Minimum timing pulley/sprocket size for a servo?


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    Default Minimum timing pulley/sprocket size for a servo?

    Hi I am trying to work out how big to make the small pulley that goes on my servo. The smaller I make it the higher the force is on the servo for any given torque.
    The Gates manual has recommendations for minimum sprocket sizes on page 8 of their GT3 manual here - http://www.gatesaustralia.com.au/~/m...nuary-2014.pdf
    My servos are:
    Nominal power: 750w / 1HP
    Normal torque - 2.39Nm
    Peak torque - 7.1Nm
    Peak power - 2250W / 3HP

    The NEMA table in the Gates GT3 pulley design guide lists a 750w motor as having no minimum sprocket size, but if I use the peak power values the minimum recommended size is 2.2"

    I cant find anything in my servo manual, they are Chinese KRS servos.

    Can anyone help?

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    Default Re: Minimum timing pulley/sprocket size for a servo?

    Quote Originally Posted by j3dprints View Post
    Hi I am trying to work out how big to make the small pulley that goes on my servo. The smaller I make it the higher the force is on the servo for any given torque.
    The Gates manual has recommendations for minimum sprocket sizes on page 8 of their GT3 manual here - http://www.gatesaustralia.com.au/~/m...nuary-2014.pdf
    My servos are:
    Nominal power: 750w / 1HP
    Normal torque - 2.39Nm
    Peak torque - 7.1Nm
    Peak power - 2250W / 3HP

    The NEMA table in the Gates GT3 pulley design guide lists a 750w motor as having no minimum sprocket size, but if I use the peak power values the minimum recommended size is 2.2"

    I cant find anything in my servo manual, they are Chinese KRS servos.

    Can anyone help?
    With a guess you would be using 5mm pitch with that size motor, so all you have to see is how small that pulley size will go and still fit your motor shaft with Key Etc, all the timing pulleys have a minimum belt bend radius, that is why they have a minimum pulley size

    It also reflects on how many teeth are engaged the smaller you go the less teeth will be engaged, so if you don't need to go to a minimum size then it has benefits, of more engagement, what are you driving with it, do you need to have it geared down, or are you using it 1:1

    Mactec54


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    Default Re: Minimum timing pulley/sprocket size for a servo?

    Our router at work has about the smallest pulley that can fit on a 2Kw servo. It's barely bigger than the shaft.
    There's no minimum size. What's important is that the overall gear ratio matches the application.

    Gerry

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    Default Re: Minimum timing pulley/sprocket size for a servo?

    Quote Originally Posted by ger21 View Post
    Our router at work has about the smallest pulley that can fit on a 2Kw servo. It's barely bigger than the shaft.
    There's no minimum size. What's important is that the overall gear ratio matches the application.
    There is a minimum size it depends on the tooth profile, all manufactures will give you what there minimum number of teeth are for any given type of timing pulley

    Here is just a quick snip of one manufacture

    Attached Thumbnails Attached Thumbnails Minimum timing pulley/sprocket size for a servo?-timing-pulley-minimum-number-teeth-jpg  
    Mactec54


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    Default Re: Minimum timing pulley/sprocket size for a servo?

    Thanks guys, I was a bit worried because to go 1:2 on my Z I'd need a 2.2" pulley then a 4.4" for the driven which seems too large. Even 1.5 ratio which I'd like for x and y gives you 3.3" which seems really big.

    They are Chinese KRS servos though not mitsubishis or anything.

    The reason I was concerned to clarify is for any given torque a smaller pulley will result in a bigger force on the shaft, and tension in the belt. I think you guys might get this, but others I tried to explain it to didn't.



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    Default Re: Minimum timing pulley/sprocket size for a servo?

    Quote Originally Posted by j3dprints View Post
    Thanks guys, I was a bit worried because to go 1:2 on my Z I'd need a 2.2" pulley then a 4.4" for the driven which seems too large. Even 1.5 ratio which I'd like for x and y gives you 3.3" which seems really big.

