HOORAH! I won!

Ahh. Useful details. What does it weigh, and how stiff are the bearings? One can get critical speed issues with springy bearings as well. Although 20 rpm would seem to be safe, regardless, unless the drum is very heavy.

I guess I don't fully visualize the setup.

Joe Gwinn

Gunner Asch fired this volley in news: snipped-for-privacy@4ax.com:

The job doesn't require the speed, torque, or precision of servos. In this case, the combined cost of the drive and motor is WAY under what a decent servo drive and motor (or unitized) would be. And then, there's the avaiable power source. But since I don't need high torque, fast acceleration, or high speed, the less-demanding steppers are fine (for the job... I prefer servos, also, for most motion tasks.)

It's a system to screen and weigh raw chemicals for a mixing cycle, and although it would be _nice_ to have the full range of speeds over which the stepper can move the screening drum, it isn't necessary. I'm just curious about how effective this new drive will be in reducing the inertia-induced resonance band.

Envision a TINY miniature version of a Stokes Oscillating Granulator, and you have the basic theme.

Lloyd

Joe Gwinn fired this volley in news:280820161500185847% snipped-for-privacy@comcast.net:

Thanks... reasonably stiff, class-5 sealed bearings -- so a tiny bit of resistance to rotation, but not the least of play.

However, each drum weighs in at about 4lb, with most of that out at the periphery; so there's a good bit of inertial moment there.

As I said to Gunner, envision a miniature Stokes Oscillating Granulator, and you have the gist of it.

Lloyd

On Aug 28, 2016, Lloyd E. Sponenburgh wrote (in article):

I?m guessing that the drum is perforated, and replaces the cylinder of multiple rods that rotate back and forth.

If you remove the drum, can the drive system manage all speeds?

Joe Gwinn

On Aug 28, 2016, Lloyd E. Sponenburgh wrote (in article):

As I sad in prior message, what happens if you remove the drum and run the system.

This repeat is to test a news client feature.

Joe Gwinn

Joseph Gwinn fired this volley in news: snipped-for-privacy@news.giganews.com:

of

No, Joe, it's a miniature copy of the old Stokes design; rods and all. Only the ratios of dimensions are changed.

With the drum off, the drive will smoothly run the stepper at all speeds in the range. With the screen off (not rubbing on the bars), it gets worse. The added friction actually improves matters. And... it gets yet a little better with chems in the hopper (more friction).

But there's still this 'middle range' where it just won't run properly, at all (jittery, stop-n-start, and some occasional reversals). Part of that is _probably_ because the chems start whirling around with the rotor, adding to its effective mass.

Yet, outside that middle range, it runs like honey. And fortunately, the mostly-used bottom speeds, and occasional higher speeds are all the application needs to accurately weigh the goods. (one super-fine chem that will almost run out the screen by itself meters out at 0.013rpm, and the fastest we ever run it is 20rpm! )

LLoyd

Gunner Asch fired this volley in news: snipped-for-privacy@4ax.com:

ACtually, steppers are in heavier use now than probably any time in the past. I _prefer_ servos, but I apply steppers where appropriate, for cost.

This particular machine has all the low-force, low-speed rotary motions done by steppers. The heavy action is either by servo or pneumatic positioners. There's also a pressing turret on the thing that weighs in at about 170lb, and gets advanced in 12 steps by pure pneumatics.

Steppers have come a long way in the last six or seven years. There are even "stepper servos" (wholly self-contained systems of motor and controller), that auto-position AS IF a servo, with acceleration and distance managed by the controller. All you have to do is supply an address, an acceleration constant and deceleration constant, and a maximum speed. Then they just 'go there', just like a servo.

Lloyd

OK.

But one gets the forbidden speed even if no chems in the machine?

Are the individual rods resonating?

From all that you've said, *something* is resonating, mechanical or electrical.

Joe Gwinn

Joe Gwinn fired this volley in news:290820160942058854% snipped-for-privacy@comcast.net:

Yep... but I figure it's the 'whole mass' of the rotor.

The rods are 1/2"dia. #316SS 4" long, and there are six of them welded to the 1/2" #316SS solid end plates. It has a 5" long 5/8" SS shaft running the length out to the front bearing and past the back bearing.

Not much there to 'flex', except perhaps the coupling between the rotor and the drive. That's a tapered fit, push-in coupling, which MIGHT have a bit of play if not fully coupled, but _shouldn't_ once the mounting screws are snug -- though one never knows. I know I cannot feel any when manually turning the rotor against a locked stepper. (yeah... I know it only takes a thou. or two...)

Lloyd

I saw a project about 40 years ago where someone removed the diodes from a couple GM 37A alternators and used them as three phase steppers. It was part of a large router for making signs. I think that it might have been in 'Mother Earth News'. My parents subscribed to it, when I was just out of high school, and I saw a few issues. Needless to say, it was a crude setup but it had plenty of torque. :)

Yes, but there also has gotta be a spring in there somewhere.

That sure sounds sturdy.

I have another idea: It may be a torsional oscillation. It's a lot easier to get low-frequency torsional oscillations with a given mass-spring-mass system.

One way to test the various theories is to insert rubber absorbers in clever places. Basically figure out what convenient points move the most for oscillation mode xx, and tie those points together with enough sorbothane to damp things down.

The problem can be finding a place where rubber is well enough mechanically impedance-matched to the metal that the rubber can steal significant vibration energy.

Joe Gwinn