When The Heat Is On, Tri-Bushing Flexes

Ameridrives and Bibby Turboflex, two Altra Industrial Motion brands, have developed a new turbomachinery bushing – Tri-Bushing. The flex element/blade triangular bushing is designed to increase the axial capability of high-performance flexible disc couplings used in turbomachinery applications.

Flexible couplings used in these applications must accommodate angular and axial movement due to thermal growth. In steam and gas turbines, the machinery shafts, casings and piping expand as their temperature rises, according to Altra.

When axial deflection is significant, spacers are purposely made short to stretch the coupling statically. Generally, a good practice consists of designing a coupling with enough flexibility to handle thermal growth, especially if that growth exceeds the coupling’s capacity by 25%, the company said. Where possible, couplings are designed to grow into their neutral, relaxed position.

In many cases, however, couplings must operate in a compressed or stretched mode to accommodate this movement. Extensive testing reveals that when approaching the axial limit of a disc coupling – while accommodating for angular misalignment – the disc packs become highly stressed in the area around the traditional round bushings, according to Altra.

The API 671 standard for “Special-Purpose Couplings for Petroleum, Chemical, and Gas Industry Services” requires couplings to accommodate 0.20 degree of angular misalignment. In most cases, turbomachinery drivetrain components are laser-aligned for maximum precision but, at 1/5 of a degree, the impingement of the flex element around the bushings does not typically result in failures due to the radius and low angle, the company said.

When selecting turbomachinery couplings, some operators prioritize axial thermal growth rather than the torque requirement. A larger coupling may be needed to handle the axial growth.

Mark O’Neil, chief principal engineer for Altra’s coupling brands, said the Tri-Bushing design came from his team of engineers from Ameridrives and Bibby Turboflex. Those engineers dealt with a variety of issues during Tri-Bushing’s development.

“The focus of our team’s attention was on the coupling’s blade/flex elements. While developing coupling solutions for the turbomachinery market, we needed to ensure that our legacy disc coupling design could be retrofitted with a new flex element,” O’Neil said. “This meant that the flex element bolt circle was locked in from the start, which limited many of the design options we could explore.”

With this constraint, the design process began, which included finite element analysis and prototype testing. Historically, dynamic testing for new coupling designs is performed for 100 million cycles. Unfortunately, during the early prototype testing, coupling failures were consistently occurring between 40 million and 50 million cycles, the company said.

The engineering team attempted to solve the problem by varying the thickness of the blade/flex elements, but the failures continued. The blade failures were all in the areas where the disc material bent around the traditional round bushings positioned at the bolt holes (see Figure 1).

Figure 1: Blade failures where the disc material bent around the traditional round bushings.

“Based on these findings, we looked at how we could eliminate or reduce the damaging impact on the disc coupling blades as they bent around the apex of the bushings at the tangents,” O’Neil said. “Ultimately, this led to the development of the … Tri-Bushing design.”

The design profile features two straight sides that reduce and redistribute the stresses that typically occur around the disc pack bushings. The FEA results shown in Figure 2 provide a comparison of the stress reductions.

Figure 2: Stress comparison of a typical round bushing and a Tri-Bushing.

“Utilizing the new Tri-Bushing design can increase a coupling’s axial capability by more than 33%,” said Oliver Doidge, Altra’s turbomachinery business development manager. “Given this significant increase in axial growth capability, in many applications, coupling designers no longer need to upsize their coupling to accommodate this growth. In addition … lateral vibration problems may be avoided with a smaller coupling. A higher safety factor results regardless of the misalignment required.”

The new Tri-Bushing design from Altra is available for use in all new Ameridrives, Bibby Turboflex, TB Wood’s and Lamiflex disc couplings. New disc packs with Tri-Bushing stress reduction technology can also be retrofitted into existing couplings as part of repair or rebuild services performed at various Altra coupling facilities around the world.