Gearboxes and Geared Motors: Selection and Integration: The Full Motor Power Range at a Glance

The Full Motor Power Range at a Glance

For your reference, here are all 11 standard motor sizes available across the jPM range:

Design Note: Each of the 5 gearbox sizes can be fitted with a number of different motors, according to the power requirement. This means there are effectively 104 power-and-speed combinations available to the designer. That's both a blessing and a responsibility.

Engineering takeaway

Twelve months after the Line 4 incident, the practitioner was promoted to lead mechanical design engineer. Not because she never made mistakes, but because she built a system that made mistakes nearly impossible.

Here's what her experience teaches you:

1. Never select on power alone. Power is necessary but not sufficient. Load classification, service factors, duty cycle, and thermal conditions all modify the effective requirement.

2. The classification system exists for a reason. It captures decades of field failure data. When you skip the classification step, you're betting against the combined experience of thousands of engineers and millions of operating hours.

3. Always check the overhung load. It's the most frequently skipped step, and it's responsible for a disproportionate number of premature gearbox failures. The formula is simple. The check takes 30 seconds. There's no excuse.

4. Thermal derating is real. If your installation runs hot, your gearbox runs weaker. Account for it or replace it early — those are your only options.

5. Document your selection. Write down every step. Every factor. Every table reference. When the next engineer looks at your specification in five years, they should be able to follow your logic completely.

Your Next Step

Pull up your most recent geared motor specification — or the next one on your desk. Run through the 5-step method above. Compare it to what was actually selected (or what you were about to select).

Did the classification match? Was the overhung load checked? Was thermal derating applied?

If the answer to any of those is "no" or "I'm not sure," you just found the gap that could save your next project.

Have a specific application you're struggling with? Walk through the tables in this guide with your actual numbers. The data is all here. The method is proven. The only variable left is whether you use it.

Bookmark this guide. Print the selection flowchart. Pin it next to your desk. Because the next time someone asks, "Why did the gearbox fail?" — you'll be the one with the answer, not the one asking the question.

Context and scope

How a junior engineer's 15,000 currency-unit mistake turned into the most valuable lesson in mechanical drive design — and how you can skip straight to the wisdom.

the practitioner stared at the smoking gearbox.

It was 2:47 AM on a Thursday. The oven conveyor at a food processing plant had seized. The worm gearbox — the one she had specified six months ago — had overheated, warped its casing, and ground the entire production line to a halt. The night shift supervisor was on the phone with the plant manager. Somewhere in the background, a forklift was already moving product that would spoil by morning.

She replayed her selection process in her head. She'd picked a gearbox that technically met the power requirement. She'd matched the ratio. She'd checked the output speed. What she hadn't done — what nobody had ever taught her — was check the thermal rating. She hadn't accounted for continuous duty. She hadn't calculated the service factor for a medium-impulsive load.

That night cost the plant roughly 15,000 currency units in spoiled product, emergency repairs, and overtime labor.

This post exists so you never have that night.

Whether you're a student opening a gearbox catalogue for the first time, an experienced engineer who wants a refresher on worm gear selection, or a procurement manager trying to understand what your engineering team is specifying — this is your complete, practical guide to worm gearbox selection. No fluff. No shortcuts. Just the methodology that separates a gearbox that works from a gearbox that works for decades.

Setting the Scene: What Exactly Is a Worm Gearbox?

Before we dive into selection, let's make sure we're speaking the same language.

A worm gearbox (also called a worm reducer or worm gear unit) is a speed reduction device that uses a worm (a screw-like gear) meshing with a wormwheel (a large gear) to achieve high reduction ratios in a compact package. They're the workhorses of industrial power transmission — found in conveyors, mixers, hoists, agitators, presses, and hundreds of other machines.

Why worm gearboxes specifically?

• High reduction ratios in a single stage. You can get ratios from 5:1 all the way up to 70:1 in a single reduction unit. Need more? Double reduction units push that to 75:1 up to 4,900:1.
• Compact footprint. The perpendicular shaft arrangement (input and output shafts at 90°) saves space compared to parallel-shaft gear trains.
• Self-locking capability. At higher ratios, the worm gear can prevent the output shaft from back-driving the input — a built-in safety feature for hoists and lifts.
• Quiet operation. The sliding contact between worm and wormwheel produces significantly less noise than spur or helical gears.

