Looking at some pricing the precision ground “shear” bolts don’t cost all that much more, so why aren’t they more widely used? A smooth bolt can take more stress because there are fewer places for fatigue to get going. OTOH the shear bolts are not plated on the shank, allowing corrosion.
Both above are 1/4" 28tpi standard aircraft bolts – in the region of $1 each.
A properly tightened bolt shouldn’t really exhibit fatigue, as the load variations in operation would be negligible compared to bolt preload. Furthermore, the surface of the plain bolt is probably work-hardened (at least to some extent) in the course of production, whereas in the machined bolt that layer is removed.
On my aircraft in common with the Vans aircraft, the main landing gear is of the Wittman type, basically a strong steel bar fitting into a socket in the fuselage. It is held in place by a perpendicular bolt through socket and gear leg. Under some unusual landing circumstances a strong twisting movement may be applied, and in this case sheared the holding bolt on both sides. They recommended using a shear bolt in this location, but it might just shift the damage to a worse place.
Simon
I wonder if “shear bolt” means multiple things? In some contexts it refers to a bolt which is precision-ground in diameter and fits into a similarly close tolerance hole, and is intended to take specific shear loads. In other contexts it refers to a bolt which is designed to shear off, in anti vandal appplications.
Shear bolts hold the blade on my mowing machine and are designed to break first.
The NAS bolts are stronger for side loads.
Simon
