Servo Direct Drive?


Page 1 of 2 12 LastLast
Results 1 to 12 of 18

Thread: Servo Direct Drive?

  1. #1
    Registered Contract_Pilot's Avatar
    Join Date
    Jan 2009
    Location
    USA
    Posts
    228
    Downloads
    0
    Uploads
    0

    Default Servo Direct Drive?

    Can servo's be Direct drive I read some place they need to be geared! Some say if sized right they can be direct drive.

    Looking at Hybrid or Actual Servo I can go with Either I have a 7I77 and a 7I76 mesa board in the bin.

    Planning on my 704 retro or a 770 locally that has a smashed electronics cab so was removed and discarded.

    Similar Threads:
    CNC Machines: Tormach PCNC 1100 CNC Mill W4th Axis | Birmingham 12X36 Lathe W/ CNC Retrofit W/Tormach BOB PathPilot & SKCD200220 VFD | My Personal Blog www.stevenrhine.com


  2. #2
    Registered
    Join Date
    Aug 2004
    Location
    spain
    Posts
    656
    Downloads
    0
    Uploads
    0

    Default Re: Servo Direct Drive?

    Direct drive works fine, if the servo is big enough.

    About 400W, 1.3 Nm continuous, and == 3.9 Nm peak, and up, should do ok on linear guide machines.
    Because of the high peak torque, it should/might also be fine on guideways as long as there is no binding.

    For comparison, 3.9Nm is what you can twist with your hand, but is fairly hard to do.
    Clearly harder than normal machine handles are turned.

    I use much bigger servos, and I gear them down 1:2.
    I do so to get more resolution and accuracy.

    10.000 count servos, 750W, 220V, at 1:2, with HTD8-30 mm wide belts.
    I don´t need all the accuracy, nor the top speed, but the extra resolution is desirable to me.

    Step size is 0.2 um.
    Step size is =/= actual machine resolution or repeatability.
    Actual incremental move size is less than one micron with 4 mm screws (32 mm D).
    Lathe.

    The smaller servos mentioned, 400W, I use on a VMC.
    At 1:3, HTD 5-15 mm.
    5000 count servos, 60V.
    A mill does not need anywhere the resolution that a lathe does.

    Industrial machines are mostly direct drive these days ..
    but they use 1 kW (older) to 2 kW (newer) servos.
    About 10.000 counts (Haas, since ==2012), older Haas was 2000 count.

    Industrially they are looking for "speed" in acceleration and top speed .. totally not needed for non-production machines.
    And the tables are easily 1000 kg or more in mass.
    My VMC table is 200 kg now, and 400 kg+ soon.

    For calcs:
    3 Nm at 1:3 == approx. 600 kgf push force on the table.

    Example for bigger servo, 750W.

    The torque is 10 Nm (peak at servo) x 2 = 20 Nm.
    Push force is T = Fl / 2 x pi x efficiency
    T - torque in Nm, 20 Nm
    F = push force in N
    l = screw lead in m, 0.004 m
    efficiency 96% (ballscrew)
    F = 2 x 3.14 x 0.96 / 0.004
    = 15072 N = 1500 kgf.

    A 32 mm ballscrew is rated at about 1400-1500 kgf push force.
    This is std on industrial machines, medium size VMCs, and is what I used for myself.



  3. #3
    Gold Member
    Join Date
    Feb 2006
    Location
    USA
    Posts
    7057
    Downloads
    0
    Uploads
    0

    Default Re: Servo Direct Drive?

    In many/most cases, using direct-drive with a servo will mean using a motor that is larger than it needs to be to get the necessary torque. Servos typically have considerable RPM capability (4000 RPM is not unusual for DC servos). With direct drive, this would result in ridiculously high rapid speeds (800 IPM with typical 5-pitch screws) which is completely unusable in a small machine. Using belt reducers (which are typically around 3:1 or 4:1 ratio) allows the use of smaller, cheaper, lower torque motors (because the reducers multiply torque, while reducing speed), and brings the peak speed down to something more reasonable. So, instead of an 800 oz-in direct drive servo capable of more speed than can be used, you can instead use a 200 oz-in servo with 4:1 reducer, and get the same usable performance for a lot less money.