The trade-off? Efficiency. Worm gears generate more heat than other gear types because of sliding friction between the worm threads and wormwheel teeth. This is exactly why thermal rating checks are so critical — and exactly what tripped up the practitioner.

The Five Faces of a Worm Gearbox: Configurations You Need to Know

Not all worm gearboxes look the same, and the configuration you choose affects everything from mounting to shaft orientation to catalogue part numbers.

There are five standard configurations:

Pro tip: The "TW" prefix indicates a single-reduction unit ("the source industrial-equipment organisation Worm"). Adding "D" (e.g., TWDU, TWDO) indicates a double-reduction unit. The last letter indicates the configuration type.

Why does this matter for selection? Because catalogue data tables are organized by configuration type. Picking the wrong configuration means you're reading the wrong table — and potentially specifying a unit that can't physically be installed in your application.

Failure trigger and engineering context

Let's revisit the practitioner's situation, because understanding what went wrong is the fastest way to understand what to do right.

Her application: an oven conveyor at a food processing plant.

The given data:

• Power required at the conveyor chain wheel: 20 kW
• Speed of the conveyor chain wheel: 15 ± 0.5 rev/min
• Chain drive reduction ratio from gearbox to conveyor: 2:1
• PCD of chain pinion keyed to gearbox output shaft: 270 mm
• Electric motor: 4-pole, full-load speed 1,460 rev/min
• Hours of operation: 16 hours/day, continuous
• Maximum ambient temperature: 45°C

What the practitioner did:

1. Calculated the required output speed of the gearbox: 30 rev/min (since there's a 2:1 chain reduction after the gearbox)
2. Calculated the reduction ratio: 1,460 ÷ 30 ≈ 48.7:1
3. Found the closest nominal ratio: 50:1
4. Looked up a gearbox in the catalogue that could handle 20 kW at that ratio
5. Specified a W12 gearbox
6. Called it done

What the practitioner missed:

• The chain drive efficiency (96%) — meaning the gearbox actually needed to deliver 20 / 0.96 = 20.83 kW
• The load classification — oven conveyors with non-uniform loading are classified as Medium Impulsive (M)
• The service factor — for a medium-impulsive load on an electric motor running 16 hours/day, the service factor is 1.33
• The thermal service factor — at 45°C ambient with continuous operation, the thermal factor is 1.485
• The overhung load from the chain pinion on the output shaft

That W12 she specified? Its thermal rating was only 8,156 Nm. The application demanded 10,117 Nm after thermal correction. She needed a W14 — one full size larger.

The cost of skipping four steps in a fifteen-step process: 15,000 currency units and a career-defining lesson.

The Complete Worm Gearbox Selection Method: 15 Steps That Protect Your Reputation (and Your Client's Production Line)

This is the method. Memorize it. Print it. Tape it to your monitor. Every step matters.

Step 1: Establish the Mechanical Data

Before you touch a catalogue, gather everything about the application:

• Input side (driver): Maximum (or design) torque, power, and speed
• Output side (driven machinery): Maximum (or design) torque, power, and speed — including any tolerance range on speed
• Duration of service: Continuous or intermittent? How many hours per day?
• Maximum ambient temperature around the gearbox

Critical notes you can't afford to ignore:

Because input and output values are inter-related through the gearbox, not all values will be known at this stage. For example, input power and output power are related by the gearbox efficiency — which you won't know until you've selected a unit.

Maximum torque or power ratings exclude shock loading or hard-start factors. Those are handled separately through the service factor.

Ambient temperature only matters for continuous duty (or intermittent duty without enough cool-down time). If the gearbox runs intermittently with adequate cooling periods, temperature doesn't affect selection.

Step 2: Calculate the Required Reduction Ratio

This is straightforward:

$$\text{Reduction Ratio} = \frac{\text{Input Speed}}{\text{Output Speed}}$$

But watch for chain drives, belt drives, or gear stages between the gearbox and the final driven equipment. If there's a 2:1 chain reduction after the gearbox, your gearbox output speed needs to be twice the final equipment speed.

In the practitioner's case:

• Final conveyor speed needed: 15 rev/min
• Chain drive ratio: 2:1
• Required gearbox output speed: 15 × 2 = 30 rev/min
• Motor speed: 1,460 rev/min
• Required reduction ratio: 1,460 / 30 = 48.7:1

Step 3: Select the Closest Nominal Ratio

From the reduction ratio tables, find the closest available nominal ratio.

