Gearboxes and Geared Motors: Selection and Integration: Coupling Selection Decision Tree

Coupling Selection Decision Tree

flowchart TD
    A[Start: Coupling Selection] --> B{Primary<br/>Requirement?}
    B -- High misalignment<br/>& vibration damping --> C[Tyre-Type Coupling]
    B -- Torsional rigidity<br/>& precision --> D[Disc-Type Coupling]
    B -- General purpose<br/>moderate loads --> E[Pin-Type Coupling]
    B -- Easy assembly<br/>& moderate loads --> F[Chain-Type Coupling]
    C --> G{Max Torque<br/>≤ 6270 Nm?}
    G -- Yes --> H[Select from Tyre<br/>Coupling Range]
    G -- No --> I[Consider Pin-Type<br/>or alternative]
    D --> J{Max Speed<br/>≤ 2900 RPM?}
    J -- Yes --> K[Select from Disc<br/>Coupling Range]
    J -- No --> L[Consider Pin-Type<br/>for higher speeds]
    E --> M[Select from Pin-Type<br/>Coupling Range]
    F --> N[Select from Chain-Type<br/>Coupling Range]
    H --> O[Select Taper Bush<br/>& Verify Bore Range]
    K --> O
    M --> O
    N --> O
    I --> E
    L --> E
    O --> P[Verify Key & Keyway<br/>Dimensions]

Power Transmission System Overview

flowchart LR
    A[Prime Mover<br/>Electric Motor /<br/>IC Engine] -->|Input Shaft| B[Worm Gearbox<br/>Speed Reduction<br/>Torque Multiplication]
    B -->|Output Shaft| C[Coupling<br/>Misalignment<br/>Compensation]
    C --> D[Driven Machine<br/>Conveyor / Pump /<br/>Mixer / etc.]

    B -.->|Overhung Load| E[Chain / Belt /<br/>Gear Drive]
    E --> D

    style A fill:#f0f0f0,stroke:#333
    style B fill:#f0f0f0,stroke:#333
    style C fill:#f0f0f0,stroke:#333
    style D fill:#f0f0f0,stroke:#333
    style E fill:#f0f0f0,stroke:#333

Key Terms Glossary

• Bore: The internal diameter of a coupling hub or bush that fits onto the shaft
• Centre Distance: The distance between the centreline of the worm shaft and the centreline of the wheel shaft in a gearbox; used as the gearbox size designation
• Double Reduction: A gearbox arrangement using two stages of worm/wheel reduction to achieve very high ratios (75:1 and above)
• Drive Application Factor (f): A multiplier applied in overhung load calculations to account for the type of drive mechanism (chain, gear, belt, etc.)
• End Float: The maximum permissible axial movement of a coupling hub relative to its mating half
• Force-Feed Lubrication: A pressurised oil supply system required when gearbox ratings exceed the sump-lubrication capacity (indicated by shaded areas in data tables)
• Keyway: A machined slot in a shaft and hub into which a key is fitted to transmit torque and prevent relative rotation
• Load Classification: Categorisation of the driven machine as Steady (S), Medium Impulsive (M), or Highly Impulsive (H), used to determine the service factor
• Nominal Ratio: The catalogue or labelled reduction ratio of a gearbox; may differ slightly from the actual (exact) ratio
• Overhung Load: The radial force acting on the gearbox output shaft due to belt/chain/gear tension from an externally mounted drive mechanism
• PCD (Pitch Circle Diameter): The effective diameter of a sprocket, pulley, or gear at which the driving force acts
• Service Factor: A multiplier applied to the design load to account for operating conditions (load type, duty cycle, prime mover characteristics)
• Setting Width: The axial distance between the two half-bodies of a coupling at the correct installed position
• Single Reduction: A gearbox arrangement using one worm/wheel stage, suitable for ratios up to 70:1
• Taper Bush: A standardised conical locking device used to secure hubs (couplings, sprockets, pulleys) onto shafts using a keyway and set screws
• Thermal Rating: The maximum continuous power or torque a gearbox can transmit without overheating, limited by heat dissipation capacity
• Thermal Service Factor: A multiplier that adjusts the thermal rating requirement based on the ambient temperature around the gearbox
• Thrust Load: An axial force acting along the output shaft, typically caused by helical gears or other mechanisms
• Torsional Stiffness: The resistance of a coupling to angular deflection under torque, measured in Nm/° — higher values indicate a more rigid coupling
• Wormshaft: The screw-like input element of a worm gearbox
• Wormwheel: The gear element of a worm gearbox that meshes with the worm; typically made of phosphor bronze

