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To mitigate the potential for disconnection of drive in drivetrains where continuous drive is required, a backup system can be implemented within the coupling of various designs. Both disc and diaphragm couplings can incorporate a dental backup drive, consisting of a set of gear teeth which have enough clearance during normal operation to avoid contact. In the event of a disc or diaphragm failure the tclearance is taken up and the teeth used to provide drive.
This article contains excerpts from the paper, “Drivetrain protection through coupling design”, presented by Oliver Doidge of Altra Industrial Motion at the 2018 Asia Turbomachinery Symposium.
Unlike a traditional gear coupling, the teeth have no form of lubrication to reduce friction and heat, therefore would be expected to wear quickly. Although this design has been utilized in many couplings seen in service, no records of failure using such a device have been published, meaning very little indication of running time can be given. There are several disc pack coupling modifications designed to protect from catastrophic failure. The simplest design incorporates the addition of an overload collar (often made of a “spark resistant” material, such as bronze or a bronze alloy) under the coupling bolt, in the event of a membrane failure the overload collar acts as a bush within the clearance hole to provide drive.
The use of overload collars is widely seen within process pumps where motor characteristics are less well known or controlled, as such many examples of their use can be identified. As with the dental backup design the overload collar has no form of lubrication, therefore the significant heat created has melted the overload collar and associated fasteners. The design of coupling was not controlled under API 671 and did not have an anti-flail device, the containment of the spacer piece being controlled only by the overload washers. With the collapsing of the material around the overload washer it is clear that the spacer could have been ejected if rotation had continued any longer.
It should be noted, that there are no “spark free” materials in the turbomachinery coupling world. Even if the flexible membrane could be designed to eliminate sparking in the event of a failure, the other components of the coupling and the machines they connect to are typically made of steel, therefore it is impossible to be classified as “spark-free”. A more robust design, commonly specified to prevent runaway of steam turbine drives in the event of membrane failure, is commonly referred to as a co-planar design. This design features interlocking driving and driven flanges that engage only in the event of a catastrophic membrane failure. In this failure mode, the driving hub engages with the mating sleeve to allow the connected equipment to be shut down without a catastrophic coupling failure. This back-up device is only intended to allow the user to shut down the train. Clearances are designed into the interlocking flange flanks so they will not come in contact during normal operation, even at maximum rated misalignment.
Another benefit of this design is that even though the coupling mass is greater, the overhung moment of the ½ coupling is often lower because its centre of gravity is further up the rotor shaft and therefore closer to the bearing. For high speed compressors where the 2nd lateral critical is a concern, this design may provide an acceptable failure mode for a disc coupling whilst offering a solution to lateral rotor dynamic issues.
A less popular, but sound design for low torque applications, adds castellated shrouds to the flanges either side of the flexible elements. The castellated shrouds are orientated so that they interlock within each other. In the event of flexible element failure, the castellation’s provide temporary drive.
With all of these methods it should be reiterated that these mechanisms have no form of lubrication to prevent wear, and as such could fail very quickly. Careful consideration to their attributes should be given to ensure suitability within the drivetrain design.