Here's a reference table showing available nominal and actual ratios for single-reduction worm gearboxes:

Key insight: Actual ratios are sometimes slightly different from nominal ratios, and they can vary by gearbox size (centre distance). Always verify the actual ratio for the specific gearbox you select — it's listed in the data tables.

If the ratio exceeds 70:1, you need a double-reduction gearbox. Double-reduction units are available from 75:1 up to 4,900:1.

the practitioner's selection: Closest nominal ratio to 48.7 is 50:1 ✓

Step 4: Calculate the Nominal Output Speed

$$\text{Nominal Output Speed} = \frac{\text{Input Speed}}{\text{Nominal Ratio}}$$

For the practitioner: 1,460 / 50 = 29.2 rev/min

This is within her required range of 30 ± 1 rev/min. ✓

Step 5: Determine the Load Classification

This is where many engineers start making mistakes. The load isn't just about how much force — it's about how the force behaves.

There are three load classifications:

Here's a partial reference for common driven machinery and their load classifications:

the practitioner's application: Oven conveyor, non-uniformly loaded → Medium Impulsive (M) ✓

Step 6: Determine the Service Factor

The service factor compensates for the severity of loading conditions. It depends on three things:

1. The type of prime mover (electric motor vs. internal combustion engine)
2. The load classification (S, M, or H from Step 5)
3. The duration of operation (hours per day)

Here's the service factor table:

For durations between listed values (like 16 hours/day), interpolate linearly.

the practitioner's case: Electric motor, Medium Impulsive, 16 hr/day → Interpolating between 12 hr (1.50) and 24 hr (1.50) = 1.33 (she needed to interpolate for 16 hours)

Step 7: Calculate the Selection Capacity

This is where the service factor does its job. You're multiplying the actual design requirement by the service factor to get the selection requirement — the number you'll use to enter the catalogue tables.

If input conditions are given:

$$\text{Selection Input Power (mechanical)} = \text{Design Input Power} \times \text{Service Factor}$$

If output conditions are given:

$$\text{Selection Output Torque (mechanical)} = \text{Design Output Torque} \times \text{Service Factor}$$

the practitioner's calculation:

First, the design output power through the gearbox:

$$\text{Design Output Power} = \frac{20 \text{ kW}}{0.96} = 20.83 \text{ kW}$$

(Dividing by chain drive efficiency to get what the gearbox must actually deliver)

The output torque (mechanical):

$$T = \frac{P}{\omega} = \frac{P}{\frac{2\pi N}{60}} = \frac{20{,}830}{\frac{2\pi \times 29.2}{60}} = 6{,}813 \text{ Nm}$$

The selection output torque (mechanical):

$$T_{\text{selection}} = 6{,}813 \times 1.33 = 9{,}061 \text{ Nm}$$

This is the minimum output torque the gearbox must handle. You'll use this number to enter the catalogue data tables.

Step 8: Make a Preliminary Gearbox Selection

Now — finally — you go to the data tables.

Go to the table for your nominal ratio (50:1 in this case) and your input speed (1,500 rev/min, the closest standard speed). Find the smallest gearbox whose mechanical output torque capacity exceeds your selection value.

For the 50:1 ratio at 1,500 rev/min input:

the practitioner's selection requirement: 9,061 Nm

At first glance, both W10 (9,847 Nm) and W12 (9,838 Nm) seem to work. the practitioner picked the W12.

But she stopped here. She never continued to Step 10.

Step 9: Verify the Actual Ratio and Output Speed

From the ratio tables, check that the actual ratio for your selected gearbox matches the nominal. For the W12 at ratio 50, the actual ratio is indeed 50:1.

$$\text{Actual Output Speed} = \frac{1{,}460}{50} = 29.2 \text{ rev/min}$$

This is within the required 30 ± 1 rev/min range. ✓

Step 10: Check the Thermal Rating ⚠️ THE STEP THAT CATCHES EVERYONE

This is where the practitioner's gearbox died. And this is where you earn your pay as an engineer.

Worm gearboxes generate heat through sliding friction between the worm and wormwheel. If the gearbox runs continuously (or intermittently without sufficient cool-down), you must verify that the thermal rating is adequate.