    Regards,
    Ray L.



  4. #4
    Registered arizonavideo's Avatar
    Join Date
    May 2008
    Location
    USA
    Posts
    1118
    Downloads
    0
    Uploads
    0

    Default Re: Servo Direct Drive?

    Sure you can.



    I have been running the Electrocraft for 4 so years direct drive with a Gecko drive and a 5mm pitch double nut ball screw. Its a 90V servo running at 75V.

    I can stall it but not with normal cutting. I wanted it to stall before breaking the bit or breaking off the end mounts and that is how it works.

    I do say that now I'm leaning toward 900oz steppers for the X and Y just because they work just as well and don't have brushes. I don't get 300 IPM but 125 IPM is just fine for there type of mills.

    Don't forget the Clear path servos from Teknic. I grabbed a CMP-SDSK-3421S-RLN to try on the z of the new PM-940M mill. If it is little weak for that it will work great for the X.

    Attached Thumbnails Attached Thumbnails Servo Direct Drive?-sany0049-jpg  
    youtube videos of the G0704 under the name arizonavideo99


  5. #5
    Registered
    Join Date
    Jun 2005
    Location
    USA
    Posts
    1350
    Downloads
    0
    Uploads
    0

    Default Re: Servo Direct Drive?

    Servos can be used to directly drive a ball screw or some other method of going from rotary to linear motion. The reason you often see gearheads on servos is the fact servos run at higher speeds than stepper motors. Most servo motors can run up to 3000 RPM. At low speeds the servo motor has less torque than a stepper motor, but at higher speeds the servo motor has much more torque where at high speeds stepper motors have much less torque. The idea is to get the servo motor geared to the middle of its speed range so you get the best of both worlds.

    Russ



  6. #6
    Community Moderator Al_The_Man's Avatar
    Join Date
    Dec 2003
    Location
    Canada
    Posts
    22917
    Downloads
    0
    Uploads
    0

    Default Re: Servo Direct Drive?

    A little misleading, for a given frame size steppers are usually have more torque, but the torque curve for a servo and stepper of the same torque, the servo is superior, this is due to the relatively flat continuous torque curve that is maximum at zero rpm right up to the max. rated rpm.
    Whereas the stepper torque curve drops off rapidly as rpm increase, there are steps to overcome this in the drive, by maintaining the rated current, but still lower top end rpm torque compared to a servo.
    As previously stated, the higher max rpm of a servo allows a more economical frame and drive size to be applied when some kind of gearing is used.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.


  7. #7
    Registered
    Join Date
    Feb 2016
    Posts
    28
    Downloads
    0
    Uploads
    0

    Default Re: Servo Direct Drive?

    I have 1kW 2500rpm 2500ppr AC servos (lcmt-10m02-80m04025). I'm trying to decide on the rest. I'm looking at 20mm 5m pitch ball screws. That would be max 492ipm. That would be about 500rpm to run 100ipm. 400rpm to run 80ipm. The machine gantry would weight out about 250lbs with dual y drives supported with sbr25 rails. That should be way enough power to travel. The force will be a spindle motor not yet selected for cutting hardwood/alumnium at times. Process time isn't important to me. My question is can i run direct drive to the ball screw? Or would I be disappointed and wasting my time?
    My other option is using 10mm pitch ball screws with a 5:1 gearbox. That would be max 196ipm. @ a more than double the cost.