Quick Revision

• Four coupling types covered: Pin-type (highest torque/speed range), Tyre-type (best misalignment/damping), Disc-type (best torsional rigidity), Chain-type (easiest assembly)
• Taper bushes provide the standardised shaft-to-hub interface; always specify both bush number and bore size
• Key dimensions are determined by shaft diameter according to the standard table
• Worm gearboxes: Single reduction up to 70:1; double reduction from 75:1 to 4900:1
• Seven gear sizes available, designated by nominal centre distance
• Selection is a 15-step process: Data → Ratio → Nominal ratio → Output speed → Load class → Service factor → Selection capacity → Preliminary selection → Verify ratio/speed → Thermal check → OH load check → Thrust check → Efficiency → Calculate unknowns → Specify
• Service factor depends on three variables: prime mover type, load classification (S/M/H), and duty duration
• Thermal check is critical for continuously operating gearboxes — the thermal rating often governs selection over the mechanical rating
• Overhung load formula: F = 2fT/d = 60fP/(πdN) — always check against allowable values
• Drive application factors: Chain = 1.0, Gear = 1.25, Vee belt = 1.5, Flat belt = 2.0
• Efficiency ranges from ~84% (high ratio, small gearbox) to ~96% (low ratio, large gearbox)
• If OH load exceeds allowable: Use an intermediate layshaft with its own bearings and a flexible coupling to the gearbox to decouple the radial load
• Higher gearbox ratings achievable via synthetic oils, oil coolers, high-tensile shafts, and dual keys

Overview

This document covers worm gearbox selection and specification, including single and double reduction configurations, and geared motor unit selection for industrial drive applications. It provides comprehensive rating data for various nominal gear ratios, centre distances, input speeds, and mounting configurations, along with a step-by-step method for selecting an appropriate geared motor unit based on application requirements such as torque, speed, load classification, overhung load, and thrust load.

Key Concepts

• Worm Gearbox: A gear system using a worm (screw-type gear) meshing with a worm wheel to achieve high reduction ratios in a compact form
• Nominal Ratio: The designed speed reduction ratio between input and output shafts (e.g., 40:1, 50:1, 60:1, 70:1)
• Centre Distance: The distance between the centrelines of the worm shaft and the wheel shaft; determines gearbox physical size and torque capacity
• Thermal Rating: The maximum continuous input power (kW) a gearbox can handle without exceeding safe operating temperatures under standard conditions
• Mechanical Rating: The maximum input power (kW) based on the strength of the gears, shafts, and bearings — typically higher than thermal rating
• Efficiency (%): The ratio of output power to input power; worm gearboxes have lower efficiency at higher ratios due to sliding contact
• Output Torque (Nm): The rotational force available at the output shaft — specified for both thermal and mechanical limits
• Single Reduction: One worm and wheel pair providing a single stage of speed reduction
• Double Reduction: Two stages of worm and wheel reduction in series for very high overall ratios
• Overhung Load (OHL): A radial force applied to the output shaft by an attached mechanism (e.g., pulley, sprocket, gear)
• Axial (Thrust) Load: A force acting along the axis of the output shaft, caused by mechanisms such as helical gears
• Drive Classification: A rating system (1 to 4) that accounts for the severity of service based on load type and operating hours
• Load Classification: Categorisation of driven machinery as Steady (S), Medium Impulsive (M), or Highly Impulsive (H)
• Geared Motor Unit: A pre-assembled combination of an electric motor and a gearbox, designed as a compact drive solution
• Force Feed Lubrication: Required for operation in shaded (high-load) areas of rating tables; uses a pump to circulate lubricant
• Oil Cooler: External cooling device that allows higher ratings beyond standard thermal limits
• Taper Lock Bush: A mechanical fastening method for mounting pulleys, sprockets, or gears onto shafts using a tapered interference fit

Single Reduction Worm Gearbox Ratings

Nominal Ratio 40:1

• Input Speed: 1800 rev/min (output 45 rev/min) down to 100 rev/min (output 2.5 rev/min)
• Centre Distances Available: 10, 12, 14, 17, 20, 24, 28 (units correspond to shaft separation)
• Key Observations:
  • At 1800 rev/min input, thermal input ranges from 28 kW (CD 10) to 216 kW (CD 28), mechanical input from 33 kW to 372 kW
  • Efficiency improves with increasing centre distance: 86% (CD 10) to 89% (CD 28) at 1800 rev/min input
  • At lower input speeds (e.g., 500 rev/min), thermal input drops significantly (10–105 kW), but mechanical input becomes proportionally larger relative to thermal
  • At 100 rev/min input, only mechanical ratings are listed (no thermal rating), with efficiency dropping to 68–77%
  • Maximum output torque (single key): ranges from 11,200 Nm (CD 10) to 72,000 Nm (CD 28)
  • Maximum output torque (standard shaft): ranges from 15,800 Nm (CD 10) to 146,400 Nm (CD 28)

