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October 12, 2018

Troubleshooting stepper motors, a “whodunit” mystery woe that applies elsewhere as well

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Stepper motors or similar devices are used to move the stopper rods or metering pins that regulate molten metal flow into the casting machine.

They serve an important function, and are utterly dependable…until they are not. Here is a lesson-learned experience of a real-world problem that were encountered, what was done to right them and a tip that applies both for this problem as well as elsewhere.

After years of providing reliable service, operations began experiencing numerous failures of the stepper motor that resulted in significant downtime. The stepper motors would just stop working during casting, and the operator had no way to control the flow, which resulted in either over-pouring of the casting wheel or under-pouring that resulted in hollow bar and bar breaks. In either case, this led to replacement of the stepper motors. What was worrisome was that this problem went from occasional, two or three times a year, to more than 50. Multiple failures per day were being experienced.

Since purchasing had not changed suppliers of stepper motors, operations and maintenance staff looked at every operational procedure to find what had changed. This included the stepper motor getting too hot from not being properly insulated from the pour pot, and mishandling by operations. The occurrences piled up and maintenance could not find anything wrong with the motors!

A maintenance program was established to check all stepper motors prior to being put into service on the casting machines. Electrical input was simulated to monitor performance in the maintenance area, and the passing stepper motors were then placed into parts storage. However, the stepper motors continued to fail. A “passed” stepper motor was removed from storage, after already having passed inspection, was retested. It too failed.

After months of failures, the root cause was finally identified. The supplier of the stepper motors had changed its sourcing of critical electronic components within the motor apparatus. Even though the new components met the supplier’s specification, they did not perform on the factory floor. The ambient heat in the storage area alone was enough to effect performance.


Tip: Many times in manufacturing, we make certain assumptions of how equipment, materials or procedures will perform, only later to find that these were mistaken assumptions. Just because a part is “new,” don’t assume it must work properly, even if it has in the past. Note the dates when problems first occur, ask suppliers early on if anything has changed on their end, including sourcing of parts within the component. Ask the supplier to go to their suppliers and ask if anything has changed.

In an age of outsourcing to either reduce costs or find alternative suppliers, a supplier may insist that a new part or material is identical or meets specification. They may well believe it, and they may well be wrong. We have seen this on electrical components, such as the stepper motor, but similar tales exist for gear boxes on filtration equipment, refractories and lubricants. The take here is to never assume something new is a given. Also, it is important to network with others that use the same or similar components and materials and ask them if they are also experiencing problems.

Additional Info

  • Company: Copper ConCast Consultants
Read 4 times Last modified on October 12, 2018
 

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