{"id":1983,"date":"2026-10-08T07:50:33","date_gmt":"2026-10-08T07:50:33","guid":{"rendered":"https:\/\/metalite.net\/?p=1983"},"modified":"2026-10-08T08:00:34","modified_gmt":"2026-10-08T08:00:34","slug":"how-to-select-worm-gearbox","status":"publish","type":"post","link":"https:\/\/metalite.net\/en_ca\/application\/how-to-select-worm-gearbox\/","title":{"rendered":"How to Select a Worm Gearbox: 5-Step Sizing Guide with Worked Example | Metalite"},"content":{"rendered":"
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Worm gearbox failures trace back to three recurring selection errors: an under-sized frame that overheats on continuous duty, an efficiency figure that was ignored when calculating output torque, and a service factor that was set to 1.0 on an application with daily shock loading. None of these errors is difficult to avoid \u2014 they require four numbers, a straightforward calculation, and one table lookup. This guide walks through that process step by step and finishes with a complete worked example that shows the method applied from motor specification to confirmed gearbox model.<\/p>\n

If you have already sized your application and know which product type you need, go directly to the relevant product page: Aluminum Series<\/a> \u00b7 WP Series<\/a> \u00b7 RV Series<\/a> \u00b7 Low Backlash Servo<\/a> \u00b7 Non-Standard<\/a>. If you are not yet sure which type fits, see our Worm Gearbox overview<\/a>.<\/p>\n

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The Four Numbers You Need Before Sizing<\/p>\n

\u2460 Motor input speed (RPM)<\/span>
\n\u2461 Required output speed (RPM)<\/span>
\n\u2462 Motor power (kW or HP)<\/span>
\n\u2463 Application shock load level<\/span><\/div>\n

If you have the required output torque directly (in Nm), even better \u2014 it skips the motor torque calculation. If you do not yet have the application shock classification, use the service factor table in Step 3.<\/p>\n<\/div>\n

\"Worm<\/p>\n

Step 1 \u2014 Calculate the Required Gear Ratio<\/h2>\n
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Formula<\/p>\n

Ratio = Input RPM \u00f7 Required Output RPM<\/p>\n<\/div>\n

Divide your motor’s rated speed by the output shaft speed your application requires. The result is the theoretical gear ratio. Round to the nearest available standard ratio step \u2014 standard single-stage worm gearboxes cover: 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 80, 100:1<\/strong>.<\/p>\n

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Rounding rule:<\/strong> Always round up<\/em> to the next available ratio when the exact theoretical ratio falls between two standard steps \u2014 this gives a slightly lower output speed that is closer to the target than rounding down would produce. For example: 72.5:1 rounds to 80:1 (output speed 18.1 RPM), not 60:1 (output speed 24.2 RPM). If fine-tuning is needed, use a VFD to trim the motor speed to achieve the exact output RPM from a standard ratio.<\/p>\n<\/div>\n

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Step 2 \u2014 Calculate Output Torque (Including Efficiency)<\/h2>\n
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Formula<\/p>\n

Output Torque (Nm) = Motor Torque (Nm) \u00d7 Ratio \u00d7 Efficiency<\/p>\n

Motor Torque (Nm) = (Motor Power kW \u00d7 9,550) \u00f7 Motor Speed RPM<\/p>\n<\/div>\n

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Critical: Do Not Omit Efficiency<\/p>\n

Worm gearbox efficiency falls significantly with increasing ratio \u2014 from approximately 85\u201390% at 5:1 to 60\u201366% at 100:1. A simple Ratio \u00d7 Motor Torque calculation overstates available output torque by 15\u201340%, depending on the ratio. An undersized gearbox selected on the basis of uncorrected torque will overheat, lose lubricant film strength, and fail prematurely.<\/p>\n<\/div>\n

Worm Gearbox Efficiency Reference by Ratio<\/p>\n

\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Ratio<\/th>\nTypical Efficiency (mineral oil)<\/th>\nTypical Efficiency (synthetic PAG)<\/th>\nUse This Value for Sizing<\/th>\n<\/tr>\n<\/thead>\n
5:1<\/td>\n85\u201390%<\/td>\n88\u201393%<\/td>\n0.85<\/td>\n<\/tr>\n
10:1<\/td>\n80\u201385%<\/td>\n84\u201389%<\/td>\n0.80<\/td>\n<\/tr>\n
20:1<\/td>\n74\u201380%<\/td>\n78\u201384%<\/td>\n0.74<\/td>\n<\/tr>\n
30:1<\/td>\n70\u201376%<\/td>\n74\u201380%<\/td>\n0.70<\/td>\n<\/tr>\n
40:1<\/td>\n68\u201374%<\/td>\n72\u201378%<\/td>\n0.68<\/td>\n<\/tr>\n
50:1<\/td>\n66\u201372%<\/td>\n70\u201376%<\/td>\n0.66<\/td>\n<\/tr>\n
60:1<\/td>\n64\u201370%<\/td>\n68\u201374%<\/td>\n0.64<\/td>\n<\/tr>\n
80:1<\/td>\n62\u201368%<\/td>\n66\u201372%<\/td>\n0.62<\/td>\n<\/tr>\n
100:1<\/td>\n60\u201366%<\/td>\n64\u201370%<\/td>\n0.60<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

