Mechanical Rating vs Thermal Power in an H/B Industrial Gearbox

An H/B industrial gearbox carries two limits, and they are not interchangeable. Mechanical rating is set by material and geometry — tooth flank and root strength, bearing life, shaft strength and stiffness — and it barely moves with the weather. Thermal rating is set by heat dissipation, and it falls the moment ambient temperature or altitude rises. The power a unit can actually transmit continuously is therefore the lower of the two:

P_allow = min(P_mech, P_th)

The shorthand: mechanical asks "can the teeth take it", thermal asks "can the oil cool down".

Why These Are Two Independent Limits

They come from different physical processes and they fail in different ways.

The last two rows are the key. A unit on a northern site in midwinter and the same unit on a southern site in midsummer have almost the same mechanical rating — but their thermal ratings can be far apart. Select on the mechanical side alone and that unit will be running hot, derated or tripping alarms by summer.

Where the Thermal Rating Comes From

The thermal rating is not a number lifted from a table. It has an explicit derivation chain.

Step 1 — the heat comes from power loss.

P_loss = P_in · (1 − η)

P_in is input power, η is efficiency, and P_loss is the power converted into heat.

Step 2 — dissipation depends on area and temperature difference.

Q_diss = k · A · ΔT

k is the heat transfer coefficient, A is the dissipation area, and ΔT is the difference between oil temperature and ambient temperature.

Step 3 — thermal equilibrium: heat generated equals heat dissipated.

P_in · (1 − η) = k · A · ΔT

Step 4 — solve for the thermal rating.

P_th = k · A · ΔT / (1 − η)

The temperature difference itself is:

ΔT = T_limit − T_ambient

T_limit is the permitted oil temperature ceiling and T_ambient is ambient temperature.

That chain settles the point: the thermal rating is not an intrinsic property of the unit. It is the capacity that unit has at a given ambient temperature. Change the ambient, and the number changes with it.

Ambient Temperature and Altitude Press Only on the Thermal Side

Two things follow directly from the P_th expression.

Ambient temperature rises → `ΔT` shrinks → `P_th` falls. This is the intuitive one: summer is harder than winter not because the gearbox got worse, but because the available temperature difference got smaller.

Altitude rises → air thins → heat transfer coefficient `k` falls → `P_th` falls. This one is easier to miss, because nothing visible changes on site — only the density of the air. A high-altitude project selected on sea-level data will end up tight on the thermal side.

Mechanical rating is largely insensitive to both. That is the entire distinction: one capacity depends on the environment, the other depends on the structure.

What Sets the Mechanical Rating

The mechanical side is four checks, and the first one to reach its limit governs.

All four are fixed at the design stage by material, heat treatment, face width, module, shaft diameter and bearing selection. They do not move with the weather.

The Service Factor System: An Institutional Corroboration

One detail of the selection system is worth stating, because it corroborates the distinction this article is making.

Under the ReducerDB selection rules:

• R/F/K series — total service factor f = f1 · f2
• S series — total service factor f = f1 · f2 · f3, where `f3` is the ambient temperature factor

R/F/K:  f = f1 · f2
S:      f = f1 · f2 · f3

Why does the S series carry one more factor? Because it turns the effect of ambient conditions on capacity into an explicit reduction. The R/F/K series describes duty through f1 (type of driven machine and hours loaded per day) and f2 (starts per hour); ambient temperature does not appear separately in that factor system.

This is not a claim that the R/F/K series ignores ambient temperature. It is a claim about the system: inside the S series, ambient temperature is formally recognised, so it must be multiplied in separately at selection.

The selection criterion is:

Choose the size from the selection tables by running power and output speed, and ensure the calculated total service factor does not exceed the service factor given in the selection tables.

The value of f3 must be looked up in the selection tables against the actual ambient temperature. It is not given here.

Which One Is in Control

The method is direct: calculate both requirements, and the larger one is the controlling one.

The intermittent case needs care. Intermittent duty means there is shutdown time to shed heat, so equilibrium may never be a problem — but if any one stretch has to run continuously, that stretch is the governing thermal case, not the average of the shift.

