“I just took over an aging shredder. How do I determine if I need a new rotor and how much life is left in it?”
Good question! First, compare the original condition to the current condition. Try to find the rotor arrangement drawing, which came with the shredder when it was new or when the replacement rotor was purchased. This drawing should show disk thickness and key dimensions.
✔️ Check that the tie rods are still holding the rotor tightly together
Measure the outside-outside end disk distance across the rotor. If it’s growing an inch or more, the spread can mean loose or broken tie rods or loose or missing end disk nuts. A loose rotor will have limited life.
✔️ Look at rotor disk thickness and edge wear
Monitor the disks diameter indirectly, by tracking the distance between hammer pin hole and edge of the disk.
✔️ Monitor disk thickness and “wash out” around the hammer swinging area about the pin hole.
✔️ Check bearing bolts for tightness and bearing housings for damage
Clean up debris and keep bearing areas clear.
Tips for maintaining the rotor to extend its life
✔️ Build up the disk diameter with weld rod; build up gradually and evenly.
✔️ Don’t let the edge of the disk get too close to the pin hole — this leads to a break out at the pin hole. Break outs can be “repaired,” but they don’t last. Better to have a new rotor on deck at that point.
✔️ Keep in mind the rotor disk is doing some of the densifying work in the mill, thus the wear. Pin protectors should extend past the end disk. They protect the end disk as well as the hammer pin as they mash and densify scrap in the mill.
✔️ Build up disk thickness where hammers might ‘wash out’ material on the disk sides. Build up evenly, attempting to keep the rotor balanced.
✔️ 7018 rod is an economic build-up rod. A better, but costlier rod is Lincoln Lincore 15CrMn or Stoody 2110 or 110. Don’t let an outside welding supplier sell you on any other fancy hardsurface welding wire or electrodes. Fancy doesn’t pay in this application.
✔️ Build up a bit, let it wear a few days, then build on top of it. You can have a problem welding on top of the previous work-hardened welding and not having it “spall” off.
Other advice
If you have a severely worn rotor, consider recording quarterly measurements.
If you see the distance outer-disk-to-outer-disk start to spread, you may be on the way to a failure. It could be months or it could be YEARS. Hard to say. That’s why you measure regularly. The key indicator is rate of change.
The amount of heavies, rotor bearing damage, and bent rotor shaft also tell the tale of the life of the rotor.
I figure it this way: If you’re going to stay in the shredding business, have the spare. The money is spent either now or once it fails. You can avoid months of lost production with a spare rotor on site. Consistent production is the key to stable revenue.
