What is a flexible coupling and why use one?

A flexible coupling connects two shafts while accommodating misalignment (angular, parallel, axial) and damping vibrations. They protect connected equipment from shock loads and torsional vibration, extending bearing and seal life.

How do I select the right service factor?

Service factors account for load characteristics. Uniform loads (fans, pumps) use 1.0-1.5. Moderate shock (compressors, mixers) use 1.5-2.0. Heavy shock (crushers, presses) use 2.0-3.0 or higher. Always check manufacturer catalogs for specific applications.

What is torsional stiffness in a coupling?

Torsional stiffness (Ct) is the torque required per unit angle of twist, measured in Nm/rad. Higher stiffness means more accurate motion transmission but less vibration damping. DIN 740 defines dynamic and static stiffness values.

Why is natural frequency important for coupling selection?

The torsional natural frequency of the coupling-shaft system must be separated from excitation frequencies (typically shaft speed and its multiples) by at least 20%. Resonance causes excessive vibration and coupling failure.

What is DIN 740 and how does it apply to couplings?

DIN 740 is a German standard for flexible shaft couplings. Part 2 covers torsional vibration characteristics, defining how to measure and specify dynamic torsional stiffness, damping, and the coupling's influence on system natural frequencies.

Free Engineering Tool #081

Flexible Coupling Calculator

Calculate the required torque rating, torsional stiffness, and natural frequency for flexible couplings per DIN 740. Select coupling type and enter operating conditions.

DIN 740 Coupling Selection Torsional Analysis

Power (kW)

Speed (RPM)

Service Factor (K_A)

Coupling Type

Inertia — Drive Side (kg·m²)

Inertia — Driven Side (kg·m²)

Quick presets

Results

Required Coupling Torque Rating

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Nominal Torque (T_n)

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Torsional Stiffness (C_t)

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Natural Frequency (f_n)

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Operating Frequency

—

Frequency Ratio (f_n/f_op)

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Nominal Torque

The nominal torque transmitted by the coupling is derived from power and speed:

P — transmitted power (kW)
n — rotational speed (RPM)
T_n — nominal torque (N·m)

Required Torque Rating

Per DIN 740, the coupling must be rated for the design torque including the service factor:

Where K_A is the application/service factor accounting for dynamic loads.

Torsional Natural Frequency

For a two-inertia system connected by the coupling:

C_t — dynamic torsional stiffness of coupling (N·m/rad)
J₁, J₂ — mass moments of inertia (kg·m²)

Service Factor Guide

K_A	Load Character	Typical Applications
1.0	Uniform	Fans, centrifugal pumps, light conveyors
1.25	Light shocks	Belt conveyors, generators, centrifugal blowers
1.5	Moderate shocks	Reciprocating compressors, positive displacement pumps
1.75	Heavy shocks	Mixers, ball mills, rubber extruders
2.0–3.0	Extreme/Reversing	Crushers, presses, shears, rolling mills

⚠️ Note: The natural frequency should be at least 20% away from the operating frequency and its harmonics to avoid torsional resonance. DIN 740-2 recommends f_n/f_op > 1.2 or f_n/f_op < 0.8.

Example — 15 kW Compressor Drive

Given: P = 15 kW, n = 1500 RPM, K_A = 1.5

T_n = 9549 × 15 / 1500 = 95.5 N·m

T_req = 1.5 × 95.5 = 143.2 N·m

Select a coupling with T_max ≥ 143.2 N·m

Flexible Coupling Calculator | DIN 740 | Free Online Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Nominal Torque

Required Torque Rating

Torsional Natural Frequency

Service Factor Guide