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IndustryArena Forum > CNC Electronics > Stepper Motors / Drives > Replacing steppers for more torque options / Advice wanted
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  1. #1

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    Replacing steppers for more torque options / Advice wanted

    Hello I’m looking to upgrade my machine to get more power onto my x and y axis.

    I already did a post about some 10A drivers, but did some more research on the steppers/servo/drivers and have some more questions.

    At the moment i have an upgraded avid pro cnc with dual Z (7kw and 2.2kw spindle).
    I mostly do cabinet work and when hitting 500ipm when cutting i feel like i’m running on the upper end of the stepper motors (x/y)ability.

    I would like to upgrade the original stepper motors and did some research of different possibilities.

    At the moment i have:
    CRP8070 (leadshine DM870) drivers
    960 OZ low inductance 2mh steppers running on a 48V power supply (wich is perfectly dimensioned for these steppers)
    The belt reduction is 3.2/1
    This translates to following numbers:
    Ipm and motor RPM are about equal / this is the most used power/rpm band.

    60 = 5NM torque
    120 = 4,6
    240 = 4
    360 = 3,6
    480 = 3
    600 = 2,4
    720 = 2
    840 = 1.6

    At the moment i see 3 possible options:

    1 ) Stepper motors with higher torque
    -Pro & cons:
    If i go to 10 Amps models i need new drivers and a new power supply
    I can gain some torque +20%
    cheapest option, but not much less expensive than option 2 (easy servo steppers)
    due to needing also a new psu unit.

    2 ) Leadshine closed loop stepper Easy Servo High Voltage 3 phase
    -easy servo motor es mh33480
    -driver es-dh2306

    -I do know that i do not need to expect too much form the closed loop system.
    At best they will stop when there is an error
    -The drivers are 230V and do not need an separate power supply
    -The system has a good torque curve and can be fitted directly onto the belt reduction system i have
    -They have 300 steps (1.2°) instead of the normal 200 steps (1.8°)

    Things i don't know any thing about:
    -3 phase does instead of 2 phase
    -They do have a high impedance, and i don’t know if this is a big problem in this situation vs the 2 phase low impedance
    -This would double the amount of torque over the usable range 0 to 1000rpm . Only doubt i have is would my machine handle this well?

    3 ) Servo System
    -Are better than steppers
    -Are more expensive
    -Need an 110V power supply (230V standart supply here)

    The main thing with the servo’s is i will need to replace the 3.2/1 belt reduction with a 1/10 gearbox to make it work.
    Personally i’m happy with the belt drive and the accuracy of my system.
    I’m a bit afraid of a gearbox setup due to price and maintenance. They wear out eventually and backlash is worse than a belt system?
    Also there is less cushioning in the system, and with my light machine i think it’s best to keep it this way.

    So at the moment i’m thinking to go for the closed loop high voltage system. Any thought on this system or other views are welcome.

    There is some documentation attached for the system in use (avid) and the leadshine easy servo steppers

    Keep safe & a happy year to come!
    Adam

  2. #2
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    Re: Replacing steppers for more torque options / Advice wanted

    I would say the biggest issue affecting torque would be the low voltage you are using. Your drives are capable of 72 volts so you are leaving 33% on the table. Doubling voltage more than quadruples torque.
    Option 4.
    Get a 72 volt toroidal power supply.
    I get 1200 ipm rapids out of 7 amp NEMA 34's running at 6 amps, 5:1 reduction, 30 mm dia pinion

    Closed loop won't help
    you can get 240v servos. eg Estun EDC servo series

    Steppers have high torque down low. The challenge is to design a system and choose a drive that allows you to harness that.
    Everybody focuses on low inductance, the more important parameter in our modelling is rotational inertia. But too big a motor gets too heavy so a lot of torque is used just turning the motors over. Also, steppers seem to loose steps when they overheat and only really need high torque when accelerating (cutting forces excluded). Running at lower amps when idle or at constant velocity is an advanced method to keep the internal motor temp cool thus avoiding lost steps... But heck, you have the drivers so get the most out of them. I upgraded from 48v and it makes a world of difference.
    Rod Webster
    www.vehiclemods.net.au

  3. #3

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    Re: Replacing steppers for more torque options / Advice wanted

    Rod,


    do you have an avidcnc machine?

    is i calculate the max voltage for the steppers i just get 48V , so i suppose the steppers i have don't handle 70V?
    32 Rootsquare 2mh = +- 48 V max?
    I'm afraid to blow up the stepper i have at the moment with a bigger PSU.

