"Stretch" & Wayne,
You're both right !! It is <almost> impossible to pull the
single bungee cord supplied by the factory tight enough to get a good
preload. There is also the problem of different tension on each wrap,
depending on the distance to the shop wall (:-) ) - this almost always
causes bungee "sag" after a few flights, even if the original appears
satisfactory. That is why we decided to go with the rings - PLUS, they
are MUCH easier to install ! Even if you let go of one, you don't lose
all of your work to that point.
'Way back there, I spoke to the company that makes the rings,
looking for the 'right' size for the Rebel - seems the #9044 is the
closest standard ring. They recommended approx. a 40 to 50 % pre-load
for best operation. Something you CAN do that will help the operation
of the bungees AND the durability of the bungee bolts, is to add a
sleeve to the bolts, between the gear tube and the penny washer. This
can be as simple as a piece of clear fuel line, or, better still, a
piece of Delrin machined to a 'spool' shape. The bolt would tighten
against this spindle, loading it in tension, which would help it
resist the side loads from the bungees. Even a small tube will let
the bungee slide over it easier than the bolt - they tend to get flat
spots at the bolt, which means each side of the loop is working alone,
rather than spreading the load over both... and the bungee is weakened.
| |
|\ ( )( ) /|| |
=| \(__)(__)/_||______|__
bolt head----> = __________|______|__ <--- bungee bolt
=| /( )( )\ || |
|/ ( )( ) \|| |
| | | | |
| | | | |
V V V |
spool bungee V
(spindle) rings gear tube
Part of the function of the bungees is to absorb energy, not store it
like a spring. The internal friction of all those little strands of
rubber sliding over each other dissipates some of the energy. The spindle
allows the strands more freedom to slide around the whole ring rather
than just stretching on one side.
There might be some advantage to making your own rings using the
bungee supplied, as it is thicker than the #9044, which is 9/16". Less
preload would still give a stronger bungee - but perhaps with less
'give'......
In any case, you don't want the bolts to be hitting the cable
EVER, in normal use. It's there in case the bungees fail.
......bobp
----------------------------orig.---------------------------------------
At 09:11 AM 4/21/00 -0400, you wrote:
Yeah it's been over 20 years since I studied this stuff too but my number was
about 1050# tension per strut static in the 3 point attitude at 1650#
gross. The
geometry gets worse as the gear flexes so 2g (on the gear) more than doubles
that force even with no crab or sideslip.
"Wayne G. O'Shea" wrote:
snip<
Good luck getting the bungees tight enough so the strut doesn't move while
taxiing. Haven't seen a set yet, that were this tight. Remember you would
need over 1200lbs of preload at gross weight to keep them tight and
non-moving!
Wayne
OK, I did a quick and dirty little experiment to see how difficult this is
going
to be.
After the first little bit of movement, the force exerted by my unused 1992
vintage MAM bungee cord is approximately linear with elongation (F=15x+20
approximately) up to 60% elongation. After that the force increases rapidly to
about 150# at 70%. I have found a reference to a test strength rating of 200#
and another for 400#. Don't know which is correct but the ramping up of the
force at 70% makes me wonder if this stuff should be stretched much more than
that.
MAM's method of using 5 wraps or loops per side of each bolt would require
about
53 to 60# of tension on the bungee cord as it is installed (about 20 or 25%
elongation) and I can see how that would be difficult.
Ron's directions for making Bungee Rings recommends about 30% elongation which
would give about 70# of preload per strand. So one ring is 2 x 70 = 140#.
Eight
rings per strut =1120#. (Six rings might be a bit low) This is in the ballpark
of our required static strut tension. I presume these things loose some tension
as they age.
My bungee attachment bolts are 7.7" apart. Using 30% pre-elongation, an
additional 3" of strut extension before the safety cable tightens, would give
about 85% elongation (The bungee cord would be 1.85 times longer than it was
before it was installed). Only fools extrapolate but that should handle more
than 2g on the gear before loading the safety cable.
Does this sound reasonable? Has anyone been unable to get sufficient preload
using bungee rings?
Bob, you seem to favour 6 rings per strut but with more preload. I couldn't
pull
hard enough in my little test to see what the result of that would be at the 3"
max strut deflection that MAM now calls for. The force seems to be ramping up
fast though in that range which would be a good thing as long as rubber fibres
don't start breaking.
There is some room for confusion here about elongation. It appears that the
cord
is manufactured with 20% or more elongation before we even purchase it. The
above comments ignore that however.
Ken
just call me "Stretch"
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