Gear rack questions

[scrambled]

Hi everyone-

I am having problems with my Flame cutter, and need some advice.

For the most part everything is working good, just not great. The reason I need it to run great is I am going to buy a Hypertherm 85 and hook to it.

The problem is only on the x axis, and this is a gantry style-- so there are two drive servos.

The problem is there is a shutter when the x axis moves in either direction. This only happens when the axis moves from a complete stop and accelerates.

I can move the x axis in the ++ direction, let it stop, then move it in the ++ direction again and the shutter happens again.

I can move it in the -- direction, let it stop, then move it in the -- direction and again it shutters.

I have looked everywhere for something loose, and can't find a thing.

I have tuned the servos, and Mach until I am blue in the face. Still shutters.

I am using Thomson duratru gearboxes that are 5 to 1, with a 2.5 diameter pinion on a gear rack.

I am also using 1125 in. oz dc brushed motors.

My gantry weights about 300-400 pounds, but the force to pull it is less then 10 pounds. I used a pull guage to measure this.

is it posible that the weight of my gantry is overpowering the servo due to inertia?

I am thinking a larger gearbox would help? Any suggestions? I have a price quote for 15 to 1 , and 25 to 1 gearboxes


I use this table for structural steel fabrication, so I don't need high rapid speeds, but I dont want to have it crawl either.


Give me some suggestions PLEASE!!!!



Steve

[WSS]

Steve, I recently had similar problems with my X shaking at 14 to 18 ipm. Leon (my tech guy from DynaTorch) had me change a setting in my tuning window. He called my problem harmonic something or other. It had to do with the mass+drive torque+???. It was way over my head, anyway what we did to solve it was up this number from 500 to 1500 and back down in increments of 250 until it started to shake again, then up 50. I ended up at 800, no more shaking! He compared it to tightening a guitar string. The down side of a higher number is the motors work harder, get hotter.

I would have to go back the screen to find out what field it was that I changed. It is known to me as "second box down, X axis" on the tuning tab page

WSS

[scrambled]

WSS-

I have tried all of the parameters in both Mach and the Granite Devices servo drives.

I was hoping someone would chime in on my gear ratios

since my final drive pinion is 2.5 inches accross, wouldn't that reduce the Thomson gearboxes by 2.5???

That is why I am leaning on the idea my ratio is two small.


What do you think?


Steve

[jcc3inc]

Sir,

First you should establish than there is NO lost motion (backlash). Try the to see if there might be some looseness between the stepper motor shaft and the output gear: it is possible that a setscrew may be almost tight but can allow the output gear to rotate slightly under load. Then check to see if you can detect lost motion under load within the gearbox itself.

Since you have brushed DC motors, you may well have a stability problem. Where does the postition feedback come from on a given axis? Do you have a separate encoder with its own separate pinion also riding on the gear rack, or is the encoder attached to the motor?

Upon re-reading your post, can you tell me how your system maintains synchronization between the two X axis drive motors? It is essential that one side be a slave to the other, and that there be some sort of absolute relation between the two in order to maintain squareness at the start and during operation. Just as a trial on this synchronizing, separate the outboard drive pinion from the rack so that it drives only from the master side, and then run it and see if you still have the shudder.

As to gearbox ratios, we used around 30:1 gearing with our DC servos with Burny CNC's. But our motors were smaller, pinions were 1.33 inch pitch diameter.

If you want to discuss further, I am at jccinc-at-owc-dot-net.

Regards,
Jack C.

[scrambled]

Jack-

I have checked all the connections on both x axis, and can not find anything loose. All of the shafts have keyways installed, and the keys are in good shape and tight.

The servo motors have the encoders mounted on the back of the motor housing.


If you are using 30 to 1 gearboxs with a 1.33 pinion, then my 5 to 1 gearbox with a 2.5 inches might be on the small side.

[jcc3inc]

Sir,

In order to evaluate your design, I would need to know the specs for your drive motors, the max voltage available from your servo amp, and the max traverse speed that you need. Some DC motors are designed for low speed operation, others run much faster at the same given voltage.

It would also be of interest to know what pitch rack you are using as well as the pressure angle. Press angle dictated the min number of teeth on the drive pinion.

Regards,
Jack C.

[scrambled]

Jack-

It will be tomorrow night before I can get you the specs.

I know the pressure angle is 14.5 for the rack and pinion.

There servo drives are made by granite devices and are at 80 or 90 vdc.


I am thinking the max rpm for true servos is 3000 RPM's


I will count the teeth on the pinion tomorrow


Steve

[scrambled]

Jack-

I have not been able to get near the Cnc table during the last week. I have a big fab job that I need to finish.

I should get the information this weekend for you.


Thanks for waiting


Steve

[jcc3inc]

Steve,

There is one more thing of importance: what is the number of counts per revolution for your encoder mounted on the end of the motor shaft? With that and the gear ratio + pinion pitch diameter we can find the system resolution in thousandths of an inch.

Thanks,
Jack c.

[Gillyshank]

think the problem probably lies with your gear ratios as jc 31inc says

if my calculations are correct

2.5" x pi = 7.8" per rev
3000 rpm top speed divided by gear box ratio of 5:1 = 600rpm

giving a theoretical top speed of 4700ipm

as gas cutting speeds are appx 20ipm the motor is barely turning (12 rpm) and the signals controlling the motor will be so small any deviation will have a massive effect

when we fitted burny servos to our machines
we would use their 3000 rpm motor with a 33.9 :1 bison gearbox and a 1.5" gear

this gives a reasonable top speed of 370 ipm

more importantly gives much more torque to move the machine about

you will obviously have to recalibrate your encoders etc if you change gearbox ratio

[scrambled]

Gillyshank-

Thanks for the reply.

I still have not been able to get to the table, but the pile of stuff that was blocking my path is getting smaller and smaller

I should be able to get the gear tooth count. and the encoder CPR tomorrow I hope.


I think my problem is the gearboxes, but that is just a hunch. I need the help with turning the hunch into fact.

So once I get the info, hopefully we can solve this puzzle!


Thanks for the help


Steve

[Torchhead]

Originally Posted by scrambled Gillyshank- Thanks for the reply. I still have not been able to get to the table, but the pile of stuff that was blocking my path is getting smaller and smaller I should be able to get the gear tooth count. and the encoder CPR tomorrow I hope. I think my problem is the gearboxes, but that is just a hunch. I need the help with turning the hunch into fact. So once I get the info, hopefully we can solve this puzzle! Thanks for the help Steve

Gear tooth count means little (unless you just need to know the gear pitch). What is important on a rack & pinion is the DP (diameter at the pitch radius). But for this exercise it's close enough to just know the pinion diameter. As stated earlier the math is simple: The 2.5 dia gear gives you approx a 8:1 SPEED INCREASE and a corresponding lose of torque and resolution. So without the gear box you are down to 1/8th of the 1125 oz in of torque (140 oz-in). Factor back in the 5:1 gear reduction and the numbers are a little better at 703 oz-in BUT the problem is physics. You have to accelerate a mass of over 300 lbs using motors that are crippled for torque. Resolution is not the greatest either. Depends on the line count of the encoders.

The analysis of the wrong ratios is spot on. About the minimum ratio for that size pinion would be 15:1. 1 run 1" pinions that only have a 3:1 step up and use a 10:1 belt reduction. My gantry is only about 160 lbs but it used for contact cutting (aluminum and composites). I never see any vibrations or jerky movement. All tuned in MACH3.

Think about a 25 hp motor in you car running it stuck in High gear and having to accelerate from a dead stop.

TOM caudle
www.CandCNC.com