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Thread: Alternative to Ball Screw or Rack-n-Pinion

  1. #21
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    Default Chain Rack

    This is the idea I was thinking about:

    The chain is fixed straight and rigid in an extrusion. The axis is moved with a sprocket the shape of which is modified to suit the straight motion of the chain.

    Attached Thumbnails Attached Thumbnails Alternative to Ball Screw or Rack-n-Pinion-chain-rack-extrusion-jpg   Alternative to Ball Screw or Rack-n-Pinion-chain-rack-jpg   Alternative to Ball Screw or Rack-n-Pinion-chain-rack-gif  
    Bill


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    Quote Originally Posted by BillTodd View Post
    This is the idea I was thinking about:

    The chain is fixed straight and rigid in an extrusion. The axis is moved with a sprocket the shape of which is modified to suit the straight motion of the chain.

    thats a good idea... i think it would actually work but i think you can make it even better. by skipping the chain and maybe using bolts and metal rods with holes down the center and bolting them down a channel like that. theoreticallyt he same idea but the chain would move even if under tension and yea it would have to be adjusted and what not.



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    Default

    how do I delete this post? don't see any delete button, can only edit
    thankx



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    Hello folks, nice animated gif Bill. (Fellow drafter?) I am constructing a table currently and found a 100' roll of rollerchain for $100 and am also thinking about this setup instead of $300 belts. With a chain there tends to be a little play between the roller and the pin however. Not sure how big of a problem it would be. If you were to tack weld the chain to a backing strip you might not need the enclosure and it also would eliminate the chance of the chain sliding around in there. Also, a guide bearing on the bottom would be a good idea (I think) to keep the sprocket from jumping up on a quick direction change. Kyle's idea in theory might work. Metal suppliers sometimes have pre-punched 1"x1/2" channel for building railings with 1/2" holes every 4" on center. If you had some of that punched with holes the entire length, and could somehow make a drive sprocket with conical rubber cleats (instead of bolts), it just might work although the cleats might get chewed up by the edges of the holes unless you were to spend alot of time deburring/softening the holes.


    here are some links to a "negative" version of this idea:
    https://sdp-si.com/eStore/Direct.asp?GroupID=1024
    https://sdp-si.com/eStore/Direct.asp?GroupID=1025
    not too much of a savings over the regular endless timing belts

    here is a material that is very affordable per foot but might be too wimpy:
    https://sdp-si.com/eStore/Direct.asp?GroupID=769
    https://sdp-si.com/eStore/Direct.asp?GroupID=171

    dreaming the impossible dream, bamwa ( prolly going belts, so far....)

    Last edited by bamwa; 01-17-2009 at 05:11 PM. Reason: add links


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    ok this is the concept i was thinking of

    also i found a rack that the grooves are round im sure making a roller pinion wouldnt be so hard.



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    I have been looking at threads concerning the Nexen RPS units. Assuming I'm willing to pay the price. How would be the best way to use the system? It's going to need some sort of reduction drive system to use economical stepper or servo drive motors. Thats going to introduces cogged belts or gears into the mix. Now you have potential backlash in a system that's sole benifit is lack of just that. So how could I direct drive this at lower the IPM that I typically use. Seems like I'd need to find the drive system first.
    Anyone though this out?



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    Quote Originally Posted by chj.sullivan View Post
    I have been looking at threads concerning the Nexen RPS units. Assuming I'm willing to pay the price. How would be the best way to use the system? It's going to need some sort of reduction drive system to use economical stepper or servo drive motors. Thats going to introduces cogged belts or gears into the mix. Now you have potential backlash in a system that's sole benifit is lack of just that. So how could I direct drive this at lower the IPM that I typically use. Seems like I'd need to find the drive system first.
    Anyone though this out?
    The notched or timing belt drives actually work quite well. I have (3) 4:1 belt reductions on my burning table. I did use 3/4" wide belts & tentioned them only snug enough there isn't any whip in the belt. I can detect no backlash at all in the system. At least visually. Nothing shows up in the work as backlash either. Gears is another story as any standard gear must have some backlash to allow for heat expansion & lubrication.

