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The Importance of Workholding Flexibility
August 15, 2024
When you offer rebuilding services to heavy industry, you have to get creative sometimes. Breakdowns are never fun and when irreplaceable machines that are a critical part of the production process fail, you have to do whatever it takes to resume operations.
Many of the lessons we learned jumping through flaming hoops of broken bits can also be applied to salvaging new components to get the delivery and budget back where they need to be.
Remember my skepticism about repair welding cast iron? Millwrights think they can weld anything to anything if the superintendent insists on it. They are frequently wrong. Many fractured cast iron housings arrived at our shop as evidence of that.
We had excellent welders too, but only rarely did they succeed where others had failed. When they did, it was usually because we had the equipment to thoroughly machine the joint area and provide a clean place for the filler metal to attach. Most of the time, we were forced to fabricate a new part out of steel.
Repairs to steel housings were handled with ease. If required we could build up damaged areas with several layers of weld bead, stress relieve the mess, and machine it so you could barely tell the repair had been made. It was not necessarily cost-effective, but downtime charges trump everything else on some projects.
A common problem was bearing races spinning in the housing bore and the shaft alignment being lost. Once alignment is lost, gear teeth and bearing rollers tend to become detached. Teeth and rollers jam the mesh and production stops.
If there is space, I prefer to re-machine those damaged bores and fit new bearing retainers that enclose the bearing’s outer race. That way the bearing carrier becomes a sacrificial item if the race spins again. The more valuable a housing is, or the more difficult it is to remove for service, the more attractive this method becomes.
For less obnoxious housings, we used a “split lining” technique. Step 1 was to determine how much material had to be removed from the bores to get 80% clean-up. This guided us on the amount of stock to remove from each side of the housing split surfaces.
After the surfaces were milled, Step 2 was to re-bore the housing to the recommended bearing fits. The bores will not clean up at the split lines, of course, but alignment is restored and, with a bit of help from some sealing compound, the bearing races will be restrained from spinning again.
Next time — saving damaged gears.
