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Within the coupling, it is possible to incorporate a number of devices to overcome the potential damage caused by a torque overload. The most common is to incorporate a mechanical shear device, either as a single spool piece, commonly referred to as a shear spacer or shear pins which have a reduced section groove on a common pitch circle. In the event of a torque overload, the reduced section groove(s) will shear, disconnecting the turbine from the driven equipment.
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.
The design of such devices needs to be tightly controlled to achieve an accurate disconnection. A device which shears below the specified torque would lead to premature disconnection causing downtime of the system, whilst high torque disconnection could cause damage to the prime mover at huge expense. To avoid failure through fatigue it is paramount to have a separation factor of 2.5 to 3 between normal running conditions and shear point of the overload device. Looking at both designs in more detail.
A shear spacer design consists of a single spool piece with reduced section groove of a calculated diameter determined to shear at a given torque. In order to maintain alignment, stop flailing of the two sheared sections and allow run-down in a controlled manner, a bearing is included. The design of this run-down bearing can differ in design with either a plain bearing manufactured from a brass material or dynamic ball or roller bearings being used.
A shear pin arrangement consists of a series of pins located in 2 flanges, the pins having a calculated groove diameter, positioned on a common pitch circle. A bearing is included to control rotation after disconnection.
To ensure disconnection accuracy, the precise shear of multiple pins concurrently is required. This relies upon the true position of the pin holes in both flanges to ensure all pins have equal torsional loading. A design incorporating 4 pins with 0.1mm (0.004 inch) true position, results in contact loading of 85%. Increasing the tolerance on hole position to 0.25mm (0.010 inch), sees this reduce to 75%, thus reducing the accuracy of disconnection. The rigidity of the shear pin arrangement is resultant upon the fit of the shear pins within the holes and the robustness of the bearing arrangement. A flexible joint can cause bending fatigue upon the shear pins and an increased face run out; imposing axial thrust loads upon attached machinery.
Whilst the shear pins are replaced after any disconnection without the need for the coupling removal the bearing arrangement is generally of a “fit and forget” design: meaning that the design has to be capable of multiple running periods, whilst also enduring the effects of long periods of non-use. For any dynamic bearing, extended periods of non-use can do significantly more damage than constant running, as rollers or balls can cause indentations into the surface of cages and races often referred to as Brinelling. This damage can cause increased wear and even bearing failure. Shear pin retention is done in 2 ways; either a radial “grub” screw acting upon a groove within the shank of the pin, or an axial retention plate is used.