Marathon BM50M: micromotor bearings inaccessible for service
What happened
A Marathon dental handpiece with a BM50M micromotor came in for repair. The collet was fixed without difficulty, but the engineers could not reach the motor bearings. First checks assumed a brushed motor; no brushes were found.
What was checked
- Checked for brushes first, following general micromotor practice — the BM50M is brushless and has none.
- Removed and serviced the collet successfully.
- Attempted to open the motor housing to reach the rotor bearings: the housing is glued shut, and access would require desoldering the windings, with Hall sensors and a magnet inside.
Cause
The model was misidentified at the start. The BM50M is a brushless motor and the manufacturer does not provide for bearing replacement: the housing is glued shut, and inside are windings that would have to be desoldered, along with Hall sensors and a magnet.
What was done
The engineers replaced the BM50M as a complete spare unit. Rebuilding the bearings was judged technically possible but a lottery — high labor and unpredictable results — so it was not attempted. Reported as solved.
Safety
Before opening a handpiece, disconnect the micromotor from the control unit and treat the instrument as contaminated after patient contact: clean and disinfect it first, and confirm electrical safety before return to use.
Lesson: Identify the exact motor type before planning a repair — sealed brushless motors are usually not designed for bearing replacement, and the sound fix may be replacing the whole unit.
Covered in the course: Dental Equipment Service — Level 2: Fault-Finding →
Written from an anonymised case shared by service engineers. It is not a manufacturer instruction and grants no authorisation. Independent note; names are trademarks of their owners.