    The closed loops just have a good torque curve and i don't need to adjust the drive belt sy
    stem to a geared one.


    BR,

    Adam

  4. #4
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    Re: Replacing steppers for more torque options / Advice wanted

    I'm running a cheap no name NEMA23 at 90 volts and it has not been an issue.
    You are not running a H bridge driver so that calculation is meaningless.
    https://www.machinedesign.com/motors...tors-explained
    Rod Webster
    www.vehiclemods.net.au

  5. #5
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    Re: Replacing steppers for more torque options / Advice wanted

    Hi,
    I suspect that if you go for higher torque steppers you will be no better off. Higher torque almost always occassions higher inductance and so all that
    extra torque disappears as the speed goes up.

    I go with rodw here, up the voltage to as high as you can get. If thats enough to satisfy your requirements all well and good....if not.....

    Manufacturers of closed loop steppers would have you believe they behave like servos......pure BS. They are still steppers and like all steppers lose torque the faster they go,
    being closed loop doesn't change that at all. Then they will tell you the drive will insert extra steps.....while that is true it still doesn't help.

    The only time a stepper misses a step is when it is overloaded at the particular speed its running at. So the drive inserts an extra pulse to keep up, but guess what, the
    stepper is (marginally) overloaded and so the extra pulse is just as likely to be missed as a regular pulse.

    There are some advantages of closed loop which cannot be denied, resolution for one and also the ability for the drive to fault out if it detects that it is lagging too far behind
    its commanded position. Additionally 3 phase steppers perform better at high speeds than two phase....but not enough to justify the cost. Note you can get 5 phase steppers as well.
    I have 5 phase Vexta steppers on my mini-mill and the Vexta drivers are 150VDC (derived directly from 230VAC) and I spin those at 2400 rpm and have not lost a step in years,
    except when I do something stupid.

    If you want genuine closed loop performance and PRODIGIUOS overload capacity get AC servos, they are light years ahead of any stepper ever made!

    I have bought three 750W Delta B2 series (160,000 cpr encoder) servos/drive/cables and they run at 3000rpm at rated torque of 2.4Nm and have an overload of 7.2Nm.....
    they are just brilliant!

    I'd recommend Delta (Taiwanese made in China) or DMM (Canadian made in China) as two high performing servos with good backup, set up software at fair prices.
    Clearpath are popular also but are expensive, a 350W Clearpath cost a few dollars more than a 750W Delta!

    There are even cheaper Chinese servos, ToAuto for example, and they are very well priced but the documentation is rubbish and backup is questionable. Additionally
    they have no setup software so you have to program them by pushing buttons on the drive like a microwave......avoid like the plague.

    If you have the budget, modern AC servos are the way to go......they'll outperform any stepper and will almost certainly outperform your expectations too, my
    Delta's have blown me away.

    Craig

  6. #6
    Community Moderator ger21's Avatar
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    Re: Replacing steppers for more torque options / Advice wanted

    32 Rootsquare 2mh = +- 48 V max?
    That's a formula that Gecko came up with, and is not a MAX voltage rating.
    As Rod said, going to 72V *should* get you 50% more speed.

    Fwiw, I'm running Nema 23's on 65V and they barely get warm.
    Gerry

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  7. #7
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    Re: Replacing steppers for more torque options / Advice wanted

    Servos:
    Why would you need to change the belt reduction to gear reduction?

    There are many more AC servos driven by 220v/230v than 110v
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)

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    Re: Replacing steppers for more torque options / Advice wanted

    Hi,
    sorry I didn't re-read OPs post.......my servo drives are 230VAC input......no need for a separate supply.

    Why would you want a 10:1 gearbox when your existing belt reduction is 3.2:1? You could just replace the existing stepper with a servo and as far as
    the machine is concerned its no different.


