Well, Phil,
then don't use Helicoils, use Keenserts/tineserts/timeserts, etc-
they uprate your stud threads to at least the next fastener size up!
And yes, loctite is your friend...
And one of the cool things about head studs-
if you REALLY care about stud preload and stretch,
you can set up a dial indicator and tension the fastener to an exact preload.
With rod bolts it can really matter in a race engine.
On head fasteners, it's overkill. Until you boost it to 50 psi, then
everything starts
to really apply!
heh heh
Toby
-------------- Original message ----------------------
From: "Hall, Phillip" <Phillip.B.Hall@nasa.gov>
> Hey List, and Roadster nuts.....
>
> Interesting discussion on torque values. We (the NESC/NASA) have been
> working these issues for a few months now. I am the lead on a Team of
> experts to find a resolution to the application of dry film lubricants
> on the bolts that hold the cap on motors on the Power Drive Unit (PDU)
> that drives the body flaps and rudder/speed brake actuators on the Space
> Shuttle. The problem is with the DFL applied to the bolts, the K factor
> decreases which (by analysis) shows we are under negative margins at the
> helicoil insert (the insert can be pulled out). By the relationship of
> T=KFD (T=torque, K=K factor, F=force or preload, and D=diameter of the
> bolt) one can see by using torque values for a non-DFL bolt on a DFL
> bolt (lower K factor), the preload can really increase. Through test we
> demonstrated we have enough margins (system is robust) to handle the
> increase in preload without pulling the insert which would result in
> lose of hydraulic pressure and vehicle. The important thing to
> remember, as you all have been discussing, the K factor is a key
> component in the system. It can vary dramatically depending on if you
> use a lubricant and which lubricant you use - as Fred found out. Oh
> yea, the K factor is the amount of friction produced in the thread and
> bolt head interfaces.
>
> The short of it. Like you guys are talking about - use the basic
> principles of a better lubricant will increase your preload so you must
> decrease your torque values. To really know what is going on in a
> system you have to use load washers to measure preload.
>
> Happy Roadstering!
>
> Phil
> SEROC
>
> PS: Next subject - the application of Loctite....
|