Nominal Ratio 50:1

• Input Speed: 1800 rev/min (output 36 rev/min) down to 100 rev/min (output 2 rev/min)
• Key Observations:
  • Thermal input ratings are lower than 40:1 at the same centre distance due to increased sliding losses
  • At 1800 rev/min, thermal input ranges from 23 kW (CD 10) to 170 kW (CD 28)
  • Efficiency is lower than 40:1: 83–86% at 1800 rev/min, dropping to 64–72% at 100 rev/min
  • At 250 rev/min input, efficiency drops to 70–78%
  • Maximum output torque values remain the same as 40:1 (they are gearbox-size dependent, not ratio dependent)

Nominal Ratio 60:1

• Input Speed: 1800 rev/min (output 30 rev/min) down to 100 rev/min (output 1.6 rev/min)
• Key Observations:
  • Further reduction in thermal input compared to 50:1: 21 kW (CD 10) to 149 kW (CD 28) at 1800 rev/min
  • Efficiency continues to decrease: 81–84% at 1800 rev/min, down to 61–69% at 100 rev/min
  • Mechanical ratings become increasingly dominant at low input speeds
  • At 250 rev/min input, efficiency is 67–75%

Nominal Ratio 70:1

• Input Speed: 1800 rev/min (output 25.7 rev/min) down to 100 rev/min (output 1.4 rev/min)
• Key Observations:
  • Lowest thermal input ratings among the four ratios: 18 kW (CD 10) to 133 kW (CD 28) at 1800 rev/min
  • Efficiency is the lowest: 78–82% at 1800 rev/min, dropping to 55–65% at 100 rev/min
  • At very low input speeds (100 rev/min), efficiency can be as low as 55%
  • Higher ratios generate more heat due to greater sliding between worm and wheel

General Trends Across Ratios

Important Notes from Rating Tables

• Ratings in shaded areas require force feed lubrication
• All ratings are based on mineral oils; synthetic lubricant ratings available on request
• Oil coolers can provide higher ratings than those listed
• Two keys must be specified for the wheel and output shaft when maximum output torque for single key is exceeded
• High tensile steel output shaft must be specified when maximum output torque for standard shaft is exceeded

Double Reduction Worm Gearbox Ratings (Mineral Oil)

Input Speed: 1450 rev/min

• Nominal Ratios Available: 75:1 to 4900:1
• Output Speeds: 19.0 rev/min (ratio 75) down to 0.30 rev/min (ratio 4900)
• Centre Distances Available: 10, 12, 14, 17, 20, 24, 28
• Key Observations:
  • At ratio 75, input power ranges from 14.7 kW (CD 10) to 156 kW (CD 28), efficiency 83–88%
  • At ratio 150, input power ranges from 13.3 kW (CD 10) to 186 kW (CD 28), efficiency 79–83%
  • At ratio 500, input power ranges from 6.2 kW (CD 10) to 68.2 kW (CD 28), efficiency 67–76%
  • At ratio 1000, input power ranges from 4.1 kW (CD 10) to 37 kW (CD 28), efficiency 59–70%
  • At ratio 4900, input power ranges from 1.2 kW (CD 10) to 12.0 kW (CD 28), efficiency 33–43%
  • Output torque reaches maximum capacity at larger centre distances (up to 165,000 Nm at CD 28)
  • Efficiency drops dramatically at very high ratios — as low as 33% at ratio 4900, CD 10

Input Speed: 960 rev/min

• Same ratio and centre distance options as 1450 rev/min
• Key Observations:
  • Input power ratings are lower than at 1450 rev/min across all configurations
  • At ratio 75, input power ranges from 11.9 kW (CD 10) to 119 kW (CD 28), efficiency 81–87%
  • At ratio 500, input power ranges from 4.6 kW (CD 10) to 44.3 kW (CD 28), efficiency 64–74%
  • At ratio 4900, input power ranges from 0.9 kW (CD 10) to 8.2 kW (CD 28), efficiency 31–40%
  • Output torque values reach the same maximums as 1450 rev/min tables
  • Efficiency is slightly lower at 960 rev/min compared to 1450 rev/min for the same ratio

Gearbox Dimensions and Configurations

Mounting Types

Unit Size Range

• Available sizes: 10, 12, 14, 17 (single reduction also includes 20, 24, 28)
• Sizes are designated by centre distance number
• Double reduction units add a second worm/wheel stage, increasing overall envelope

Key Dimensional Parameters

• A — Overall length (mm)
• B — Height to shaft centre (mm)
• C — Width across mounting feet (mm)
• D — Mounting foot length (mm)
• F — Mounting hole pattern (format: pitch × number / bolt size)
• G — Output shaft details (bore, keyway, etc.)
• H, J — Additional envelope dimensions
• K — Bolt hole size (for mounting)
• Oil Capacity — Approximate litres of lubricant required
• Weight — Approximate mass in kg (quoted without oil)

Wormshaft and Wheelshaft Details

• E1 — Wormshaft diameter
• V1 — Wormshaft length
• W1 — Wormshaft bearing span
• X1, X2 — Wormshaft extension details
• Tapped Hole — Thread size for input shaft connection (e.g., M20×42, M24×50, M30×60)
• E2 — Wheelshaft (output) diameter
• V2 — Wheelshaft length
• W2 — Wheelshaft bearing span
• Y1, Y2 — Wheelshaft extension details

Dimensional Data Summary (Single Reduction — Foot Mounted Underdriven)

Important Configuration Notes

• Non-reversible units require a sprag clutch backstop to be fitted
• Units with central mounting pads use a bolt hole diameter designated as dimension K
• Shaft-mounted types use output sleeve details instead of foot mounting dimensions
• For units with flange mounting motors, refer to separate motor-specific data
• Double reduction units have two output keys as a standard feature
• Second reduction units may have blank central mounting pads

Geared Motor Units

Overview

• Designed for relatively low power applications (approximately 0.1 to 4.0 kW motor power input)
• Only foot-mounted geared motor units are covered; output is via bored bush with key or output shaft extension
• Units can also be specified as a free-standing unit (gearbox without motor) coupled to a designer's chosen motor
• Five gearbox sizes available (designated by frame size number: 11, 17, 22, 26, 30)
• 12 gear ratios available per size, ranging from 5:1 to 70:1
• All units fitted with 4-pole motors (nominal speed 1400–1420 rev/min), giving output speeds from 288 rev/min down to 20 rev/min
• 11 motor sizes available, ranging from 0.12 kW to 4.0 kW
• Each gearbox size can be fitted with multiple motor sizes according to power requirements
• Total of 104 power and speed combinations available

Note: The smallest motor size (0.12 kW) is not a preferred size and may have extended lead times.

Geared Motor Unit Selection Method

Step 1: Establish Mechanical Data

• Determine the type of output (driven) machinery
• Establish maximum (design) torque, power, and speed (including tolerance range on speed if given)
• Determine duration of service: continuous or intermittent
• Determine average number of hours per day of operation

Step 2: Determine Load Classification

• From the load classification table, identify whether the driven machinery is:
  • S (Steady) — e.g., conveyors (uniformly loaded), fans, generators
  • M (Medium Impulsive) — e.g., car dumpers, dough mixers, machine tools, textile dryers
  • H (Highly Impulsive) — e.g., crushers, hammer mills, rubber mills, tumbling barrels

Step 3: Determine Drive Classification

• Cross-reference:
  • Load classification (S, M, or H) from Step 2
  • Average hours per day of operation (Under 3 hours, 3–10 hours, Over 10 hours)
• This yields a Drive Classification number from 1 to 4

Step 4: Select Unit from Data Tables

• Using the drive classification from Step 3, go to the appropriate data table
• Select the unit based on required output speed and output power (or torque)
• The data tables provide: output power (kW), output torque (Nm), and recommended gearbox size for each motor power and gear ratio

Step 5: Check Overhung Load

• If a gear, pulley, chain-wheel, flywheel, or other mechanism is directly attached to the output shaft, calculate the overhung load (radial force on the shaft)
• Use the approximate formula:

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

Where:

• F = overhung load (N)

• T = output shaft torque (Nm) — use design value, not selection table value

• P = output shaft power (W) — use design value, not selection table value

• d = pitch circle diameter (PCD) of pulley, sprocket, or gear (m)

• N = output shaft speed (rev/min)

• f = drive application factor:

  • 1.0 for chain drive or toothed belt
  • 1.25 for gear drive
  • 1.5 for vee (wedge) belt
  • 2.0 for flat friction belt

• Compare calculated overhung load to the allowable overhung load from the capacity table

• Overhung load capacities assume the load is applied mid-way along the shaft (at dimension A)

