Mechanical Design Data and Machine-Element Reference: Mounting Configuration Comparison

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]

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

Couplings & Worm Gearbox Selection

Overview

This document consolidates mechanical design reference data for shaft couplings and worm gearboxes, two fundamental power transmission components. It covers selection criteria, dimensional data, performance ratings, and a step-by-step gearbox selection methodology with a worked example. The material is drawn from an industry-standard mechanical design data manual and is intended as a practical engineering reference for component specification and selection.

Key Concepts

• Shaft Couplings connect two rotating shafts to transmit torque, while accommodating varying degrees of misalignment (angular, axial, or parallel)
• Taper Bushes are standardised locking devices used to mount couplings (and other components) onto shafts, using a taper-lock mechanism for secure, keyway-based attachment
• Worm Gearboxes provide high reduction ratios in a compact form, using a worm (screw) and worm wheel (gear) arrangement — suited to high-power, low-speed applications
• Service Factors adjust theoretical ratings to account for real-world operating conditions such as load type, duty cycle, and ambient temperature
• Overhung Loads are radial forces imposed on a gearbox output shaft by belt, chain, or gear drives — a critical check in gearbox selection

. Coupling Types

Four main coupling families are covered, each suited to different application requirements:

1.1 Pin-Type Flexible Couplings (Taper Bore)

• Construction: Two half-bodies joined by elastomeric-bushed pins; taper bore accepts standard taper bushes
• Product code convention: Code ending in /77 denotes one taper-bored half body; code ending in /88 denotes a second taper-bored half body; a code ending in /78 combines one of each to form a complete coupling
• Key parameters: Number of pins (3–12), power rating at 100 RPM (kW), nominal torque (Nm), normal maximum speed (RPM), taper bush number, bore range, and setting width
• Bore range: Minimum 32 mm to maximum 5.000" (125 mm) depending on size
• Speed range: 2200–6800 RPM (decreasing with increasing coupling size)
• Torque range: 194–18,536 Nm across the size range
• Note: At maximum bore, keyways may be shallower than standard

1.2 Tyre-Type Flexible Couplings

• Construction: Two flanged half-bodies connected by a flexible rubber tyre element, providing high misalignment capacity and vibration damping
• Available body types: "F" type (smaller sizes, ~40–60 range) and "H" type (larger sizes)
• Taper bore variants: Available with product code /77 (F-type half body) and /88 (H-type half body)
• Key parameters: Bore dimensions (A, B), setting distance (M), flange diameter (E), tyre width (W), clamping screw torque, and mass
• Bore range: 12 mm minimum up to 150 mm maximum bore
• Tyre widths: 67–274 mm depending on coupling size
• Clamping gap: 2–6 mm (important for tyre installation and removal)
• Note: Clamping screws must be withdrawn to release tyres; wrench clearance for taper bush screws is needed when the bush large end is outboard

Tyre Coupling Ratings Table

1.3 Disc-Type Flexible Couplings

• Construction: Two hubs connected by a flexible disc element (typically stainless steel or composite laminate), providing torsional rigidity with angular and axial misalignment capacity
• Key parameters: Taper bush number, power at 100 RPM (kW), nominal torque (Nm), maximum speed (RPM), bore range, and end float
• Bore range: 12 mm minimum up to 110 mm maximum
• Speed range: 900–2900 RPM (normal maximum speeds with 1° angular malalignment; higher speeds require manufacturer consultation)
• Torque range: 71.6–4298 Nm
• Misalignment tolerance: Maximum 1° angular, maximum 0.5 mm axial
• Size designations: Use letter-number codes (e.g., D41N, D52S, D71W, D89N, D108W, D127S) where the letter suffix indicates the disc type (N = normal, S = standard, W = wide)

1.4 Chain-Type Flexible Couplings

• Construction: Two sprocket-like hubs enclosed by a duplex roller chain and a cover/housing; the chain allows for angular, axial, and parallel misalignment
• Key parameters: Taper bush number, power at 100 RPM (kW), nominal torque (Nm), maximum speed (RPM), bore range, dimensions (B through F), and malalignment tolerances
• Bore range: 12 mm minimum up to 140 mm maximum
• Speed range: 700–3500 RPM (higher speeds require manufacturer consultation)
• Torque range: 52.5–8595 Nm
• Misalignment tolerance: 1° angular, 0.25–0.5 mm axial
• Taper bore variants available for selected sizes

