From: RWolf99@aol.com
Message-ID: <24.c1df4b5.27263e45@aol.com>
Date: Mon, 23 Oct 2000 21:22:13 EDT
Subject: Ball Lock Shackle Pins -- The Whole Story
To: lancair.list@olsusa.com
Reply-To: lancair.list@olsusa.com
Mime-Version: 1.0

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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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