    If it works.....Don't fix it!


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    Default resurrecting this thread

    I have searched cnczone first before posting this, so I'd like to resurrect this thread.

    Roller Chain as rack has been itching my mind for a few months now. everywhere I look, rack and pinion is expensive.

    I am heartened by the design that BillTodd put in this thread, but what are the implications of using a regular sprocket in the design? I threw this example together in sketchup to illustrate to a relative what I was trying to accomplish(not to proportion or scale):


    the idea is to lay out the chain, space the links, then CLAMP them into place, so they are immobilized indefinitely. another option instead of clamping, but involves a lot of drilling is to buy chain attachment links, which come with tabs and can be used to anchor the chain one length at a time.


    now, I know there are accuracy issues and speed issues between teeth, but I think they could be mitigated by either:
    smaller chain and more teeth on the sprocket
    arranging 2 sprockets and chains in helical offset.
    potting the chain rack in epoxy.

    I've seen video and pictures of rollerchain cnc in standard configuration, but I have yet to see this show up in real life besides nexen.

    does anyone have pictures or video of something like this design in action?



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    I salute you all for your exploring of ideas!
    But...
    Using chains for movement is way old, you're reinventing the wheel.
    Conveyor chains are heavily used in the industry and lets say a "close family member" has been quality manager for a big company providing chains since the 50's.
    First off, they don't move and they don't flex enough to need to be fixed. Bolt them in the ends and make one of the ends adjustable so the chain can be tensed. There are backlash free sprockets for chains as well. In heavier applications the chains is stacked, it looks like a bunch of chains clamped together but they are just one single chain. Check the lifter frame on a fork lift and you'll get the idea.

    So, why don't we see more of these?
    They're heavy.
    They're not cheaper than belt drives, that are easier to make backlash free (and smoother in motion).
    For example Harley Parkinson's are driven by 1"-belts, maintanence free and stronger than a chain.

    So, why aren't heavy applications done with belts?
    Because the industry is somewhat conservative.
    When reaching high loads where weight isn't critical, the chain is still cheaper.
    A belt that takes the equal load of a chain is wider and need more space in width, the chain grows in height.

    I now a guy who made a CNC-controlled circular saw driven with Motorcycle chains. Works like a charm.



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    It was over 3 years ago now that I was exploring ideas of driving a CNC. So I may be incorrect here.

    I remember exploring your exact idea of using roller chain as a "rack". As I remember it the teeth of the sprocket extend through the chain to a point the trough for the chain could not have a flat bottom. I think it may be a workable idea, but I also think it will become labor intensive to either fab up a trough or machine one from key stock.

    I opted to go with rack & pinion. I haven't regretted it at all.

    Best of luck & please keep us posted.

    If it works.....Don't fix it!


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    yeah, I'm not trying to be unique. I know people have done it on this site, and have had positive experiences with it. I haven't seen photos or videos of one. I'm not sure why more people haven't tried it or even posted experience with it.

    10 feet of 24 plus both 1 inch sprockets for x and y would run me 50 bucks from mcmaster.

    has anyone seen this from SDP? pricing seems pretty reasonable.
    https://sdp-si.com/eStore/PartDetail...83&GroupID=764
    I'd only want light applications, like plotting, paper cutting, light materials

    Mike Everman's setup looks pretty sweet, albeit out of my price range. ahh well. I do enjoy a challenge...



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    I am heartened by the design that BillTodd put in this thread, but what are the implications of using a regular sprocket in the design?
    Hi Joe,

    Apart from the tooth projecting slightly through the chain as mentioned above, the main issue is that the roller does not fully engage with the standard sprocket until it is almost in the centre of the motion, so there is considerable backlash (potentially).

