Gearhead Torque Calculator | Gear Ratio, Output Speed and Quote Check

Ratio used, required ratio, output speed, theoretical and usable torque, margin, output power and comparable price by quote
Quote	Ratio used	Required ratio	Output rpm	vs required	Theoretical (N·m)	Usable (N·m)	Torque margin	Output (W)	Comparable price
Supplier A · 60 W, same rpm Does not meet requirement	30.00 actual	30.00	53.33	0.00%	7.842	7.842	-33.33%	43.8	185,000
Supplier B · 90 W, 81% efficiency Checks passed	30.00 actual	30.00	53.33	0.00%	13.05	13.05	+10.96%	72.9	248,000
Supplier C · imported 50 Hz Does not meet requirement	25.00 nominal	30.38	64.80 50 Hz rated 54.00	+21.50%	10.74 estimate · 50 Hz rated 12.89	10.74	-8.68%	72.9	198,000

Stated output speed and torque versus calculation, implied ratio and implied efficiency by quote
Quote	Stated rpm	rpm gap	Implied ratio	Stated torque (N·m)	Torque gap	Implied efficiency	Max permissible (N·m)
Supplier A · 60 W, same rpm	53.30	-0.06%	30.02	7.845	+0.04%	73.03%	9.807
Supplier B · 90 W, 81% efficiency	53.30	-0.06%	30.02	13.00	-0.40%	80.67%	15.00
Supplier C · imported 50 Hz	54.00	0.00%	25.00	14.00	+8.60%	87.96%	—

Confirmed or unconfirmed supplier checklist items by quote
Quote	Duty rating, service factor and life basis	Output shaft, keyway, flange and mounting fit	Output rotation direction (same as or opposite to the motor)	Permissible overhung and thrust loads	Ambient temperature, low-temperature efficiency and supply voltage
Supplier A · 60 W, same rpm	Confirmed	Confirmed	Confirmed	Confirmed	Confirmed
Supplier B · 90 W, 81% efficiency	Confirmed	Confirmed	Confirmed	Confirmed	Confirmed
Supplier C · imported 50 Hz	Unconfirmed	Confirmed	Unconfirmed	Unconfirmed	Unconfirmed

What goes wrong when a replacement gearhead only matches rpm

When a conveyor, mixer, automatic door or small lift driven by a gear motor fails, most people look at the nameplate speed and ratio and search for a match.
Output speed, however, is only motor rpm divided by the gear ratio, so a smaller motor produces exactly the same rpm.
A replacement that matches only rpm may stall or overheat under the real load, and you pay for a second purchase plus the downtime.

This gearhead ratio, output speed and torque quote check calculator takes each supplier’s motor rpm, output or torque, gear ratio and efficiency and calculates output speed and theoretical transmitted torque.
It compares them with a requirement back-calculated from the existing drive nameplate, checks whether supplier figures agree with the calculation, and flags missing data such as maximum permissible torque or the 50/60 Hz basis.
The results can be saved as a checklist to send to the suppliers.

The gear formulas are universal, but the 60 Hz default supply frequency, the KRW example prices and the VAT handling are based on Korean rules effective in 2026.
Outside Korea, set the site supply frequency to your own mains (50 or 60 Hz) and enter every price in one currency without tax.

Who this helps

Workshop and small-factory owners comparing two or three quotes for a replacement gear motor
Maintenance staff considering an imported product sold with a 50 Hz catalog rating
Operators changing conveyor speed with a new ratio who want to confirm torque first

Key features from requirement to supplier checklist

Requirement from the old nameplate

Enter the old motor’s rated speed, output in W or rated torque, gear ratio and efficiency, and the same formulas give the required output speed and torque.
If an engineer has set a load torque, switch to direct entry instead.

Output speed and transmitted torque

For every quote it calculates output speed, theoretical torque, usable torque capped at the maximum permissible torque, and output power.
Motor output in W is converted to torque, and each quote can use N·m, mN·m, kgf·cm, kgf·m or lbf·in.

