Complete Overhaul of Mitsubishi Motors HF-H54S-A51 and HF-H104S-A51

26.11.2025 by Viktor Siebert

Complete Overhaul of Mitsubishi Motors HF-H54S-A51 and HF-H104S-A51

A long-standing industrial customer faced an important decision. The machine tool he had relied on for more than fifteen years was showing increasing signs of aging. More and more often, the CNC reported axis and servo faults; sometimes the errors disappeared after a restart or after a new calibration run. Over time, however, these intermittent failures became more frequent, and two servo amplifiers of the MDS-D series had already failed. The machine was still an essential part of his production schedule and was expected to remain in operation for several more years. Because of this, the customer decided on a complete technical overhaul of all servo motors.

He sent us all motors from the machine, including the two larger axis units HF-H54S-A51 and HF-H104S-A51. Already during unpacking, the typical picture of a heavily used machine appeared: strong contamination, coolant ingress, corrosion, hardened seals and, in some cases, very noticeable bearing noise. Despite their condition, all motors were still electrically functional. The robustness of the HF-H series became evident: even after such a long service life, the motors still ran mechanically, although clearly beyond acceptable wear limits.

Before taking anything apart, we tested each motor individually on our MDS-DH test bench. For this purpose, we used MDS-DH-V2-1010 and MDS-DH-V2-2020 drives and checked current consumption, commutation, torque behavior, brake functionality and encoder feedback. Several motors showed unstable encoder signals from their OSA105S5 units. It was particularly noticeable that some encoders had absorbed oil, a very typical damage pattern when housings are not fully sealed, when seals harden over time or when the motor has operated many years under coolant exposure. Such conditions can lead to feedback errors (41, 42, 43), matching exactly what the customer had been experiencing on the machine.

After completing the incoming inspection, we began the full disassembly process. Each motor was completely dismantled: housing, stator, rotor, encoder mount, brake unit (where applicable), connectors and terminal covers. All old coatings were removed, every component was cleaned ultrasonically, and bearing seats as well as sealing surfaces were inspected. Many bearings had reached the end of their service life. The races showed clear pitting and the noise at higher speeds was unmistakable. The seals were hardened, some brittle, which allowed coolant to enter the motor interior. In some cases, contamination had reached as far as the encoder compartment.

The mechanical refurbishing was carried out with precision: installation of new industrial-grade bearings, new shaft seals, cleaned shafts and corrected bearing seats, reworked mating surfaces and replacement of corroded screws. Motor connectors, often affected by coolant mist, were also replaced.

The housings received our standard protection system: full stripping, primer coating and then a multi-layer industrial repainting to ensure long-term resistance.

The most sensitive part concerned the OSA105S5 encoders. These were opened, cleaned, fitted with new internal bearings and recalibrated. Each encoder was tested individually before being mounted back onto its motor.

Finally, the functional tests were carried out on our test bench:

reference run
constant-speed simulation
load simulation
temperature monitoring
tests under various acceleration profiles

Only after all tests passed were the motors packed and shipped back to the customer. In total, our team needed seven working days to overhaul all motors, while simultaneously handling other customer orders. This was an excellent performance from our workshop team.

The result: the customer’s machine now runs smoothly again, without intermittent axis faults. The previously failed servo amplifiers were repaired and returned to operating condition as well. By overhauling the entire drive system, the customer significantly extended the machine’s operational life an economically sensible decision compared to purchasing a new machine.