FEATURE MACHINE BUILDING & FRAMEWORKS
TORQUE LIMITATION for heavy plant protection
The UEP range of modular torque
Andrew Bargh of Bibby Turboflex, part of Altra’s Couplings Division, looks at the technologies available to protect machinery against overloads
P
rotecting machinery and systems against expensive failures and costly
down-time is critical in today’s competitive market, but what are the relative benefits of electrical solutions versus mechanical in high torque applications. Large machinery and rotating systems can generate enough rotating energy (inertia) to cause significant machine damage during a jam, crash stop or during uncontrolled braking due to, for example, a power failure. The inertia generated within such machinery is dependent upon the speed and the mass of the rotating system. A large mass at slow speed could do more damage than a smaller mass at high speed during a jam or crash stop for example. At very low speeds systems can develop a huge amount of unnecessary torque, which can seriously damage drive system components such as shafts, gearboxes, chains and couplings.
ALTERNATIVES So what alternatives might we use to protect machinery from the mechanical overload generated by a jam or a crash? These include shear pins, friction clutches, sensing devices and electronic control devices. Electrical torque limiting solutions, such as the final two examples above, are becoming increasingly popular in many industries as they are seen to provide higher degrees of accuracy and reduce down-times in the event of overload. When a sensor or control detects a problem, corrective action can include stopping and reversing the drive, engaging a brake or simply shutting down the machine. The protective functions integrated into today’s sensing and control units are an expedient supplement to the mechanical torque limiting clutch, but not a replacement. Thus, mechanical torque limiters still make sense, but not necessarily for every application. As with the design of the drive itself, engineers should consider numerous factors before determining the best means of protection.
26 MARCH 2014 | AUTOMATION
However, relying only on electrical/electronic devices will most certainly result in equipment and machinery that is inadequately protected and prone to overload induced damage and down-time. When torque increases
relatively slowly, there is enough time for
electrical/electronic devices to detect problems and initiate corrective actions prior to reaching the clutch’s set torque. In these instances the solution is ideal as it prevents damage to the drive system and requires little or no down-time to reset the equipment. However, if the collision torque rises quickly (a ‘hard’ collision) due to the speed or mass of the rotating components, the electronic control may not be able to react before significant damage occurs. At these higher rates of increase there is virtually no time for an
electrical/electronic device to recognise the problem and begin corrective action before the torque reaches the set-point of the clutch. In this situation, only a mechanical overload device can react quickly enough to prevent damage. A mechanical safety coupling, for example, completely disconnects the drive from the load within 3-5ms – one-third of the time needed by an electronic cut-off.
REMOTE AREAS The complexity of each system should also be considered, especially in applications which are remote or have limited access. Electronic monitoring systems need multiple sensors for data - between the monitoring system and all of its sensors and other components, you have a system with multiple possible points of failure. Once installed, a mechanical limiter can be left in-situ with minimal maintenance, and yet offer complete reliability in the case of overload.
Above and below: Bibby Transmissions is part of Altra Industrial Motion, a designer, producer and marketer of a wide range of mechanical power transmission products
limiters from Bibby Turboflex, has been developed to provide optimum protection against overloads and deliver maximum safeguarding against expensive failures and prolonged down- time on machinery. It offers users the benefits of simplified installation; accurate release torque repeatability, achieved with minimal variation between static and dynamic release; simple manual re-engagement; and low cost maintenance. The UEP range of modular torque limiters was designed specifically to offer an alternative to shearpin protection for high torque drives where it’s accurate release torque repeatability, coupled with a fast re-set feature, offers a significant reduction in down-time. Also, being relatively tamper-proof, they prevent unauthorised personnel from changing the release torque setting. The design of the UEP torque limiters
also means that installation is flexible, enabling the centre section of the torque limiter to be removed without the need to move the motor or gearbox. Similar design versatility ensures that any maintenance of the modules is a straightforward operation, and that recalibration is a simple removal operation achieved without having to replace the complete torque limiter unit. When considering how best to protect machinery from overload it is important to consider the different scenarios where this might occur. Electrical monitoring or sensing equipment can initiate corrective actions prior to overload during a slow torque increase. However, there are times when the
increase is too fast and the
electrical system may be too slow to prevent damage to other components. By speaking to a specialist supplier of
complete power transmission solutions it is possible to learn about the many different torque limiting solutions available on the market and specify the solution that is most suitable to your application.
Bibby Transmissions
www.bibbytransmissions.co.uk T: 01924 460 801
Enter 211 /AUTOMATION
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