    It would be easy enough to grind a little off the teeth to fix the protrusion.

    It may be possible to fix the back-lash using a second sprocket pre-loaded against the other (this should be fairly easy and inexpensive, if you are also adding a belt drive reduction to the motor)


    Bill

    Bill


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    Hi all, as this thread is "an alternative to ball screw or rack and pinion" and the current batch of posts are on rack and pinion type variations, I'm going to digress and pose the design of the wire drive as a possible solution........it has linear anti-backlash features that, given the strength in wire gauge/thickness to drive a loaded table, would seem to be the ultimate in a no backlash solution.

    However the thickness of the wire means that it has to be wrapped round a shaft or drum without deforming the wire and introducing a bending stress that would cause it to eventually break, and it has to be of a one strand construction or stretch and spring is produced.

    The wire would wind on and off of the drive shaft drum as the table was driven from end to end.

    To make the drive non slip, the wire must be attached to the drum and allow it to coil and uncoil and stack side by side without crossing the coils......all this of which has been discussed before....but it does work and is used for t/c grinder table drives where backlash is totally eliminated.

    The diam of the wire, for the tensile strength required without stretching and kinking, is important as this will determine the size of the drum where the wire will coil onto to prevent bending stresses, and this will also impact on the ability of the drum diam to create the resolution required.

    I envisage instead of the wire, a length of fine pitch chain that can roll round a drum/shaft and also bend slightly sideways to allow the chain to stack side by side, maybe in helical grooves cut on the shaft.

    A normal brand new unused bicycle chain, when coiled, will lay side by side when the coil diam gets down to 150mm, so enter the chain/wire drive without the need to have sprockets or rigid guide channels, just guide paths to stop the chain from sagging as it entered and left either side of the drum.

    A 'bike chain size pitching that has been used for a bit has quite a bit of sideways ability to deflect from the straight path, but does not display any significent end movement once tensioned, and I think this would make a drive that fulfills the need for backlash freedom and resolution needs.

    The ideal "chain" would not need to even be a regular drive chain at all, just a regular two links with one middle link and the ability to have links that give bending flexibility without springing in the linear plane when tensioned.....in other words a potential solid wire cable of approx 6mm diam that can wrap round a shaft which a solid wire of 6mm diam is impossible to achieve, but the link chain with 6mm cross sectional area can, and it can wrap round a shaft of quite a small diam as well to give not only flexibility but resolution.

    I've seen multi link chains for suspending counterweights on boring mill heads and the chains in this instance have about 8 or 10 links side by side (without the need for rollers)and roll round a pulley of about 150mm.

    It is possible that you would get a cyclic effect as the chain links wrapped round the drum, but that could be simply tested with a regular bicycle chain test set-up.....at any rate the links would have the inner faces that contact the shaft slightly curved to suit the shaft diam.

    In the hoist industry where wire rope is used, as in mining etc, the rope drums are cut with a half round thread, corresponding to the wire rope diam, to lay the rope coils side by side and keep them from overlapping and cutting the coils.
    Ian.



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    There's a whole thread about cable drives here Ian:

    http://www.cnczone.com/forums/linear...ble_drive.html

    Bill

    Bill


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    Hi Bill, yes I'm fully aware of the wire drive posts.....the method I proposed here was to offset the stretch that occurs due to having wire that has to be thin enough to wrap around a shaft without kinking and breaking due to the bending stress, but at the same time strong enough to allow tensioning to prevent stretch.

    The method I had in mind was for a chain type linkage that could be tensioned very tightly and wrapped round a small diam as it is a chain and as such not subject to a spring action or kinking on tensioning.

    Referring to my previous post I advocated a chain made with two links and one link in the middle...no roller....which makes a very strong tension device that can also wrap round a small shaft diam and at the same time deflect sideways to lay alongside the links as you would have with a cable, but without the stretch or kinking in a cable.