The Cooling Decision Tree

When the calculation gives P_loss > Q_diss — heat generated exceeding heat dissipated — four routes exist:

P_loss > Q_diss ?
  ├─ no  → thermal side is not the constraint; select on mechanical capacity
  └─ yes → four options:
       ├─ 1. larger housing (increase dissipation area A)
       ├─ 2. add a fan (increase heat transfer coefficient k)
       ├─ 3. add an oil cooler (raises k substantially; needs supporting hardware)
       └─ 4. reduce load or improve ventilation (lower P_loss or raise k)

The trade-offs:

Order of attack: check first whether ventilation can be improved (cheapest), then consider a fan, then whether a larger housing is unavoidable, and only last an oil cooler (highest system complexity).

Condition Table

Field Checklist

At selection:

1. Establish the running power P and the output speed.
2. Establish the driven machine type and the hours loaded per day (for f1).
3. Establish the starts per hour (for f2).
4. Confirm whether the unit falls under the S series system — if so, the ambient temperature factor f3 must be included.
5. Collect the ambient temperature range, using the maximum, not the average.
6. Confirm the altitude.
7. Confirm the mounting arrangement and ventilation (indoor, outdoor, enclosed space).

At verification:

1. Select the size on mechanical capacity and verify flank and bearings.
2. Check thermal power at the most adverse ambient temperature.
3. Compare P_loss with Q_diss to decide whether cooling is needed.
4. Check output radial and axial loads separately.

After commissioning:

1. Record actual oil temperature under summer or high-altitude conditions as the basis for later review.

Common Mistakes and Where This Article Stops

Common mistakes:

1. Selecting on mechanical capacity only, with no thermal calculation. It can look fine in winter and at low altitude, and then fail in summer or on a plateau.
2. Calculating thermal power with the average ambient temperature. The most adverse — the maximum — is the one to use.
3. Ignoring altitude. Lower air density weakens dissipation directly.
4. Treating the thermal rating as a fixed property of the unit. It is a capacity at a given ambient; change the ambient and it changes.
5. Sizing an intermittent-duty unit on continuous duty (over-designed), or a continuously running unit on intermittent duty (under-designed).
6. Improving ventilation without rechecking fan type and dust maintenance.
7. Ignoring radial and axial loads. They are a separate limit outside mechanical capacity and must be checked on their own.

Where this article stops:

• The two-limit distinction, the formulas, the ambient-sensitivity analysis and the cooling decision apply to selection checks on H/B industrial gearboxes and general-purpose reducers.
• This article gives no mechanical rating values, no thermal power values, no heat transfer coefficient or dissipation area values, and no altitude correction figures.
• It gives no numerical value for the `f3` ambient temperature factor — that must be looked up in the selection tables against the actual ambient temperature.
• It does not quantify where selection problems "mostly" come from; it gives the distinction between the two limits and the method for judging which one applies.
• A specific selection must follow the product selection tables and the requirements of the individual machine. This article does not replace a check calculation for a specific project.

Selection Input Checklist

Items 7 and 8 are the two most easily missed: neither affects mechanical capacity, both press on thermal power — so a mechanical check alone shows nothing about them.

About This Article and the Publisher

This article was written by the Chengfa Reducer (Changzhou) Co., Ltd. (CHINFAR) technical team and released after technical review. The H/B series industrial gearboxes are supplied by Chengfa Reducer (Changzhou) Co., Ltd.; CHINFAR products are built and delivered on the joint manufacturing system shared with HUIGUANG.

Related pages:

• H/B series industrial gearboxes
• H/B series industrial gearbox application overview
• Gearbox overheating: tracing a temperature fault back to its cause

The overheating article is diagnosis: the unit is already hot, so why. This one is selection: which of the two limits applies, and how each is established. The two run in opposite directions and complement each other rather than overlap.

Next Step

Send the eleven items above to sales@chinfar.com. If the project sits in a hot region or at altitude, state items 7 and 8 explicitly — they leave the mechanical check untouched while pressing only on thermal power, and a mechanical check alone will never show them.

One point to be clear about: this article gives no rating values and no environmental correction factors. A specific selection must follow the product selection tables and the requirements of the individual machine, and output radial and axial loads still require a separate check.