    I'm guessing that you are reading servo specs and see, say, rated torque of a 400W servo as 1.27Nm, and that seems way less than the 5Nm that your existing stepper has?
    Firstly the servo has three to four times rated torque as overload, so it will in fact have nearly the same torque as your stepper when needed. The real difference is that the servo
    will still be producing 1.27Nm (3.9Nm overload) right up to 3000rpm.

    Personal experience says that a 400W servo will beat the pants off your stepper any day of the week. I very much doubt you'll need a gearbox at all.

    My existing mini=mill has 5pahse Vexta steppers running through low lash (<2 arc.min) 10:1 planetary gearboxes. They were second hand when I got them six
    years ago and I've used them heaps since and never so much a touched them, and they are swilling in coolant, dust and debris all the time. Good gearboxes
    have lash smaller than I cam measure and yes they are expensive but I doubt you'll ever wear them out even if you do need them.

    My 750W servos are direct coupled to 5mm pitch 32mm diameter ballscrews, and it accelerates a 115kg cast iron axis bed at 0.25g to 15m/min (250mm/sec) in100ms!!!!
    Using field weakening mode, a clever technique to lower the back EMF to allow the servo to go faster than rated speed, I have had my servo doing 5000rpm for axis speeds
    of 25m/min. The servo remains dead quiet, stays perfectly cool and throws a great big lump of cast iron around with ease. Funnily enough if anything gets warm its the ballscrew.
    I'm guessing that the preload of the ballscrew must result in energy transfer, and when they are spinning at 5000rpm there must be quite a bit! After several minutes going back and forth
    at 5000rpm (equivalent to 25m/min) the screw is slightly warm to touch. I've been told about and read about ballscrew heating but it the first time any machine of mine has anything like the
    speed to exhibit ballscrew heating. That speed and brute power comes from using servos, steppers wouldn't come close.

    Craig

  9. #9

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    Re: Replacing steppers for more torque options / Advice wanted

    Thanks all for the input!

    Will look into the 72V PSU, why toroidal?

    While i'm learning something, a couple more questions:

    The reason i've added the closed loop steppers was not for having a closed loop, but because their torque curve was very appealing!
    They produce more torque than any other stepper system with the same dimensions.

    If running servo's on a 3.2/1 reduction, i'm just able to use just a part 0/1000 rpm of what the motor i capable off? Most of the time it will run at 600 rpm or less. This is no problem for the servo's?
    Therefore i thought it was needed to get a bigger reduction.

    If choosing servo's do i need to take the braked or unbraked versions for my application. I see delta has both versions.

    The servo route is appealing but i guess it will be overkill for my machine pricewise... I've seen some delta servo's and drivers on alieexpress but am hesitant for counterfeind products there. Als support would be zip.

    Adam

  10. #10
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    Re: Replacing steppers for more torque options / Advice wanted

    Toroids have a lot of advantages for steppers which don't require regulated power. Cost is a big one at higher voltages. Something like this might suit you (without knowing your motor specs.)
    https://www.antekinc.com/ps-10n70-10...-power-supply/

    With some drivers, like the Lams I am using now, they accept AC voltage so it eliminates the need for the regulator stage so that reduces the failure points. In fact, Lam have a 110V driver so in the US, no toroid would be required.

    When I upgraded my machine I used two 32 volt off the shelf transformers to give 64 volts AC @ 20 amps. See pic. There is a bank of 8 fuses there as per the Lam recommendations and they go straight to the drives.

    AC converts to DC at a factor of around 1.4 so the motors get about 90 volts.

    here is a bit of random background
    https://www.steppermotorcanada.ca/st...canada_013.htm

    Re servos, it takes a bit to get your head around a servo torque curve after using steppers. A said you get constant torque right through to about 3000 rpm or so then it falls to nothing. So the guys are probably right when they say reductions are not required. This does make designing with servos much easier. We found with steppers, pinion diameter, pinion reduction ratio and motor inertia were the key design parameters. Inductance not so much but the ratio between resistance and inductance had some impact.
    Rod Webster
    www.vehiclemods.net.au

  11. #11
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    Re: Replacing steppers for more torque options / Advice wanted

    Hi,

    If running servo's on a 3.2/1 reduction, i'm just able to use just a part 0/1000 rpm of what the motor i capable off? Most of the time it will run at 600 rpm or less. This is no problem for the servo's?
    Therefore i thought it was needed to get a bigger reduction.
    You could just run the servos to 1000rpm, no problems. You could use a 10:1 reduction and get even more thrust and torque, but as you've already pointed out such gearboxes, good ones
    anyway, are not cheap.

