Gearboxes and Geared Motors: Selection and Integration: Worm Gearbox Operating Parameter Relationships

Worm Gearbox Operating Parameter Relationships

flowchart LR
    A[Higher Gear Ratio] --> B[Lower Output Speed]
    A --> C[Lower Efficiency]
    A --> D[Higher Heat Generation]
    A --> E[Higher Output Torque per kW Input]

    F[Larger Centre Distance] --> G[Higher Torque Capacity]
    F --> H[Higher Power Capacity]
    F --> I[Larger Physical Size]
    F --> J[Higher Oil Capacity]
    F --> K[Higher Weight]

    L[Lower Input Speed] --> M[Lower Thermal Rating]
    L --> N[Lower Efficiency]
    L --> O[Mechanical Rating Dominates]

Overhung Load Calculation Decision Flow

flowchart TD
    A[Is a mechanism attached to the output shaft?] -->|No| B[No overhung load check required]
    A -->|Yes| C[Identify mechanism type]
    C --> D[Determine drive application factor f]
    D --> E["f = 1.0 (chain/toothed belt)"]
    D --> F["f = 1.25 (gear drive)"]
    D --> G["f = 1.5 (vee belt)"]
    D --> H["f = 2.0 (flat belt)"]
    E --> I["Calculate F = (2 × f × T) / d"]
    F --> I
    G --> I
    H --> I
    I --> J{F ≤ Allowable OHL for unit size and speed?}
    J -->|Yes| K[Selection OK — proceed]
    J -->|No| L{Options}
    L --> M[Select larger gearbox unit]
    L --> N[Use intermediate layshaft with bearings and flexible coupling]

Key Terms Glossary

• Centre Distance (CD): The perpendicular distance between the worm shaft axis and the wheel shaft axis; determines the gearbox frame size and directly affects torque/power capacity
• Thermal Rating: Maximum input power under continuous duty without exceeding safe temperature limits, based on standard ambient conditions and mineral oil lubrication
• Mechanical Rating: Maximum input power based on gear tooth strength, shaft strength, and bearing capacity — typically exceeds thermal rating
• Worm: A screw-shaped gear that meshes with a worm wheel; the driving element in a worm gearbox
• Worm Wheel: A toothed wheel that meshes with the worm; the driven element producing the speed reduction
• Overhung Load (OHL): A radial force perpendicular to the shaft axis, generated by belt tension, chain pull, or gear mesh forces on an attached drive element
• Axial (Thrust) Load: A force acting parallel to the shaft axis, commonly produced by helical gears or inclined conveyor drives
• Drive Application Factor (f): A multiplier applied to the overhung load formula to account for the dynamic characteristics of different drive types (belt, chain, gear)
• Taper Lock Bush: A split, tapered sleeve used to clamp a pulley, sprocket, or coupling to a shaft without keyway modification; allows easy mounting and removal
• Sprag Clutch Backstop: A one-way mechanical device fitted to non-reversible gearboxes to prevent reverse rotation under load (e.g., inclined conveyors)
• Force Feed Lubrication: A system using an oil pump to circulate lubricant to gearbox components, required for high-load operating conditions
• Layshaft: An intermediate shaft supported by its own bearings, interposed between the gearbox output shaft and the driven mechanism to absorb overhung or thrust loads
• PCD (Pitch Circle Diameter): The effective diameter of a pulley, sprocket, or gear at which the driving force acts
• Service Factor (f_d): A multiplier applied during selection that accounts for the severity of the application, prime mover type, and hours of operation
• Free-Standing Unit: A gearbox supplied without a motor, intended to be coupled to a separately sourced motor by the system designer

