bakerocb
Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA
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 Posted: Mon Dec 15, 2014 8:45 pm Post subject: Bolt Preload |
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12/15/2014
Hello Scott, Thank you for your very pertinent posted comments on bolt
preloads copied below. Please allow me to pick some nits / raise some
questions. You wrote:
1) “This requires a continuous joint and not leaving any part of the bolt in
the open air as in the factory landing gear installation."
I either don't understand or I disagree with the above statement. In a
normal installation the pre load tension force on the bolt is applied at two
locations, the bolt head and the nut, normally by rotating the nut while
holding the bolt head from rotating. The entire length of the bolt between
those two points is then loaded to a certain stress level created by the
torque that has been applied to the nut. Obviously there must be some
material (the items being held together by the bolt) that prevents the nut
from being rotated until it contacts the bolt head. But that stacked up
material (usually a minimum of two pieces with maybe some additional pieces
being squeezed between them) does not need to extend along and cover the
entire length of the bolt, from nut to bolt head, when proper torqueing of
the nut is completed. An air gap in the stack up will not, by itself,
prevent a proper bolt preload from being created.
Such an air gap exists even in your stack up and can be seen in the photo in
your original email where the bolts will pass through open air along side of
the aluminum gear cross piece. See the picture in the link here (need to
click on search by image):
http://www.kisbuild.r-a-reed-assoc.com/KIS-TR1-Web/ScottSterns/landinggear1.JPG
2) "The bolts clamp the steel plate onto the flox pads and allow the bolts
to be under a constant preload."
Not quite correct. Here is the probable entire stack up sequence as I see it
starting from the interior of the fuselage and working our way down.
(Realize that not everything in this stack up is placed immediately on top
of each other, but all parts or pieces are being ultimately clamped between
the bolt head and nut):
a) Bolt head.
b) Probably a large area steel washer.
c) Fuselage bottom shell (either fiberglass or carbon fiber). May be
reinforced in the vicinity of the landing gear cross slot.
d) Flox pad.
e) Rubber pad.
f) Aluminum landing gear cross piece.
g) Steel plate (with four bolt holes).
h) Probable steel washer.
i) Nut (either self locking metal or nylon, or castellated with a cotter
key).
How close did I come?
3) "The rubber pad goes around the landing gear and not under the bolts."
The rubber pad may not be directly under the bolts, but the pad definitely
is in the stack up and is being ultimately clamped between the bolt head and
nut.
4) "The bolts clamp the steel plate onto the flox pads and allow the bolts
to be under a constant preload."
It is interesting to speculate just how much relevant preload (stretch -- in
the conventional sense of preloading a steel bolt) can be created on a steel
bolt that has a rubber pad as part of the stack up between the bolt head and
nut.
5) "The grove parts are a better option today though since they have a built
in radius that allows the gear to flex so you don't need the rubber pad."
I would take the position that any time you are clamping a hard metal
object, steel or aluminum, against an epoxied fiberglass structure or flox
pad that a flexible interface (rubber or elastomeric) pad is desirable in
order to avoid crumbling any high spots or uneven areas.
OC
=================================================================
From: Scott Stearns
Sent: Monday, December 15, 2014 4:04 PM
To: kis-list(at)matronics.com
Subject: Re: TR-1 Incident down under
Hi Mike,
The proper use of a bolt is to clamp parts directly together. Preloading a
bolt is what we really do when we tighten a nut down to a certain torque
value. The bolt stretches as you tighten the nut and the bolt provides a
constant clamping force (preload) as long the load on the joint never
exceeds the clamping force provided by preloaded bolt.
A great example is the bolts that clamp the main rod bearings around the
crankshaft of an engine. Lycoming is very specific on the procedure for
torquing these bolts right down to specifying a specific thread lubrication
fluid. You can imagine if half the nut torque is going into moving a burr
through the threads and not into stretching the bolt then you will not get
the correct bolt preload. In this case the joint will not be clamped
together with a load greater than the load from moving the piston and the
joint will open slightly and stretch the bolt with each load application.
Now the load on the bolt is cycling instead of the bolt being under a
constant preload keeping the joint clamped together. Eventually the bolt
will fatigue and fail. As I recall this is what happened to the 400 hour
since factory new engine in the original factory demo Cruiser leading to an
off field landing.
The landing gear bolts of course do not go through anywhere near the load
cycles that the main bearing bolts do, but it is still good engineering to
have bolted joints under a constant preload rather than cycling with every
load cycle. This requires a continuous joint and not leaving any part of
the bolt in the open air as in the factory landing gear installation.
Here is a picture of my landing gear installation on my TR-1. The bolts
clamp the steel plate onto the flox pads and allow the bolts to be under a
constant preload. The grove parts are a better option today though since
they have a built in radius that allows the gear to flex so you don't need
the rubber pad. The rubber pad goes around the landing gear and not under
the bolts.
http://www.kisbuild.r-a-reed-assoc.com/KIS-TR1-Web/ScottSterns/landinggear1.JPG
View on www.kisbuild.r-a-reed...
Anyhow, this was probably more than you were asking for, but I hope it
helps.
Cheers,
Scott
From: Mike Pienaar <mjpienaar(at)shaw.ca>
To: kis-list(at)matronics.com
Sent: Sunday, December 14, 2014 5:54 PM
Subject: Re: TR-1 Incident down under
Thanks Scott,
What do you mean by "they allow the bolts to be properly preloaded"
Thanks for your help
Mike
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