Torque vs Tension
Posted: Sat Mar 05, 2022 3:48 pm
Every wonder why sometimes stuff falls apart?
Couple thoughts here. First of all, your fancy torque wrench is probably
out of calibration. Check it with a weight (cheap). The pointer type torque
wrenches are actually more accurate, though mechanics like to sh1t on them.
Try hard not to drop them. The expensive clicky ones, anyways. You can probably
drive over the pointer torque wrenches with a tractor and they'd be ok.
Next, torque is a really bad way to tighten up something, but often it's all
we have. Check carefully - are the threads supposed to be dry or lubricated
for the given torque spec? What lubricant?
Next is how you torque. Realize that sometimes 85% of the torque is required
to break the static friction. This means (esp with dry threads) that you if you
torque to 40 ft-lbs and stop, then torque to 50 flt-lbs and it doesn't move, you
would not have rotated as much as if you had directly applied 50 ft-lbs without
stopping. Oops. Completely different tension, but same torque value on your
mega-buck Snap-On clicky/beeping torque wrench.
Torquing is really stupid and often alarmingly decoupled from tension.
The correct way of measuring fastener tension is by measuring it's length, but
this is not always possible or economical.
See Young's Modulus: stress vs strain curve.
Anyone that does anything with metal needs to know the above curve.
You may either want to tension elastically (no permanent deformation) or plastically
(permanent deformation). Anything plastically deformed (past the Yield Strength) is
"use-once" - throw it out and replace it with new, every time you assemble. IIRC the
Lycoming Table of Limits specifies a length range for your crucial engine through bolts
which must be checked at overhaul. Think about that.
The right way to torque connecting rod bolts is with a caliper, btw. If the big end on
a con rod lets go, you're going to have a ventilated crankcase like that 205 in Oregon.
I know fasteners are boring and stupid but a lot of people died at Reno a few years
back when a P-51 crashed into the crowd, after a small fastener failure. The size of
the end of your pinky.
Next up: How to torque after cylinder replacement. It's a lot of work to do it right,
which is why everyone always does it wrong in the field, and Mike Busch says that
only engine shops ought to change cylinders.
Couple thoughts here. First of all, your fancy torque wrench is probably
out of calibration. Check it with a weight (cheap). The pointer type torque
wrenches are actually more accurate, though mechanics like to sh1t on them.
Try hard not to drop them. The expensive clicky ones, anyways. You can probably
drive over the pointer torque wrenches with a tractor and they'd be ok.
Next, torque is a really bad way to tighten up something, but often it's all
we have. Check carefully - are the threads supposed to be dry or lubricated
for the given torque spec? What lubricant?
Next is how you torque. Realize that sometimes 85% of the torque is required
to break the static friction. This means (esp with dry threads) that you if you
torque to 40 ft-lbs and stop, then torque to 50 flt-lbs and it doesn't move, you
would not have rotated as much as if you had directly applied 50 ft-lbs without
stopping. Oops. Completely different tension, but same torque value on your
mega-buck Snap-On clicky/beeping torque wrench.
Torquing is really stupid and often alarmingly decoupled from tension.
The correct way of measuring fastener tension is by measuring it's length, but
this is not always possible or economical.
See Young's Modulus: stress vs strain curve.
Anyone that does anything with metal needs to know the above curve.
You may either want to tension elastically (no permanent deformation) or plastically
(permanent deformation). Anything plastically deformed (past the Yield Strength) is
"use-once" - throw it out and replace it with new, every time you assemble. IIRC the
Lycoming Table of Limits specifies a length range for your crucial engine through bolts
which must be checked at overhaul. Think about that.
The right way to torque connecting rod bolts is with a caliper, btw. If the big end on
a con rod lets go, you're going to have a ventilated crankcase like that 205 in Oregon.
I know fasteners are boring and stupid but a lot of people died at Reno a few years
back when a P-51 crashed into the crowd, after a small fastener failure. The size of
the end of your pinky.
Next up: How to torque after cylinder replacement. It's a lot of work to do it right,
which is why everyone always does it wrong in the field, and Mike Busch says that
only engine shops ought to change cylinders.