Quick Revision

• Worm gearbox efficiency decreases as the gear ratio increases (40:1 ≈ 88% → 70:1 ≈ 80% at high input speeds)
• Thermal ratings limit continuous duty; mechanical ratings limit short-duration peak loads
• Centre distance is the primary determinant of gearbox size and torque capacity
• Shaded areas in rating tables require force feed lubrication; oil coolers can extend ratings further
• Two keys are required when single key torque limits are exceeded; high tensile shafts when standard shaft torque limits are exceeded
• Double reduction achieves ratios from 75:1 to 4900:1 but with significantly lower efficiency (as low as 31%)
• Geared motor unit selection follows an 8-step process: mechanical data → load classification → drive classification → unit selection → OHL check → thrust check → shaft dimensions → specification
• Overhung load formula: F = 2fT/d — always use the design torque value, not the selection table value
• Drive application factors: 1.0 (chain), 1.25 (gear), 1.5 (vee belt), 2.0 (flat belt)
• Load classifications: S (Steady), M (Medium Impulsive), H (Highly Impulsive) — determined by the type of driven machinery
• Drive classifications (1–4) combine load classification with daily operating hours
• If overhung or thrust loads exceed allowable values, either upsize the gearbox or use a layshaft with its own bearings
• Non-reversible units must have a sprag clutch backstop fitted
• Motor sizes range from 0.12 kW (D63 frame) to 4.0 kW (D112M frame), all 4-pole at nominal 1400–1420 rev/min
• Maximum output torques are gearbox-size dependent, not ratio dependent: single key up to 72,000 Nm, standard shaft up to 146,400 Nm at largest centre distance

Context and scope

How a junior mechanical engineer's worst nightmare became a masterclass in worm geared motor unit selection — and why you need this method before your next project.

Setting the Scene: What Are Worm Geared Motor Units, and Why Do They Matter?

Before the practitioner could fix the problem, she needed to understand what she was actually working with.

A worm geared motor unit is a compact, integrated package: an electric motor directly coupled to a worm gearbox. The worm gear mechanism converts the motor's high-speed, low-torque input into low-speed, high-torque output. They're everywhere — driving conveyors, mixers, agitators, packaging machines, fans, pumps, and hundreds of other industrial applications.

The specific units covered here are based on the well-established jPM Worm Geared Motor and Speed Reducer range (Renold). These units are designed for relatively low-power applications, typically between 0.1 kW and 4 kW motor power input.

What Makes These Units So Popular?

• Compact design — motor and gearbox in a single, bolt-down package
• Wide speed range — output speeds from as low as 20 rev/min up to 288 rev/min
• High gear ratios available — 12 nominal ratios from 5:1 up to 70:1
• Self-locking capability — at higher ratios, the worm gear naturally prevents back-driving, which is a safety feature for lifting applications
• Simple mounting — foot-mounted configuration bolts directly to frames and bases

Five Sizes, Endless Combinations

The range consists of five gearbox frame sizes:

Each gearbox size can be fitted with multiple motor sizes, and each offers all 12 gear ratios. That means there are effectively 104 power-and-speed combinations available to the designer. The motor sizes range from 0.12 kW (frame D63) to 4.0 kW (frame D112M), covering 11 standard motor sizes in total.

Key Insight for You: More combinations means more ways to get it right — but also more ways to get it wrong. A systematic approach isn't optional. It's essential.

All geared motor units in this range are fitted with 4-pole motors operating at a nominal input speed of 1400–1420 rev/min. Output is via either a plug-in shaft (hollow bore with keyway) or a solid output shaft with key.

Step 1: Classify the Load

Every driven machine creates a specific type of load on the gearbox. The classification system uses three categories:

The comprehensive Load Classification by Application table (Table 1 in the reference data) lists hundreds of specific machine types and their classifications. Here's a representative excerpt:

Pro Tip for You: When in doubt, classify conservatively. If your application sits between two categories, choose the heavier one. The cost of a slightly larger unit is trivial compared to the cost of a catastrophic failure.

Step 2: Determine the Service Factors

Here's where things get more nuanced. The service factor accounts for three critical variables simultaneously:

a) Prime Mover Type (Driver Characteristics) Different power sources deliver different levels of smoothness:

b) Duration of Service How many hours per day does the machine operate?

c) Driven Machine Load Characteristics Whether the load is steady, medium impulsive, or highly impulsive.

