Overview of Coupling Types
Couplings connect two shafts, transmit torque, and compensate for manufacturing and assembly tolerances. VDI 2240 systematically classifies them into non-switchable and switchable couplings. Among non-switchable couplings, a distinction is made between rigid couplings (e.g., flange couplings per DIN 116, with no backlash or elasticity but with full alignment accuracy), torsionally flexible couplings (e.g., jaw couplings with an elastomer star, which compensate for radial, axial, and angular misalignment and dampen vibrations), and torsionally rigid, compensating couplings (e.g., metal bellows couplings, which combine high stiffness with precision). Among engageable couplings, a distinction is made between externally actuated (e.g., electromagnetic couplings) and self-acting types (e.g., freewheel, slip, or safety couplings); they connect or disconnect the shafts—either externally actuated or self-acting (depending on direction, speed, or torque).
Rated Torque and Service Factor
A coupling is selected based on the torque to be transmitted. The rated torque is derived from power and speed and is then multiplied by an operating or shock factor (S_B), which accounts for load peaks, start-ups, and shocks. For flexible shaft couplings, DIN 740 specifies the design procedure and the service factors:
M_N = 9550 · P / n
M_dim = M_N · S_B
M_N = Rated torque (Nm) · P = Power (kW) · n = Speed (min⁻¹) · S_B = Service factor (guideline values: smooth operation 1.0–1.5 · moderate shocks 1.5–2.5 · severe shocks 2.5–4.0)
Five selection criteria
- Torque: Design for a value ranging from 1.3 to 1.5 times the continuous drive torque.
- Alignment tolerances: radial 0.1–1.0 mm, axial 0.2–2.0 mm, angular 0.5–2.0°.
- Speed/Centrifugal Forces: Centrifugal forces become a factor at speeds of >5,000 min⁻¹—lightweight, compact designs are preferred.
- Zero backlash: specified in arcminutes (arcmin); 1° = 60 arcmin.
- Temperature range: Elastomer couplings typically range from −20 to +80 °C; for higher temperatures, metal bellows or flange couplings are used.
Mnemonic
Worked example
Given
A motor has a power output of P = 5.5 kW at n = 1,450 min⁻¹; the application involves moderate shocks (S_B = 2.0).
Calculation
- M_N = 9550 · 5.5 / 1,450 ≈ 36.2 Nm
- M_dim = 36.2 · 2.0 ≈ 72.4 Nm → the coupling must be rated for at least ~72 Nm
- Marked point: Example: 72.4 Nm
- Marked point 1: Example: 72.4 Nm
Description and values of the figure
The design moment M_dim=M_N·S_B increases linearly with the shock factor S_B. The three zones (different patterns rather than just colors) indicate the duty type: steady-state (S_B 1.0–1.5), moderate shocks (1.5–2.5), and severe shocks (2.5–4.0). The worked example with P =5.5 kW and n =1,450 min⁻¹ yields a rated torque of M_N≈36.2 Nm, and with S_B=2.0, a design torque of approximately 72.4 Nm.
| Shock factor S_B | M_dim (Nm) |
|---|---|
| 1.0 | 36.2 |
| 1.5 | 54.3 |
| 2.0 | 72.4 |
| 2.5 | 90.6 |
| 3.0 | 108.7 |
| 4.0 | 144.9 |
Knowledge check
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