Question and Answer:
What is the bend a cable
can withstand when wrapped around a pulley?
Submitted
by Bob Lake,
EAA number, rglake@nexicom.net
I have looked in the standard methods book and
the EAA website, home builders section, but
can't find information on how much radius is
permitted with a 1/8 inch cable for various
diameters of pulleys. Do you know where I can
look for info. I want to have a 1/8 inch cable
change direction by 90 degrees. How large should
the diameter of the pulley be?
ANSWER:
There seems to be very little specific info
on your question, so it took a bit of
digging to find a published reference. But
none the less a reference has been found!
As you mentioned, there was nothing specific in AC 43.13, nor in any
other readily available “handbook”. Further, there was no specific
mention in CAR 3, CAR 4, or 14 CFR Part 23 for standard category
aircraft. The only ready reference I found was in Tony Bingelis’
book The Sportplane Builder. However, I knew Tony had to
have gotten his guidance somewhere, so the search continued.
I finally found a document published in 1974 by the Defense
Technical Information Center, a unit of the Department of Defense at
Ft. Belvoir, VA (http://www.dtic.mil/dtic/).
Their studies indicate that a pulley diameter ration of at least 20
to 1 (that is, the pulley diameter is at least 20 times the diameter
of the cable) is desirable in order to provide maximum cable
endurance. This would lead us to determine that the minimum pulley
diameter for a 1/8 inch cable would be 2.5 inches.
That being said, I have seen instances on many aircraft, including
standard category types, where a slight deflection (45° or less) are
handled by smaller pulleys. However, for direction changes
approaching 90° I would stick with the 20 to 1 formula. For
direction changes approaching 180° you might consider going to an
even larger pulley.
Hope this helps! Let us know if you have further questions.
