Re: Servo drives- Mariss

I'm glad you brought up inertia but you are making a good thing sound
like a bad thing.:-)

The reflected moment of inertia does go down as the square of the
gear ratio. This good from the motor side of things. Ideally you want
to have the load inertia equal the motor's moment of inertia to
maximize power rate (1:1 inertial ratio). Things still work with a
10:1 mismatch; beyond that big problems start to happen. Mount a
bicycle wheel to a step motor or servo to see what I mean.

Load inertias are normally much larger than the motor's driving them.
Inertia increases with the 4th power of radius and proportionally
with length on a cylinder. Compare your leadscrew diameter and lenght
to the motor's rotor or armature dimensions. A gear reduction is
often used not just to increase torque but to bring this mismatch
more into line.

Mariss




--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Leslie Watts"
<leswatts@a...> wrote:

> I have been watching this thread a bit and I see no mention of

inertia...

> so I will bring it up briefly.
>
> Reflected inertia from the load of a geared drive is the SQUARE of

the

> motor gear ratio.
>
> If you use a big ratio like 10:1 to get the most power out of a

servo it's

> inertia will be 100 times larger looking from the load!
>
> Another way of saying this is that motors running at high rpm will

take a

> lot
> of time to change speed. They are pretty good flywheels.
>
> Because of this and other reasons servos are often run at low rpm

and low

> power density
> to achieve the desired machine acceleration in the most economical

fashion.

>
> Les
>
> Leslie M.Watts
> L M Watts Furniture
> Tiger Georgia
> (706) 212-0242
>
> Main page:
> http://www.lmwatts.com
> Engineering:
> http://www.lmwatts.com/shop.html
> Cnc surplus for sale:
> http://www.lmwatts.com/forsale.html
> Carved signs:
> http://www.lmwatts.com/signwp.html