Mayf, Listers,
Continuing in the same spirit, I had asked myself the question, whether
torquing the rod bolts tighter really helped, or was it just the ultimate
tensile strength of the rod bolts that counted. First, it turns out that the
bolts break at the same point, whether you torque them tight or loose. But,
and this is an important "but", the clamping force generated by torquing the
rod bolts is what holds the end cap tight on the rod when the tensile forces
Mayf is talking about are at work. Obviously, if a gap develops between the
rod and end cap, this is bad - very bad. When the tensile force of the rod
exceeds the clamping force of the bolts, this is when a gap starts to
develop. You can very easily figure the clamping force of each bolt, times
two equals the total clamping force. BTW, as ARP and others suggest, the
best way to torque a bolt is to measure stretch, or as engineers call it
"strain". From Young's modulus, if we know strain, we can calculate stress -
the tensile force exerted by the bolt. In practice, it's a whole lot easier
just to use a torque wrench. The error with a torque wrench is the variable
"friction", which depends on the state of the threads, what lubrication is
applied, etc. This can cause an error as big as a factor of two in how much
stress and strain are applied to the bolt. If you know Young's modulus of
the steel your bolt is made of, you can measure it's length before and after
you torque it; i.e., the strain and figure the stress.
If you want to know more about this subject, check out the following Web
pages:
http://www.avweb.com/articles/bogustor.html
http://www.mech.uwa.edu.au/DANotes/threads/fatigue/fatigue.html
These are just a couple, and there are many more out there for those who are
so inclined.
TTFN,
Bob
-----Original Message-----
From: owner-tigers@autox.team.net [mailto:owner-tigers@autox.team.net]On
Behalf Of DrMayf
Sent: Saturday, February 24, 2001 9:38 AM
To: Derek White; DBabcock@wecocbc.com
Cc: tigers@autox.team.net
Subject: Re: Connecting rod torque specs
No flames, here, just some thoughts and this was a good place to attach
them. I agree with Derek. Use the best rod bolt you can buy and install them
correctly for longevity...
Hmmm, I tend to think of the load on rod bolts as pure tensile. Here is
why..when the piston is going down in the power stroke the load is
completely on the saddle of the rod at the big end. When it is on the
exhaust stroke it is pushing the gas out so there is residual load on the
top of the piston, so the rod may or may not be in compression with the load
on the saddle. When it is on the intake stroke the load is on the cap and
the bolts are in tension. When in the compression stroke, it is back to
being a compressive load on the saddle of the rod. The worst loading, in my
opinion, is when we blap the throttle, to ahem, clear the plugs or some
other such. In that case the butterflys in the carb are closed, the pistons
essentially have nothing but inertial loads. And when we rev these puppies
up like that, the the load gets pretty big on the rod caps (pistons want to
keep going on the up stroke).
Sent: Saturday, February 24, 2001 5:09 AM
Subject: Re: Connecting rod torque specs
mayf (b9471136)
> I put in ARP bolts and none of my rods had to be resized (we checked them
> all.) I think the torque was 30lbs with their moly lube or higher without.
> If you ever think of the forces on rod bolts, the mind boggles and the
best
> bolts get bought. cheers, derek
|