How to Inspect the Drag Slat Conveyor
BY A Valued OEM
Your conveyor has served you well all season, but it’s been a few years since its last rebuild. Now you need to balance your seasonal downtime schedule and component replacement costs with the chance for chain failure next season. Here are a few basic guidelines to help you decide what to do.
While you still have the conveyor in operation, use an infrared thermometer to take temperature readings of all bearings, the motor and gearbox. High temperatures indicate excessive wear. If you are in doubt about a temperature reading, contact the manufacturer for a recommendation.
It’s time to lock out and tag out the drag slat conveyor before your ground personnel go any further. The remaining steps take place while the plant is out of operation.
The drag chain is generally the first determining factor when deciding to rebuild. There are several indicators of remaining chain life.
First, take a look at your records to determine the tons of material that the chain has conveyed, and compare that to the average life reported by the manufacturer. Second, make a thorough inspection of the chain rollers. As the rollers wear they take on a saddle shape, as most of the wear is in the center of the roller. Chains can be run until the point the rollers begin cracking and even breaking apart, but you do not want to go into a new season with any cracked rollers.
Additionally, as chain wears it elongates, which some personnel refer to as “chain stretch.” The chain doesn’t actually stretch, but as the pins and bushings wear, each chain joint is allowed to pull further apart. If a chain has 3 percent elongation it must be evaluated closely, and probably replaced. If it has 5 percent elongation it’s gone. If you keep accurate track of the number of links that have been removed, you can calculate the amount of elongation. Few people, however, have that kind of accuracy in their records. Therefore, the best thing to do is remove a pin and check for wear.
You will notice that the removed pin only wears on one side—or half of its circumference. The depth of hardness varies by pin size, but if you contact your supplier, the company should be able to give you that information—it will typically be something less than 1/8-inch.
If you show more than 50 percent wear on the depth of hardness you will want to replace the chain because, once it wears through the hardened surface, it fails rapidly. Sprockets are similar to chain in that they are surface-hardened. Once you wear through the hardened section—typically about 1/8-inch—the sprocket wears very rapidly. The most common mistake made by contractors is not changing head shaft sprockets frequently enough, as worn sprockets greatly accelerate chain wear and can even result in chain failure.
Slats must be checked for wear and for bending. Any bent slats should always be replaced. Normal slat wear occurs mostly on the ends, and results in material build-up in the conveyor box. Again, excessively worn slats should be replaced.
The final inspection for a conveyor rebuild is the floor plate. In our industry chrome carbide overlay plate or NiHard castings are the standard. Chrome carbide is the easiest to check because you can see if any areas have worn through. NiHard will crack and break long before it wears through. Some manufacturers provide wear indicators cast into the plate to let you see how much wear is left. If you don’t have wear indicators your best indicator will be the tonnage that you have run compared to the average wear life offered by your supplier. If in doubt, replace the floor plate before installing a new chain.