Selecting the right ball screw Size and system


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    Default Selecting the right ball screw Size and system

    Hi there

    I am trying to build a new system, and I have a question on selecting the linear drive system. question is as follow;

    this is for a 2 meter * 4 meter wood cutting Router.
    I know if I use a Rack and pinion system because of the nature of the pinion, I gain speed but I lose power.

    what I was thinking is to use a Ball screw system rather than Rack and pinion, But In order to compensate for the speed that I lose,

    I want to use Screw with Pitch distance of a bout 2 or 3 Cm.

    Do you recommend such system? What would be the right crew Diameter for the Y axis of about 4 meter?
    And if you know the proper brand name that has such screws please let me know. Finally do you recommend the Double Nut or ball on it?

    regards
    johnansaro

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    Community Moderator ger21's Avatar
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    Default Re: Selecting the right ball screw Size and system

    Neither rack and pinion or ballscrews is more powerful. It just depends on your gearing. You're entire drive system needs to be matched together to work the best. You should take into account what motors you plan on using when you make your selection?
    Personally, I think rack and pinion is a much better choice for a 4m axis.
    For a ballscrew that long, I'd use something 32mm-50mm in diameter, with a 32mmpitch, and a fixed screw with spinning nut.

    Gerry

    UCCNC 2017 Screenset
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    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)


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    Default Re: Selecting the right ball screw Size and system

    4 meters is over 13 feet long.

    Please use rack and pinion with an appropriate spring loaded gear reduction system. For servos, a low backlash planetary gearbox or belt reduction system 4:1 to 10:1 depending on pinion size. For steppers, a belt and pulley gear reduction system 2:1 or 3:1. Use at least Nema 34 if using steppers.

    If you are stuck on using ballscrews, then consider a rotating nut where the ballscrew does not spin.

    If you persist in using ballscrews that spin, a minimum of 25mm lead, perhaps 40mm lead. I'd really need to do the math considering a number of factors like the torque vs speed graph of the motors and the weights of the parts to give you a better answer. Also if the screw diameter is too large, you will need larger motors to accelerate and decelerate such a large screw. A number of factors come into play here, with how big, how heavy, how fast you want the top speed and acceleration.

    I assume that this is because of your last build where you had problems with missing steps and rack and pinion? Use gear reduction and a properly spring loaded pinion and you will have less problems. Best idea.

    2nd Best idea. Use a rotating nut.

    3rd Best idea. minimum of 25mm lead, maximum of 40mm lead. Not sure what the shipping cost would be on a 13 foot long ballscrew though. Probably 25mm diameter, no larger. That's my best guess without doing any math and not knowing all the factors.



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    Default Re: Selecting the right ball screw Size and system

    Hi There

    Thanks so much for your most valuable reply.
    But correct me if I am wrong, that a properly sized and designed Ball and screw system is better than a Properly sized and designed Rack and pinion system.

    For the simple fact of that pinion is only partially in contact with Rack compare to Total Contact of Ball and screw.

    The only draw back that I can see for Ball screw system is the size of my Table, which makes it hard to find and install a 4 meter Long Screw.
    (keeping it straight, and avoiding it to bend, I think is the challenge)

    From what I know the only advantage of Rack and Pinion, (besides that fact that they come in Length of 2 meter) is the Speed that they can get,
    and this can be Achieved by increasing the Length of Pitch Lead in Ball and screw systems.

    Therefore at it's best scenario a R&P Spring loaded System with 3:1 gear reduction. is a equivalent of a 32 mm Lead pitch.

    Also thanks for your recommendation of the Fixed screw and Rotating Nut System(some thing that I did not know about it),
    which makes a lot of sense, since I do not need to spin the 4 meter Long Screw, and All I have to spin is the Nut,
    but the draw back is that I need to move the motor just like the R&P System.

    Can you provide any link or Info's on all the types of Fixed screw and Rotating nut Systems?

    Regards
    johnansaro

    Last edited by johnansaro; 09-10-2017 at 12:12 AM.


