[Brass_Machine]

I was watching a couple videos (on YouTube) on a converted X2 to CNC. In one of the videos, the guy had direct drive on all 3 axes and stated he was going to "upgrade to gear reduction". This has me slightly confused, why is going to gear reduction an upgrade?

Thanks!

[Al_The_Man]

If you do not have the size of motors capable of direct drive for the high accel/decel rates you would like, then gearing is a solution, gearing increases the torque be the ratio of the reduction and decreases the motor/load inertia ratio by the square of the reduction.
Al.

[Brass_Machine]

If you do not have the size of motors capable of direct drive for the high accel/decel rates you would like, then gearing is a solution, gearing increases the torque be the ratio of the reduction and decreases the motor/load inertia ratio by the square of the reduction.
Al.

So if I have appropriately sized motors, I don't need gear reduction?

Thanks for the answer!

[Al_The_Man]

As it applies to servo motors, yes.
You only have to look at some of the Big early Mazaks, they often had direct drive ballscrew with very large DC servo's driving the ball screw.
And like I said, your desired accel/decel has a large bearing on sizing and gearing.
Al.

[Brass_Machine]

But for something tiny, like an X2 mill with steppers? Would that be a waste?

[Al_The_Man]

I do not use steppers so i cannot advise you in those cases.
Al.

[jsheerin]

Any time people change something, it's an 'upgrade'.

Steppers driving screws typically don't use belt reduction because steppers have more torque at low speed and the screws provide enough gear reduction to get the needed speed of the machine parts. So if you pick your motors and screws correctly for what your machine needs to do, you usually don't need additional gear reduction. On the other hand, servos need to be geared to run at their max rpm to get the most power out because they have the same torque at all speeds (roughly speaking). So if you didn't gear them to be able to use their full rpm range, you'd never take full advantage of their capabilities.

[Al_The_Man]

Actually if you look at actual data sheets, both steppers and servo's have maximum torque at zero rpm, I think this belief comes about because as a rule steppers have a higher torque rating in the same package or comparable NEMA size.
Typically the the torque curve for servo's go from maximum at zero and is flat or taper down slightly to the end of the rpm range.
Al.

[jsheerin]

True, but servos' torque is usually flat up to some high rpm, like say 2k rpm for a max 3k rpm motor, while steppers instantly start losing torque as speed increases, some of them very sharply. The better steppers now have a much shallower decline of torque, but I think they're closer to constant power devices while servos are closer to constant torque.

[lgalla]

I have seen a million times in posts people saying gearing down their steppers,say 3/1 and get 3 times the torque.Is this right?Looking at torque curves for steppers it looks like gearing down places the motor in a low torque zone at the upper rpm.Can the stepper actually have less torque with gearing?
Larry

[ger21]

Can the stepper actually have less torque with gearing?

Yes and No. It'll have 3 times the torque at any given rpm. However, it may end up operating at an rpm where it has less than 1/3 the torque, negating any gains the reduction may have given.

What you really need to do when using steppers, is design the machine for a given max stepper rpm. Depending on drives and voltage, most steppers will have a usable max rpm somewhere around 800-1000 rpm. Larger motors can be even less, and smaller motors can be higher.
You pick a max velocity for the machine, and gear it so that the stepper is at it's max usable rpm.

One other thing with steppers, though, is that as velocity increases, resolution decreases. As velocity goes up, you may reach a point where you need to switch to servos, with higher count encoders to get more resolution.