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....
-----Original Message-----
From: owner-datsun-roadsters@autox.team.net
[mailto:owner-datsun-roadsters@autox.team.net] On Behalf Of
gregs672liter@netzero.net
Sent: Monday, August 14, 2006 11:07 PM
To: sandhoff@csus.edu
Cc: datsun-roadsters@autox.team.net
Subject: Re: ARP, the result
As I understand it, "torque" really relates to a measure of the degree
of resistance or friction. The goal is to stretch the bolt to about 75%
of its capacity. But since we cannot actually measure the stretch
itself, it is related to the amount of torque necessary to acheive this
degree of stretch.
Since different lubes effect the degree of resistance, that subsequently
effects that actual torque number used. For moly lube, the torque
should be 60-65ft lbs. ARP was not sure what a synthetic oil torque
would be, but it would be less than regular oil. Using regular 30
weight oil they suggest 80ft lbs. Based on my experience (now),
synthetic oil should be less... probably 70lbs. I did a torque to 70
before going to 80 without problem. I broke the 4th bolt of my final
80lb torque sequence. However, the first 3 bolts, though they took the
80lbs, may very well be over torqued (or stretched) and weak now, so I
replaced them all to be sure. Now the my brain is working again, I
clearly remember using moly lube and 60lbs the other 3 times I torqued
these bolts, always without problem. That will teach me to make sure I
write this stuff down!
Greg
-- "John F Sandhoff" <sandhoff@csus.edu> wrote:
[off-list]
> ...seems that I did over torque the bolt due to the synthetic oil...
So, I'm curious... the type of oil (dino vs synthetic) affects the
desired torque? I can see oil vs no oil changing things, but I hadn't
realized synthetic was that much slipperier - interesting...
-- John
John F Sandhoff sandhoff@csus.edu Sacramento, CA
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