These three factors combine into a single Service Factor (f_D) using Table 2:

Mechanical Service Factors (Table 2)

Now let's revisit the practitioner's conveyor:

• Prime mover: Electric motor → Steady input
• Duration: 16 hours/day → Over 10
• Load: Non-uniform belt conveyor → Medium impulsive (M)
• Service factor = 1.50

The original engineer used a factor of 1.00. That's a 50% underestimate on severity. No wonder the unit was dying.

Thermal Service Factors (Table 3)

Operating temperature also affects gearbox capacity. At elevated ambient temperatures, the lubricant degrades faster and thermal capacity drops.

Critical Note: At 40°C ambient, you need to derate the gearbox capacity by a factor of 1.35. In hot climates or enclosed, poorly ventilated spaces, this factor alone can push you into the next gearbox size. A substantial increase in thermal rating is available when using synthetic lubricants — consult the manufacturer for details.

Starts/Hours Factor (Table 3 — Additional)

If your application involves frequent starting and stopping, an additional factor applies:

Step 3: Determine the Drive Classification

This is where it all comes together. Based on the service factor, you land in one of four drive classifications:

the practitioner's conveyor: Service factor 1.50 → Drive Classification 3 (Heavy Duty).

The original selection was made from Classification 1 tables. That's a two-class error — a potentially catastrophic miscalculation.

The Deep Dive: Reading the Selection Tables Like a Pro

Once the practitioner showed the practitioner how the classification system worked, the actual selection from the data tables became almost mechanical. Here's exactly how you use them.

The Master Selection Tables

For each of the four drive classifications, a complete selection table exists showing every combination of:

• Nominal output speed (rev/min)
• Nominal gear ratio (:1)
• Motor power (kW)
• Gearbox size (jPM11 through jPM30)
• Actual output power (kW)
• Actual output torque (Nm)

Sample Data — Drive Classification 1 (Light Duty)

How the Data Changes Across Classifications

The key difference between classifications is the derated output capacity. A jPM22 gearbox that delivers 0.66 kW at 144 rev/min under Classification 1 conditions will deliver less rated power under Classification 3 conditions because the service factor demands more conservative selection.

Here's a direct comparison at one operating point (144 rev/min, 10:1 ratio):

Notice the pattern: As classification severity increases, the same output requirement often pushes you into a larger gearbox size. A 0.75 kW motor at 10:1 ratio needs only a jPM17 under light duty — but requires a jPM22 under heavy and extra-heavy duty. This is exactly the mistake that killed the practitioner's conveyor.

The 5-Step Geared Motor Selection Method

Step 1 — Establish Your Mechanical Requirements

Gather the following data for your driven (output) machinery:

• Type of output machinery (conveyor, mixer, pump, etc.)
• Maximum (or design) torque — including tolerance
• Required output speed — including acceptable speed range
• Power requirement at the output shaft
• Duration of service — continuous or intermittent?
• Average hours per day of operation

Step 2 — Classify the Load

Use the Load Classification by Application table (Table 1) to determine whether your load is:

• S = Steady
• M = Medium Impulsive
• H = Highly Impulsive

Step 3 — Determine the Drive Classification

Use Table 2 (Mechanical Service Factors) to find your service factor based on:

1. Your prime mover type (electric motor, I.C. engine, etc.)
2. Your load classification from Step 2
3. Your hours per day of operation

Then map the service factor to a Drive Classification (1, 2, 3, or 4):

Service Factor ≤ 1.00 → Classification 1 (Light Duty) Service Factor ≤ 1.25 → Classification 2 (Medium Duty) Service Factor ≤ 1.75 → Classification 3 (Heavy Duty) Service Factor > 1.75 → Classification 4 (Extra Heavy Duty)

Step 4 — Select from the Appropriate Data Table

Go to the selection table for your drive classification (pages differ per classification). Find the row matching your required output speed and gear ratio. Read across to find a unit that meets your power and torque requirements.