    The servo route is appealing but i guess it will be overkill for my machine pricewise...
    There is no doubt that good servos are still more expensive. The price gap between steppers and servos has narrowed but they are still expensive.

    I've seen some delta servo's and drivers on alieexpress but am hesitant for counterfeind products there. Als support would be zip.
    I bought from:

    https://www.fasttobuy.com/

    and they proved perfectly reliable. Remember Delta is a Taiwanese brand but the servos are actually made in China, so Chinese suppliers like FastToBuy get them direct from
    Deltas warehouse. I got two unbraked 750W servos for X any Y axes but anted up a bit more for a braked unit for the Z axis. Unless the Z axis is really heavy you probably
    don't need a braked servo. I plan to build another (third) spindle using a big servo motor and that would be heavy enough that a braked servo is advisable, so I got one.

    I have been told that Delta's backup is good, but in truth I've never had any need for it, so I cannot personally attest to it.

    With the nonsense US is putting itself through I'm not sure that I would rely on anything out of that cesspit, I rather suspect the Chinese are a better bet, they
    certainly have treated me very well.

    As for toroidal power supplies they are just so much more robust and forgiving than a switch mode supply. A switch mode supply will go straight into shut-down, if it
    doesn't blow up, on overload whereas a toroidal transformer power supply will 'grunt down' a bit but still deliver the current.

    Craig

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    Re: Replacing steppers for more torque options / Advice wanted

    Hi rodw,

    A said you get constant torque right through to about 3000 rpm or so then it falls to nothing
    .
    That's not quite the best description of what actually happens.

    The low inertia model Delta servos which I have are broadly very similar to other low inertia model from different manufacturers are rated to 3000 rpm.

    That is to say 'rated torque (2.4Nm, in the case of a 750W servo) is maintained up to 3000rpm'. You can set a parameter in the drive that allows the servo to go to
    its maximum rpm, in the case of my Deltas that is 5000 rpm. Presumably 5000 rpm is the highest safe speed. Between rated speed (3000rpm) and 5000 rpm
    the torque decreases linearly so that at 5000 rpm the power output is still 750W but the torque is reduced:

    power= torque x speed so:
    torque =power/speed where power =750W and speed is 5000/60 x 2 x pi =523.6 radian/sec
    torque= 750 / 523.6
    =1.43Nm

    So in Field Weakening mode, the wrinkle that allows you to cheat the servo into going faster the Back EMF really allows, occurs with a reduction in torque but it doesn't crap
    out altogether either.

    As an example of how this would work in practice I will program may machine for 25m/min G0's, ie taking full advantage of Field Weakening to run the servo at 5000rpm.
    Given that there is no cutting thrust required for a G0 move the somewhat reduced torque is no problem. G1, or cutting moves, will be restricted to rated speed, that is 3000rpm
    or 15m/min where full rated torque and therefore thrust is available to counter cutting forces.

    You may have seen advertising material for industrial machines, I call it CNC porn, where the rapids are 36m/min and yet max cutting speed is 25m/min say. This is an example
    of why the two specifications differ, the manufacturer is making use of Field Weakening.

    As good as servos are....and modern servos are great, they are still a big investment, so will depend on OPs budget, or maybe what his wife will let him away with.

    Craig

  13. #13

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    Re: Replacing steppers for more torque options / Advice wanted

    Thanks for the added info! Will look into it and do my homework.

    I would only upgrade x and Y axis (3 motors) The Z axis are sufficient.
    The motor that comes free i would use for a home build 4th axis.

    Adam

  14. #14

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    Re: Replacing steppers for more torque options / Advice wanted

    I see that delta has A2 and B2 type servo's big difference?

