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Homework Help: Direction of forces

  1. Jan 8, 2012 #1
    Dear people,

    Please look at the attached figures and tell if the direction of forces have been drawn correctly?

    The force-vectors are starting from a part, which is bolt-tightened to a rotating drum.

    The pictures represent three situations: a) the rotation is accelerating (a motor is driving the drum); b) the speed is constant; c) the speed is slowing (the slowing motion is caused by friction and gravity only, no brakes are used).

    Looking forward to your opinions,

    Attached Files:

  2. jcsd
  3. Jan 8, 2012 #2

    Simon Bridge

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    It is usually better if you show us your reasoning.
    Hint: If you convert the pdf to a jpeg (or other image) you can display the attached file in the body of the post :)
    <looking at the diagrams>

    The spokes-thing is the drum right?
    The bolt in question seems to be perpendicular to one of the spokes, but extends quite a way, is this correct? (Need better labels on your diagram - maybe a blown-up section showing the bolt and thus which forces are important to you.)

    It is not clear which forces you are trying to show.
    I don't see any centripetal or tangential forces.
    The forces labelled FT for example are not tangential as drawn and the ones labelled FR are not radial.

    I think you need to clear up your diagram. Try drawing as a free body perhaps?
  4. Jan 8, 2012 #3
    Hello Simon,

    Actually the sketches have been drawn off from a wood hacking machine, which has a wood-hacking blade bolted under a holder. Actually the real reason I am asking this is to get some idea about the forces that impact the blade during different stages of the work.
  5. Jan 8, 2012 #4

    Simon Bridge

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    http://static3.album.ee/files/1050/18/large_26254635_RnOS.jpg [Broken]

    OK - so you want a crossection of that.
    I see the bolts are not radial. Looks like the whole structure below the blade rotates, is this correct? It wood expected to feed into the grooves/holes for hacking as the whole structure rotates? It also looks like the machine does not rely on the bolts to hold the blade fixed in place: it looks like it fits against a groove or lip just above in the pic.

    A force analysis would be quite complex - what is it you want to find out, exactly?
    The effect on the bolts? The cutting force? What?

    That will guide the analysis.
    Last edited by a moderator: May 5, 2017
  6. Jan 8, 2012 #5
    I hope this drawing is better:
    http://static3.album.ee/files/1050/21/orig_26255323_cSKJ.jpg [Broken]
    The Blade goes under the holder: both blade and holder are removable parts if all nuts are removed. The reason I am asking about the force-vectors is that the wood-hacker got damaged and now is the question if it was foreign object or the Blade moved a little bit out and touched the Anvil (anvil is seen on the picture).

    I wouldn`t give out many details about what happened with the machine, because in other forum where I have asked help and given such info, then all we get as a reply is that: "didn`t you find the residues of foreign object, were all the nuts tightened, etc." All these questions have been taken care and now I would like to get some idea about possible movement of the Blade.

    It is enough for me to understand and see the force-vectors. I understand that also some calculations can be made (we have the blade weight, its distance from the center, drum rotation speed), but it would be enough just to see the vectors (are the ones shown on the first post correct or not; or where could I read more about it).

    In my understanding the blade can not move out under the holder (in the same direction as the drum is rotating), because the drum rotation is either accelerating or at a constant speed. There is no braking system on this machine (when it is turned off, then it stops by free-spinning).
    Last edited by a moderator: May 5, 2017
  7. Jan 8, 2012 #6

    Simon Bridge

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    So what you care about is how the wood impacts the blade (as the most likely reason the blade moved a bit).

    Just to get this out of my system... bear with me:
    Presumably you want to know before operating the machine with a new blade - that would be an expensive way to find out :) The simple and fast way to see if the blade impacts the anvil is to add a dummy blade and rotate the drum slowly. If it hits, you have your answer.

    This would leave that there is some give in the system which allows the blade to get pushed off-line ...

    The blade goes past like a razor across your face - the impact force will act perpendicular to the radial-line through the tip of the blade.

    This will create a torque clockwise about the butt-end (the back - the not-sharp edge) of the blade in the diagram which is balanced by the bolts securing the blade-holder. It would not be correct to display the securing force at the bolt-position since the holder distributes this along the width of the blade (otherwise the blade may bend or shear-off at the bolts). Perhaps include the holder as part of the blade for this bit.