    I would have to say that this is by no means a new idea as I first came across it in 1950 when i obtained a box of old pocket watch parts, and amongst them was a thin chain made as I described.

    The chain was approximately .5mm X .2mm thick, and consisted of two links and a middle link riveted together so making a chain approx 50mm long.

    The chain was used in the watch to drive a fusee which had a tapered barrel with grooves that went from a small diam to a large diam in a spiral form with approx 4 or five grooves.....common practice in bigger mantlepiece clocks at the turn of the last century etc, and compensates for the weakening of the mainspring drive as the mainspring winds down.

    The fact that it is a chain makes it very strong and also very flexible so allowing it to wind round a small diam without bending and kinking as a wire would.

    The fact that a chain wrapped round and attached to a shaft does not have any backlash as would occur with a rack and pinion drive layout either with a gear and rack or chain sprocket and chain laid in a channel.

    You can not have a thick wire to wrap round a smallish shaft without it kinking and breaking from bending stress loads, unless the wire was thin and that makes it prone to stretch and breakage......for comparison a bike chain can wrap round a shaft of about 2" diam or thereabouts, and deflect to lay side by side with the coils.....and a 2" shaft gives you approx 6" of travel per revolution.
    Ian.



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    Default drill bits used as cable worm gears.

    HandleWanker, you sparked an interesting idea about the wire drives with me. instead of going smaller, how about going bigger, and inverting the process as with a chain rack? how about wire rope or cable as rack?

    bear in mind, this is still armchair problem solving and we are talking about alternatives. at 1.50 a foot, it's still pretty darn cheap to test out, even if too unconventional.

    McMaster-Carr

    1/4 inch cable has amazing rigidity when tensioned axially. you could sandwich a worm gear or a large drill bit at an angle between two of them. this is also a reduction drive as well, if you think about it. and the motion of the worm would also make it self cleaning.



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    Armchair engineering......I like that.....the horizon is boundless and allows you to explore without reservation of inhibition anything imaginable.

    The 1/4" cable drive does have some limitation...as it's a twisted bundle of wires the application of tension is enough to make it stretch like a rubber band, even if not quite as much, but in a drive situation you would have the cable attached to ends of the machine frame and provided the frame did not flex the only other lost motion is the flexing in the cable under load....the longer the cable the more "backlash" or stretching appears.

    If'n you get .02mm (.001") per 300mm (12 inches) elongation from the stretch in the cable....then you have backlash each time you reverse direction and it is accumulative with the cable length.

    You don't get stretch so much in a solid wire, but you have the problem that as a solid high tensile wire increases in thickness, to offset breaking under tension, it becomes too rigid to wrap round a smaller shaft, drum or pulley, and the larger the driving shaft diam the less resolution you have.

    You could have a number of solid wires of smaller diam laid side by side to enable them to wrap round a smaller pulley, but as the length increases the sag if not supported would cause problems.....and as it is desirable to wrap the wire round a shaft to get travel length, this then makes multi wires laying side by side a problem.

    I toyed with the notion of a chain as it is capable of flexing round small diams, but that is only an armchair solution.

    The neater rack and pinion method has already been noted to be successfull, so all roads lead to Rome eventually.

    In any form, a rack and pinion drive would have to be hardened to make it proof against pinion/rack tooth impact wear...not apparent at first but as cratering develops so backlash starts to appear.

    I have never seen a geared tooth set up using soft gears that withstood the pounding that the teeth are subject to in impacting on each other in a drive situation.

    Alternatives are fine as long as they go one step further in the path of progress as opposed to doing it the same way but differently.

    When all things are equal the simplest is always the best.

    Equal means achieving the SAME results with different methods.

    On the scale of complexity and cost, we have at the top of the ladder..the ball screw, next comes Acme screw drive followed closely by rack and pinion then cable and pulley.

    It goes without saying, one man's meat is another man's poison...and we all would like the sheer accuracy, freedom from backlash and simple fixing of the ballscrew, but not the cost.

