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What I mean by this is a motor with a large hole in the middle, instead of a shaft. Some motors I found allow to easily remove the shaft, but the bore/shaft diameter I need (at least 4 cm) and RPM (700-900) are not even close. Where can I find such a motor?

(The motor has to be small. Why it is needed: http://i.imgur.com/eoL79BU.jpg )

A ready DC motor + gears + bearing assemply like this will also work. But I'd prefer not to as teeth will produce more noise and wear off faster. I'm also skeptical something like this will allow about 900 rpm. enter image description here

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    \$\begingroup\$ I'm voting to close this question as off-topic because it's not electrical engineering \$\endgroup\$ Aug 8, 2015 at 14:42
  • \$\begingroup\$ Where should it be then? \$\endgroup\$
    – Leo Ervin
    Aug 8, 2015 at 15:57
  • \$\begingroup\$ Probaby engineering.stackexchange.com \$\endgroup\$ Aug 8, 2015 at 16:07
  • \$\begingroup\$ Though shopping questions are probably off topic there, to. \$\endgroup\$ Aug 8, 2015 at 16:08
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    \$\begingroup\$ It's not a shopping question. There's a difference between asking what kind of component to use VS where to buy said component. I know in my question I say "where" I can get it, not "what", but you're giving a hard time to the user with such restrictions and this much attention to wording. This is not the first time someone asks to close a topic because it is supposedly a "shopping suggestion" in StackOverflow and then answers are added as comments instead. Seriously, stop it. Or at least just suggest to reword something instead of voting right off. \$\endgroup\$
    – Leo Ervin
    Aug 8, 2015 at 16:48

3 Answers 3

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look at modern PMDC washing machine drum motors they often have the all the working out at the rim, and could probably be modified to have a large bore, in a normal machine shop fairly easily. I don't know if spin cycle exceeds 900 RPM or not,

something like this: http://www.fp.fisherpaykel.com/direct-drive-motors/common/pdf/4876_NZ_Motors_BRO_HR.pdf

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Specific product and product recommendations are off-topic here, but I think if you search for "hollow shaft motor" you will quickly find some candidates that are close.

Edit: As @Jasen says, a lathe spindle bore can easily be in that size. Probably unsuitable because they're made to take large loads for the bore diameter, but you might be able to borrow the construction, which is similar to the rendering you added). This is the arrangement in the tiny Sherline lathe I have- bore is very small (only 10mm) in the case of the Sherline, but that's a matter of the bearing specifications- the pulley diameter is >50mm. Drive is via a urethane belt, and this particular one can have spindle RPM up to 2800 RPM.

This may be easier or cheaper than finding a suitable motor that can be used directly.

enter image description here

Edit2: There may be automotive wheel bearings that are big enough, and you can certainly find suitable tapered roller bearing sets.

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  • \$\begingroup\$ I'm not really asking for a specific brand/model, just what they are called at least, so I'll know what to search for. BTW I have searched for "hollow shaft motor" before. The bore/shaft diameter is too narrow. \$\endgroup\$
    – Leo Ervin
    Aug 8, 2015 at 12:17
  • \$\begingroup\$ A lathe typically has a bore like that behind the chuck, standing a full-size lathe on its end and keeping the librication working would be tricky though, \$\endgroup\$ Aug 8, 2015 at 12:39
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Here's another way...

First find a ballrace of large enough diameter for your purpose and rated at the speed you require. In larger diameters, searching eBay for "lazy susan bearing" can be fruitful, though the larger size may not be capable of 900rpm.

Second, attach a ring or short cylinder to its inner race (e.g. turn its OD down to a press fit in the inner race). This should extend below the ballrace for mounting to the main structure, and above it to hold the stator coils.

Third, attach a ring or short cylinder to its outer race (e.g. turn its ID down to a press fit over the outer race). This should extend above the ballrace to form the rotor.

Now adapt the instructions for building a "DIY brushless outrunner motor" (useful search term, example result here). The rotor magnets attach inside the outer (rotor) ring with epoxy. The stator coils are attached to the inner ring. The instructions should allow you to select a suitable number of rotor and stator poles.

If you don't need high motor power, you don't need a laminated iron stator. An insulating material can be used, with slots milled around its periphery for the windings. It will be reasonably efficient (because there's no conductive material for eddy currents) but with lower torque than the originals, but you will need to increase the number of turns to compensate for the lower inductance of a coreless stator. This style is successfully used in reverse for home made wind turbines.

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