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Since "Blitz" Fox mentioned that I was an authority on Avibank Ball Lock
pins, I thought I'd better become one fast! My friends at Scaled Composites
were nice enough to let me xerox the appropriate 3 pages from the Avibank
Catalog, which I will scan and e-mail to Marv when I get back to the office.
As Blitz says, Avibank Manufacturing on Victory Blvd in Burbank CA (area code
818) is the place to get these. I don't know the phone number or the price.
I have used many Avibank pins throughout my career, including the ball lock
shackle pins. They're pretty nice.
The part number you need is BL-5-SP-20, which means "ball lock shackle pin,
5/16 diameter, 2.0 inch grip"
SIZING THE PIN
The TD-1 tie down rings I bought from Vern this year (three of them) have a
steel tube welded to a metal plate. It is chrome plated and looks pretty
spiffy, which is weird because you'll never see it again once you install it.
The tube has a 5/16 diameter hole and the tube is about 2 inches long. The
internal end of the tube (not the end that's flush with the skin but the
other end) has a deep chamfer to allow a good fit to the shackle pin locking
balls. The tubes on the two I have not installed are 1.996 inches long and I
suspect that the one I have installed is the same length. This is how the
grip length is determined. So, 5/16 diameter, 2 inch grip.
FIXING THE BURR
The TD-1 is threaded partway up the inside of the tube -- about halfway. In
my opinion, this allows the option of using either the threaded eyebolt that
it ships with, or a ball lock shackle pin. The first one I got would not
allow me to insert a 5/16 inch transfer punch all the way through due to a
burr on the inside. I cleaned this up by running a 5/16 inch drill bit
through the hole (probably by hand but I don't recall). The burr was not
large but precluded the transfer punch from going through. The other two
would not easily allow the transfer punch through but I was able to force it
through with gentle-to-moderate hand pressure and it slides easier now, so
this means it was a smaller burr. I'll still spin the drill bit through it,
by hand, to completely clean it up. What this suggests is that the inability
to insert a ball lock pin all the way might be due to the burr, which on two
specimens was at the threaded-unthreaded junction halfway down the tube. (Of
course, if the end of the tube is covered with micro and paint, your pin is
probably too short. No need to buy expensive pins to check -- get cheap
nonlocking ones from the hardware store, verify the required length, then buy
the expensive locking shackle pin)
LOAD CAPABILITIES
There is test data on the load capabilities of the shackle pin in three load
directions. 180 degrees (pulling it directly out of the hole), 90 degrees
(pulling sideways), and 135 degrees (like a tiedown rope angling 45 degrees
towards the ground). The specified ultimate load is the average failure load
of test specimens divided by 1.15, and it should be stressed that the 180
degree case (pulling it straight out) assumes a hardened tool steel shoulder
bushing, which is essentially what you have with a steel TD-1. If you use an
aluminum tube (i.e., if you made up your own TD-1 out of aluminum) the
failure mode will be local deformation of the aluminum where the balls dig
in. (It might even jam up enough that you'll never get it out, so use the
steel TD-1.)
For the 5/16 diameter pin, the ultimate loads are 2390 pounds at 180 degrees,
1570 pounds at 135 degrees, or 1040 pounds at 90 degrees (which is not a
realistic angle in this application). These numbers are larger than a
standard 2-ball quick release pin, since the shackle pin has 4 balls.
Jim Frantz is correct that these are weaker than a pin loaded in double
shear, where a 5/16 pin can withstand12800 pounds (calculated, not test
data). However, two wing tie downs angled at 45 degrees gives a combined
capability of 2*1570 = 3140 pounds, which should be more than enough. How
strong a rope are you using, anyway?
USE THE RIGHT GRIP
Always good advice for any manly endeavor! If the pin is too long, the pin
will be loaded in bending rather than single shear. Avibank specifies a
maximum allowable gap of 0.031 inches between the shackle pin shoulder and
the face of the fixture. In theory, you could have a tube-plus-micro that's
2.010 inches long and the smallest pin that fits is 2.100 long, leaving a gap
of 0.090 inches. In practice, I wouldn't worry over that -- just use the
smallest 1/10 inch increment that will fit. (No, a washer doesn't help, I
don't think.)
MY PERSONAL GRUDGE
Everyone calls these quick release ball lock pins "pit pins". I think
they're called "pip pins".
- Rob Wolf
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Please send your photos and drawings to marvkaye@olsusa.com.
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