. Taper Bushes — Range of Bores

Taper bushes are the standardised interface between shaft and coupling (or sprocket, pulley, etc.). The metric range includes:

* Asterisked bores indicate non-standard or shallow keyway variants

• Keyway note: When ordering, specify both the bush number and the bore size required
• Shallow key depth: Some bore sizes use a shallow key depth variant (marked with asterisk)

. Key and Keyway Dimensions (Metric)

Keys and keyways conform to the relevant industrial standard (originally referenced as BS 4235: Part 1: 1972). Parallel keyways are supplied as standard unless otherwise specified.

. Worm Gearboxes

4.1 General Information

• Application: Designed for relatively high-power applications requiring significant speed reduction
• Types available: Both single-reduction and double-reduction configurations
• Mounting configurations: Underdriven, overdriven, shaft-mounted, vertical, and agitator types (note: agitator-type data is typically excluded from standard catalogues and requires manufacturer consultation)
• Single-reduction ratios: Available from 5:1 to 70:1
• Double-reduction ratios: Available from 75:1 to 4900:1
• Gear sizes: Seven standard sizes are available, designated by a number code (e.g., W10, W12, W14, W17, W20, W24, W28) where the designation letter indicates a worm gearbox and the number represents the nominal centre distance between the worm shaft and wheel shaft in inches
• Input speed range: Standard catalogues cover eight input speeds from 1800 to 100 rev/min for single-reduction; for double-reduction, only 1450 and 960 rev/min are typically listed (higher speeds up to 2800–3000 rev/min possible with manufacturer consultation)

4.2 Rating Basis

• Power and torque ratings are based on mineral oil lubrication and standard steel shafts with a single key
• Higher ratings can be achieved through: synthetic oils/additives, oil coolers, high-tensile steel shafts, and two keys (requires manufacturer consultation)
• Actual vs nominal ratios: Actual ratios may differ slightly from nominal ratios; always use the actual ratio for accurate speed calculations
• Efficiency: Listed as efficiency at rated (maximum) power; at very low power, efficiency drops slightly; for normal operating conditions, efficiency can be treated as constant

4.3 General Specification

. Worm Gearbox Selection Method

A systematic 15-step procedure is used to select a suitable worm gearbox:

Step 1 — Establish Mechanical Data

• Input (driver): Maximum (or design) torque, power, and speed
• Output (driven): Maximum (or design) torque, power, and speed (including tolerance range if given)
• Duration of service: Continuous or intermittent, and average hours per day
• Maximum ambient temperature around the gearbox

Important notes:

• Input and output values are inter-related by the gearbox — not all will be independently known at the start
• Maximum torque/power excludes shock loading or hard-start factors (these are handled by the service factor)
• Ambient temperature does not affect selection if the gearbox operates intermittently with sufficient cooling time between runs

Step 2 — Calculate Reduction Ratio

• Reduction ratio = Input speed ÷ Output speed

Step 3 — Select Nominal Ratio

• From the reduction ratio tables, select the closest nominal ratio to the required value
• If the ratio exceeds 70:1, a double-reduction gearbox is required

Step 4 — Calculate Nominal Output Speed

• Nominal output speed = Input speed ÷ Nominal ratio

Step 5 — Determine Load Classification

• Classify the driven machine load as: Steady (S), Medium Impulsive (M), or Highly Impulsive (H)
• Use the load classification table (see Section 6 below)

Step 6 — Determine Service Factor

• From the mechanical service factor table, find the factor based on: prime mover type, load classification, duty duration (hours/day), and whether operation is continuous or intermittent

Step 7 — Calculate Selection Capacity

• If input conditions are given: Selection input power = Design input power × Service factor
• If output conditions are given: Selection output torque = Design output torque × Service factor

Step 8 — Preliminary Gearbox Selection

• Go to the gearbox data tables for the relevant nominal ratio and input speed
• Select the smallest gearbox with a capacity greater than the selection capacity from Step 7

