[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]

Re: Waiter, there's a fly in my soup!

On Mon, 08 Dec 1997 20:27:33 -0400, geo wrote:

>Having replaced the timing belt on the 5kq I5 with the Paul Heneghan
>method of calibrating my wife, ie. placing my 110
>lb. green eyed beauty three feet out on the bar to get 330 lb/ft of
>torque on the damper bolt, I now have a new variable.
>>[ ... ]
>> Geo,
>> Read your post re: 310 lbs of torque...
>> My recollection is this is not enough.  The 310 lbs is the spec for when
>> using a torque multiplier.
>> You really want about twice that much.  i.e. a 200 lb person on the end of
>> a three foot pipe.
>> Dave C.
>>[ ... ]
>  It has been many years since I was in a physics class, but I'm assuming
>force = weight x radius with the axis being
>known.  I really don't want to go at this sucker again, at least for another
>60k!  Any help on this would be greatly appreciated.

Disclaimer: Fools go where angels fear to tread.

Apology: My belated apology to the list for being a smartass jerk the last
time this thread came around. Temperatures were running high then.

You are real close, Geo. The torque is the product of the force applied
(at a right angle to the radius of application) and the radius itself.
The trouble - and I do mean trouble - occurs when you add the
confusion of trying to measure torque directly with an instrument
calibrated for a different radius (moment arm) than the one you are
effectively using.  If you can stay focused on your original
assumption, it will help you through the potential ensuing melee.

If a torque wrench - there, I've said it - is used in combination
with an extension to give greater leverage, a little math can
easily convert the torque readings to actual applied torque and
the other way round (provided the wrench and the extension are
kept in a nice straight line. Otherwise -gasp - it's trigonometry time).
This must appear in every shop math book in the world.

Let Lt be the effective length of the torque wrench, drive to handle mid-grip
 "   Lx be the effective length of the extension, drive center to drive center
 "   Tt be the torque reading from the wrench
 "   Tx be the torque generated at the business end of the extension
 "   F = the force applied (arbitrary amount for this problem) at mid-grip

Tx = F(Lt + Lx)

Tt = F(Lt)

The torque ratio is found by dividing one expression by the other.

Tx/Tt = (Lt + Lx)/Lt     The force term cancels and does not appear
                                   in the ratio.

The Audi documentation has generated some controversy in the
past because it gives a torque reading to be obtained using a special
extension tool (2079) without reference to the torque wrench
length. This seems to imply that the extension tool converts input
torque to output torque by simple arithmetic multiplication. Our
little torque ratio relationship above shows that this cannot be
since Tx/Tt is not a constant.

Fortunately, the raw torque at the bolt is also given as 450 Nm
(332 lb-ft) so you can always work out the correct wrench reading,
such as Tt = 332 Lt/(Lt + Lx) lb-ft. Phil has provided the appropriate
references from the RR engine manual. The torque readings with
and without the extension tool are 350 and 450 Nm. From this
torque ratio, 450/350 = 1.286, we can compute that the wrench
to extension length ratio is 3.5. Anybody got a 2079 lying about?

DeWitt Harrison    de@aztek-eng.com
Boulder, CO
88 5kcstq