mgeoffriau

iTrader: (7)

What you're describing is static friction (or stiction), and I believe torque specs take it into consideration, as instructions will often warn that a lubed bolt should not be torqued to the full spec.

NA6C-Guy

iTrader: (1)

Exactly, and all I'm trying to say is by adding flex (extensions, especially thin ones) you are making for a more broad margin of error in your final torque. You may be shooting for 200lb/ft, but you don't know for sure if it made it to 200lb/ft (though with friction, the bolt probably never makes it all the way to 200) or if friction caught it at 175lb/ft, and it never broke free for that final little bit, and the load on the extension simply gave you that extra force to click the wrench. Remove the extension, remove the margin for error. Or go to a thicker extension with far far less flex.

Just admit what I'm saying makes sense so I can go to bed.

Oh ,and yes, static friction is taken into account and probably is factored into specs, but as i said, it's not a laboratory or a perfect world. The bolt head or surface could be scratched, adding more friction, a thin layer of dirt or build up under the bolt could add to the friction, or temperature of the materials, more or less lube, ect. Those things could throw it off by 10%, maybe 20% or more. If you have no parts to flex, you put in 1/8 turn at the handle, you get 1/8 turn at the fastener. So those things don't matter. But, you add flex on the work end, and your 1/8 turn may only equate to 1/4 at the fastener, leaving you short. The extension "ate" 1/4 of a turn, and roughly half of the torque applied.

samwu8k

iTrader: (3)

just re-torqued everything with a 4 inch long 1/2 drive extention + adapter. diddnt move at all...or maybe a very un noticable amount. but yea ill definitely be able to sleep @ night now. ill be re-checking the torque after i drive the car the first time and the engine reaches normal operating temps.

thanks for your input everyone!

NA6C-Guy

iTrader: (1)

Shouldn't need to recheck/re-torque, since it's an MLS gasket, at least I assume, unless something is different with yours than I have seen with every Miata I have dealt with. As long as you are sure they are correct now, they should stay that way.

Also, to clarify with everyone, I'm not arguing this to be a dick, or making it a big deal. I just enjoy this kind of talk. If I am wrong about this somehow, I want to understand why. Though I don't think I am at all.

Joe Perez

iTrader: (8)

I completely understand what you're saying.




(If this forum supported the use of the <flash> tag, I'd have used it, too.)


Take your hypothetical about the tack weld on the bolt head. PLEASE READ THE WHOLE THING, AND THINK ABOUT EACH PARAGRAPH CAREFULLY BEFORE YOU PROCEED TO THE NEXT, AS I SUSPECT THAT YOU HAVE BEEN SKIMMING.

You take a torque wrench, slap a 100 foot long 1/4" extension on it, and twist it until the wrench reads 100 ft/lbs. You will probably have to rotate the handle several times to get there as the extension twists up, but it'll eventually get there. So now the wrench reads 100 ft/lbs, and 100 ft/lbs is being applied to the bolt head.

This is not enough to break the weld, so the bolt does not rotate, and as a result, the bolt remains "loose" insofar as the stretching and tensioning of the bolt shaft is concerned.

Now you remove the extension and do the same thing. This time, you will only have to turn the wrench a few degrees before you hit 100 ft/lbs, but the net result will be the same. The weld is not going to break, and so the bolt is not going to become tensioned.

Do we agree on this?


Ok, now say that we are going to apply enough force to break the weld. We'll start without the extension this time. You apply the wrench to the bolt and start leaning on it. At the instant you hit 500 ft/lbs, the weld breaks free. Assuming you then release the wrench, the bolt will have only rotated a few degrees, and will not become properly tensioned.


Now go back and repeat with the 100 foot long extension in place. As before, you start twisting and twisting until the wrench reads 500 ft/lbs. When it reaches that point, the socket will also be applying 500 ft/lbs to the bolt head, and as before, the weld will snap.

Ironically, when this happens, all of that pent-up energy in the twisted extension will rapidly start to be released in the form of rotating the now un-stuck bolt, and unless you release your grip on the wrench immediately and allow it to spin freely as the extension un-winds (which will probably cause it to hit you in the face, shattering your jaw and knocking you off of the 100 foot tall ladder whereupon you fall to your death, realizing just a few milliseconds before impact that, despite the bragging rights, purchasing the 100 foot long extension which was on sale for $7.99 at Harbor Freight might not have been the smartest decision you ever made), the bolt will most likely be massively over-torqued and snap.

Of course, if it were possible for you to magically remove the socket from the bolt head at the instant that the weld broke, you'd avoid the whole gruesome death thing, and exactly as in the extentionless scenario, the bolt would not be properly tensioned.


