There isn't much information available regarding vault emergency
doors so I thought I'd try to kick up some discussion and maybe
more images. These seem to have gone out of style by the
1950s/1960s.

Rationale: If the main vault door malfunctions then there is
significant hardship on the bank and customers. A second way
to enter was desired but not at the cost of a second full-sized
door, so emergency doors were usually much smaller in diameter.
But to maintain security they were just as thick and controlled
with combination locks and time locks. Today main doors are trusted
enough and breakdowns are rare enough to be an acceptable risk.

Pros: Business will not be seriously disrupted if the main door
fails.

Cons: They are small, requiring somebody to crawl through. This
would likely be a technician to repair the main door but customers
would not be expected to make the trip. They are costly, require
attention similar to the main door (open/close daily, set the time
locks; periodic maintenance), and they occupy space inside the
vault which prevents use such as safe deposit boxes.

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Above you can see that a small-diameter door that is just as thick
as the main door gives it an odd perspective. It also means the
crane hinge must allow a larger swing angle to allow the door to
clear its opening.

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Above is an image that clearly shows a crane hinge design which
allows more swing ("hingeward") than most main doors.

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A frame grab from the Travel Channel's "Off Limits" program showing
the door at the First National Bank Building in Saint Paul. Note
the boltwork is rectangular; hence my question, are there any patents
just for emergency doors?

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Finally, a postcard of a Remington & Sherman vault, you can just make
out the emergency door in the background. It looks like it even has a
day gate! Also note there was no penetration of the vault for
electrical power (lights), a cord is strung through the door when open.

The only patent I know of is US 901,047 which shows a main door with
an emergency door built into it which solves the problem of wasted
space inside the vault. I don't know if one was ever constructed.

Does anybody have imagery, patents, stories, etc.?