Supplier specification check

Stated output speed and torque are compared with the calculation against your tolerance.
The implied ratio and implied efficiency worked back from the specification reveal hidden differences such as a peak torque or a 50 Hz basis.

Supplier checklist and lowest price

Five items are tracked per quote: duty rating, mounting dimensions, rotation direction, shaft loads and operating environment.
The lowest VAT-excluded price among quotes that passed every check is shown, and a TXT checklist is saved in your browser; no input is sent to a server.

Match the meaning and units of each input first

Rated motor speed: the rated (loaded) speed on the nameplate or catalog.
Entering a no-load value such as the 1,800 rpm synchronous speed of a four-pole induction motor overstates the output speed.
Motor output in W or rated torque: Korean quotes often show only watts, which the calculator converts to torque.
If the catalog lists rated torque, entering it is more accurate; never enter starting or peak torque.
Nominal and actual ratio: for 1:30 enter 30.
If the actual ratio is unconfirmed, the nominal ratio is used and the item stays on the checklist.
Gearhead efficiency: the manufacturer value for that ratio, in percent.
Lower efficiency means less output torque from the same motor and ratio.
Stated output and maximum permissible torque: the supplier’s output speed, output torque and gearhead torque limit.
Enter all three in that quote’s torque unit; blanks skip the related check and are listed as missing.
Choose the torque unit before typing the numbers.
Changing the unit converts the values already entered to the same magnitude, so if you typed them in the wrong unit, switch the unit and then re-enter the numbers.
Site supply and rating frequency: an AC motor on the mains runs at different speeds on 60 Hz and 50 Hz, so set the supply frequency of the machine (60 Hz in Korea) and the frequency each catalog rating uses.
When they differ, output rpm and torque are converted to the supply frequency before judging; choose the drive option when an inverter, DC, BLDC or servo drive sets the speed.
The old nameplate has its own rating frequency too, so a 50 Hz-only nameplate on imported equipment running at a different site frequency converts the requirement the same way.

Gearhead output speed and torque formulas

Output speed and transmitted torque

Output speed n_G = n_M ÷ i
Theoretical torque T_G = T_M × i × η
Motor torque T_M = P × 60 ÷ (2π × n_M) (when output in W is entered)
Usable torque = min(T_G, maximum permissible torque)
Supply conversion k = f_supply ÷ f_rating → output rpm × k, torque ÷ k (only when k > 1)
Output power (W) = usable torque × n_G × 2π ÷ 60

The ratio i is how many times the motor speed is reduced, and η is the gearhead transmission efficiency.
The speed and torque formulas come from Oriental Motor’s gearhead specification data and glossary, and its power formula P = 1.047 × 10⁻¹ × T × N is solved backwards to turn watts into torque.
Because the load torque a gearhead can transmit is limited by its gear material and size, any calculated torque above the permissible value is counted only up to that limit.
For a quote rated at a frequency other than the supply, speed is scaled by k following synchronous speed 120f/P; when it runs faster, torque is estimated at constant output as torque ÷ k, and when it runs slower, torque is not raised.
The supplier specification check still compares figures at the rating frequency they were stated for.

Specification check metrics

Specification gap = (stated − calculated) ÷ calculated × 100
Implied ratio = motor rpm ÷ stated output rpm
Implied efficiency = stated output torque ÷ (motor torque × ratio) × 100
Torque margin = (usable torque ÷ required torque − 1) × 100
Required ratio = motor rpm × k ÷ required rpm

Conversion factors from each torque unit to N·m
Unit	Meaning	1 unit = N·m
N·m	Newton metre (base unit)	1
mN·m	Millinewton metre (small motors)	0.001
kgf·cm	Kilogram-force centimetre (Korean catalogs)	0.0980665
kgf·m	Kilogram-force metre	9.80665
lbf·in	Pound-force inch (US catalogs)	0.112984829

Reference transmission efficiency of Oriental Motor GN gearheads by number of stages
GN gearhead stages	Reference efficiency
One reduction stage	90%
Two reduction stages	81%
Three reduction stages	73%
Four reduction stages	66%

These reference efficiencies describe one manufacturer’s gearheads, so they are never used as defaults.
Use them only to understand that efficiency falls as higher ratios add stages, and enter the catalog value of the product actually quoted.