  8. #8
    Registered
    Join Date
    Jun 2005
    Location
    USA
    Posts
    1350
    Downloads
    0
    Uploads
    0

    Default Re: Servo Direct Drive?

    quickjam,

    Those are very large servos. You can connect them directly to the ball screw with a coupler. The picture below is a 750W Panasonic motor attached to one of my ballscrews. The second picture is a 200W motor connected to a harmonic drive with a 11:1 reduction on one of my machines. What helps determine which way to go is the torque and speed requirements. A stepper motor has more torque at slower speeds, but a servo motor has more torque and more at higher speeds. Most servos range between 2500-3000 rpm. In a direct connection depending on the pitch of the ballscrew you can sometimes limit the speed of the motor to 200-300 RPM. For optimal performance again based on your ballscrew pitch you need need a gearhead like you said. In my case I started direct connected and still have that arrangement on my smaller 24"x24" machine and it works perfect. On my larger 60"x60" machine the ballscrews I found on ebay were designed for rapid movement so to get in the mid-range of the servo I need to add a gear head. If you do with a gear head you need to find one with no backlash. Those are more expensive and that could determine you approach on price alone.

    Russ


    Servo Direct Drive?-dsc01252-jpg

    Servo Direct Drive?-dsc01085-jpg



  9. #9
    Gold Member
    Join Date
    Feb 2006
    Location
    USA
    Posts
    7057
    Downloads
    0
    Uploads
    0

    Default Re: Servo Direct Drive?

    Quote Originally Posted by quickjam View Post
    I have 1kW 2500rpm 2500ppr AC servos (lcmt-10m02-80m04025). I'm trying to decide on the rest. I'm looking at 20mm 5m pitch ball screws. That would be max 492ipm. That would be about 500rpm to run 100ipm. 400rpm to run 80ipm. The machine gantry would weight out about 250lbs with dual y drives supported with sbr25 rails. That should be way enough power to travel. The force will be a spindle motor not yet selected for cutting hardwood/alumnium at times. Process time isn't important to me. My question is can i run direct drive to the ball screw? Or would I be disappointed and wasting my time?
    My other option is using 10mm pitch ball screws with a 5:1 gearbox. That would be max 196ipm. @ a more than double the cost.
    At roughly 1.3HP, those motors are MASSIVELY over-sized for ANY benchtop machine - probably by a factor of 2 or more. One problem you're going to have is, if you ever crash it, you WILL do serious damage. Using reducers will do little more than increase the amount of damage.

    Building a GOOD CNC machine means actually doing the engineering calculations to DESIGN an appropriate drive system matched to the machine hardware and performance requirements. Simply buying parts based on guesswork, and bolting them together, particularly grossly over-size parts, will not yield a good result...

    Regards,
    Ray L.



  10. #10
    Registered
    Join Date
    Feb 2016
    Posts
    28
    Downloads
    0
    Uploads
    0

    Default Re: Servo Direct Drive?

    Oh this will be a rare build for sure. But no table top will hold. 3 tons and counting. I'm sure to find a weakest link somewhere. And that I will learn from. TX



  11. #11
    Gold Member
    Join Date
    Feb 2006
    Location
    USA
    Posts
    7057
    Downloads
    0
    Uploads
    0

    Default Re: Servo Direct Drive?

    Quote Originally Posted by quickjam View Post
    Oh this will be a rare build for sure. But no table top will hold. 3 tons and counting. I'm sure to find a weakest link somewhere. And that I will learn from. TX
    OK, so you're building a 3 TON machine, and asking for design advise on a forum where a G0704 is considered a large machine? What could possibly go wrong....

    Regards,
    Ray L.



  12. #12
    Registered
    Join Date
    Feb 2016
    Posts
    28
    Downloads
    0
    Uploads
    0

    Default Re: Servo Direct Drive?

    Tx CNCMAN.
    I will try direct with 2505 screws and see how it goes from there.



Page 1 of 2 12 LastLast

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  


About CNCzone.com

    We are the largest and most active discussion forum for manufacturing industry. The site is 100% free to join and use, so join today!

Follow us on


Our Brands

Servo Direct Drive?

Servo Direct Drive?