Fans--specifically, EMD road deisels--and more exactly, fan shrouds

[M636C]

I'd agree with dacort, but I think that the smaller 36" fans only came in the unshrouded type - look at the GP/SD 35 which had two flat top 48" fans and an unshrouded 36" fan. There were also low profile versions of the flat top fan used on some Eastern road locomotives and on some demonstrators, like at least one SD45X, to allow running on Eastern lines with tighter clearances.

M636C

[dacort]

Making an educated guess here but I think some of those terms may be describing the same thing. I would think that button-top, cap-top, disk-top, and pan-top all refer to the fans with the disk/hub insert in the center (I'm assuming EMD here). I'm not totally sure about when the flat top fans were introduced, but I think it may have been on the GP30 and later models.

[Anonymous]

I used to think fans (fan shrouds) come in two types: round-top and flat-top. Flat-top fans had a higher rim or collar and may or may not have had a flat central "hub" which was about half the diameter of the fan and may or may not have extended slightly above the rim. Round-top fans had little or no rim or collar, a slightly concave slope, and a smaller central rounded (convex) nub.

More recently I have come across the following terms to describe fan tops:
button-top
cap-top
disk-top
open-top
pan-top
in addition to the generic:
flat-top
round-top

I am completely confused. I know that the original manufacturing was not completely consistant, that fans types were frequently swapped during routine maintenance, and that wholesale replacement was common during rebuilds. Can anyone give me guidance about which is which. What dates they were used or on which models. Or where I can get more info. Searches of Trains.com forums has yielded zip. Thanks.