• If the allowable value is exceeded, either choose a larger unit or use an intermediate shaft (layshaft) with its own bearings and a flexible coupling

Step 6: Check Thrust (Axial) Load

• If a helical gear or other mechanism creates an axial load on the output shaft, verify it does not exceed the allowable axial load from the capacity table
• If exceeded, choose a larger gearbox or use an intermediate shaft with bearings to absorb the axial load

Step 7: Check Output Shaft Dimensions

• Verify the output shaft diameter is suitable for the attached mechanism (e.g., bore of pulley or sprocket)
• Check mounting bolt sizes, bolt hole locations, and centre distances from the dimension data

Step 8: Specify the Unit

• Specify: gearbox size, gear ratio, output speed, and motor details (frame size, power rating)
• Motor data includes: rated speed, current at rated voltage, moment of inertia, and rotor mass

Motor Ratings and Performance Data

Motor Flange and Shaft Dimensions

Overhung Load Capacities (N)

• Load capacities assume resultant load applied mid-way along the shaft (dimension A)
• Dimension A varies by gearbox size: 60 mm (Size 11), 75 mm (Size 17), 95 mm (Size 22), 115 mm (Size 26), 140 mm (Size 30)

Service Factors

Starts per Hour Factor (f_s)

Worked Example: Geared Motor Unit Selection

Problem: An industrial textile dryer operates for 12 hours/day, driven by a geared motor unit via a wedge belt drive. The wedge belt pulley has a PCD of 160 mm and will be attached to the output shaft via a taper lock bush with a maximum bore size of 50 mm. The torque at the pulley is 200 Nm and the required speed is 35 ± 2 rev/min.

Solution:

1. Output torque = 200 Nm at 35 ± 2 rev/min

   • Required output power: P = T × ω = 200 × (π × 35 / 30) = 0.733 kW

2. Load classification: Textile dryer = M (Medium Impulsive)

3. Drive classification: Load M, 12 hrs/day (over 10 hours) → Drive Classification 3

4. Unit selection: From Drive Classification 3 data table, select a unit with output speed 36 rev/min, ratio 40:1, output torque 225 Nm — output speed 36 rev/min is within the tolerance of 35 ± 2 rev/min. Unit size: Size 30.

5. Overhung load check:

   • F = (2 × f × T) / d = (2 × 1.5 × 200) / 0.16 = 3750 N
   • Allowable OHL for Size 30 at 50 rev/min = 6900 N; at 25 rev/min = 7000 N
   • 3750 N < 6900 N → OK, no interpolation necessary

6. Thrust load: No helical gear → No axial load → OK

7. Output shaft diameter: Size 30 output shaft = 40 mm (nominal)

   • Maximum pulley bore = 50 mm → 40 mm < 50 mm → OK
   • Bolt holes: 14.5 mm diameter → use M14 bolts
   • Bolt hole centre distances: 160 mm (side) and 130 mm (end)

8. Specification: Size 30, Ratio 40:1, Output Speed 36 rev/min, D90S frame motor (1.1 kW)

Single vs Double Reduction Gearboxes

Efficiency vs Ratio (Single Reduction, at 1800 rev/min Input, Approx. CD 20)

Mounting Configuration Comparison

Geared Motor Unit Selection Process

flowchart TD
    A[Step 1: Establish Mechanical Data] --> B[Determine output torque, power, speed]
    B --> C[Determine service duration and hours/day]
    C --> D[Step 2: Load Classification]
    D --> E{Driven Machinery Type?}
    E -->|Uniform/Light| F["Steady (S)"]
    E -->|Moderate Impact| G["Medium Impulsive (M)"]
    E -->|Heavy Impact| H["Highly Impulsive (H)"]
    F --> I[Step 3: Drive Classification]
    G --> I
    H --> I
    I --> J[Cross-reference: Load Class × Hours/Day]
    J --> K[Drive Classification 1–4]
    K --> L[Step 4: Select Unit from Data Table]
    L --> M[Match output speed and power/torque]
    M --> N[Step 5: Check Overhung Load]
    N --> O{F ≤ Allowable OHL?}
    O -->|Yes| P[Step 6: Check Thrust Load]
    O -->|No| Q[Choose larger unit OR use layshaft]
    Q --> P
    P --> R{Axial Load ≤ Allowable?}
    R -->|Yes| S[Step 7: Check Output Shaft Dimensions]
    R -->|No| T[Choose larger unit OR use layshaft]
    T --> S
    S --> U[Verify bore, bolt sizes, centre distances]
    U --> V[Step 8: Specify Unit]
    V --> W[Size + Ratio + Speed + Motor]