Coupling Design Guide

As the leading manufacturer of small precision couplings Huco is committed to continuous development of its products and manufacturing systems. It is this commitment that has created a comprehensive range of couplings, both standard and custom, and provides the reassurance that Huco products can be relied upon across a wide range of applications.

In conjunction with this pocket guide the Huco range offers designers the opportunity to create the perfect combination of coupling and application, whether to compensate for misalignment or to ensure high-precision positioning in sophisticated control systems.

Introduction to Couplings

Precision Shaft Couplings

In the simplest of terms a coupling’s purpose is to transfer rotational movement from one shaft to another. Reality is somewhat more complicated, though, as flexible shaft couplings have also to compensate for misalignment between the two shafts. This ability must be balanced with the need to be pliable in the planes of misalignment while still having the torsional strength to carry out the coupling’s main function is known as the Compliance Mechanism where compliance is the capacity for allowing relative displacement. Several factors should always be taken into consideration when looking to specify flexible shaft couplings. These are torsional stiffness, backlash, torque, life and attachment system. All of these have a bearing on coupling selection.


This is the measure of resistance to torsional rotation in the coupling, and in applications such as closed loop velocity and motion control systems it needs to be high. Whereas in systems where the transmission is subject to shock loads, the coupling requires a low torsional stiffness, sometimes referred to as torsional damping.


The free play between input and output shafts is commonly referred to as backlash. If rotation is constant then backlash has little impact. However, if the system requires changes in rotational direction, a dwell is created which in high- speed, short cycle applications can create noise and instability. In open-loop systems backlash will also cause loss of accuracy.


A coupling’s torque capacity can be defined in several ways including nominal torque, reversing torque and peak torque. As far as Huco’s products are concerned a coupling’s capacity to transfer rotation under load is qualified by its peak torque rating. This figure is determined through Huco’s testing procedures and is the maximum reversing torque applied over at least one million cycles without loss of performance. More information on Torque can be found on page 14


The life expectancy of any flexible coupling is dependent on the individual application. Therefore, published performance values, which are based on extensive simulations, are intended as a guide. For instance, where perfect shaft alignment is the case, a coupling can sustain its peak torque value almost indefinitely. However, where misalignment extends beyond the recommended limits, failure can be induced in disc, bellows and helical beam couplings, while wear will be accelerated in universal joints and displacement couplings. Aluminium beam-type couplings will always have a finite fatigue limit when an alternating load is applied.


The simplest and most cost-effective method of attaching a coupling to a shaft is to use set screws which locate on flats or dimples on the shaft. Clamps may also be used and have the advantage that as the shaft diameter increases so does traction. For high integrity drive systems a key and keyway system should be employed.