Purpose of washer crown on square nuts?

[pip123]

Subject item: square nut, flat on the bottom and washer crowned (either a chamfer or radius) on the top.

Question: what is the purpose of the washer crown? Shouldn’t the flat bottom of the nut be in contact with the washer for maximum surface area contact?
Using the washer crowned side towards the washer would reduce area contact.

Origination of question: The company I work for recently purchased a company that makes fastener hardware; in particular square head machine bolts and nuts. The engineering drawings of these products show a majority of the radiused side of the nut towards either a flat washer or flat mating part.

Thanks in advance,
Ron

 

[evelrod]

Way back in the day, 'square' headed fasteners were used as 'erection' bolts on structural steel bldgs, bridges, etc. These were subsequently changed to rivets (or specialized bolts and washers). The problem with erection bolts is that they are used sans washers. Without the raised 'washer' section on the nut it makes it extremely difficult to tighten the fastener sufficiently with a 'spud wrench' and can also damage the connection. These erection bolts were normally re used several times.
Bethelem Steel was the last of the big erection companys (as far as I recall) to convert to modern bolts, something like the early 70's, just before they went out of the steel erection business.

Rod