Coupling selection is crucial to reliability
A
Huco Dynatork product story
Edited by the Engineeringtalk editorial team
Dec 23, 2004
For many power transmission systems, the selection of the most appropriate coupling can indeed be made on the basis of torque, speed and duty cycle requirements.
For many power transmission systems, the selection of the most appropriate coupling can indeed be made on the basis of torque, speed and duty cycle requirements.
However, the experience of one company serves as a salutary lesson that these parameters alone are not necessarily sufficient in every case.
The Huco coupling it chose certainly met the specifications for the job but it quickly failed.
So too did a larger size version of the same coupling that replaced it.
So what went wrong? The initial, Huco size 34, Unilat coupling was specified for a US manufacturer of rail track maintenance machines.
The machines are widely used on the UK's railways with their management and maintenance the responsibility of a large contractor.
Their role is one of preventive maintenance.
They are designed to grind sections of track to remove surface cracks which, if left, can propagate through the rail and cause catastrophic failure.
By routinely grinding the rail, its life is extended and costly downtime and replacement greatly reduced.
The machine is equipped with a number of grinding units, each with 16 grinding wheels mounted to match the profile of the track.
It can travel at speeds up to 13km/h regrinding several kilometres of track per night.
The Uni-Lat coupling chosen completed the connection between the vehicle axle and an encoder, mounted outside the wheel.
The encoder signal tells the system the precise position/number of rotations of the wheel.
Described as a universal/lateral offset coupler, the Uni-Lat has a sound reputation for reliability, particularly in applications such as pulse generator drives.
A unique property is its resistance to axial motion.
This makes this model ideal for light push/pull duties and anchoring axially restricted shafts.
Huco first became aware of a problem with the rail grinding machine when the contractor requested the size 41 version of the Uni-Lat to replace the failed size 34.
However, this coupling also began to show signs of premature wear and as a precaution, a service engineer replaced it with a section of rubber hose and two jubilee clips to keep the train running while the coupling problem was resolved.
A visit to the contractor's site in the Midlands by a Huco sales engineer soon identified incorrect coupling selection as the cause.
Although the torque and speed were well within the capability of the Uni-Lat, the coupler was being subjected to not only axial but also lateral movement, a combination it is not designed to accommodate.
The cause of this two-way motion was the result of the encoder being fitted to the wheel on three flexible rubber mounts.
The inertial mass of the encoder bouncing on its mountings created an excessive loading that the Uni-Lat simply could not tolerate.
The solution was simple and has been wholly successful.
Huco recommended the switch to a size 20 Huco Flex-P coupling.
Exceptional flexibility and toughness provided by the moulded elastomeric element are the principle characteristics of this general-purpose coupling.
Capable of handling torque up to 18Nm and up to 15 degrees of angular misalignment the Huco Flex-P can easily accommodate the fluctuating axial loads that are inherent in this application.
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