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Crowfoot Wrench - Torque Settings

Discussion in 'Lounge' started by oldtech, Feb 2, 2020.

  1. I'm having an argument with a mate about using a crowfoot with a torque wrench .

    Looking at the picture and imagining the distance between the centre of the 3/8" drive and the centre of the bolt head ( to be tightened ) I reckon the extra lever effect is going to put additional torque on the bolt head ....
    .... before the torque wrench clicks .

    My mate reckons LESS torque will be applied to the bolt .... and I won't have it
    Now I'm in danger of overthinking the whole thing !

    I know there are some excellent engineers on here ....
    What sayeth thou ?

    .... and yes , I could have googled it ....:sleeping:

    crowfoot wrench.png .
    #1 oldtech, Feb 2, 2020
    Last edited: Feb 2, 2020
  2. It's more torque with the spanner in line with the torque wrench. Put it at right angles and the torque setting will be nearly correct . There is an equation you can use if you want to have the spanner in line.
    The spanners for old style ducati heads are cranked so the head of the spanner is directly below the torque wrench drive so no problems there.
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  3. This is an interesting puzzle for a Sunday.

    I would have thought that the force thru the wrench to the extension bar remains constant (because of the pre set torque) regardless of angle of the spanner, and the offset from the extension bar to the centre of the bolt remains constant. So, the torque on the bolt will remain constant regardless of angle of the wrench handle.


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  4. I have had this discussion a few times regarding tightening fuel injection pipes on large marine diesels. Could never figure out why the guys were damaging the fittings until I realised they were using the crowfoot in line with the torque wrench and not re-calculating the torque setting.
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  5. OK - so the bolt is certainly going to get more torque than what is set on the torque wrench BUT the amount of extra torque will depend on the angle of the torque wrench , when tension is applied ??

    There's always going to be some slack , when you add bits-and-bobs between the torque wrench and the bolt , and this seems to be why my mate believes that torque will somehow be "lost" .

    I know exactly which tool you're talking about ...
    I knocked one up many years ago .... " in me shed "

    BTW - I'd already taken one of my crappy photos because I'd planned to start a new thread
    " Home-made Ducati tools , knocked up in yer shed "

    Head nuts.jpg
  6. If the tool is used as in the picture, then there is an increase in torque on the fastener, negligible as it may be. When using crowfoots, having them at 90 degrees to the wrench gets the torque as accurately as possible.
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  7. Here you go.
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  8. I'm setting myself up here I can feel it, but I still don't get it.

    If the torque wrench is set to say 20 ft bis it will click off at 20 ft lbs regardless of angle to the crows foot spanner.

    That 20 ft lbs doesn't change if the spanner is at 90 degrees.

    I understand that more than 20 ft lbs will be applied to the bolt because of the offset to the extension bar in the crows foot but the force on the bolt must remain the same regardless of the angle of the crows foot because the torque wrench is applying the same force (when it clicks off ) to the extension bar?

  9. The vid I just put up explanes it quite well.
  10. I don't think it matters how long the wrench handle is it will click off at 20 ft lbs, even if the handle was 10 feet long, still clicks off at 20 ft lbs.

    That 20 ft lbs is always applied the same distance from the centre of the bolt, turning it thru 90 deg doesn't change that.



    P.S. the video logic applies if there was no torque wrench.
  11. Leverage :thinkingface:
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  12. Don't we use torque wrenches to negate leverage.

    Isn't the leverage what force goes thru the extension bar to the crows foot , which the torque wrench ensures is always say 20 ft lbs.

    I think I'd better stop now.


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  13. The video explains it very well. It’s all about the turning moment or force around the centre line of the fastener. With the crows foot at 90 degrees to the wrench, the distance around which the force is being applied is the same. In line, you’re adding to the length of the lever, so 10kg over that added distance means an increase in applied torque.
    In truth, in real life, crows foots rarely get used on small fasteners where the increase could cause a problem. On a M10 fastener for instance, no matter where the foot is, providing the wrench is set mid range for the bolts tolerance, you’ll still be in range.
    It’s worth bearing in mind though if you’re on critical fasteners.
    V4 sump plug is a case in point. It’s too tight for a socket and the wrench head, mine anyhow, so I use a crows foot. These can easily be over tightened and crack the sump, so I’m very careful with foot position on the torque wrench.
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  14. Maybe I'm nit-picking here , but strictly speaking I think we should be talking about " mechanical advantage " , not "leverage " .

    Also , these are rotational forces , not linear ones .

    @nelly is on the right track IMO

    PS - the wiki page has good diagrams and illustrations

  15. That page explains it in diagrams with Levers :eyes:
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  16. Torque wrenches are only "guessing sticks" anyway ;);)
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  17. I agree with what I have highlighted but we aren't talking about the length of the torque wrench, imagine the crows foot is 3ft long, the force applied by the torque wrench would be far greater at the end of the crows foot sticking straight out wouldn't it.
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  18. Leverage? :worried:
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  19. Yes , but further down there is the animation of meshing gears ... :pensive:
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  20. Is that leverage then :(

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