Each cell in the table gives you three pieces of information:

• Gearbox size (jPM11 to jPM30)
• Output power (kW)
• Output torque (Nm)

Select the unit whose output power and torque meet or exceed your requirements.

Step 5 — Check the Overhung Load

If there is a gear, pulley, chain-wheel, flywheel, or other mechanism directly attached to the output shaft that produces a radial or overhung load, you must verify it doesn't exceed the allowable value.

Use Table 6 — jPM Overhung Load Capacities to check.

The Overhung Load Check: The Step Most Engineers Skip

This was the step that sealed the practitioner's reputation as a thorough engineer. Most people stop at Step 4. the practitioner never did.

What is an overhung load? It's the radial force (perpendicular to the shaft axis) created by a belt, chain, gear, or other drive component mounted on the output shaft. If this force exceeds the gearbox's bearing capacity, the bearings fail prematurely — exactly what happened on Line 4.

Overhung Load Capacities (Table 6)

Note: The overhung load capacities assume the resultant load on the output shaft is applied mid-way along the shaft, at the position given by Dimension A.

The Overhung Load Formula

If you need to calculate the actual overhung load on the shaft, use this formula:

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

Where:

Drive Application Factor (f) Values:

If the calculated overhung load exceeds the allowable value for your selected gearbox size, you have two options:

1. Choose a larger gearbox with higher overhung load capacity.
2. Use an intermediate shaft (layshaft) with its own bearings, coupled to the gearbox via a flexible coupling. This prevents the overhung load from being transmitted to the gearbox bearings.

A Complete Worked Example: the practitioner Gets It Right

Six months later, a new project came across the practitioner's desk: specify the geared motor unit for a screw conveyor transporting variable-density granular material, driven by an electric motor, running 12 hours per day.

Requirements:

• Output speed: ~96 rev/min
• Output power needed: 0.45 kW
• Output torque needed: ~45 Nm
• A chain sprocket (PCD = 0.15 m) is mounted on the output shaft

Here's exactly how the practitioner worked through it:

Step 1: Mechanical Data ✓

• Machine type: Screw conveyor (non-uniformly fed)
• Speed: 96 rev/min → Gear ratio 15:1 (from 1420 ÷ 96 ≈ 15)
• Power: 0.45 kW at output
• Torque: 45 Nm at output
• Operation: 12 hours/day, continuous

Step 2: Load Classification ✓

From Table 1: Screw conveyor = S (Steady) to M (Medium Impulsive) depending on loading. Non-uniform feed → M (Medium Impulsive)

Step 3: Drive Classification ✓

From Table 2:

• Prime mover: Electric motor (Steady input)
• Duration: 12 hours/day (3–10 bracket? No — 12 is over 10)
• Load: Medium Impulsive
• Service Factor = 1.50 → Drive Classification 3 (Heavy Duty)

Step 4: Select from Classification 3 Data ✓

Looking at the Drive Classification 3 table at 96 rev/min (15:1 ratio):

the practitioner needs ≥0.45 kW output and ≥45 Nm torque. The 0.55 kW motor with jPM17 gives 0.49 kW and 49 Nm — both exceed requirements.

Selected: jPM17 gearbox, 0.55 kW motor, 15:1 ratio.

Step 5: Overhung Load Check ✓

The chain sprocket creates an overhung load. Calculate:

F = (2 × f × T) / d F = (2 × 1.0 × 49) / 0.15 F = 98 / 0.15 F = 653 N

From Table 6, the jPM17 at 96 rev/min (interpolating near 100 rev/min):

• Allowable OHL = 1,900 N
• Actual OHL = 653 N ✓

653 N is well within the 1,900 N allowable. The selection passes.

the practitioner submitted the specification. The unit was installed, commissioned, and ran flawlessly for the next three years without a single bearing failure or overheating incident. the practitioner signed off with a simple note: "Properly specified. Good work."

