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I'm trying to analyze a simple shaft supported by two bearings and subject to a single point load. But I keep coming across assumptions that only consider the vertical reactions and neglect the moment reactions of the bearings.

This article states this assumption in examples: 5-73 and 5-86. I've tried to find an explanation for this but keep finding examples that just state the assumption.

I'm at a loss not having my textbooks with me at the moment but from looking at examples I've noticed in some cases that the bearings take the form of a simple pin support hence the bending moment is zero at those points. See here for an example (although no assumption stated).

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    $\begingroup$ Is this for an exercise or are you analyzing a real case? Assuming no moment reactions from bearings is common in exercises, otherwise you need information regarding the friction between the shaft and the bearings. $\endgroup$
    – Wasabi ♦
    Mar 3, 2016 at 10:58
  • $\begingroup$ It represents a portion of a steering system. From my understanding, including the two moment reactions would make it a statically indeterminate system which could be solved using superposition. But it seems like neglecting it is fine to do so in some cases. $\endgroup$
    – Dom W
    Mar 3, 2016 at 11:18

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Many bearings (single row ball bearings for example) are meant to allow slight misalignment of angle without significant counteracting torque. This allows them to be used in situations where there are two bearings supporting a shaft without having to have the alignment between the two bearing supports be extremely critical.

This means that unless there is significant bending in the shaft, the torque applied by the bearing would be negligible compared to the side or axial loading.

Many bearings that are meant to handle torque (double row angled ball bearings for example) are in reality composed of two (or more) bearings that are really using radial loads to apply the torque, so even these bearings could be modeled without using bending moments.

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