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Clarification about a Power screw's specifications please

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  1. Apr 9, 2017 #1
    Hello, I have a power screw. According to the specifications, it is M8x1.25. Does that mean the effective diameter of the thread is 8 mm while the pitch is 1.25 mm? Is the screw radius just 4mm?

    From the internet, it looks like the lead angle is atan(Lead/(pi*effective diameter of the thread)) or atan(number of threads*Pitch)/(pi*effective diameter of the thread).

    Could you please let me know how to find the number of threads and the lead (m/rev)? What is the lead angle in this case? Thanks in advance.
     
  2. jcsd
  3. Apr 9, 2017 #2
    To your first paragraph: yes and yes.
    The "nominal diameter" would be 8mm. That's the outer edge of the helix.
    The pitch would be 1.25mm. That's the distance from one turn to the next on the helix.

    I am not sure what you mean by "lead angle". If it is the angle that helix would make with the surface being screwed, it would be atan(pitch/(pi*diameter)).

    In the simple case, the "lead" as you describe it, would be the pitch - except that you would have to convert from mm/rev to m/rev.
    In the less simple case, the screw may not actually penetrate at the full rate if the hole it is moving through has not been pre-drilled.
     
  4. Apr 10, 2017 #3

    CWatters

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    If you have one perhaps measure it :-)
     
  5. Apr 11, 2017 #4
    What instrument do you recommend to measure the lead angle and the thread angle?

    I want to know the type of screw thread I have. It looks like I can tell the type if I can measure the thread angle.


    Thanks. By "lead angle", I mean the one mentioned in:

    https://en.wikipedia.org/wiki/Lead_(engineering)
    http://www.mitsubishicarbide.com/en...c_threading_formula_top/tec_threading_formula

    I think both refer to the same thing but please correct me if I am wrong.

    For your reference, here is a photo of the thread I have:

    https://www.flickr.com/photos/149564621@N02/shares/9878y6
     
    Last edited: Apr 11, 2017
  6. Apr 11, 2017 #5

    Nidum

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  7. Apr 11, 2017 #6
    Yes, they are the same. For the M8x1.25, there is only one helix.
     
  8. Apr 11, 2017 #7

    CWatters

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    Can use a micrometer to measure the diameter and just count the threads over say 10 or 20mm to work out the pitch.
     
  9. Apr 12, 2017 #8
    I know. What about the lead angle and thread angle? Can they be measured by some kind of instrument that can be bought at hardware stores?
     
  10. Apr 12, 2017 #9
    Thanks. Do I need to measure the thread angle to find out if it is ACME, Trapezoidal, Vee, Buttess, Round or square thread, etc? From the attached photo, can one tell what type it is? I got the power screw in Japan.
     
  11. Apr 12, 2017 #10

    Nidum

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    Yes . Almost certainly it is a 60 deg V thread .
     
  12. Apr 12, 2017 #11

    Nidum

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    M8 x 1,25 ISO Thread form . v2.png

    That's a CAD image of a screw with true ISO 60 deg V thread form . See the similarity to the thread form on your screw ?
     
  13. Apr 12, 2017 #12
    They look quite similar but the one on top seem to have sharper crests and troughs. Do I have to concern about this?

    I need to find the coefficient of kinetic friction, coefficient of static friction, velocity threshold (m/s) and low pass filter constant.
     
  14. Apr 13, 2017 #13

    Nidum

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    In simple theory the coefficients of friction only depend on the materials of screw and nut and/or on lubrication conditions .

    You'll have to explain to me what the velocity threshold and low pass filter constant for a screw thread are .
     
    Last edited: Apr 13, 2017
  15. Apr 13, 2017 #14
    Thanks. Cannot find the coefficient of static and kinetic frictions for brass with aluminum. Anybody knows the values? For the velocity threshold, I was told that it is related to the continuous stick-slip friction model. Please refer to:

    https://www.flickr.com/photos/149564621@N02/shares/97aB49
     
  16. Apr 15, 2017 #15

    Nidum

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    Brass against Aluminium is a very poor materials combination to use in any kind of bearing application .

    For your brass screw a non metallic nut would be better . Consider Nylon or PTFE .
     
    Last edited: Apr 15, 2017
  17. Apr 15, 2017 #16
    Thanks. That is interesting but I am modelling a product that I bought. The screw is brass and the nut is aluminum.
     
  18. Apr 16, 2017 #17
    There are some methods like 2wire,3 wire measuring methods for measuring the depth and dia of the screw threads. And these vary for thread to thread. That is type of the thread
     
  19. Apr 16, 2017 #18
    Yes you are correct. 8 is the nominal diameter in mm, 1.25 is the pitch in mm and M stands for metric. Lead is the distance traveled by the nut measured parallel to the screw axis in 1turn. Lead angle is the inclination of the thread with the screw axis. If you don't mine, I suggest you a square thread for Power transmission rather than others. Because square threads are ment for power transmission only. I doesn't mean that other screws can't transmit the power. Yes, they can but as not efficient that those of square ones can!!!
     
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