Step 9 — Verify Actual Ratio and Output Speed

• From the reduction ratio tables, obtain the actual ratio for the selected gearbox size
• Calculate actual output speed = Input speed ÷ Actual ratio
• Verify the output speed falls within the required tolerance range
• If not, consider an alternative mechanical drive system (e.g., chain drive) in conjunction with the gearbox

Step 10 — Check Thermal Rating (Continuous Operation)

• If the gearbox operates continuously (or intermittently without sufficient cool-down time), obtain the thermal service factor from the thermal service factor table
• The thermal service factor depends on ambient temperature

Step 11 — Verify Thermal Capacity

• Multiply the thermal service factor by either the design input power or output torque
• Compare this selection thermal value against the gearbox's thermal rating from the data tables
• If the gearbox thermal rating is insufficient, either:
  • Select the next larger gearbox size (recheck actual ratio), or
  • Use auxiliary cooling (synthetic oil, oil coolers)

Step 12 — Check Overhung Load

• If a belt, chain, gear, or other mechanism is attached to the output shaft, calculate the overhung (OH) load
• OH load formula: F = (2 × f × T) / d, or equivalently F = (60 × f × P) / (π × d × N)
  • Where: F = overhung load (N), T = output shaft torque (Nm, design value not selection value), P = output shaft power (W, design value), d = PCD of pulley/sprocket/gear (m), N = output shaft speed (rev/min)
  • f = drive application factor: 1.0 (chain drive or toothed belt), 1.25 (gear drive), 1.5 (vee belt), 2.0 (flat friction belt)
• Compare calculated OH load against the allowable value from the gearbox overhung load tables
• If exceeded, select a larger gearbox or use an intermediate layshaft with its own bearings and a flexible coupling to the gearbox

Step 13 — Check Thrust Load

• If a helical gear or other mechanism produces axial thrust on the output shaft, verify this does not exceed the gearbox's allowable thrust load
• Alternative: use an intermediate shaft (layshaft) with its own bearings to absorb the axial load

Step 14 — Determine Efficiency and Calculate Unknowns

• Read efficiency from the gearbox data tables at the selected ratio and input speed
• Use efficiency to calculate any remaining unknown values:
  • Output power = Input power × Efficiency
  • Input power = Output power ÷ Efficiency
  • Input torque = Output torque ÷ (Ratio × Efficiency)

Step 15 — Specify the Gearbox

• Specify the mounting type: underdriven, overdriven, shaft-mounted, or vertical
• Read key dimensions from the manufacturer's dimension tables: input/output shaft diameters, centreline distances, bolt hole locations, etc.

. Load Classification and Service Factors

6.1 Load Classification by Application (Partial List)

Note: Applications marked with an asterisk (*) in original tables require specific manufacturer consultation.

6.2 Mechanical Service Factors (Table 2)

Note: Linear interpolation is acceptable for service hours between those listed.

6.3 Thermal Service Factors (Table 3)

Note: A substantial increase in thermal rating is achievable using synthetic lubricants (consult manufacturer).

. Reduction Ratios — Actual vs Nominal

7.1 Single Reduction — Nominal & Exact Ratios

7.2 Double Reduction — Nominal & Exact Ratios

. Gearbox Data Tables (Selected Nominal Ratios)

The gearbox data tables provide thermal and mechanical ratings for each combination of input speed, gear size, and nominal ratio. The tables include: input kW (thermal), output torque (Nm, thermal), input kW (mechanical), output torque (Nm, mechanical), and efficiency (%).

Key to input speeds for standard electric motors (used for directly coupled motor input):

• 4-pole motor: 1500 rev/min (actual ~1450 rev/min)
• 6-pole motor: 1000 rev/min (actual ~960 rev/min)
• 8-pole motor: 750 rev/min (actual ~720 rev/min)

Note on shaded areas in data tables: Ratings in the shaded area require force-feed lubrication (not standard sump oil).

Example Data — Nominal Ratio 50/1 (Single Reduction, 1500 rev/min Input)

. Overhung and Thrust Load Capacities

9.1 Output Shaft Overhung Load Capacities (Newtons)

Values vary by ratio, output speed, and centre distance. Sample data at 1450 rev/min input speed:

9.2 Output Shaft Thrust Load Capacities (Newtons)

At both 1450 and 960 rev/min input speeds, thrust load capacities are constant across centre distances for ratios ≥ 20: