Planetary Gearbox Overheating —
Root Causes, Diagnosis and Prevention

A planetary gearbox overheating event is not simply uncomfortable — it initiates a cascade of degradation mechanisms that accelerate failure at every level of the gearbox simultaneously. Understanding exactly what happens inside the housing when temperature exceeds rated limits explains why the Arrhenius-based life prediction is so unforgiving, and why even brief temperature excursions compound over a machine’s lifetime.

Every Korea Ever-Power EP gearbox has a maximum input speed rating — the highest rotational speed at which the internal gear mesh, bearing system, and lubrication can maintain a normal operating temperature. Exceeding this speed does not immediately fracture gears; instead, it produces a rapid temperature rise driven by two mechanisms operating simultaneously.

First, bearing centrifugal forces increase with the square of rotation speed — at double the rated speed, centrifugal forces on the bearing balls quadruple, squeezing the lubricant film between balls and raceway and increasing friction heat by the same factor. Second, gear mesh frequency (the number of tooth-to-tooth engagements per second) increases linearly with speed — at double speed, every heat-generating tooth engagement occurs twice as often, doubling the mesh heat generation per unit time.

Common trigger in Korean industry: Variable-frequency drives (VFDs) allow servo motors to run above their nameplate speed. A Korean packaging machine upgraded from 80 CPM to 120 CPM by increasing VFD frequency from 50 Hz to 75 Hz runs the motor — and the gearbox input shaft — at 1.5× rated speed. Unless the gearbox was originally specified with headroom for this speed increase, the gearbox begins overheating within days of the upgrade.

Overheating Diagnosis — Root Cause 1 Signature

Torque overload generates heat through the direct relationship between friction losses and transmitted power. A planetary gearbox operating at 97% efficiency dissipates 3% of its input power as heat. At rated torque and speed, this heat is within the gearbox’s thermal capacity — the housing surface area radiates and convects it fast enough to maintain steady-state temperature. When applied torque exceeds the rated value, friction power increases in proportion, and the housing temperature rises until either a new thermal equilibrium is reached or the maximum seal/bearing/grease temperature is exceeded.

The worm-to-planetary replacement trap: A Korean food packaging line replaces a worm reducer (η=60%) with an EP-BPG planetary (η=97%) to save energy. The facility engineer notes that the planetary is more efficient — and chooses a motor with the minimum power for the planetary’s running torque at 97% efficiency. What the engineer misses: the motor is now also more efficient, delivering more torque per ampere than before. The conveyor that previously ran at 80% of the worm reducer’s rated torque (limited by motor heat) now runs at 95% of the planetary’s rated torque — and on heavy material days, briefly exceeds it. The gearbox overheats within weeks.

Korea Ever-Power EP series gearboxes are factory-filled with sealed grease designed for the full gearbox service life — no periodic re-greasing is required or recommended under normal operating conditions. Overheating from grease degradation occurs through three mechanisms: natural end-of-life oxidation (at the normal usage rate, after approximately 20,000 hours), accelerated oxidation from previously overheated grease, and contamination from external sources breaching the seal.

The over-greasing failure mode is specific to Korean industrial practice and deserves special attention. When a maintenance team adds grease to a sealed planetary gearbox — either because they believe it needs routine lubrication or because they misidentify a seal leak — the added grease increases internal pressure, forces the existing grease against the seals, and may introduce incompatible grease types. Internally pressurised grease creates churning losses that directly add to operating temperature. Korean field cases confirm that over-greased EP gearboxes can reach housing temperatures 20–30°C above normal within one operating shift after the incorrect greasing.

A gearbox that operates within temperature limits in March may overheat every August — not because anything has changed in the machine, but because Korean summer ambient temperatures add directly to the gearbox’s steady-state operating temperature. This “ambient stacking” effect is the most frequently overlooked root cause in Korean industrial overheating cases, and the one that produces the most frustrating pattern: the gearbox runs fine for eight months of the year and fails in summer.

The steady-state gearbox housing temperature is approximately: T_housing = T_ambient + ΔT_operating, where ΔT_operating is the temperature rise from friction losses above ambient — typically 20–40°C for a correctly sized gearbox. If a gearbox produces ΔT_operating = 40°C and the Korean factory ambient is 18°C in March, the housing reaches 58°C — well within the 90°C grease limit. In August, the same Korean factory with poor ventilation may reach 38°C ambient — the same gearbox now reaches 78°C. Add a partial load increase from summer production surge, and the housing exceeds 90°C.

The steady-state housing temperature can be estimated before installation using the thermal equilibrium model: at steady state, the heat generated by friction losses equals the heat dissipated through the housing surface by natural convection and radiation. Solving for the housing temperature above ambient gives the operating ΔT.