    This means there will be a force along the width of the blade (your pink arrow - but from the blade cutting-edge, not the bolt) which is opposed by the bit of the drum abutting the blade. (Is there a gap there? Probably not good to have the bolts holding this as well.)

    From this, cutting wood would tend to make the blade+holder want to peel away from the drum radially - the metal would bend or try to break off. Since blades are usually pretty brittle, I'd expect it to shatter before it could bend more than a fraction of a millimeter ... you'd see big crescent-shaped chunks carved out of it. Kinda favors the foreign object hypothesis.

    Metal fatigue would show tear-lines through the the bolt-holes. The older part of the tears would be dull, and the new parts shiny. If you see something like that then the blade just got old. (There would also be tears from small nicks in the cutting edge ... now I think of it. How am I doing?)

    I'd still try a dummy blade JIC the damage has knocked something out of line.

    OK - that's probably used up my arrogance quota for tonight :)
    Do you have a pic of the damaged blade?
    Last edited: Jan 8, 2012
  8. Jan 8, 2012 #7
    Thank You very much Simon!

    Your last response was very helpful!!!
    You gave us the confirmation about foreign object hypothesis. Here is also picture of the damaged blade holder and below this you may see the remainings of the blade that was sitting under the holder (this machines has two blades, second passed with minor damages - see third picture).
    http://static3.album.ee/files/1050/23/orig_26255869_Me9g.jpg [Broken]
    http://static3.album.ee/files/1050/23/orig_26255873_FJHO.jpg [Broken]
    http://static3.album.ee/files/1050/23/orig_26255857_Svqf.jpg [Broken]
    Second picture is of the Blade - I noticed you had some questions/ideas about the blade vs. bolt. See third pic., the size of the bolt slots, these slots are needed to have clearance for the perfect blade adjustment.
    And you are also right about the blades`brittle material: there was no bending, just a lot of small particles.

    :) You can remove this from your system!

    Last edited by a moderator: May 5, 2017
  9. Jan 8, 2012 #8

    Simon Bridge

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    So it's a twin-blade razor? The top one was on the outside?

    See the crescent bite out of the lower blade - that was the sort of damage I was expecting. I get to feel smug about the shear-lines through the bolt-holes (slots) <smug> OK that's enough :)

    Looks like you have a hard obstruction (the bottom blade hit it on the big chip) and a lot of metal fatigue. Can't be sure without getting my hands on it though.

    Um - if the outer blade hit the anvil, it would be a glancing blow tending to push the blade inwards along it's whole length. I don't know what it would be pushed against - I thought there was only one blade resting flush against a solid surface. I'd have expected a long break like that to be along a support edge - but that's also where you'd get the most fatigue too. <thinks> lifting would break along the bolt-arches, lowering would break further forward since I don't imagine a support being further back than the bolts so no I think we're in the clear.

    I suspect you've had hard stuff jam in the blades before and you just stopped the machine to clear the obstruction?

    I'd still rig a dummy blade - you can probably mock up something the correct dimensions. JIC.
  10. Jan 9, 2012 #9
    Ok, the last one: this is the place where the blade together with blade-holder was ripped and only tiny fractures were remained from the blade (picture of the blade particles in my previous post). The second blade&holder remained attached (my first picture) - the second blade luckily just was pushed inwards. I will not probably post anymore pictures what had else happened...

    This was a nightmare, especially considering the fact that the apparatus on the truck (only the wood-hacker) costs around 150K € and although you have the highest possible insurance, these guys still are looking for reasons for not to cover the repair costs :(
    If this was a FOD, then the insurance will cover the costs; if it got damaged due to bad maintenance or loose blade, then it won`t be covered - simple as that. But we will probably hire the manufacturer and its engineers to give their opinion&report.

    http://static3.album.ee/files/1050/28/large_26257119_abNg.jpg [Broken]

    What is your opinion: based on the fracture signs in which direction this Blade-tip has broken (according to picture up or down)?

    http://static3.album.ee/files/1050/30/large_26257535_8WES.jpg [Broken]
    Last edited by a moderator: May 5, 2017
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