    I wonder if'n a hydraulic cylinder to shift the table and a linear encoder to know where you were would be the ultimate in positioning and accuracy......there could be no backlash and the encoder would tell you to 100th of a mm where you were, and speed of drive is in the hydro pump capacity.

    Many years ago when I worked a Kearns horizontal borer, the table movements were all indicated with light verniers, and you can't beat a vernier for knowing your exact precise position, even with high speed positiong drives going back and forth.....you only have to read them.....and backlash is non existent....and resolution is in the vernier engraving increments.

    Supposing we had a hydraulic drive with a digital read out etc etc....someone with knowledge of encoders could come in on this and explain the workings of a linear encoder to position the table and the problems encountered in the reading at speed.

    Would you lose position if'n the encoder missed reading the linear encoded table scale, or does it actually read the position on a scale as in a calibrated vernier...IE, the position is read as opposed to counting the number of lines that flash past the reading head....miss a line and you are forward or back of position, whereas with a calibrated vernier you are at one and only one position at any time.
    Ian.

    Last edited by handlewanker; 09-28-2011 at 11:14 AM.


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    by soft gearing I'm assuming you mean the delrin gear rack I posted from SDP-SI?

    I figured that the shear loads on those gears would be the same amount as 5 start acme with acetal nut. I'd have to do the math to be sure, but from a visual standpoint, I think it would be in the envelope. my design right now is just plain chain/timing belt drive, more because I would be able to mill aluminium if I wanted to. most of my materials would be either foam, cast urethane, cardboard or pvc with the occasional mdf board thrown in for good measure.

    so.. what about using a drill bit at an angle to 'worm' along a series of rods, like kylestrong87's idea for rack? heck, what about finding a gear size that fits perforated DIN rail?



    or using a 36" drill as a leadscrew or ballscrew?

    all roads lead back to a rack and pinion or chain when it comes to simplicity. But to me, it seems that the market is cornered simply because very few people are thinking farther outside the box. everyone wants cheap and accurate and scaleable. there just needs to be that one spark that can push everyone to an inexpensive alternative that alway's existed, but nobody thought to look for.

    btw, I think BillTodd should cut his sprocket design and test it out.



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    The inescapable fact is the more exotic you go the more complicated the solution.

    With a resolution need of +- 1mm you can just use good old Allthread and use a common hex nut to back it up to cut down the backlash......it will work for a while till the middle of the thread wears and you attempt to adjust the back up nut and find it gets tighter when you move to the ends, and still has backlash in the middle.....so you go to a spring loaded back up nut and this is OK provided you don't overcome the spring pressure on reverse drive.

    Then the need escalates to 1/10mm resolution and the Allthread gets changed to Acme thread.....with back up nut etc etc.....same time you want to do a bit of Alluminium cutting and the resolution, while good for wood at .1mm is now required to go to .o2mm....the expectation is further satisfied by going to a ball screw, and so the machine, whatever has come of age and does sophisticated work with tolerances held to .02mm in every plane.

    The problem is some of us want .01mm resolution and no backlash using Althread and Delrin nuts......the pigs, having been fed, will soon be ready for flight.

    First lesson to be learned...you can't use soft metal on soft metal and have no wear.
    Second lesson to be learned......you only get what you pay for.
    Third lesson to be learned.......the law of physics is exact and absolute.
    The fourth and final lesson to be learned is...business is business and pleasure is pleasure.....the two can never meet.

    I quite support any method that can be thought of to drive a load along a plane path, but at the same time I would like to know what resolution the proposed design could provide.....eg maybe class 1------ .002mm (.0001"), Class 2-------.02mm (.001"), Class 3-------.25mm (.010") and class 4-------1mm (.040") etc etc.

    This will seperate the ball screw from the wrapped string design and make the proposer think as to what it will best be suited for.
    Ian.



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