john
And yet you can have the most accurate Torque Wrench in the world and the end result be inaccurate.
Bolting is interesting subject, and still, relatively, a rather new technology. Books have been written on it.
The end result you are trying for is Pre-Load (the tension in the bolt). Using a Torque value is a means to achieve this end result, it has no other significance.
The main issue: When torque is applied to a bolt, most of that torque (90-95%) goes into overcoming friction, only 5-10 % go into preload. That means if the friction is off a little, maybe a little galling / burr, or maybe slippery coating, etc, the pre-load can be off whole lot (personal testing, off easily by half to double ! ). Its actually been shown that a experienced machinist / mechanic with just a normal wrench can be just as accurate.
So why even use a torque wrench? Well it is a cost effective / convenient method. And it will get you in the ballpark (especially if you are new to a new bolting situation), and of course in the case of certain things, like clamping plastic or carbon, etc, it can be warning method to prevent too much bolt tension.
If conditions are the same, same thread condition, same lubricate, same bolt head diameter, etc, etc, the method can be precise (ie bolting a cylinder head, definitly want a good consistent torque wrench, among other things.) And finally the Torque method can even be accurate if preload/torque ratio is established beforehand for a particular setup and then maintained.
I don't have any carbon parts, but i would think the most critical thing when clamping round carbon parts, is a perfect fit of the clamp interface, so that the clamp pressue is spread out nice and evenly. Thus i would probably never use a carbon handlebar with say an old steel welded stem, but only with a precisely machined, perfectly round stem. I would probably only use a carbon seatpost with a perfectly reamed and sized seattube diameter. Most steel frames (due to warpage when welding) would not qualify. I would tighten only to where things are nice, snug and will never slip, but not much more then that.
Bikes are weird with fasteners... most of the bike's-bolt's-preloads are not limited by the bolt, but rather the clamped items or threaded component... I rarely use a torque wrench, but of course have in the past and occasionly still do. If on a bicycle, I usually use a simple beam type (simple, durable, never loses calibration).
Just remember no matter how accurate the torque wrench, it's still just a ballpark tool, and there is no substitution for feel, observation, common sense, experience, etc.
john m schmidt, pe
portland oregon
--- On Tue, 7/22/08, Seth Hosmer wrote:
From: Seth Hosmer
Subject: Re: [OBRA Chat] Tool envy: Effetto Mariposa Giustaforza Torque Wrench
To: obra@list.obra.org
Date: Tuesday, July 22, 2008, 7:32 AM
There are a few retailers of this product in PDX, my business being one of
them...if anybody wants to try it out, swing on by...
As a recovering overtightener, I've found this torque wrench very helpful
to help me from breaking things and using ez-outs to get out the bolts I've
over-tightened/stripped..
Seth Hosmer, DC, CSCS
Health & Performance Chiropractic
www.hpchiro.com
503.227.2279
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