  15. #15
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    Re: Replacing steppers for more torque options / Advice wanted

    Hi,

    I see that delta has A2 and B2 type servo's big difference?
    No, not a huge difference. The B2 series has a 17bit incremental encoder, actually 160,000 count per rev, and is in effect Deltas entry level model.
    The A2 series is a later design, a few, maybe as much as $50 more than a B2 servo of the same power. The A2 series have 20 bit incremental encoders, actually
    1,280,000 count per rev. One advantage of the A2 series is that it has a 'load sensing encoder input channel'. Like any servo it has a built-in rotary encoder, and it is required
    for the servo control to work, but the A2 can have a second position sensing encoder, say a linear glass scale, and the position loop closes on that encoder.
    Thus you have dual loop, and could be extremely useful for a machine with backlash for instance.

    Its common in the semiconductor manufacturing industry to use laser interferometers for sub-micron precision motion. The A2 series servos are perfect for that style of thing.
    I did not realize that the A2 had that feature, otherwise might have paid a bit more and got some. Having said that I have a rigid machine with zero lash ballscrews and I don't really need
    A2 'goodness', in fact I'll not really be pushing the envelope with the B2 series servos I have got either!

    The A3 series is later again and has 24bit multi turn absolute encoders. I don't know whether it has load sensing also, but I suspect it does. The great thing about a multi turn encoder
    is that it can count a certain number of complete revolutions plus interpolate to 16 bit precision within the current revolution. I understand also that the encoder has a battery
    (maybe supercap??) backup so that when you power up the machine it knows exactly , (number of complete turns and 16 bit within this turn) without having to home
    the machine.

    The latest model is the B3 series, and just what goodies its got going on inside I don't know yet, I think it has the same 24 bit multiturn encoder, but more than that I don't know.

    If you really want some 'servo porn' check out Yaskawa. Way too expensive for me, and I'd never fully exploit all that its got....but nice to idle away a wet afternoon.

    Craig

  16. #16

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    Re: Replacing steppers for more torque options / Advice wanted

    I’ve been taking in some more info about the subject.
    First thing i’m gonna do is fitting a bigger psu and see what this brings.
    I received the curves for 72V and these look promissing. I hope the higher voltage doesn’t bring more resonance in my (already light) system.

    I’ve decided that for my machine the servo drives are overkill pricewise.
    In the near future i would like to add closed loop steppers for the fact that they stop the machine if i screw up big time.
    It would be an added safety i would like to have.
    Another hopefully benefit will be less resonance in the system due how closed loop steppers work?

    When researching on howto dimension the steppers for cnc machines i found a lot of useful info on the forum.

    Here is the info i’ve gathered so far:
    My machine:
    125kg on the Y gantry singel motor (which takes the biggest forces)
    220 kg on the x axis double motors
    R&p Belt drive 3.2/1. (20T/64T gearing) and 1” (25,4mm) 20T pulley
    -Cutting forces set to 320NM (found some info on the forum again) I would like to cut 1 pass mdf 17mm and do a second 1mm onion skinn pass. This is with a 1/2” PCD compression bit. For now i settle with 1 extra pass (first cut 12mm DOC).

    I’ve also found a calculation spreadsheat on the forum, see link below for the interested. Many thanks to Toby (thegolfer) for posting this!
    https://www.cnczone.com/forums/linea...culations.html
    Attached are my calculations for the steppers, and they are in line with what i have now for motors (X asis sufficient, Y axis is pushing it with 3NM instead of the 3.42NM calculated)
    If my calculations are right i need a torque of 3.42NM at 500rpm=500ipm. For the Y axis. I would use the same motors on the X axis, so these would be more than sufficient.

