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Thread: Questionable torque wrench settings

  1. #1
    Join Date
    Apr 2009
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    Default Questionable torque wrench settings

    I have the Volvo S70, V70 & C70 1996 to 1999 Petrol manual.

    I have been comparing the newton-meter and pound-feet torque settings for several values and discovered that they don't all properly convert. For example. on page 2A-2:

    Crankshaft pulley-to-sprocket bolts: Nm 25 +30 degrees LbFt 18, whereas in reality 25.08333 Nm = 18.5 LbFt and 18 LbFt = something less than 25 Nm.
    Cylinder head lower section to block: Nm 60 +130 degrees LbFt 44, whereas 44 LbFt, again, is something less than 60 Nm!

    So my questions are 1) where do these torque setting values come from? and 2) should I trust the Nm values and recalc the LbFt values or trust the LbFt values and recalc the Nm values?

    Thanks in advance.

  2. #2
    Join Date
    Jan 2007
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    4,850

    Default

    Hi burnsey,

    The specifications come directly from the manufacturer. I agree, while applying the common conversion factor to the Newton Meter value will yield a torque that is slightly different than their listed foot-pound values, it isn't enough to matter. When you consider that torque wrenches themselves aren't perfect (even when comparing two identical torque wrenches), and the fact that even if the wrench is perfectly calibrated, your arm also enters the equation. Have you ever used a digital torque wrench that beeps and vibrates when the desired torque is reached? The digital display doesn't just stop when the desired torque is reached - it keeps registering actual torque until the operator stops applying force. It's not uncommon for a couple of extra foot pounds to be applied, even if extreme care is taken. The threads of the fastener and its corresponding hole or stud also make things less than perfect. If you really want to start splitting hairs, so does temperature!

    Combine all of this with the second part of applying torque to the fastener: the amount of rotation in degrees. A couple of degrees more or less will affect the end result. So will the use of an extension (due to twist). I would suggest that this step introduces far more of a chance for an inaccurate application of torque than the discrepancy in the conversion factor does.

    In the end, what we're talking about here is 0.43 foot-pounds extra on the first spec you listed, and 0.2 foot-pounds on the second. Not enough to matter one way or the other. What IS important is that all of the fasteners on a certain component (take the cylinder head, for example) are all in the ballpark and equal to one another.

    Good post though. Better to be safe than sorry.

  3. #3
    Join Date
    Apr 2009
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    3

    Default Okay, but...

    A very comforting and practical reply that I will accept at face value.

    But, for the record, here's the rub...
    Take the example of 25 Nm plus 30 degrees. I have to assume that the extra 30 degrees is less than 1 Nm, otherwise they would have specified 26 Nm plus some amount of extra turn. So assume that the 30 degrees is one-sixth of a Nm (about one-eighth LbFt) or maybe one-half Nm (about one-third LbFt). Why would they specify this to such a high degree of precision if it were unimportant? And why would the Nm settings have this extra precision and none of the LbFt settings? Frankly, it doesn't make sense.

    I guess I have to confess to some level of ignorance here. I have known other engineers who have spec'd parts with additional unquantified torque, but it's unclear to me why. I've helped design and build some fairly high tolerance systems in the past (satellites, Stealth bomber, other MIL-spec stuff) and I've never seen a need for this. I actually thought it might have something to do with a materials issue (aluminum case, steel bolts) but even that seems unlikely. I have the technical ability to set these things to a "measured" torque setting of about +/- 0.08 LbFt, but it hardly seems worth the trouble in light of your "don't worry about it too much" response.

    Guess it's time to stop whining and jus' start slappin' it back together. Thanks for the help.

  4. #4
    Join Date
    Jan 2007
    Posts
    4,850

    Default

    Hey again burnsey,

    NO! We would never advocate "just slapping" anything back together. Torque is important, but not to the tolerances you're referring to. It is comforting, though, to see someone who cares about the quality of their repair - good on ya for that!!

    The torque setting + additional rotation is commonly used on many fasteners these days. The engineers have calculated the amount of stretch that the fastener undergoes during the rotation phase. I'll admit I don't know how they do this, or why in some cases, except that my first encounters with this method was because the manufacturers began using "torque-to-yield" bolts that were designed to stretch a certain amount, but they could only be used once because the bolt would deform when tightened properly. Other uses of this method were because of fasteners whose threads had a slight "interference fit" so they would tend to not loosen up on their own. These kinds of fasteners are commonly used on suspension and brake components, and a traditional torque setting couldn't be used because the drag on the fastener, especially after a certain amount of torque was applied, would create an inaccurate torque reading.

    The problem here is your assumption that the 30-degree additional rotation would equate to something as small (insignificant) as an eighth of a foot pound. In reality, the rotation step of tightening is what usually imparts most of the torque to the fastener; usually much more than the initial (step 1) setting.

    For example, take the vibration damper bolt on Ford 4.6 liter V8 in a Mustang. The spec for a 1996 model is 114 to 121 ft-lbs. On 1997 and later models it's 28 to 31 ft-lbs, plus an additional 90-degrees, plus an additional 90-degrees - a half-turn twist from the initial, fairly low, setting (and the bolt is essentially the same size as on the earlier model).

    I have several friends in the aerospace industry - three at Boeing (the Shuttle), one at The Aerospace Corporation, one at Dukes - with whom I've helped repair cars, build and race cars, motorcycles, high-end bikes, etc., and they're all OK with manufacturers when they furnish torque specifications which span a certain range, even when, in my mind, the range seems a little too broad! So, essentially, the manufacturers really don't supply torque specs to "such a high degree of precision" that you're assuming is necessary.

    And, like I alluded to before, most consumer-grade torque wrenches aren't capable of such precise measurements even if it was necessary.

    The most irritating torque-related issue that I've experienced was due to a torque wrench that apparently had "seen better days" and snapped off a couple of bolts for me, even after I was sure it was set correctly. Sure enough, the new torque wrench didn't do that . . .

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