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IndustryArena Forum > Mechanical Engineering > Linear and Rotary Motion > Best belts for 750w servos and 1.8kw spindle? Mill retrofit
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  1. #1
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    Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    Hi guys, is there a 'go to' belt option for servos that provides minimal backlash and is not out of this world expensive? Perhaps what the 'pros' use on some VMCs?
    I have read through some of the gates catalogue and I just don't want to read through lots of catalogues (like goodyear might be next) working out each manufacturer's sizing process before I arrive at some universally agreed on best option like "just use HTD 5 (a belt type I have seen a bit of)" and "read this catalogue".
    My servos are three 750w 3000rpm and my mill is a RF-45 (I know a little big for the table).

    Also, for spindle I bought a 1.8kw 3000rpm servo, I did some sizing calcs using the gates catalogue for a 'v' belt but then realised I couldn't do rigid tapping. Should I use a timing belt for the spindle then? Any recommendations here for a quieter option?

    Thanks, any pointers appreciated!

  2. #2
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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    You can get a lot of GT2 x 6mm belts and pulleys on eBay. They are FAR more powerful than you might think.
    The older HTD belts have backlash; the GT2 ones don't.
    Yours servos sound just fine for the RF-45.
    Forget rigid tapping: you need a much more $$ machine for that. Experiment with thread milling instead. I use that from M3 too M24, very happily. And trying to manually tap an M24 thread is really really hard work!
    So most anything will do to drive the spindle.

    Cheers
    Roger
    Sydney

  3. #3
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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    Quote Originally Posted by RCaffin View Post
    You can get a lot of GT2 x 6mm belts and pulleys on eBay. They are FAR more powerful than you might think.
    The older HTD belts have backlash; the GT2 ones don't.
    Yours servos sound just fine for the RF-45.
    Forget rigid tapping: you need a much more $$ machine for that. Experiment with thread milling instead. I use that from M3 too M24, very happily. And trying to manually tap an M24 thread is really really hard work!
    So most anything will do to drive the spindle.

    Cheers
    Roger
    Sydney
    Thanks Roger, but I think I can do it, the RF-45 will be hand scraped before assembly so everything should be pretty schmick. Linux cnc controller, servos, automatic oiler, rails on z axis, etc. I won't try it immediately though.
    If I'm wrong, I guess I'll learn lol.... But even if I can't rigid tap, I believe a timing belt may still be advantageous as it would allow me to maintain constant rotation of the tools during changes - always grabbing tools at the same spot on a mill is better, I read on cnc cookbook. I plan on going to a TTS style system down the road.
    I did read GT2 had lower backlash on the net.
    But the other thing was I checked the GT2 catalogue and the HP limit for GT2 for low rpm was below my 750w per axis, hence my doubts about GT2. I read in another thread a mod saying 'go for T5 belts, backlash won't be a problem" though I'm not quite sure exactly what those are.
    Maybe though since my servos are 750w @ 3000rpm, and this is within spec for GT2, it might be ok, since at lower revs you'll draw less power. Although.... the max torque a servo can put out for short bursts is much larger - but in those cases you might break tools before belts.
    I'd like to see some bigger machines using GT2, that would reassure me.

    edit: I just realise people do rigid tapping with VFDs and encoders on the spindle, that is probably using v-belts, so maybe I can do it with a v-belt. The art is in syncing the z to the spindle.

  4. #4
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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    Oilers ... ?? .... oh, dovetails. yeah.

    GT2 has essentially zero backlash. (GT3 is the same as GT2.) Definitely better than the older designs.
    Attachment 386996
    GT2 9 mm (up from 6 mm) on Z axis. Can probe to 1 micron. Also on Y axis, replacing something like HTD. Haven't done X axis YET as it is very hard to reach.

    Drawback: I had to make the pulleys to get exactly what I wanted at a price I could afford.
    Attachment 386998
    But maybe prices have come down?

    Cheers
    Roger

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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    Very nice, what cutter did you use for the pulley profiles? I have a dividing head and another mill so I could do that, but it's not cnc so it will be time consuming I guess.

