KWS Manufacturing

Analyzing Different Types of Screw Conveyor Failures

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Question

I have about 30 screw conveyors in my plant for conveying various bulk materials. Most of the screw conveyors work great with very little maintenance or down time. However, sometimes a screw will fail at the end of the screw near the internal collar and coupling bolts. Sometimes the failure occurs where two screws are coupled together at a hanger or even at the drive end. The center pipe of the screw looks like it has been ripped open like a beer can! What the heck is going on?

Answer

Screw conveyors are the ideal solution for conveying thousands of bulk materials. The failure that you have described is caused by the internal collar breaking free from the center pipe of the screw. Once the internal collar breaks free, the center pipe of the screw will fail at the coupling bolt holes. Please refer to the photo below of a typical screw failure.

Analyzing Different Types of Screw Conveyor Failures - KWS Manufacturing

Typical Screw Failure Due to Internal Collar Breaking Free from Pipe

The torque of the drive unit rotates the screw to convey the bulk material. Under normal design and operating conditions, a screw conveyor is designed to handle full-motor torque with no mechanical failures. The center pipe of the screw and internal collars are designed for full-motor torque when the two become an integral welded assembly.

Internal collars must be properly welded into each end of the center pipe of a screw section. For properly manufactured CEMA standard screws, the internal collars are plug welded to the center pipe. KWS provides two plug welds on each side of the pipe at 180-degrees apart for a total of four plug welds per bushing. Please refer to the photo below for proper plug welds and location.

Analyzing Different Types of Screw Conveyor Failures - KWS Manufacturing

Proper Size and Location of Plug Welds

Plug welds provided on KWS screws are permanent and will not break loose. Bushings or internal collars are permanently fixed to the pipe to provide an unbreakable weldment. We use the following steps to ensure a perfect plug welded bushing. First, ½-inch diameter holes are burned in the center pipe using a hand-held plasma torch. Please refer to the photo below.

Analyzing Different Types of Screw Conveyor Failures - KWS Manufacturing

Internal Collar Fits Snugly in Pipe After Holes are Burned

Four holes are cut in the pipe with two on each side 180-degrees apart. Then, the inside of the pipe is cleaned up and the internal collar fitted into the pipe. The internal collar fits snugly in the pipe with the edges flush with the end of the pipe. Using a wire-feed MGAW welder, the holes are filled with weld material. The plug weld penetrates the internal collar and is joined to the center pipe as the hole is filled. The process is repeated for all four plug weld holes.

A properly designed and manufactured screw will provide many years of uninterrupted production. Even when a screw conveyor is subjected to an upset condition and the drive unit generates full-motor torque, there should not be a mechanical failure of any component in the screw conveyor. Once the motor reaches locked-rotor amps, the motor heaters or a circuit breaker will “kick out” and power will be immediately turned off to the motor.

As part of our commitment to Customer satisfaction, KWS always supplies CEMA standard screws with a minimum of four plug welds per screw that are guaranteed for the life of the screw.

Analyzing Different Types of Screw Conveyor Failures - KWS Manufacturing

Properly Designed and Manufactured KWS Screws Ready for Shipment