Physical Dimensions: Getting the Unit to Fit

Beyond performance, you need to know if the unit physically fits in your installation. Here are the critical mounting dimensions for foot-mounted (Type 2) configurations:

Foot Mounting — Type 2 Dimensions

Dimension values vary with motor frame size (D71M through D112M).

Gearbox Shaft and Key Dimensions

Overall Gearbox Body Dimensions

Special Considerations You Can't Afford to Ignore

Free-Standing Gearbox Option

The jPM gearbox can be specified without a motor — as a free-standing unit coupled to any motor of the designer's choice. This gives you flexibility but adds complexity, since you're now responsible for motor-to-gearbox alignment, coupling selection, and the combined system's performance envelope. The selection procedure for the gearbox itself remains identical, but the motor-to-gearbox interface follows the same principles as standalone gearbox selection.

The 0.12 kW Motor Exception

The smallest motor option (0.12 kW, frame D63) is not a preferred size and may not be available off-the-shelf. Expect extended lead times if you specify this motor. Where possible, step up to 0.18 kW (frame D71M) for better availability.

Speed Tolerance

Actual output speeds depend on the full-load speed of the motor and the exact gear ratio. The nominal speeds listed in the tables are approximate. If your application requires tight speed control, verify the actual output speed with the manufacturer before committing to installation.

Thermal Considerations in Hot Environments

For installations where ambient temperatures exceed 20°C — which includes most real-world industrial environments — apply the thermal service factor from Table 3:

Quick Reference — Thermal Derating:

20°C ambient → Factor 1.00 (no derating needed) 30°C ambient → Factor 1.16 (16% derating) 40°C ambient → Factor 1.35 (35% derating) 50°C ambient → Factor 1.62 (62% derating) 60°C ambient → Factor 1.97 (nearly double derating!)

If you're designing for tropical climates, desert installations, or enclosed machine rooms, thermal derating is not optional. It's the difference between a unit that runs for 10 years and one that fails in 10 months.

When the Overhung Load Is Too High: The Layshaft Solution

If your calculated overhung load exceeds the allowable capacity, don't just upsize the gearbox blindly. Consider an intermediate shaft (layshaft) arrangement:

1. Mount the layshaft in its own bearing housing
2. Connect it to the gearbox output via a flexible coupling
3. Mount the chain sprocket, pulley, or gear on the layshaft instead

This completely isolates the gearbox bearings from external radial loads. It's a proven solution used in thousands of industrial installations worldwide.

The Complete Selection Flowchart

Here's the decision tree in one visual flow:

START │ ▼ [1] What does your driven machine need? → Power (kW), Speed (rev/min), Torque (Nm) → Hours per day, Continuous or Intermittent? │ ▼ [2] What TYPE of load does the machine create? → Table 1: Find your machine → S, M, or H │ ▼ [3] What is your SERVICE FACTOR? → Table 2: Prime mover type × Load type × Hours/day → Result: f_D value (0.90 to 2.50) │ ▼ [4] What DRIVE CLASSIFICATION does this give you? → f_D ≤ 1.00 → Class 1 → f_D ≤ 1.25 → Class 2 → f_D ≤ 1.75 → Class 3 → f_D > 1.75 → Class 4 │ ▼ [5] SELECT from the correct Classification table → Find your speed/ratio row → Read across to find motor power ≥ your need → Note gearbox size and output torque │ ▼ [6] CHECK OVERHUNG LOAD (if applicable) → Calculate F = (2 × f × T) / d → Compare with Table 6 allowable value → If F > allowable → larger gearbox OR layshaft │ ▼ [7] CHECK THERMAL CONDITIONS (if ambient > 20°C) → Apply Table 3 thermal factor → May require upsize │ ▼ [8] VERIFY PHYSICAL FIT → Check mounting dimensions vs. available space → Confirm shaft configuration (plug-in vs. solid) │ ▼ SELECTION COMPLETE ✓