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    Default Re: Selecting the right ball screw Size and system

    Quote Originally Posted by johnansaro View Post
    But correct me if I am wrong, that a properly sized and designed Ball and screw system is better than a Properly sized and designed Rack and pinion system.
    At the size of machine you're talking about, I don't think so. Also, you are building a wood working machine. Not a VMC. Have a look at all of the very successful commercial wood working machines of comparable size. I'm talking about Biesse, something like that. Very high end top of the line machines. Coincidentally, those kind of machines also cut aluminum well. So find a large, high quality, top end commercial machine, of the same size you want to build that uses a ballscrew. How many are there that do that? Rack and pinion is the better choice IMO, for a machine that large.

    Quote Originally Posted by johnansaro View Post
    From what I know the only advantage of Rack and Pinion, (besides that fact that they come in Length of 2 meter) is the Speed that they can get, and this can be Compensated by increasing the Length
    I assume you mean increasing the lead of the ballscrew? You'd also have to use more powerful motors to account for the inertia of the long ballscrews.

    Quote Originally Posted by johnansaro View Post

    Therefore at it's best scenario a R&P Spring loaded System with 3:1 gear reduction. is a equivalent of a 32 mm Lead pitch.
    It is in that ball park, but it's not a best scenario, it depends on the radius of pinion that you use, and there are different sizes you can choose from.

    I am not an expert on rotating nuts, perhaps someone else can help you there, or you will have to search online. I only know that for a machine that large, the only way I would consider a ballscrew is if it had a rotating nut. But that's just my opinion. It would be more costly to do it that way compared to rack and pinion. How easy it would be, I don't know.

    Your best bet is to copy a design that is easy to build and has worked well for other people, and meets your requirements for top speed, acceleration, and depth of cut. Those are of course parameters that you must balance cost and time against performance.



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    Default Re: Selecting the right ball screw Size and system

    Hi John,
    Have you seen nexen if you want to overcome limitation on using traditional gear and rack... but maybe very expensive ...

    http://www.nexengroup.com/



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    Default Re: Selecting the right ball screw Size and system

    Hi there
    My particular thanks to NIC 77 , and asuratman for your reply.

    The nexen product that asuratman has mentioned seem very interesting and promising,
    I have to check to see that, if I can find them in my neck of the wood.

    Have any one had some experience with them? Please let me know.

    But NIC 77 , I know that possibly most of the most Professional wood cutting machines
    that have been made use R&P Systems, off course as you know that

    They mostly use Servo Motor with some kind of Gearing system, which is expensive.
    Not that I do not want to go that path, But i am just curious,
    that If one does not want to spend that much. Can one put together a Descent enough Machine
    using step motors? and knowing the limitation of step Motors in higher speed,
    I came up with Idea of using Long pitch Lead Screw.

    I just want to make it as simple as possible, Basically the system that I am thinking
    has only Step motors mounted directly on the Screw,
    OR for the case of fixed screw through a pulley system.

    I am just concerned on using that 4 meter long possibly with 25 mm or 30 mm diameter Screw,

    Has any one had such a experience? Is it physically possible to use such a
    Long screw with out being bent,/ curved, and causing some kind of malfunction on it's behavior?

    regards
    johnansaro



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    Default Re: Selecting the right ball screw Size and system

    Quote Originally Posted by johnansaro View Post
    But NIC 77 , I know that possibly most of the most Professional wood cutting machines
    that have been made use R&P Systems, off course as you know that
    Of course I know that. I'm just trying to get you to do some searching at what other people have made to be successful. Also, I'm not an expert on the subject, so perhaps you will find something I haven't.

    Quote Originally Posted by johnansaro View Post
    They mostly use Servo Motor with some kind of Gearing system, which is expensive.
    Not that I do not want to go that path, But i am just curious,
    that If one does not want to spend that much. Can one put together a Descent enough Machine
    using step motors? and knowing the limitation of step Motors in higher speed,
    I came up with Idea of using Long pitch Lead Screw.
    If you're looking at using Nema 34 steppers, yes, I believe that you can make a decent enough machine for a rack and pinion system. For a ballscrew of that length, I don't think so. For rotating nut ballscrew design, quite possibly.

    If you want really fast accelerations and top speeds of 1000 IPM, then no. But a decent enough machine, yes.

    Quote Originally Posted by johnansaro View Post
    Has any one had such a experience? Is it physically possible to use such a
    Long screw with out being bent,/ curved, and causing some kind of malfunction on it's behavior?
    Whether or not you can buy such a long screw from China without it being bent, I don't know. To buy it from somewhere else, really expensive.

