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Every so often when I'm looking through industrial supply websites looking at various fractional hp electric motors there appears to be a class of motors that are referred to as "hubless". Attempts to find diagrams or explanations of these types of motors only yields references to various 2-wheeled conveyences with large open wheels. So, what is the defining characteristic and operating principal of a hubless electric motor, and where are they used?

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  • \$\begingroup\$ A link to one of those industrial supply websites would be great. \$\endgroup\$
    – vini_i
    Commented Jan 26, 2021 at 1:58
  • \$\begingroup\$ Here's an example:emotornations.com/products/… Note that the shaft-dimension is "hubless" so does it have a shaft? What exactly is turning? \$\endgroup\$ Commented Jan 26, 2021 at 2:17
  • \$\begingroup\$ They also seem to be called "unit bearings" or the bearing type is "unit". \$\endgroup\$ Commented Jan 26, 2021 at 2:26
  • \$\begingroup\$ Frameless is another word for it \$\endgroup\$
    – Voltage Spike
    Commented Jan 26, 2021 at 3:03

2 Answers 2

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They're essentially a linear motor wrapped into a circle.

You want hubless motors and (hollow shaft motors , and hollow shaft encoders, for that matter) when you need or want to have something else occupy the space where the hub would be.

Many times the center of the axis of rotation in an instrument or machine is a prime spot with many contenders vying for a chance to sit in there.

Edit: Based on your newly supplied link, I'm not sure that is an accurate description of the motor. Most of the similar products are called "unit bearing motors" and are shaded-pole fan motors with an integral sealed bearing arrangement.

For example this one.

enter image description here

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  • \$\begingroup\$ I'm not buying your answer for the moment until someone explains what other items or objects would be placed in this prime hub area in the example motor I give a link to above. \$\endgroup\$ Commented Jan 26, 2021 at 2:29
  • \$\begingroup\$ I think the center also becomes valuable in some cases passing working fluids through a machine. It's easier to pass something through the center of a rotating shaft than the side. \$\endgroup\$
    – K H
    Commented Jan 26, 2021 at 2:50
  • \$\begingroup\$ Yes, compressed air is one that we have used. \$\endgroup\$ Commented Jan 26, 2021 at 2:51
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If you are asking about "hollow bore motors" which have a driven hole exists where the shaft should be:

enter image description here

Taken from: https://www.posital.com/en/industries/electrical-motors/hollowbore-directdrive.php

They are useful when you want to:

  1. drive a shaft rather than sticking something onto a driven shaft.
  2. drive a very long shaft
  3. drive a shaft in the middle
  4. drive a specially machined shaft

One example that comes to mind is the joints on a robotic arm are more straightforward with a hubless motor than a shafted motor.


If you are asking about a motor that has a rotating face with a bolt pattern on it rather than a protruding shaft (the example still has a hole in the middle but it is not driven and has no accommodations to rigidly a fix a shaft to it):

enter image description here

From T-Motor

Such a motor is useful whenever you are mounting something with a flat face to it. Having a shaft would mean extra work. Such examples are:

  1. Turn tables
  2. Thin objects
  3. UAV camera gimbals. The camera gimbal is often just bent sheet metal. It is much lighter and easier to just drill the bolt pattern into the sheet metal arms and bolt it to the motor. Having a shaft means you need a bunch of junk to fix the shaft rigidly to the gimbal arms and then a collar to stop the shaft from sliding out.
  4. Very large two-blade quadcopter propellers that are molded in two halves rather than a single piece. This precludes a hub through which a shaft can mount. Instead you bolt the end of each blade to each half of the bolt pattern.

Some things are just much more convenient when you can bolt it to the motor's rotating face directly than if you had to fix it to a shaft.

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  • \$\begingroup\$ Why wouldn't they spec the diameter of the hole? Wouldn't that be useful to know? And how in the world would you balance it unless the shaft you were sliding into this hole had a very tight fit? (and stop that I know what you're thinking). \$\endgroup\$ Commented Jan 26, 2021 at 2:35
  • \$\begingroup\$ @PeggySchafer They should spec the diameter of the hole the same way they spec the diameter of the shaft. And balancing is an issue for shafted motors as well so that is nothing special. There should be accommdations for setscrews or flats, or similar things to rigidly fix the shaft to the hole. Press fits, unless they are cryogenic press-fits (which are seriously special stuff...like military or NASA special) are not going to work on anything but the smallest motors. \$\endgroup\$
    – DKNguyen
    Commented Jan 26, 2021 at 2:37
  • \$\begingroup\$ @tomnexus Oh, we might be talking about different things. Post a an example photo. Are you talking about motors with a hole in the middle? Or motors with a solid center but no protruding shaft? \$\endgroup\$
    – DKNguyen
    Commented Jan 26, 2021 at 2:40
  • \$\begingroup\$ @tomnexus WHat about the one I posted? \$\endgroup\$
    – DKNguyen
    Commented Jan 26, 2021 at 2:58
  • \$\begingroup\$ Yes that's perfect. Probably wouldn't attach to a shaft, it would go directly onto a housing or gear, also with a large hole for encoders, slip rings etc. \$\endgroup\$
    – tomnexus
    Commented Jan 26, 2021 at 3:20

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