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IndustryArena Forum > Mechanical Engineering > Linear and Rotary Motion > Screw diameter and strenght relation
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  1. #1
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    Screw diameter and strenght relation

    Hi guys

    If a screw has a pitch of 5mm and diameter of 25, does it push the load with more force than a 16mm screw of the same pitch? The load moves the same 5mm per revolution, but the thread is in a lot lower angle on the big screw so wouldn´t that affect on the resistance hence giving "more force" to the system?

    At least it should take more force to make the bigger screw turn by the movement of the load (lets imagine the motor is not present and does not apply holding force).

    Confused here!

    -PropellerHat

  2. #2
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    Re: Screw diameter and strenght relation

    hy, obviously, bigger is better / stronger

    arround here is a guy that can tell exactly how much strength/resistance/whatever? you get by increasing dimensions ... he is ( at least ) into screws & nuts mechanisms, powered by hydraulics

    so short answer is yes / kindly
    Ladyhawke - My Delirium, https://www.youtube.com/watch?v=X_bFO1SNRZg

  3. #3
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    Re: Screw diameter and strenght relation

    Hmmm... Not convinced in here.

    Sure a big one is sturdy but that's not my question.

  4. #4
    Community Moderator Jim Dawson's Avatar
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    Re: Screw diameter and strenght relation

    If you question is about the load back driving the screw, then yes, I think the larger screw would be somewhat more resistant to turning due to the lower thread angle.

    Consider that a thread is an incline plane wrapped around a cylinder. So if you unwrap the incline plane you have a wedge, and I think the smaller angle would be more resistant to movement when a force is applied perpendicular to the wedge.

    Is it enough difference to affect anything in the real world? I don't know.
    Jim Dawson
    Sandy, Oregon, USA

  5. #5
    Community Moderator ger21's Avatar
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    Re: Screw diameter and strenght relation

    Also be aware that a larger screw will have much more inertia, and require a lot more power to accelerate it than a smaller screw. This would likely cancel out any gains made by the shallower angle of the threads.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  6. #6
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    Re: Screw diameter and strenght relation

    Not convinced in here
    hy, my answer was not specific, since i am not into this field, thus i can not pin-point the related phenomen, because i don't know what you are after

    helix angle is related to friction : for example, if you lift a heavier mass, then you need a bigger screw .... but i don't know if this is relevant for you

    same pitch on a bigger screw allows more travel, since the bigger screw will buckle harder ... is this ok ?

    At least it should take more force to make the bigger screw turn by the movement of the load
    let's take a 'very' small and a ' very ' big screw with same pitch, and put them vertically :
    ... the big one will rotate and raise the mass, and rotary motion can be stopped and continued any time
    ... the little one, if stopped, may start to rotate in reverse, since mass will start to travel down

    now let's take a ' very ' big screw, and a ' 95%very ' big screw, thus both are big enough to hold the mass at ' idle ' ; which one requires more force to be turned ? the smaller requires more force, because of some trigonometric math .... google " inclined plane phisychs, or forces, or trigometry etc "

    now let's take those 2, again, and put them horizontally : again, the smaller will require more force ...

    ... in all these cases, is considered that without a mass, the screws can rotate without oposing force

    i don't know, just saying ... draw something, highlight the force that you wish to debate / kindly
    Ladyhawke - My Delirium, https://www.youtube.com/watch?v=X_bFO1SNRZg

  7. #7
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    Re: Screw diameter and strenght relation

    me again : imagine a car jack with a big screw, and another with a tiny screw : at any moment, you can lift up/down with the big one, but the small one, if you take your hand from him, may start to reverse back, because machine mass is pushing against him too hard

    big jack requires less force, and is more 'idle' resistant



    what would you do with a meat grinder with a tiny screw ? even if you go to the gym, you may not get meat out of it, because it will simply can not push ....but if you use a bigger screw inside the meat grinder, you will also be able to rotate it 'like a boss', with less efort

    was this relevant ?
    Ladyhawke - My Delirium, https://www.youtube.com/watch?v=X_bFO1SNRZg

  8. #8
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    Re: Screw diameter and strenght relation

    Quote Originally Posted by PropellerHat View Post
    Hi guys

    If a screw has a pitch of 5mm and diameter of 25, does it push the load with more force than a 16mm screw of the same pitch?
    Looking at the basic math, that doesn't include preload drag (which will vary with each screw design and isn't necessarily dependent on the diameter), or how the ballnut bearings react to the load.....then if you were using the screw to provide a static force on another object, the same motor torque, they would give you the same force.

    But...you might damage the 16mm one. You need to consider things like Euler Buckling, end fixity of the ballscrew, the length of the ballscrew, and manufacturer specifications for the maximum input torque of each ballscrew. Also the life expectancy of the ballscrew, when selecting which ballscrew to use.

    Now let's consider if the ballnut is moving while providing the force like in a CNC router or mill. In this case the ballscrew is providing a force while accelerating at the same time. The larger diameter ballscrew has a larger rotational inertia and requires more torque to accelerate it. The motor will have less left over torque to apply as a "force" against something. In this case, the smaller diameter ballscrew can provide more cutting force. If moving at a constant speed then they each would be capable of providing the same force. Once again, this is a simplification, things like preload drag on the ballscrew are not included.