Step-by-step use

Photograph the motor and gearhead nameplates of the failed drive and note rated speed, output in W and gear ratio.
Find the old gearhead efficiency in its catalog; if it is unavailable, enter the required output torque confirmed by an engineer.
Set the tolerance for required speed and for specification agreement, and the site supply frequency.
The default 5% is only a starting point; tighten it for speed-sensitive equipment.
Enter up to four supplier quotes.
Choose the torque unit and rating frequency as printed in each catalog, then enter motor rpm, output or torque, ratio, efficiency, stated output, maximum permissible torque and price.
Tick the items you have confirmed with each supplier.
Leave duty rating and service factor, shaft and flange dimensions, rotation direction, permissible shaft loads, and temperature and supply voltage blank until confirmed.
Read the status and items to check, then save the supplier checklist as TXT.
Send it to the suppliers, collect the missing data and confirm the replacement after an engineer’s approval.

A fictional example checked by the calculator

Three quotes for a conveyor drive

The old nameplate shows a 60 Hz, 1,600 rpm, 90 W motor with a 1:30 gearhead at 73% efficiency.
Motor torque is 90 × 60 ÷ (2π × 1,600) = 0.5371 N·m, so the requirement is 53.33 rpm and 11.76 N·m (119.95 kgf·cm) at the output.

Supplier A keeps ratio 30 but uses a 60 W motor, so its output is the same 53.33 rpm while theoretical torque is only 7.842 N·m, a torque margin of −33.33%, and it does not meet the requirement.
A’s stated 80 kgf·cm (7.845 N·m) differs from the calculation by just 0.04%, so the quote is honest but the product is too weak for this machine.

Supplier B combines a 90 W motor with an 81% efficient gearhead for 13.05 N·m, a +10.96% margin, and its figures differ by −0.06% in rpm and −0.40% in torque, inside the tolerance.
Supplier C is rated at 50 Hz and 1,350 rpm, so its rated output is 54.00 rpm, but on Korea’s 60 Hz supply it runs at 64.80 rpm, 21.50% too fast, and its estimated torque at constant output falls from 12.89 N·m to 10.74 N·m, a torque margin of −8.68%.
Its stated 14 N·m is 8.60% above the 12.89 N·m calculated at the 50 Hz rating, an implied efficiency of 87.96%, and matching the required speed at 60 Hz would need a ratio of 1,620 ÷ 53.33 = 30.38.
Although C’s 198,000 KRW is cheaper than B’s 248,000 KRW, it does not meet the requirement and is never shown as the lowest price.

Output speed, theoretical torque, torque margin and verdict for each fictional quote
Quote	Output (rpm)	Theoretical torque (N·m)	Torque margin	Verdict
Supplier A · 60 W, same rpm	53.33	7.842	−33.33%	Does not meet requirement
Supplier B · 90 W, 81% efficiency	53.33	13.05	+10.96%	Checks passed
Supplier C · imported 50 Hz	64.80	10.74	−8.68%	Does not meet requirement

These values come from the calculator’s own pure functions; they are fictional and are not real product specifications or market prices.

How to read the comparison

Does not meet requirement

Output speed outside the tolerance, or usable torque below the required torque, means the quote fails.
Use the required ratio in the output table, motor rpm at the supply frequency ÷ required rpm, to ask the supplier for a ratio or motor size that fits.

Needs review

The requirement is met, but the actual ratio, permissible torque, stated figures or checklist items are missing, or the specification disagrees with the calculation.
Each item to check is a ready-made question, so send the checklist, get the answers and enter them again.

Capped at permissible torque

When theoretical torque exceeds the maximum permissible torque, usable torque is reduced to that limit.
The motor could then overload the gearhead, so an engineer must confirm the load never exceeds the permissible value.

