Fine Material Screw Washers: The Good, the Bad and the Ugly

Must-Know Operating and Maintenance Recommendations To Keep Your Screw Washers Operating At Their Best
Many aggregate plants use Fine Material Screw Washers for dewatering slurries of minus 1/4-inch by 0 fines for the production of varying specifications of sand. The feed to a Fine Material Screw Washer could be dry screened minus 1/4-inch material, slurry directly from a wet vibrating screen, sand classifying tank, or other wet process system that is able to control the top size of the material and classify the sand to the desired grading.

Five Steps to Success

Making sure that a Fine Material Screw Washer is operating at an optimum setting and retaining the product gradation required can be simply evaluated by following these five steps.

1. Make sure that the weirs allowing the overflow of water and undesirable fines are level and positioned to retain the fine sand desired and remove undesirable excess fine sand. For example, if excess fines are in the product, raising the side weirs and dropping the back weir often allows for a maximum removal of undesirable fines.
2. Rising current classification water beneath the pool area of the screw washer also may need to be adjusted. If product sized fines are overflowing, the screw washer and any rising current water is being injected beneath the pool area. Make sure you reduce the flow or shut it off completely.
3. When desired product sized fines are being lost and are overflowing the screw washer, sometimes correcting an improper sand-slurry feed entry helps. The pool area in back of the baffle plate near the overflow needs to be as calm as possible so turbulence is reduced to a minimum, allowing for fine-sized product sand to settle. Directing the sand- slurry feed to the discharge side of the baffle plate will allow there to be minimum turbulence at the rear of a Fine Material Screw Washer for optimum fine sand retention.
4. In many instances, screw shaft speed is set by the manufacturer for the original application, but it may need to be adjusted — particularly if the screw is turning too fast for the fineness of the sand. Generally, when dewatering average concrete sand that has up to 20% of the product passing through 50 mesh, the peripheral speed for a Fine Material Screw Washer should not exceed 150 feet per minute or 75% of full speed. Keep in mind that a reduction in speed often needed for fine mason sand will result in a capacity reduction.
5. To ensure a sand product that is as dry as possible discharges from a Fine Material Screw Washer, the proper amount of washback or flushing water needs to be injected in the dry deck dewatering area.

Simple Care Includes Safety

While the Fine Material Screw Washer may be among the simplest machines to maintain in an aggregate plant, appropriate safety precautions must always be taken including a lockout/tagout of the motor starters.

Proper lubrication of the bearings supporting each end of the screw shaft should provide long bearing life since the speed of a shaft is very slow in comparison to other types of process equipment. Lubrication needs of seals on submerged rear bearings, if a part of the manufacturer’s design is less than the bearing itself, assure proper protection of the rear bearing. A non-lubricated seal most always leads to more frequent rear bearing failures and subsequent downtime with a loss of production.

If the discharge end of the screw shaft is supported by a standard pillow block bearing, it is often recommended that a lube line be run down the support to near ground level so that maintenance personnel can regularly supply grease to the bearing if a service platform is not at its level.

Typically, a motor/V-belt drive and shaft-mounted reducer turn the screw shaft. Periodically changing the reducer gear oil in accordance with a manufacturer’s recommendation usually provides long life for this component. If the reducer on the machine has been in service for several years, periodic evaluation of the gear oil for contaminates is warranted to avoid an untimely failure in the middle of a critical production period.

Periodic inspection of V-belts for both wear and tensioning also is recommended to reduce downtime and an unexpected failure.

Do I Need to Replace It?

The need to replace wear shoes can easily be determined by visual inspection. Replacement shoes are typically available in the manufacturers’ standard iron, urethane, or rubber types. The best abrasion-resistant material for wear shoes may be dependent upon the chemical composition of the sand being dewatered and its physical structure.

The most abrasive sand may be manufactured (granite-like) sand that can abrade or gouge synthetic wear shoes, often not providing suitable life when compared to their costs.

Doing your own check-up or getting the assistance of the local manufacturers representative can often provide more product-sized sand retention and less downtime, providing the highest possible availability for the operation of a Fine Material Screw Washer.