Just because the extension twists does not mean that any torque is being "wasted". You apply torque at the top, and the same amount of torque "comes out" at the bottom. You can "waste" work, but you cannot "waste" force.

jasonrobo02

iTrader: (13)

Joe is correct. To convince yourself of this, think about your engineering, physics, etc. classes and draw a free-body diagram.

You can treat the bolt as a fixed end condition and apply a moment at the wrench end of the extensions (100 ft-lbs). When the wrench clicks at 100 ft-lbs, you are in static equilibrium. Thus the sum of forces and moments must equal 0. If your only input is the 100 ft-lbs moment, your reaction moment at the bolt must equal 100 ft-lbs.

fooger03

iTrader: (2)

Thanks for that guys, I thought I was going to have to jump through the monitor and choke Jesse. :P

And yes, it doesn't matter if it's 1/16th of an inch of hardened steel, or 300 miles of fishing line; if you're putting 100 ft-lb of torque on one end, 100 ft-lb of torque is being applied to the other end - completely regardless of how much twist is in the medium.

NA6C-Guy

iTrader: (1)

I'm fully understanding your end of the argument, you can't get around physics. All of what I am saying boils down to precision of input. With all of that twisting, the bolt has more... room for error (not the correct word), as the twisted extension is not linear, but more like a progressive spring. With no extensions, you have a more precise adjustment and no "room for error". If you make an input, the bolt immediately responds. Where as with an extension, you load the extension. Static friction becomes more of a... something. I just woke up, brain not working yet.

Surely none of you argue this. In a long, round about way, this is all I am trying to say. No extension precise feel over a 3/8'' extension and a numb feel.

*and I really don't think the point I am trying to make is understood completely whether you guys say you understand or not. It may be my fault for not wording it correctly. It's all about working on an imperfect bolt, in an imperfect environment, with imperfect tools. All about static friction. I have another example, but it will have to wait till after work. Then all of you can want to choke me again.



*also, I am not arguing physics so much as practical application I guess you could say. I'm not arguing that 100lb/ft in results in 100lb/ft output

ALSO, to continue this never ending rambling... can't rotational force be converted into linear force, where a connection is made between an extension/adapter to the wrench or socket? Say the extension attachment end is worn out and tapered slightly. It's not only going to be applying force that is rotational but also a linear, up and down force (trying to push your hand and tool away from the bolt), which is wasted energy in the direction we want it to go, is it not?

mgeoffriau

iTrader: (7)

Here's what I don't get about your argument:

You are claiming that, even though the 100 lb/ft is fully applied through the twisted extension, somehow it won't be enough to overcome the stiction in the bolt because of the flex in the system.

But if 100 lb/ft isn't enough to overcome the stiction in the bolt at the end of the twisted extension, then it won't be enough to overcome the stiction even without the extension.

In other words, if the bolt stiction is greater than can be broken with 100 ft/lb of force, it doesn't matter whether that force is applied through a flexy extension or not (assuming you are turning the wrench in a fairly smooth manner as one does when torquing bolts).

If no-extension can break the stiction of the bolt when extension can't, all that means is that by using no-extension, you've managed to momentarily exceed the target torque specification.

NA6C-Guy

iTrader: (1)

It's not BECAUSE of the stiction (if that's what it's called). I'm simply saying that the load of the twisted extension could fool you into thinking the fastener has achieved full torque, when it fact the extra x amount of torque you were seeing was actually just the twist of the extension acting as a torsion bar. If there is no place in the work end of the torque wrench to twist, you will be sure all of your rotational force is transferred into the fastener. If you don't overcome the stiction without an extension, you would know it, because your wrench wouldn't move... see what I'm saying? Whereas with an extension, you would have extra movement of the wrench as see extra torque that actually isn't turning the fastener. Making for a broad "arc of error".

fooger03

iTrader: (2)

You need help.

Joe Perez

iTrader: (8)

Savington

iTrader: (31)

Do you seriously think that you can twist a 3/8" extension 1/8 of a turn (that's 45 degrees) with 50ft.lbs of force?

Do the math: http://www.engineeringtoolbox.com/to...fts-d_947.html

or

do the experiment: Chuck up a 6" 3/8" extension in a sturdy vise, attach a torque wrench, and figure out how much force you have to apply to torsionally bend the extension 45 degrees (1/8 of a turn). Just do me a favor and wear some chain mail so you don't give yourself internal organ damage when the extension shears at around 400ft.lbs (after deflecting maybe 10 degrees), slinging the torque wrench into your gut at break-neck speeds.

EO2K

iTrader: (37)

In for video!

NA6C-Guy

iTrader: (1)

Joe...

You guys are no fun, I expected more from this thread. I'll go troll elsewhere then.