Episode #7: Types of Rotary Mechanics

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The Motion Control Show

We're continuing to talk about terminology and this time we're talking about rotary mechanics.  I’m Corey Foster of Valin Corporation.  Let's talk about this.

Typically, when people think of rotary mechanics, they're talking about rotary tables.  Here's a classic picture of a rotary table on the right.  Now, this is a worm gear-driven table.  So, you have to have an external motor.  Here's the motor mount over here on the right-hand side.  That could be a rotary servo or stepper, even a DC motor.  But, that goes through a worm gear to a screw and then that turns this table.  This table is where the load gets mounted onto it.  Now there's some bearings in here and there might be some integral limit switches.  There might be a rotary encoder or even a linear encoder depending upon how the mechanics are set up.  So, this is a typical rotary table. 

If we take that and make a motor into a rotary table here, these are called direct drive rotary motors.  When we do a cut away, the motor is actually cut into it.  It is very similar to a regular rotary motor except that there's no shaft going through it, so you have this hollow bore.  That allows you, rather than to couple two shafts together, it allows you to mount a load directly to it.  The advantage of this is you can have a much larger load both from the size of the load but also the inertia of the load.  The inertia to rotor matching can be much higher.  This is really important when it comes to tuning a servo motor, but that is another conversation.  But, you can have that larger load.  It can be a big overhanging load.  So, these have that advantage.  All these rotary mechanics have some sort of advantage like that.  So, that’s a direct drive.

Another alternative is to use a gearhead.  Now, typically when we talk about gearheads, we’re talking about gearheads with a shaft.  And we’re not going to support a load with a gearhead on that shaft.  They're not designed for it.  But, a gearhead that has a face with a mount like this, again where the load can be directly mounted to it, can be used that way.  Now, remember, you still have to have an external motor just like the worm gear-driven rotary table, so you might have again a rotary servo or stepper, even a DC and maybe even an AC motor on here, but this is again where the load gets mounted onto it.  These are good for, here you can see it on one of our YouTube videos, for a gantry system where it's mounted to a custom bracket, and this flange here is facing down, so the load could be mounted to it rather than like a regular gearhead with a shaft or direct drive motor.  It's just a regular rotary motor with a gearhead where the load gets mounted to this face.

When we look at these 3 basic options, two of them you still have to mount an external motor to it.  And therefore, you still have, in both of those cases, a coupler and a shaft that have to be mounted together.  You still have some compliance.  So, those have external bearings and they can support the larger loads.  That's nice.  The direct drive motor has the advantage of the motor being directly bolted so now there's no compliance between the load on the motor and that's where it can handle that much higher inertia.  The other ones can handle the larger loads, too, but not nearly as high of an inertia as the direct drive so again these options all have the pluses and minuses. (NOTE: check out Episode 70 for a clarification on this comparison of inertias for worm-gear driven and direct drive rotary stages.) They'll have different price points, different speed capabilities, different resolutions, some different things.  But those are some basic terms.  I hope that helps.  If you have any questions, definitely give us a call at Valin.  We're happy to help you.

Contact Valin today for more information at (855) 737-4716, or fill out our online form.