is sensitive to feed size due to the composition and design of the wear parts. If you are experiencing frequent parts breakage you may want to lower the feed size or RPM. If this does not stop the breakage, there may be a fit problem on the impeller table and you should consult the factory immediately.
In most cases, VSI crushers are run in closed circuit with a vibrating screen. In this case, it is very important that the screen be sized to handle the production of the VSI plus the recirculating load. If not, the fine material will be carried over the screen and be sent back to the VSI. This will result in higher wear, HP draw and lower than capable production due to the increase in recirculating load.
The recirculating material can be checked to see if there is more that 5% of less than product sized particles in the material. If so, the screen is not large enough or is not operating efficiently.
Improving Wear Life
Reducing moisture and fine material in feed will increase wear life. However, if the crusher is not being fed at a higher rate or the feed contains a large amount of material below 3/4″, the wear pattern on the impellers and anvils may narrow.
This can sometimes be improved by using a tapered or conical feed disc, which spreads the flow of material on the impeller, widening the wear pattern.
In addition, in some models the anvil ring can be adjusted up or down on any unused parts of the anvil. When doing so we recommend wearing the bottom of the anvil first, because the material will follow the existing wear patter, and will be directed downward.