    They are Chinese KRS servos though not mitsubishis or anything.

    The reason I was concerned to clarify is for any given torque a smaller pulley will result in a bigger force on the shaft, and tension in the belt. I think you guys might get this, but others I tried to explain it to didn't.
    I would not worry to much about the extra shaft force unless the shafts are wimpy and not designed correctly ( size for the job they are have to do, belt tension is normal and will handle anything you can throw at it

    If the small pulley is on the motor your ratio will be 2:1 I don't think you will be using it the other way round, 1:2

    Your pulley size will depend on what Pitch you go with, you have not said what pitch you are using for these numbers

    You can use a GT 5mm pitch 15mm wide belt, if you can get a wider belt go with it, but this will work fine for your size motor, this will get your pulley diameter smaller a 24 tooth pulley and a 48 tooth will be smaller than what you are looking at

    Mactec54


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    Default Re: Minimum timing pulley/sprocket size for a servo?

    Quote Originally Posted by mactec54 View Post
    I would not worry to much about the extra shaft force unless the shafts are wimpy and not designed correctly ( size for the job they are have to do, belt tension is normal and will handle anything you can throw at it

    If the small pulley is on the motor your ratio will be 2:1 I don't think you will be using it the other way round, 1:2

    Your pulley size will depend on what Pitch you go with, you have not said what pitch you are using for these numbers

    You can use a GT 5mm pitch 15mm wide belt, if you can get a wider belt go with it, but this will work fine for your size motor, this will get your pulley diameter smaller a 24 tooth pulley and a 48 tooth will be smaller than what you are looking at
    I think I am going to use GT 5mm pitch, 9mm wide, does this seem too small? Do I base it on instantaneous or maximum HP or the rated HP? Since the 750w servos are about 1HP but can operate at 3HP for short bursts.
    From the gates table at 3200RPM a 21 tooth pulley is rated to 3.09HP for 15mm belt width.
    But at 3200RPM a 21 tooth pulley at 9mm belt width is rated to 1.67HP.



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    Default Re: Minimum timing pulley/sprocket size for a servo?

    Quote Originally Posted by j3dprints View Post
    I think I am going to use GT 5mm pitch, 9mm wide, does this seem too small? Do I base it on instantaneous or maximum HP or the rated HP? Since the 750w servos are about 1HP but can operate at 3HP for short bursts.
    From the gates table at 3200RPM a 21 tooth pulley is rated to 3.09HP for 15mm belt width.
    But at 3200RPM a 21 tooth pulley at 9mm belt width is rated to 1.67HP.
    9mm is not enough for your motor which has a max of 3hp , you are forgetting you are running at 2:1 so even 15mm belt is marginal, don't go smaller than a 24 tooth pulley 26 tooth is better if you have the room

    Mactec54


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    Default Re: Minimum timing pulley/sprocket size for a servo?

    Quote Originally Posted by mactec54 View Post
    9mm is not enough for your motor which has a max of 3hp , you are forgetting you are running at 2:1 so even 15mm belt is marginal, don't go smaller than a 24 tooth pulley 26 tooth is better if you have the room
    Thanks, I will do that then.

    One more question - I was thinking of ordering the ballscrews with machining to support a little bit more coupling room to attach a manual handle. The handle may have to be geared to, but it would allow my mill to be manual/cnc which for me would be an advantage in my small space. OK idea?



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    Default Re: Minimum timing pulley/sprocket size for a servo?

    Quote Originally Posted by j3dprints View Post
    Thanks, I will do that then.

    One more question - I was thinking of ordering the ballscrews with machining to support a little bit more coupling room to attach a manual handle. The handle may have to be geared to, but it would allow my mill to be manual/cnc which for me would be an advantage in my small space. OK idea?
    Balance of the handles, is very important when you add handles, they will shake the hell out of your machine if not balanced, regular manual milling machine handles are not good for doing this with

    Mactec54


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Minimum timing pulley/sprocket size for a servo?

Minimum timing pulley/sprocket size for a servo?