    The next thing to consider is the critical speed of the ballscrew. Larger diameter screws can be rotated at faster speeds before they become unstable and start to whip. This is not an exact figure and factors like how straight the ballscrew was to start with come into play.

    Many of the parts we buy are the inexpensive Chinese variety....I haven't seen any manuals for those giving detailed specs. Typically ballscrews are selected on best practices and depend on the length of the machine, and I can't give you exact answers on what you should select. For example, I don't think it would be a great idea to use a high torque Nema 34 with a 16mm Chinese ballscrew, but that's just my opinion.

    The lead of the ballscrew is also huge in selecting what you need. A long CNC router might need a large diameter (to prevent whip), large lead (like 40mm), being turned by a servo using gear reduction. Either that or a rotating nut design.

    Also to consider in machine design are the effects of the ballscrew lead. For example, typical of a stepper driven system, where the torque falls off with RPM, at a given acceleration and top speed, the 10mm lead ballscrew may have more cutting force available than the 5mm lead of the same diameter. This is because there is more motor torque available at lower RPM, and because you only have to accelerate the ballscrew to half of the RPM to achieve the same top speed.

    Typically CNC routers driven by steppers using ballscrews of 5mm lead will have an enormous amount of cutting force available at lower RPM's, that you could never use, but with decent acceleration values, will be very limited in speed. At the top speeds the 5mm is capable of, the 10mm lead would be able to provide more "force". This is based on the ability of the stepper motor to accelerate and decelerate at the higher RPM of the stepper motor, which is the worst case scenario, and still provide "force" with the left over torque.

    For servo systems, where the rated torque is constant, a 2:1 or 3:1 belt driven gear reduction driving a 10mm lead might give you the best results.

    The short answer is that the two screws will theoretically provide the same force on a static load, but if you're using this information to design a CNC, there are many other factors that need to be considered.

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    Re: Screw diameter and strenght relation

    Thanks for profound replies.

    I am building a machine but this is not really a question for that, more "just a thought" as I stumbled upon the fact that pushing a load uphill on a steeper climb does take more effort and we basically have this scenario on different diameter screws if the pitch is the same, with the difference that one rotation on both screws of course results in same displacement. All the rest of the affecting elements (like friction and inertia) left aside.

    If the holding force is different then why not the (dynamic?)force too?

    -Propeller

  10. #10

    Re: Screw diameter and strenght relation

    Generally,
    Static load capcity for 16x5 ball screw is around 1240kgf,dynamic load capacity is around 765kgf
    For 25x5 ball screw,
    The static laod capacity is around 2300kgf ad for dynamic load capcity is around 980kgf.
    This is not the standard dates since different factory may have different design,but obviously 25x5 model can carry more larger load.

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    Re: Screw diameter and strenght relation

    Maybe I should reformulate the question...

    Imagine 2 different 5mm pitch screws, one 16mm diameter and the other 25mm, both pushing a load of let´s say 500Kgf, slowly (no effect of inertia) and let´s also assume the friction on both screw´s ball circuits and bearings is the same.

    Does the motor of the smaller diameter screw have more resistance to overcome than the motor of the big screw?

    I would guess yes, since the angle of the thread is steeper in the 16mm screw.

    Please correct me if wrong.

    Edit: Now I got it... The benefits of slacker thread angle are lost in the "longer lever" due to the bigger diameter, of course

    But what if it is a rotating nut design....?

  12. #12
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    Re: Screw diameter and strenght relation

    Hi Prop Hat - An easier way to think about it is the concept of "work". No matter what the pitch of the screw if the load moves the same distance the "work done" is the same. Rotating nut or not. Now work=force x distance so you can see that if the distance is the same the force is the same, neglecting inertial forces ie going slow. Prior posts have mentioned dynamic ratings. These do not relate to inertial loads but relate to lifetime calculations of the bearings. Static ratings relate to single event strengths, dynamic ratings relate to loads that wear the parts out over time eg fatigue. cheers Peter

    you ask about friction: small screw vs big screw - in general friction is not related to any size or area. It is however related to load and how the two surfaces interrelate. In general empirically, Friction load = friction co-efficient x load. For ballscrews we usually lump all these losses (rolling friction, contact friction, lubrication losses, misalignment) together in an efficiency factor which the manufacturer quotes. Typically ~0.9 for ballscrews or better for better quality screws. So 10% of the "work" is lost in heat due to friction, this is why things get warm in use... 90% does what we want and 10% goes up in smoke....

    So why do we have different size screws if same pitch provides same force?? ? mainly to prevent buckling which is a function of length and force. Look up Euler buckling which has been mentioned before...

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    Re: Screw diameter and strenght relation

    Thanks peteeng, this was a lot of good info. Yeah I guess if one of the screws was more efficient the less efficient one would have to get a lot hotter.

    Obviously the main benefit of a big screw is the added rigidity and I´ve heard of the 90% efficiency of ballscrews but this "ramp angle" got me tangled with it

    cheers

  14. #14
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    Re: Screw diameter and strenght relation

    Hi - Here's the equation for the force created by a screw. You will notice diameter is absent. Peter

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