  6. #6
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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    Quote Originally Posted by j3dprints View Post
    Hi guys, is there a 'go to' belt option for servos that provides minimal backlash and is not out of this world expensive? Perhaps what the 'pros' use on some VMCs?
    I have read through some of the gates catalogue and I just don't want to read through lots of catalogues (like goodyear might be next) working out each manufacturer's sizing process before I arrive at some universally agreed on best option like "just use HTD 5 (a belt type I have seen a bit of)" and "read this catalogue".
    My servos are three 750w 3000rpm and my mill is a RF-45 (I know a little big for the table).

    Also, for spindle I bought a 1.8kw 3000rpm servo, I did some sizing calcs using the gates catalogue for a 'v' belt but then realised I couldn't do rigid tapping. Should I use a timing belt for the spindle then? Any recommendations here for a quieter option?

    Thanks, any pointers appreciated!
    For your spindle a Poly-V-Belt a J-8 will be enough for your motor, for your other axes drives GT-2-5 5mm being the pitch and 15mm wide belt would be your best choice for the 750w servo motors

    If you want to do rigid tapping and have the control that can do it, then yes you will need to use a timing pulley setup, so the belt for that would be the GT2-5 with a 25mm wide belt, you will most likely have to custom make the spindle pulleys, timing pulleys only if you are using the servo motor encoder for Rigid tapping, if you are using a spindle encoder for Rigid Tapping, then you can use the poly-v-belt
    Mactec54

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    gfdgdfgdh
    sory, I am vrong post.....

  8. #8
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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    I use the GT2 2 mm pitch, either 6 mm wide or 9 mm wide. It has plenty of strength.

    I use 1.1 mm ball end cutters or 1.2 mm ball end cutters, in multiple passes to get as close as possible to the GT2 profile. I have made both aluminium and plastic pulleys, and both work just fine. Yes, plastic (nylon and acetal) work fine because the load is distributed.

    That said, a caution is needed over the GT2 profile. It was invented by Gates and is/was covered by patents, and they don't hand out the specs for the profile (although you can get the patent). There are tons of Chinese 'GT2' belts and pulleys available, but they do NOT match the Gates design. The Chinese profiles are different. Well, that could be as you might expect ... and I think the GT2 patent has expired now.

    However, while there are lots of Chinese bits available, they seem to be aimed solely at the 3D printer market, so the range is quite limited. I could not get what I wanted from the Chinese sources so I bought the belts from SDP in USA, used micro-photography to get a fairly good copy of the profile, and went from there. Of course there are deviations from whatever Gates sell, but they are down in the 10 micron range. I figured that was close enough.

    I wrote my own programs for this, including the taper-lock fittings behind the brass nuts. It may be that the latest version of the Gearotic SW can also make the profile. Very clever guy, Art F.

    Cheers
    Roger

  9. #9
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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    Quote Originally Posted by RCaffin View Post
    I use the GT2 2 mm pitch, either 6 mm wide or 9 mm wide. It has plenty of strength.

    I use 1.1 mm ball end cutters or 1.2 mm ball end cutters, in multiple passes to get as close as possible to the GT2 profile. I have made both aluminium and plastic pulleys, and both work just fine. Yes, plastic (nylon and acetal) work fine because the load is distributed.

    That said, a caution is needed over the GT2 profile. It was invented by Gates and is/was covered by patents, and they don't hand out the specs for the profile (although you can get the patent). There are tons of Chinese 'GT2' belts and pulleys available, but they do NOT match the Gates design. The Chinese profiles are different. Well, that could be as you might expect ... and I think the GT2 patent has expired now.

    However, while there are lots of Chinese bits available, they seem to be aimed solely at the 3D printer market, so the range is quite limited. I could not get what I wanted from the Chinese sources so I bought the belts from SDP in USA, used micro-photography to get a fairly good copy of the profile, and went from there. Of course there are deviations from whatever Gates sell, but they are down in the 10 micron range. I figured that was close enough.

    I wrote my own programs for this, including the taper-lock fittings behind the brass nuts. It may be that the latest version of the Gearotic SW can also make the profile. Very clever guy, Art F.

    Cheers
    Roger
    The GT2 patent did expire, so they protected the same patent by calling it a GT3 which you will see in there listings the GT3 is the same as the GT2 for anyone else that needs to know

    The belts you used are not suitable for his use, the 750w servo motor would tear those belts apart
    Mactec54

  10. #10
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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    Thanks fellas, now the million dollar question - what is the GT2 profile I need to cut? Anyone got an image or CAD file?