Checks passed

The requirement and specification agree within tolerance and every checklist item is confirmed.
Inertia, start frequency and shock loads were still not calculated, so get an engineer’s approval before ordering.

Korea’s 60 Hz supply and motor efficiency rules

Article 18 of the Electricity Business Act (MST 284383, effective 2026-09-18) requires electricity utilities to maintain supply quality, and Article 18 and Annex 3 of its Enforcement Rule (MST 291405, effective 2026-10-16) set the standard frequency at 60 Hz within ±0.2 Hz.
The same annex sets standard voltages of 220 V (±22 V) and 380 V (±38 V).
AC motors connected straight to the mains must therefore be compared on a 60 Hz rating, while a 50 Hz rating changes speed by about 1.2 times because synchronous speed is 120f/P.
That is why the calculator’s site supply frequency defaults to 60 Hz.

The Regulation on the Operation of Efficiency-Managed Equipment (Notice 2026-137, effective 2026-05-27) covers three-phase induction motors rated from 0.75 kW to 375 kW and measures full-load efficiency under KS C IEC 60034-2-1.
If the replacement motor falls in that range, ask whether it is a registered efficiency model.
Article 30 of the Value-Added Tax Act (MST 276117, effective 2026-01-02) sets VAT at 10%, so prices that include VAT are divided by 1.1 before comparison.
The calculator does not judge electrical compliance or certification, and the legal texts were checked through the Korean National Law Information open API on 2026-10-25.

Three practical situations

Replacing a failed gear motor in a hurry

When a discontinued model or long lead time forces a different brand, build the requirement from the old nameplate and enter the quotes side by side.
Quotes that match rpm but lack torque stand out immediately, which prevents a stalled machine and a second order.

Reviewing imported or online products

Overseas catalogs often give only 50 Hz values or state torque in lbf·in or kgf·cm.
Choosing the unit and rating frequency per quote puts everything on the same N·m basis, and a 50 Hz quote is judged on the speed and estimated torque it will actually have on a 60 Hz supply.

Changing conveyor speed with a new ratio

Switch the requirement to direct entry and enter the new target speed as the required rpm, and the required ratio is calculated for each quote.
Lowering the ratio to go faster also lowers output torque in the same proportion, while raising it may push torque past the permissible limit, so watch the margin either way.

Limits of the calculation and quote-checking tips

This is an arithmetic check at the rated operating point.
It does not calculate starting torque, load inertia and acceleration, service factor for start and stop frequency, permissible overhung or thrust loads, shock loads or brake capacity.
It does not select motors, and it does not design or approve lifts, hoists or other equipment with a falling-load hazard.

Depending on the ratio, the output shaft may turn in the same or the opposite direction to the motor.
A reversed direction means rewiring or another ratio, so confirm it on the checklist.
At low temperatures, thicker grease can reduce both transmission efficiency and output torque.
For cold stores or outdoor winter equipment, ask for characteristics at the operating temperature.
Continuous or intermittent duty changes gearhead life at the same torque.
Tell the supplier the daily running hours and start count and ask whether the selection includes a service factor.
A different shaft diameter, keyway or flange bolt pattern prevents mounting even when performance matches.
Send the existing drawing or measured dimensions with your enquiry.

Frequently asked questions

If the output speed matches, can I swap in any gearhead?

Output speed is simply motor rpm divided by the gear ratio, so a smaller motor gives exactly the same rpm.
Output torque is motor torque × ratio × efficiency, so a 60 W and a 90 W motor at the same rpm differ in torque by almost 1.5 times.
Check the required output torque, maximum permissible torque and duty rating together with rpm to avoid buying twice.

The quote lists only motor output in watts. What do I enter?

Choose “Enter motor output in W” and the calculator converts rated torque with T = P × 60 ÷ (2π × n).
For example, a 90 W motor at 1,600 rpm has a rated torque of about 0.5371 N·m.
The formula holds only when output and speed describe the same rated operating point, so enter the rated (loaded) speed, not the synchronous speed.

Which efficiency value should I use?

