TechArchive

Original article date: February 2000

Need a flexible coupling? Worried about longevity? Tommy Miller has been finding out about a coupling that indicates the applied torque and whether wear is starting to occur

Flexible shaft couplings come in many shapes and sizes. Some are cumbersome and heavy, whilst others are complex, expensive affairs that are difficult to use. It is refreshing therefore to see a new coupling being introduced that is simple yet innovative, offering positive benefits for the machine designer/builder as well as the end user.

Gates’ Eurogrip flexible couplings were designed in response to the increasing use of electronic speed controls in industry, the range of couplings covering standard motor sizes. The design consists of a rubber sleeve, rather like a small toothed belt, linking two metal toothed end pieces that are secured to the driving and driven shafts.

Available for shaft diameters of 19, 28, 42, 48 and 60mm, the couplings are bored to suit a taper bush or a plain bore and keyway.

The Eurogrip offers a number of benefits over more traditional type couplings. It is classed as a zero backlash coupling – there being no free movement or ‘play’ occurring between the two connected shafts when subjected to a reversing movement. Although it will deform elastically while turning, on reversing it will return to the starting point – this feature being a requirement for such equipment as positioning drives and actuators where the final position is critical.

Damped vibrations

Because the drive is carried solely through the high performance elastomeric sleeve, it gives a high vibration-damping capacity, making the coupling especially suitable for direct drive applications in pumps and compressors. The high compliance will be greatly appreciated by designers of speed control systems, where resonance can be a problem, the damping effect actually increasing with load to prevent resonance.

Although the zero backlash and vibration damping features are excellent, it is the coupling’s built-in torque and wear indicator feature that is perhaps the most innovative.

Printed on the outside surface of the coupling sleeve are curved ogee lines (gentle S-curves). As the coupling is driving and naturally twists, the curved lines are designed to become straight under given torque conditions. By using a stroboscope to view the coupling during running, an estimate may be made of the load by observing which line is straight. The lines are nominally set for zero, nominal and overload torque for both directions of twist.

These same lines also act as an indicator that the coupling is nearing the end of its working life, allowing it to be changed before a breakdown occurs. This is achieved by the torsional characteristics of the coupling changing as the sleeve nears the end of its design life, leading to the lines straightening at lower torque values than previously.

The sleeve design also provides a ’shear-pin’ function, enabling the coupling to act as a predictable fuse in the system and therefore give a built-in safety measure.

Despite having very compact dimensions (even the largest coupling in the range for 60mm shafts is only 150mm in diameter with an overall length of 105mm), motor outputs of up to 55kW and a peak design torque of 850Nm can be accommodated. Durability should also be very good – Eurogrip conforms to the requirements of DIN 740 Part 2 and has an operating range of -25degC to +100degC.

Installation is straightforward, as the couplings are designed to operate with up to 5deg of axial misalignment. The end pieces themselves are made from aluminium alloy to reduce weight and inertia, and are anodised to increase their wear resistance and strength. The taper bushes that lock into the end pieces are quickly secured to the shaft by tightening just two grub screws, and are just as easily disassembled for maintenance by utilising two jacking screws to ‘break’ the taper joint.

To assist designers who need to select the correct coupling for their application, Gates has produced a comprehensive design manual. This provides a step-by-step guide to the specification and design of the coupling, complete with example calculations and a guide to service factors. Also included are installation instructions to ensure that the life of the coupling is not compromised by incorrect fitting.

• Gates

February 2000