This calculation reveals that the gearbox at 300 N·m (its rated value) would exceed safe operating temperature without forced ventilation — meaning Korea Ever-Power’s published rated torque assumes a ventilated installation or intermittent duty cycle. Always confirm the duty cycle (continuous vs intermittent) and ventilation condition when selecting gearbox frame size for continuous high-load Korean conveyor and packaging applications.

Always calculate T_rated = T_running × SF (1.25–2.5). Never specify at running torque alone. SF absorbs startup peaks, seasonal load variation, and material surges that cause transient overload.

Confirm n_motor × (f_VFD/50) ≤ n_max_gearbox. Always recheck when VFD frequency is changed. This step prevents the most common Korean conveyor/packaging overheating cause.

Use T_ambient = 38°C as the summer design basis for Korean indoor factories without air conditioning. Verify T_housing = 38 + ΔT_operating ≤ 80°C (conservative), not ≤ 90°C (limit).

Instruct maintenance teams explicitly: sealed Korea Ever-Power EP series require no re-greasing. Over-greasing is a direct cause of overheating in Korean field experience. Post a label on the gearbox if necessary.

Gearboxes must not be enclosed in cabinets without ventilation. Minimum 50 mm clearance on all faces for natural convection. Directional fan air preferred for continuous high-load applications.

Continuous S1 duty and intermittent S3 duty have different permissible torques for the same gearbox. Verify rated torque in the catalogue corresponds to your actual duty cycle before final frame size selection.

During annual maintenance, record housing temperature at steady state and compare to previous year. A 5°C year-on-year rise indicates beginning grease degradation — plan replacement in the next maintenance window.

그만큼 EP-KF/KH hypoid series 0°C minimum is not a cold-start limit — it is the operating minimum. Using KF/KH in environments where temperatures reach 0°C produces grease viscosity that generates excess heat from churning at low temperature. Paradoxically, a “cold” KF/KH can overheat from cold-temperature churning losses.

큐
My EP-AB gearbox ran fine for 2 years and then started overheating with no machine changes. What is the most likely cause?

A gearbox that ran correctly for two years and then begins overheating without machine changes is exhibiting natural end-of-life grease degradation. At three-shift Korean operation (~6,300 hours/year), two years of operation equals approximately 12,600 hours — past the halfway point of the 20,000-hour design life. The grease is beginning to lose its viscosity-temperature characteristics from normal oxidation aging. This manifests as a gradual, steady temperature rise over weeks or months rather than a sudden step change. The correct action is to schedule gearbox replacement in the next planned maintenance window — typically at the next annual shutdown. Do not attempt to add or replace grease in the sealed unit; the housing must be replaced. For simple conveyor and agitator drives where continuous torque is modest, the Economic Line sealed-grease construction provides the same zero-maintenance principle at lower cost.

큐
Is it safe to continue running the gearbox at elevated temperature while waiting for the replacement unit to arrive?

This depends on the temperature and root cause. If housing temperature is 80–90°C (10–20°C above the 70°C typical operating point), short-term continued operation is possible with active monitoring — increase monitoring to every two hours and watch for any rapid temperature increase (more than 5°C per hour), unusual noise, or visible grease leakage. If housing temperature exceeds 90°C, Arrhenius life reduction is accelerating rapidly and continued operation should be minimised — reduce load or speed if possible to lower temperature. If temperature exceeds 100°C, stop the gearbox immediately; bearing and seal damage may already be irreversible, and continued operation risks a complete in-service failure that is more disruptive and dangerous than a planned shutdown.

큐
Can I install a cooling fan on the gearbox housing to solve overheating?

Adding forced cooling can address Root Cause 4 (ambient temperature stacking) and provide temporary relief for mild Root Cause 2 (modest overload). A 2 m/s airflow over the gearbox housing can reduce steady-state ΔT by 30–50%, providing meaningful temperature margin. However, forced cooling cannot address Root Cause 1 (overspeed) or Root Cause 3 (grease degradation/contamination) — in those cases, the problem source remains active regardless of cooling added externally. Forced cooling applied to an overloaded gearbox (Root Cause 2) reduces the symptom while gear tooth fatigue accumulates unseen inside the housing. The correct sequence is always: (1) diagnose root cause, (2) fix root cause, (3) add cooling as supplementary margin if ambient conditions justify it.

큐
For a multi-axis Korean gantry machine, should I connect CV shafts to gearbox outputs that run hot?

If a gearbox in a multi-axis gantry system runs at elevated temperature, it is particularly important to diagnose and correct the root cause before adding coupling components. An overheating gearbox bearing produces shaft runout as the bearing clearances change with thermal expansion — a hot gearbox output shaft may oscillate in position by 0.01–0.05 mm with thermal cycling. When a CV drive shaft connects two axes that need synchronisation, thermal position drift in one gearbox introduces gantry synchronisation error that compounds with actual trajectory errors. Maintain normal gearbox operating temperature before integrating precision shaft couplings in synchronised multi-axis gantry configurations.