Use the transmission efficiency the supplier publishes for that gear ratio.
Oriental Motor’s technical data gives GN gearheads 90% for one stage and 81% for two stages, falling to 73% and 66% as stages are added.
Those figures are only a reference; efficiency varies with gear type, ratio, temperature and grease, so the calculator never fills them in by default.

If theoretical torque exceeds the maximum permissible torque, is that extra strength?

No.
The maximum permissible torque is the largest load torque the gearhead can take, set by the material and size of its gears and bearings.
Even if the motor could produce more, the gearhead cannot transmit it, so usable torque is capped at the permissible value and you are told to keep the load below that limit.

Can the nominal and actual gear ratios differ?

Some products list a nominal ratio such as 1:30 or 1:50 while the tooth counts give a slightly different actual ratio.
If you enter the actual ratio, output speed and torque are recalculated with it and the gap from the nominal ratio is shown.
If the actual ratio equals the nominal one, enter the same value so the item counts as confirmed.

Can I use a 50 Hz catalog rating in Korea?

The Korean standard frequency is 60 Hz under Annex 3 of the Enforcement Rule of the Electricity Business Act.
An AC induction or synchronous motor follows synchronous speed 120f/P, so a 50 Hz rating runs about 1.2 times faster at 60 Hz and, at the same output, its torque falls to about 1/1.2.
With the site supply frequency set to 60 Hz, the calculator converts a 50 Hz quote’s output rpm and torque to 60 Hz before judging and never passes it automatically, so ask the supplier for the 60 Hz rating and re-enter it.

What does an implied efficiency above 100% mean?

Implied efficiency is the stated output torque divided by motor torque × gear ratio.
Efficiency cannot exceed 100%, so this warning usually means the stated torque is a peak or momentary value, or that it belongs to a larger motor.
Confirm which operating point the supplier figure refers to before comparing.

Is the lowest price among passing quotes a recommendation?

No.
It only shows the cheapest quote whose entered figures meet the requirement and whose checklist is complete.
Prices that include VAT are divided by 1.1 for the 10% Korean VAT rate under Article 30 of the Value-Added Tax Act, and the final choice still needs an engineer to review mounting and load conditions.

Sources and next steps

Sources were checked on 2026-10-25.
Gear ratio, maximum permissible torque, transmission efficiency, synchronous speed and the power formula follow the Oriental Motor glossary, and the output torque formula and low-temperature note follow the Oriental Motor gearhead specifications.
The Electricity Business Act and Annex 3 of its Enforcement Rule, Article 30 of the Value-Added Tax Act and the efficiency-managed equipment regulation were checked in force through the Korean National Law Information open API.
Enter motor and gearhead figures, efficiency, permissible torque and prices from each supplier for every quote.

Enter your quotes above and save the supplier checklist to confirm maximum permissible torque, duty rating and load conditions.
Once the missing data is in and an engineer approves, you can confirm the replacement quote.

Gearhead Ratio, Output Speed and Torque Quote Check Calculator

1. Requirement from the existing drive

2. Supplier quotes (up to 4)

Comparison results

Requirement

Output speed and torque comparison

Supplier specification check

Supplier checklist

Supplier A · 60 W, same rpm · Items to check

Supplier C · imported 50 Hz · Items to check

What goes wrong when a replacement gearhead only matches rpm

Who this helps

Key features from requirement to supplier checklist

Requirement from the old nameplate

Output speed and transmitted torque

Supplier specification check

Supplier checklist and lowest price

Match the meaning and units of each input first

Gearhead output speed and torque formulas

Output speed and transmitted torque

Specification check metrics

Step-by-step use

A fictional example checked by the calculator

Three quotes for a conveyor drive

How to read the comparison

Does not meet requirement

Needs review

Capped at permissible torque

Checks passed

Korea’s 60 Hz supply and motor efficiency rules

Three practical situations

Replacing a failed gear motor in a hurry

Reviewing imported or online products

Changing conveyor speed with a new ratio

Limits of the calculation and quote-checking tips

Frequently asked questions

Sources and next steps

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