When to check thermal rating:

• The gearbox is single-reduction AND operates continuously
• OR operates intermittently but without sufficient time for the lubricating oil to cool down

The Thermal Service Factor Table:

Interpolate for intermediate temperatures.

the practitioner's thermal check:

Ambient temperature: 45°C → Thermal factor (interpolated): 1.485

Selection output torque (thermal) = Design output torque × Thermal factor:

$$T_{\text{thermal}} = 6{,}813 \times 1.485 = 10{,}117 \text{ Nm}$$

Now compare this against the thermal torque rating of the W12 at 50:1 ratio, 1,500 rev/min input:

W12 thermal output torque = 8,156 Nm

10,117 > 8,156 💀

The W12 cannot dissipate enough heat. It will overheat. It will fail. And it did.

The W14's thermal output torque: 11,696 Nm. That's what she needed.

The lesson: A gearbox can have enough mechanical strength to handle the load but not enough thermal capacity to survive the heat. Always check both.

Step 11: Compare Thermal and Mechanical Ratings

After computing both the thermal and mechanical selection requirements, your chosen gearbox must satisfy both.

The gearbox must pass both checks. The W14 is the correct selection.

Note: If the thermal rating is the limiting factor (as it often is for continuous-duty applications), you have two options: (1) select a larger gearbox, or (2) use auxiliary cooling — synthetic oil, oil coolers, or external fans can substantially increase the thermal rating. Consult the manufacturer for these options.

Step 12: Check the Overhung Load

If anything is attached to the output shaft that creates a radial (sideways) force — a chain sprocket, belt pulley, gear, or flywheel — you must check the overhung load.

The Overhung Load Formula:

$$F = \frac{2fT}{d} = \frac{60 \cdot f \cdot P}{\pi \cdot d \cdot N}$$

Where:

• F = overhung load (in Newtons)
• T = output shaft torque in Nm (design value, NOT selection value)
• P = output shaft power in Watts (design value, NOT selection value)
• d = PCD (pitch circle diameter) of sprocket, pulley, or gear in metres
• N = output shaft speed in rev/min
• f = drive application factor:

the practitioner's overhung load calculation:

With f = 1 (chain drive), zero slack side tension, and PCD = 270 mm = 0.270 m:

$$F = \frac{2 \times 1 \times 6{,}813}{0.270} = 50{,}470 \text{ N}$$

From the overhung load capacity tables for the W14 at ratio 50:1 and 1,450 rev/min input speed:

Allowable overhung load for W14 = 99,100 N

50,470 < 99,100 ✓ — The overhung load is well within limits.

Step 13: Check the Thrust Load

If a helical gear or other mechanism directly attached to the output shaft creates an axial (along the shaft) thrust force, verify it doesn't exceed the gearbox's thrust load capacity.

the practitioner's case: A chain pinion doesn't create significant thrust. No thrust load to check. ✓

If thrust loads exist, the allowable values are listed in the catalogue tables. If exceeded, select a larger gearbox or use an intermediate (layshaft) with its own bearings.

Step 14: Calculate the Complete Input/Output Data

Now that you've selected the correct gearbox (W14), calculate all the unknowns using the gearbox efficiency from the data tables.

For the W14 at 50:1 ratio, 1,500 rev/min input, the efficiency is 84%.

Step 15: Specify the Complete Gearbox

Your specification must include:

• Gearbox model: TWO 14 (overdriven configuration in this example)
• Nominal ratio: 50:1
• Input shaft diameter: 75 mm
• Output shaft diameter: 120 mm
• Side bolt hole centre distance: 597 mm
• End bolt hole centre distance: 431.8 mm
• Mounting configuration: Foot-mounted, overdriven

