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Thread: Clarification on Power screw's specifications please

  1. #1

    Clarification on Power screw's specifications please

    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. #2
    Administrator Kelly Bramble's Avatar
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    IS there a specification with that thread? See: http://www.engineersedge.com/hardwar...ead-sizes1.htm
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  3. #3
    Thanks for the table but I don't seem to find the parameters I am looking for.

  4. #4
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by bbq_bbq View Post
    Thanks for the table but I don't seem to find the parameters I am looking for.
    Actually, it is...

    So, what is the thread controlling specification?
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  5. #5
    Perhaps I don't know how to read the table. Probably effective diameter of thread and either the lead or the pitch and number of threads are the controlling specifications.

    The number of threads is measured by counting the number of thread crests in one inch. From
    [link removed], M8x1.25 has about 20.32 threads per inch. So, Lead = 20.32*1.25mm = 25.4 mm/rev = 0.0254 m/rev. Am I right?

    To obtain the lead angle, one uses atan(Lead/(pi*effective diameter of the thread)).

    I don't know what effective diameter of the head is. Somewhere on the internet somebody mentioned that the effective diameter is the pitch diameter but another person said it is not. In case they are the same, should I use the max or the min value?

  6. #6
    Administrator Kelly Bramble's Avatar
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    The link had the same information (actually less) as engineers edge table data.... Also, the angle is given within the image included on the webpage... The thread angle is 60 degrees

    If you have a lead of 1.25 mm thread Apex to Apex (pitch to pitch) that would mean 1.25 mm * (1in / 25.4 mm) or 0.04921 inches Thread Apex to Apex.

    1 in / 0.04921 in = 20.32 threads in an inch.

    But, what you're looking for is the lead or advance per each turn which quite simply is equal to the pitch.

    1.25 mm lead = 1.25 Pitch which means 1.25 mm advance per. turn (360 degree rotation of the screw) or 0.04921 inches / turn.
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  7. #7
    Thanks Kelly. From the image included in the website you cited and the one below, how to obtain the lead angle from the thread angle?


    As for the pitch diameter, should I choose 7.16 (max G0) or 7.042 (min LO)?

  8. #8
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by bbq_bbq View Post
    Thanks Kelly. From the image included in the website you cited and the one below, how to obtain the lead angle from the thread angle?
    The one cited is industry standard and it appears to be ISO68.

    What are you doing here? Designing a custom thread? What's the goal?
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  9. #9
    I am trying to model the self locking behavior of this M8x1.25 thread in computer simulation. To do that, I need to obtain the following parameters:

    a) Coefficient of Kinetic Friction
    b) Coefficient of Static Friction
    c) Velocity Threshold (m/s)
    d) Lead (m/rev): 0.00125 (m/rev) from your previous post
    e) Screw Radius (m)

    In my case, the nut is aluminum and the thread is copper I think as it is in gold color.
    Last edited by bbq_bbq; 04-09-2017 at 06:43 PM.

  10. #10
    Administrator Kelly Bramble's Avatar
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    I'm still trying to figure out to what type of thread/standard you are modeling..

    ACME, Trapezoidal, Vee, Buttess, Round, square thread. You seemed to be looking at a vee thread initially..

    You need to narrow down the industry standard specification or if you're inventing a thread then..
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  11. #11
    Technical Fellow jboggs's Avatar
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    To the OP: When you said "power screw" I assumed you meant Acme form. But you should clarify that. I would expect the self-locking characteristic of an Acme tooth form to be different from a standard ANSI form, and different from a square form, etc. To address your original question - M8 means outer diameter of 8mm. 1.25 pitch means thread-to-thread distance. You can calculate the helix angle calculating the outer circumference and applying the thread pitch.

  12. #12
    Thanks Kelly and jboggs. I sent an email to the company asking them about the screw. Still waiting for their reply. I will post once I have heard from them. Thank you.

  13. #13
    Haven't heard from the company yet. Here is a photo of the power screw I am trying to model.

    Attached Images Attached Images

  14. #14
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by bbq_bbq View Post
    Haven't heard from the company yet. Here is a photo of the power screw I am trying to model.

    Looks like a standard v-thread to me...

    I just remembered (searched this website) that there is a webpage Power Screw Design Equations and Calculator on Engineers Edge.

    It is for ACME and square power screw threads.
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  15. #15
    Technical Fellow jboggs's Avatar
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    It is very unusual to use that V-type thread configuration as a power screw because of its friction characteristics. It is designed to pull a bolt or nut up tight and stay there, not to freely move a nut along an axis. Acme and square threads are much better for that type application.

  16. #16
    Quote Originally Posted by Kelly Bramble View Post
    Looks like a standard v-thread to me...

    I just remembered (searched this website) that there is a webpage Power Screw Design Equations and Calculator on Engineers Edge.

    It is for ACME and square power screw threads.
    Thanks. I think the "lead angle" mentioned in this link is the one I talked about. So, according to the equation, it will be
    atan(1.25/8pi) = 2.85 degrees?

    The store in Japan told me that the screw is made of brass and the red frame (the nut) is made of aluminum. The contact area is dry.
    I need to know the coefficient of kinetic friction and coefficient of static friction between brass and aluminum. Engineering Edge has a table for static coefficient but it does not contain the data I need. The tables posted on the internet usually pair brass with cast iron or steel. Anybody knows the values for brass with aluminum?

    http://www.engineersedge.com/coeffients_of_friction.htm
    Last edited by bbq_bbq; 04-13-2017 at 07:53 AM.

  17. #17
    Administrator Kelly Bramble's Avatar
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    There is information here: coefficient of kinetic friction, coefficient of static friction

    In general, use the highest coefficient to can find, ensure an adequate FOS and you'll be fine. I doubt whatever friction number you locate and use in your calculations is going to bullseye the application when tested and in the field. This is design engineering - not school.

    So.... you want to put brass and aluminum together, slide them against each other and create heat and friction under a load? Sounds like a galling and corrosion experiment to me...

    Wrong material for continuous service at whatever load you think it will handle.
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  18. #18
    Technical Fellow jboggs's Avatar
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    Ditto. And the V thread form will just amplify those problems. In most industrial power screw applications the thread form is Acme, the screw is steel, and the nut is brass or plastic. Have not seen brass driving aluminum, ever.

  19. #19
    Quote Originally Posted by Kelly Bramble View Post
    There is information here: coefficient of kinetic friction, coefficient of static friction

    In general, use the highest coefficient to can find, ensure an adequate FOS and you'll be fine. I doubt whatever friction number you locate and use in your calculations is going to bullseye the application when tested and in the field. This is design engineering - not school.

    Thank you. What is FOS? From the table, the coefficients vary a lot. For example:

    Aluminum - Aluminium: 1.1-1.35
    Brass - Steel: 0.5
    Bass - Cast Iron: 0.28

    Do you mean try to set the coefficient of static and kinetic friction to 1.35?

  20. #20
    Administrator Kelly Bramble's Avatar
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    There's a search box at the top Left of the forums main page and at the top right of of most webpages on this site (60,000+ webpages).

    http://www.engineersedge.com/analysi...ety-review.htm
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