DIY Large Bore Hollow-shaft servo motor

[Eric R.]

Dear All,

I am currently designing a large moving gantry milling machine. For the X-axis (approx. 5' / 1.5m) I will use servo driven rotating ballscrews (double axis configuration). For the Y-axis (approx. 10' / 3.0m) I want to use a fixed ballscrew with a rotating nut. As I prefer not to use belts, I would like to use a hollow-shaft servo motor with a large bore (approx. 2" / 50mm). The only problem is that these are hard to find and if available, way too powerful and very expensive (> 2000 USD). Therefore I am planning to build my own hollow-shaft motor based on a frameless motor kit. Anybody out there that already has experience with this? Or would anybody be interested in such a hollow-shaft motor? In that case I am more than willing to share my ideas.

Kind regards,

Eric

[svenakela]

There's a thread or two about DIY-darn-heavy-duty-servo here, don't remember where though but you should find it with some searching.

Regards,
Sven

[dberndt]

I have a large panasonic servo motor. Its admitedly very large for your application, 1000rpm max, 24n/m continuous torque. The shaft hollow is ~1.8".

I have no use for it and would be willing to sell it real cheap. Shipping wont be fun though, it must weigh atleast 50lbs, probably more.

[Zumba]

Hey Eric, how much does a frameless motor kit cost? The concept seems pretty straight foward... you have a big toroid shaped stator (press fit into a housing?) and I guess you machine some female threads in the hollow rotor to attach the ballnut?

Personally, I don't have anything against belts, but the integrated motor/nut is definitely much more sleek.

[RICHARD ZASTROW]

Zumba, The hollow servos are available as frameless and within housings. Danaher calls their housed servos "cartridge motors". Yaskawa, Indramat and IMA (among others) also sell them. I have designed a few machines using these and used keyless clamping sleeves to attach hollow shaft servos to shafts and shafts to gears and sprockets. Keyless clamping sleeves are available from Spieth, Ringfeder and Fenner Drive. None of these require taper boring. If you do the research, you will find that when machined within specs, the keyless connectors on keyless shaft/bores is a stronger setup than using keys. Also it is adjustable axialy and finely radialy for runout correction if needed. Also, these make great integral spindle, shaft, motor heads.

[Eric R.]

Hey guys, sorry for the delay in replying. I am currently on a brief vacation, skiing in the Rockies. I am just having a short break, so I will keep it short for now.

dberndt: thank you for your kind offer. The motor seems rather big for my application, but nevertheless I would appreciate it if you would send me a brief message, preferbly with some more specs of the motor, a few pictures (especially regarding the hole) and your selling price.

Zumba: when I am back home I will send you a link from a site that I found that lists prices of Parker frameless kit motors. Typically prices are about 30-50% cheaper than regular (non-hollow shaft) housed servo motors with comparable specs. Compared to hollow-shaft or direct drive motors these frameless kits are even up till 70% cheaper! Like Richard states, many servo motor manufacturers have frameless motors available. On many of their websites you can also find User Manuals and Installation Manuals that provide information on how to handle these motors. For my application I was thinking of using the so called cold-fit methode: heating up the rotor (up till 300F / 150C) and cooling the hollow shaft (-4 F / -20 C or lower). More info can be found in the manufacturer documentation of e.g. ETEL (www.etel.ch).

Richard: thank you for your follow-up on Zumba and the additional info.

[Zumba]

Thank you both for the info.

I'm currently working on building my own heavy machine tools with high capacity. Belt-less servo spindles for lathes and machining centers would be the cat's meow.

[Eric R.]

Hi Zumba,

I survived the ski slopes of Breckenridge. Here is the link for frameless motor pricing:

http://www.electromate.com/products/...ries_id=101224

[Zumba]

Hey Eric, thanks for the link. These motors look very promising. They seem to be priced based on stack length, so smaller motors cost roughly the same as large ones with the same stack length.

The K500 series is pretty cool... tons of torque, up to 6000 oz-in, and 2" rotor bore. What kind of a servo controller would you need for such a large motor?

[Eric R.]

The higher the torque, the more power it requires and therefore the bigger the drive needs to be. And BIG usually means lots of MONEY. So, I would first decide on what torque is really needed and then select a suitable motor and drive.
Btw... please notice that the K500 series does not come with the integral commutation board. Therefore I am looking at the K127 series.