    I found the BELT profile but I'm guessing the pulley profile is slightly different to accomodate for stretch / other parameters, and also varies slightly with diameter?

    Thanks a lot... I'll keep searching for the GT2 pulley profile but I've turned up nothing yet.

  11. #11
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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    The belts you used are not suitable for his use, the 750w servo motor would tear those belts apart

    Not so sure about that, for several reasons.
    First off, the GT2 belt design has a power rating double that of the HTD. It's up in the kW range. The fibres are a bit stronger. The design is a LOT more advanced. They replace chain drves with these belts.

    Second, I doubt that the 750 W servos would ever have to deliver that power. I run slightly smaller servo motors through a 3:1 reduction by GT2 2 mm x 9 mm wide, and I can't SEE the current in the ammeters on the drives. OK, that's on rails rather than dovetails, but even so. I would be very surprised if the XY servos drew as much as 1/4 A: that's ~13 W under normal conditions.

    To be sure, OP could go to GT2 5 mm pitch for greater width. Or to GT2 8 mm pitch - up in the many kW range there! Not needed.

    I'm guessing the pulley profile is slightly different to accomodate for stretch / other parameters, and also varies slightly with diameter?
    The belts do not stretch - not enough to worry you anyhow. That's part of the deign spec.
    The profile of the grooves themselves does not change with pulley diameter. After all, every pulley of whatever diameter must be able to handle the same GT2 teeth. Can't do that unless the profile of the groove is fixed. Now, the curve between the grooves - yes, that is proportional to pulley diameter - or more accurately, to the pulley pitch circle diameter, which is not quite the same thing.

    Images and CAD files - PM me.

    Cheers
    Roger

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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    Any suggestions as to how to mount one to the servo? The 750w servos have a 20mm bore, and a ~6mm (5.90 measured) width keyway. Total "diameter" of keyway + servo shaft is ~21.9mm.
    I did some searching for 20mm bore GT3 pulleys on sdp-si and all they went up to was about 14mm bore.

    Hmm, these HTD look good, maybe I could buy a few and once my cnc is up and running make some GT2... though I'd prefer off the shelf, done right first time - https://www.aliexpress.com/item/LUPU...819607822.html

  13. #13
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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    Quote Originally Posted by j3dprints View Post
    Thanks fellas, now the million dollar question - what is the GT2 profile I need to cut? Anyone got an image or CAD file?

    I found the BELT profile but I'm guessing the pulley profile is slightly different to accomodate for stretch / other parameters, and also varies slightly with diameter?

    Thanks a lot... I'll keep searching for the GT2 pulley profile but I've turned up nothing yet.
    That would be what everyone wants to know, how to cut the correct profile, the best place to find what you need to make your timing pulleys is from Gearotic , they have the profiles to make your pulleys

    "Gearotic Motion Gear design Software"
    Mactec54

  14. #14
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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    Quote Originally Posted by RCaffin View Post
    To be sure, OP could go to GT2 5 mm pitch for greater width. Or to GT2 8 mm pitch - up in the many kW range there! Not needed.
    If you read my post that is what I posted, that he would need to use GT2-5mm and a 16mm wide belt

    Just remember the information I sent to you on the pulley profile was not to be shared, it just shows you who you can trust, it was a good thing I did not give you any of the math behind it as to how the profile is generated

    You are completely wrong in what you are saying about the profile not changing, it's way more complicated than you think, look at gear cutting and this profile follows the same practice
    Mactec54

  15. #15
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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    I did some searching for 20mm bore GT3 pulleys on sdp-si and all they went up to was about 14mm bore.
    EASY.
    Use the existing bore, whatever size, to centre the pulley (lathe or mill), then bore it out carefully. Not a problem.

    As for mounting on the shaft - when it gets that big you could easily consider a taperlock fitting. That too would need to be MYOG. Not hard, works well.

    Cheers
    Roger

  16. #16
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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    Hi Mactec

    In the end I did not use the information you sent me confidentially. I used micro-photography instead, off Chinese belts and pulleys and off a genuine Gates belt. The attached jpg shows how I did it.
    Attachment 387054
    Yeah, I had to read the Gates patent several times to get some idea of what to do. They did try hard to be obscure.