    Some questions i have at the moment is:

    1. Would the cutting forces at 320NM be a good guesstimate (3/4 mdf or plywood with 1/2” compression bit one pass at 500ipm)
    2. Looking at motor’s torque curve some are in full steps other in half steps. On my machine there are 1/10 microsteps configured on my drivers.
    do i need to take in account the microstepping, do i loose some torque with microstepping? in the spread there is already a factor 2 added for safety.
    3. Rotor inertia, these differ largely form brand to brand,(between 2700 to 4500 g/cm2). Larger will accelerate less and stop better? With my light machine the biggest issue i have is vibration at high speed short movements. Any motor better to reduce this or would this not make any difference.
    4. inertia mismatch, iv’e read alot about but because i’m matchematically inclined i can’t figure out howto calculate my machine load inertia.

    Again all pointers are welcome!

    Keep Safe,

    Adam

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    Re: Replacing steppers for more torque options / Advice wanted

    anybody that can help me out?

  18. #18
    Community Moderator ger21's Avatar
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    Re: Replacing steppers for more torque options / Advice wanted

    1) What is 320NM? NM is a torque rating. If it's 320 Newtons, that's about 72 lbs, which is higher than you'll likely ever see.

    2) It really depends on the drives, and without a torque curve chart using the exact same settings, it's impossible to know.
    Some drives (Geckos) switch to full step mode at higher speeds. Other may as well, but don't advertise it. If you're already using a 2x safety factor, you should be fine using the half step values.

    3) If it takes longer to accelerate, it'll take longer to decelerate. I'm no expert, but I don't think rotor inertia is going to make a lot of difference.

    With my light machine the biggest issue i have is vibration at high speed short movements
    This is more likely a machine rigidity issue than a motor issue, due to the violent nature of linear acceleration.
    Gerry

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  19. #19

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    Re: Replacing steppers for more torque options / Advice wanted

    Gerry,

    Thanks for the reply,

    yes it's 320 N, was a typo.

    I'm sure vibration is a machine rigidity issue, the short movements make it vibrate. I don't have motor issues at the moment. But looking into the different specs is saw that rotor inertia differs largely from one to another brand.

    If someone can tell me more about rotor inertia and load inertia please do.

    Adam

  20. #20
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    Re: Replacing steppers for more torque options / Advice wanted

    Hi,
    I posted this a while back, it details a calculation of angular momentum of my new build mill:

    Hi,
    I've done the calclations for the acceleration potential of my machine including rotational inertia. I've had my calculation checked
    by peteeng, a professional mechanical engineer. Additionally I have Hiwins calculation formulas and can double check all the calculations
    and they all point to the same conclusion.

    My previous calculations are widly optimistic.

    The full calculation is:
    Torque= Ieff. dw/dt

    dw/dt is the angular acceleration.
    Ieff is the total effective first moment of inertia.

    Ieff is the sum of the individual components.

    1)The rotational inertia of the armature of the servo. For my 750W 34size Delta servo that is (from the spec sheet)=1.13 .10-4 kg.m2
    2)The rotational inertia of the ballscrew. For my 32mm dia screw of 650mm length =5.252 .10-4 kg.m2
    Note that I got this figure from the THK spec sheet but both I and peteeng calculated it from first principles and all agree.
    3)The effective rotational inertia implied by the axis mass. This forumla I derived from first principles and agrees with the theoretical treatment
    published by Hiwin and also agrees with peteengs analysis.
    Ilinear= m.(p/2pi)2 where p=pitch in meters and m is the axis mass.
    Using numbers for my machine, p=0.005m and m=110kg:
    Ilinear=110. (.005/2 x 3.141)2
    =0.69 kg.m2

    Ieff is the sum of these components:
    Ieff= (1.13 + 5.252 + 0.69) .10-4
    =7.07 .10-4 kg.m2

    dw/dt= 2.4 /7.07 .10-4
    =3395 rad/s2

    And to convert that to linear acceleration:
    Alinear= dw/dt x p/2pi
    =2.7 m/s2

    This figure is less than 1/10th of what I had calculated previously, and is my error. Note how despite the heavy axis, its contribution
    to the first moment of inertia is small, even the first moment of the servo armature exceeds it, but both are dwarfed by the first moment of the ballscrew.
    I had not made allowance for that factor previously.

    The take away feature is that with my machine at least 'the rotational mass of the ballscrew dominates the acceleration equation'.

    Craig
    Post some details of your machine and the moment of inertia of your steppes and do the calculation.

    Craig

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