* Use the lower-bound efficiency figure (left column) for conservative sizing on mineral oil \u2014 do not use the synthetic figure unless you have confirmed synthetic lubrication. Actual efficiency also depends on operating temperature, input speed, and load \u2014 these are representative values for initial sizing.<\/p>\n

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Step 3 \u2014 Apply the Service Factor<\/h2>\n
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Formula<\/p>\n

Design Torque = Output Torque \u00d7 Service Factor (SF)<\/p>\n<\/div>\n

The service factor accounts for the difference between the steady-state torque calculated from motor nameplate data and the actual peak load the gearbox will experience in service \u2014 due to shock loads, start-up surges, cyclic loading, or extended continuous duty. Select the service factor from the table below based on your application’s actual operating conditions:<\/p>\n

\n\n\n\n\n\n\n\n\n
Load Character<\/th>\nSF (up to 10 h\/day)<\/th>\nSF (10\u201316 h\/day)<\/th>\nSF (over 16 h\/day)<\/th>\nTypical Applications<\/th>\n<\/tr>\n<\/thead>\n
Smooth, uniform load<\/td>\n1.00<\/td>\n1.25<\/td>\n1.50<\/td>\nFans, centrifugal pumps, light conveyors<\/td>\n<\/tr>\n
Moderate shock<\/td>\n1.25<\/td>\n1.50<\/td>\n1.75<\/td>\nGeneral conveyors, mixers, packaging machines, screw feeders<\/td>\n<\/tr>\n
Heavy shock<\/td>\n1.50<\/td>\n1.75<\/td>\n2.00<\/td>\nCrushers, hammer mills, conveyor under crusher discharge, vibrating equipment<\/td>\n<\/tr>\n
Extreme shock \/ reversing<\/td>\n2.00+<\/td>\nConsult<\/td>\nConsult<\/td>\nFrequent full-load reversals, impact loading, multi-start \/ stop cycles under full load<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

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Step 4 \u2014 Select Frame Size from the Torque Rating Table<\/h2>\n

Select the smallest frame size whose maximum rated output torque equals or exceeds your calculated Design Torque from Step 3. For the product type (Aluminum\/RV, WP, Servo), use the corresponding specification page torque table.<\/p>\n

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Aluminum \/ RV Series<\/p>\n

Frames 025\u2013150 \u00b7 Torques 4\u20131,760 Nm \u00b7 For food, pharma, wet, OEM compact drives<\/p>\n

View Spec Table \u2192<\/a><\/p>\n<\/div>\n

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WP Series<\/p>\n

Frames 40\u2013250 \u00b7 Torques 30\u20134,500 Nm \u00b7 For heavy-duty industrial 24\/7 service<\/p>\n

View Spec Table \u2192<\/a><\/p>\n<\/div>\n

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Low Backlash Servo<\/p>\n

Sizes S1\u2013S8 \u00b7 Torques 10\u20131,500 Nm \u00b7 For servo motors and precision positioning<\/p>\n

View Spec Table \u2192<\/a><\/p>\n<\/div>\n<\/div>\n

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Sizing tip:<\/strong> If your design torque is close to the maximum rating of the frame size (within 10%), step up to the next frame size. The small cost difference buys a meaningful improvement in thermal margin and service life. Never select a gearbox at exactly its rated torque for a real application \u2014 always leave headroom.<\/p>\n<\/div>\n

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Step 5 \u2014 Verify Thermal Rating for Continuous Duty<\/h2>\n

For applications running more than 4\u20136 hours continuously per cycle, the thermal power rating \u2014 not the mechanical torque rating \u2014 is often the binding constraint. A gearbox has two ratings: mechanical (the maximum torque the gear pair can transmit) and thermal (the maximum continuous input power the housing can dissipate as heat without oil temperature exceeding the lubricant’s safe limit).<\/p>\n

The thermal power rating decreases sharply with increasing ratio because higher ratios have lower efficiency and therefore generate more heat from the same input power. At 80:1 with 62% efficiency, 38% of all input power becomes heat \u2014 every kilowatt of motor input generates 380 W of heat in the gearbox oil. For continuous duty at high ratios, check the thermal rating in the product’s full specification sheet (contact our team if not shown online) and confirm that your input power does not exceed it.<\/p>\n

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Thermal Rating Checks \u2014 When Required<\/p>\n