    To explain the jpg: microphotography of the profile, then import jpg into Autosketch, then start fitting a complex curve to the shapes. If any of them come out similar to what you sent me, fair enough. After all, they should be similar if we are talking about the same profile. And yes, it took quite a while to get a good match.

    Cheers
    Roger

    EDIT: some companies will actually provide you with 'engineering' drawings of the pulleys. You have to look rather closely at them: some of those drawings actually have the tooth profiles in some quite accurate detail. Yeah, arc radii, arc centres, etc. Very helpful. I also used them.

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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    Thanks guys,

    I was just running some preliminary numbers for sprocket sizes.
    I looked at the GT2 / GT3 manual and it has a NEMA table that recommends for 1hp (~750w) motors a ~2" sprocket diameter is recommended and 2.2" for 3hp at ~3000rpm. So I am thinking 2.2" sprocket minimum to account for occasional servo 'over torques' though it may not be necessary.
    Then a 1:1.5 ratio would get me max ~400ipm for a 5mm pitch ballscrew, with a resolution of ~1.33 micron (lol?).

    Does this seem reasonable? At SDP-SI the 36 56 tooth pulleys are $17.75 and $26.96.
    Hub diameter is 38.1mm, and my ID would be 21mm. I'll have to pull out an old uni textbook to do my torque calculations but I'm guessing it will be fine.
    Then I just need a 6mm broach to make a keyway, I might practice on a blank first, I've never done that.

    Just found simpson36s video who has probably the best RF-45 conversion I've seen, I wonder what belts these are:
    https://www.youtube.com/watch?v=20l-HVgJtcc

  18. #18
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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    Sounds fine to me. Give it a try.

    Cheers
    Roger

  19. #19
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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    [QUOTE=j3dprints;2152770]Thanks guys,

    I was just running some preliminary numbers for sprocket sizes.
    I looked at the GT2 / GT3 manual and it has a NEMA table that recommends for 1hp (~750w) motors a ~2" sprocket diameter is recommended and 2.2" for 3hp at ~3000rpm. So I am thinking 2.2" sprocket minimum to account for occasional servo 'over torques' though it may not be necessary.
    Then a 1:1.5 ratio would get me max ~400ipm for a 5mm pitch ballscrew, with a resolution of ~1.33 micron (lol?).

    Does this seem reasonable? At SDP-SI the 36 56 tooth pulleys are $17.75 and $26.96.
    Hub diameter is 38.1mm, and my ID would be 21mm. I'll have to pull out an old uni textbook to do my torque calculations but I'm guessing it will be fine.
    Then I just need a 6mm broach to make a keyway, I might practice on a blank first, I've never done that.


    Yes you are on the right track, you will probably never run your machine over 200IPM so you have plenty to spear, those belts look like the same as I suggested 16mm, you want your timing pulleys to have the Flanges Everyone that does these machines use this size belt

    You sure you need a 6mm broach, most 750w have 5mm keys
    Mactec54

  20. #20
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    Re: Best belts for 750w servos and 1.8kw spindle? Mill retrofit

    Quote Originally Posted by RCaffin View Post
    Hi Mactec

    In the end I did not use the information you sent me confidentially. I used micro-photography instead, off Chinese belts and pulleys and off a genuine Gates belt. The attached jpg shows how I did it.
    Attachment 387054
    Yeah, I had to read the Gates patent several times to get some idea of what to do. They did try hard to be obscure.

    To explain the jpg: microphotography of the profile, then import jpg into Autosketch, then start fitting a complex curve to the shapes. If any of them come out similar to what you sent me, fair enough. After all, they should be similar if we are talking about the same profile. And yes, it took quite a while to get a good match.

    Cheers
    Roger

    EDIT: some companies will actually provide you with 'engineering' drawings of the pulleys. You have to look rather closely at them: some of those drawings actually have the tooth profiles in some quite accurate detail. Yeah, arc radii, arc centres, etc. Very helpful. I also used them.
    The method you used gets you close enough to do what you wanted to do, not perfect by any means

    Information has always been available for most other profiles, there are only a few that are protected, Goodyear has one also, which I used to make some of, I have often thought of using the GT2 profile and creating the Goodyear Design together the two would be an incredible drive system
    Mactec54

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