Key takeaways
- The signs: noise, heat, leaks and metal in the oil
- Every part is cleaned and inspected before decisions are made
- Housing bores and shaft fits are often the real cause
- Worn fits are rebuilt, then machined back to size
- Bearings and seals go in new at reassembly
We rebuild gearboxes of any brand or size. People are often surprised by how much of a rebuild is metalwork on the housing and shafts instead of the gears. Here is the sequence, in the order it happens.
Signs a gearbox needs rebuilding
The box tells you before it quits. Watch for these:
- A new whine, growl or knock that changes with speed or load
- Heat at the case, or oil that discolors quickly
- Oil leaks at shaft seals or the case joint
- Metallic flakes or glitter on the magnetic drain plug or in a sample
- Shafts with end play or movement you can feel by hand
Step 1: teardown and cleaning
The box is drained and taken apart with attention to how each part sat, so assembly can follow the same order. Gears, shafts, bearings and the case are cleaned so wear marks and cracks are visible. A dirty part hides what is wrong with it.
Step 2: inspection
Each part gets a careful look. Gear teeth are checked for pitting, scoring and broken or worn faces. Bearings are checked for roughness and play. Shafts are checked for journal wear, damaged keyways, grooved seal areas and straightness. The case is checked for bore size and round at every bearing seat and for the alignment of the bores to each other.
On steel gears, shafts and cases that have failed or run hot, Magnaflux can show cracks that are invisible by eye. Our guides on repair or replace cover how those findings change the plan.
Step 3: the findings call
With the parts inspected, you decide what to repair and what to renew. Gears worn through are replaced. Bearings and seals are renewed as a rule. Shafts and housings that are salvageable are repaired. We have rebuilt gearboxes for the mining industry, saw mills, power companies and the printing industry.
Step 4: housing and shaft repair
This is the machine shop's part of the job. A bearing bore worn oversize is built up by metal spray, a sleeve or weld and bored back, using align boring where several bores have to share one line. A shaft journal or seal area is rebuilt and turned to size. Keyways are recut. Faces that seal against covers are trued so the joint seals.
Step 5: reassembly and checks
New bearings and seals go in. Gears are set to the right contact and backlash, shafts are checked for free rotation, and the case is closed and filled. Rotating parts that need balancing are balanced by an outside balancing shop that we arrange after machining.
A word on downtime
Plan the gearbox rebuild with the plant or machine schedule in mind. Ask for the findings call as early as you can, since decisions on gears and bearings drive the rest of the job. Having the old oil sample and the nameplate photos ready speeds that first conversation.
What to send
Photos of the nameplate and the case, the symptoms, and what the box drives. If you can open an inspection cover, photograph the gears. Call (775) 359-5570 or attach the photos to the quote request. See also the gearbox repair page.
Sources
- AGMA, American Gear Manufacturers Association (2026-10-18)
- Thermal spraying (Wikipedia) (2026-10-18)