    I have seen a video of someone using a long screw but they had to make a contraption to prevent whip and support the screw in the middle then move out of the way as the ballsnut passed by. It seemed very precarious to me, I'm not sure it's the best idea.

    I've got a .pdf file from THK for rolled ballscrews, it gives the ballscrew root diameter for their different rolled ballscrew sizes. I assume all rolled ballscrews will be fairly similar from different manufacturers, so I just use this file for calculations.

    If we look at the Nook Industries Critical speed calculator (which can be over conservative depending on how straight the screw is, but it is what they recommend)

    Metric Critical Speed Calculator | Nook Industries

    Using a bearing to bearing distance of 4000mm and an end fixity of "B"

    25mm ballscrew, about 22mm root diameter gives a critical speed of 166 RPM

    32mm ballscrew, about 28mm root diameter gives a critical speed of 212 RPM

    so for a 40mm lead, 212 RPM x 40mm per rev = 8480 mm / min = 333 IPM

    And a 32mm ballscrew, 4 meters long, will need a larger motor than rack and pinion or rotating nut. If you go with larger than 32mm then the cost will be much larger as well, and the size of the motor needed will go up, and the relationship isn't linear.

    Of course, the Nook calculator can be a bit over conservative, and perhaps the 4 meters was overall length and not length of travel.

    I have found some videos of fast moving ballscrew driven machines from China on YouTube, but they don't tell you if it's rotating nut, or what the lead is, or what the motor size is. It could be a 2" diameter high lead screw driven by a 1kw servo with belt reduction for all I know. Lots of $$$ to do that.

    So for a 40mm ballscrew, about 35mm root diameter, you are looking at a critical speed (according to Nook) of 264 IPM

    and for a 50mm diameter ballscrew, about 44mm root diameter, 332 RPM

    So for a 4080, 264 RPM x 80mm = 21120 mm / min = 831 IPM, but you're not going to get away with Nema 34s on that. Perhaps Nema 42 or a large servo with belt reduction, I'd have to do the math, sufficeive to say, it's a high budget option.

    For rotating nut, perhaps someone else can help you, I've never tried to design a rotating nut machine, or have even looked into it very much.

    Good luck.



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    Default Re: Selecting the right ball screw Size and system

    Keep in mind that the bigger the motor, the slower it's going to spin.
    A rack and pinion system with CNC Router Parts carriages is going to be the least expensive by far, and the best performing. Imo, there's no way to get good performance from a 4m screw and steppers. Remember, with steppers, as you increase speed, you lose resolution, because steppers have a fixed resolution, and more speed usually means spinning them slower.
    A spinning nut will work, but you'll need a good design for one, and the ability to fabricate the components.

    I use a 4m long machine with a ballscrew every day. It uses a large AC servo, and spinning nut. And it still requires a support in the middle of the screw, to prevent sagging. The support moves out of the way when the carriage passes it.
    It's a $150,000 machine.

    Gerry

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

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

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: Selecting the right ball screw Size and system

    Hi ger21 , and NIC 77

    Thanks a lot for your reply.

    Could you please provide a picture or a video link of that Moving support, that you mentioned.

    Is this moving support a must to have or is it a good optional Item?

    otherwise if I do not have it, am i going to have a sagged screw?

    reagards
    johnansaro

    Last edited by johnansaro; 09-11-2017 at 05:29 PM.


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    Default Re: Selecting the right ball screw Size and system

    Hi ger21

    Also about your machine's screw, could you tell me, what is the Length of your pitch Lead?

    and what is it's Diameter?

    and do you use any gearbox with your servo motors, and if not, and you use only a pulley System,

    is it a simple pulley (1:1 ratio) or is it a speed reduction of some sort(like 3:1 ratio)

    Regards
    Johnansaro



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    Community Moderator ger21's Avatar
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    Default Re: Selecting the right ball screw Size and system

    I won't be able to check for a few days.
    It's between 30-50mm pitch, and the screw is about 32-40mm.
    It uses a roughly 3:1 belt reduction on the spinning nut.

    The support is just a spring loaded support in the center, that gets pushed aside as the machine passes. Yes, the sscrew would sag a lot without the support.
    Also, the screw is under tension. It's installed with a dial indicator, and tightened until it moves about .03". It's a LOT of tension.

    Gerry

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

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

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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Selecting the right ball screw Size and system
Selecting the right ball screw Size and system