Improvement method and result

After the factory incident, the practitioner rebuilt her selection process from scratch. She created a personal checklist — a simplified version of the 15-step method — that she runs through for every gearbox specification:

the practitioner's Gearbox Selection Checklist

• Gather ALL mechanical data — input AND output sides, duty cycle, ambient temperature
• Account for external drive efficiencies (chain drives, belt drives between gearbox and driven equipment)
• Calculate the actual reduction ratio needed at the gearbox
• Find closest nominal ratio from the catalogue; verify it's ≤70:1 for single reduction
• Determine load classification — Steady, Medium, or Highly Impulsive
• Look up the service factor based on prime mover type, load class, and hours of operation
• Multiply design values by the service factor to get the selection capacity
• Select the smallest gearbox whose mechanical rating exceeds the selection capacity
• CHECK THE THERMAL RATING if continuous duty — multiply design values by the thermal factor
• Verify the gearbox passes BOTH mechanical and thermal checks
• Calculate and check the overhung load if sprockets, pulleys, or gears are on the output shaft
• Calculate and check the thrust load if helical gears or similar are on the output shaft
• Verify the actual ratio and output speed fall within tolerances
• Calculate complete input/output data using the gearbox efficiency
• Fully specify the gearbox — model, ratio, mounting, shaft dimensions, bolt patterns

Deep Dive: Understanding Efficiency — The Hidden Variable

Gearbox efficiency isn't just a number in a table. It's the reason your motor draws more power than the driven equipment consumes. And for worm gearboxes, efficiency varies significantly with operating conditions.

Key facts about worm gearbox efficiency:

Efficiency increases with speed and decreases with ratio. A worm gearbox running at 1,800 rev/min with a 5:1 ratio might achieve 88-89% efficiency. The same gearbox at 100 rev/min with a 70:1 ratio might only achieve 61-68% efficiency.

The stated efficiency is at maximum (rated) power. At very low power inputs (below maybe 20% of rated capacity), efficiency drops slightly. But for typical operating ranges, catalogue efficiency values are reliable.

Efficiency matters for motor sizing. If you need 20 kW at the output and the gearbox efficiency is 84%, your motor must supply at least 20/0.84 = 23.8 kW. Round up to the next standard motor size.

Here's how efficiency typically varies across the operating range:

The takeaway: Always use the efficiency value from the specific data table row for your selected gearbox, ratio, and speed — not a generic "worm gears are about 80% efficient" assumption.

The Physical Gearbox: Construction and What to Specify

Understanding what's inside your gearbox helps you make better decisions about installation, maintenance, and troubleshooting.

Gear Case

Close-grained cast iron. All joints and bearing bores are accurately machined to ensure oil tightness and precise gear location. The ribbed exterior helps with heat dissipation.

Wormshaft and Wormwheel

• Wormshaft: Integral with its shaft — alloy steel, case-hardened, ground, and polished on the thread profiles
• Wormwheel rim: Phosphor bronze (cast centrifugally, secured to a cast iron centre by electron beam welding on larger sizes)
• The tooth form (Holroyd gear form for the source industrial-equipment organisation class units) corresponds to British Standard recommendations and includes a modification to both worm threads and wheel teeth that ensures true uniform angular velocity under all loading conditions. This modification also creates a tapered oil entry gap between teeth — promoting oil-borne friction (hydrodynamic lubrication) rather than metal-to-metal contact.

Shafts

Standard shaft extensions are metric dimensions. Imperial shaft extensions are also available. Wheelshaft is carbon steel (standard) or high-tensile steel for demanding applications. Double extension wormshafts or wheelshafts are available on request.

Bearings

Standard metric taper roller bearings throughout (10", 12", and 14" units), arranged face-to-face on both worm and wheel to maximize stiffness. Larger units use a matched set of tapers at one end with a deep groove ball bearing at the opposite end — this allows thermal expansion of the wormshaft while still handling radial and thrust forces.

Seals and Lubrication

• Viton oil seals are standard
• Lubrication: Oil bath from the sump for underdriven and overdriven units. Vertical and agitator types require grease lubrication for the wheeline bearings. Lower speeds may require grease for all bearings — consult the manufacturer.

Cooling

Maximum heat dissipation by air cooling via a radial fan directing air over the ribbed gear case. In applications where airflow is restricted, units can be supplied without a fan.

Backstop

A sprag clutch backstop can be fitted internally or externally with manual tension release. Essential for any non-reversible application (e.g., inclined conveyors, hoists, lifts).

Seven Gearbox Sizes: What They Mean and When You Need Each

Worm gearboxes come in seven sizes, designated by the nominal centre distance between the worm shaft and wheel shaft (in inches):

Note: The "typical power range" is highly dependent on ratio and input speed. A W10 at 5:1 ratio handles far more power than a W10 at 70:1. Always check the specific data table.