By Zak Khan
Couplings and bearings must mount to components in a system. Because of this, they come with tolerances and assembly concerns. Tolerances refer to manufacturing tolerances and tolerances in installation and mounting. Tightening torque refers to couplings that screw onto shafts or other components in a system.
Tightening torque is the torque required to correctly mount a coupling. When couplings screw onto a shaft, they must be appropriately tightened. If this is not done correctly, the bearing or coupling will not function properly. An overly-torqued, over-tightened bearing can cause over-stress in the bearing and cause it to fail prematurely. If it is under-torqued or under-tightened, the bearing may slip or there might be too much “play.” This can cause inefficient operation or vibration. At its worst, under-tightening can cause catastrophic system failure.
When mounting bearings and couplings, tightening torque should always follow the manufacturer’s specifications. Most manufacturers provide the amount of torque that should be used to fasten a bearing or coupling to a component. Trying to gauge if the assembly is tight enough by “feel” or “intuition” is not a good practice and engineers should avoid such assembly techniques. If the information regarding tightening torques cannot be found in documentation provided with a coupling or bearing, ask the manufacturer.
Fitting tolerances refer to the variation between the actual size of the shaft and the rated values a coupling can accommodate. For example, if a coupling mounts to a 5 ± 0.1 mm shaft, then the tolerance is 0.1 mm. Usually couplings that flex and bend have larger tolerances, as they can accommodate greater distortions. Make sure to check if the range in which movement occurs falls into these tolerances. Also make sure to measure the shaft with a degree of precision comparable to the tolerance. Attempting to use a coupling beyond its tolerances can cause it to fail.
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