495 oz motors

[lamicron]

I'm building a 4' x8' plasma table , the gantry is about 100 kilos, can I drive it with the 495 oz motors and the 4030 drivers from kelling? I 'll use two directly (no pulleys reduction) to gear racks.
Thanks for help
Luis

[Robin Hewitt]

The pinion PCD will determine your trade off between speed and acceleration.

I calculate your theoretical max acceleration will be 18 ft/s/s divided by the pinion PCD in inches.

[lamicron]

Ok, but the question is: will these motors and drivers work for this weigth?
Luis

[Robin Hewitt]

You didn't mention rolling resistance so I assume you have that well covered.

If so, anything will move it.

OTOH it is extremely heavy so the question seems to be how quickly you can get it up to speed. 100kg is about 220 lbs which is a lot of metal to throw around. Is this a router/plasma bed?

I also assume that's pull in torque rather than holding torque for the motors, if it's holding torque you will lose a lot of acceleration.

[lamicron]

Thank you Robin, It is only for plasma and torch operation,no contact. I think I can lower the weigth of the gantry.
Luis

[energyforce]

I wouldnt bother lowering the weight until you do the math.

My gantry is big and heavy (see www.ubcustom.com/cnc) and on top of this im turning two 6foot long 1" shafts. I barely have any friction from the 8 OPN pillow block bearings (4 each side) but i even accounted for that. My I think 400 or 600 oz in (i forget) motor can turn that no problem. Its all about the reduction and acceleration that you need.

Here try this.... find out how much HP your motor is rated at. If you have a chart that came with your motor look at the torgue vs speed curves. Power = torgue times rotational speed (i.e. rpms). Check your units!!!! Be wary of the torgue vs speed curves. If you find the maximum (rated HP) by looking at the curve you are forgetting that your motor will probably not even operate at this speed hence not give your the torgue u want. Now that you have determined the power (or HP) number you are comfortable carrying around in your pocket move to step 2. This is your motor HP.

Step 2 is calculating the linear horsepower you will require.

power = work / time = force x distance / time = acceleration x mass x (distance / time) = acceleration x mass x (average velocity)

Now say you want to pep your machine up to a velocity of 1000ipms in 4 seconds (no laughing please) you get these numbers....

acceleration = (1000ipm) / 4 sec = _____
mass = 220lbs/32.2
average velocity = (1000ipm - 0ipm) / 2 <<EDIT THANKS ROBIN>>

Chance your velocities from ipms to feet per second
do the math and your answer will be in lbf/sec
Use the unit converter and find out your required HP

If your required HP is more than your motor HP you are pretty much screwed. Either change your acceleration or reduce your weight. Keep repeating this step until your motor will work. Or get a bigger motor :-p

Someone please check my theory. I ALWAYS make mistakes!

[Robin Hewitt]

power = acceleration x mass x velocity

Perhaps a rather complicated way to do the sum?

1 hp = 550 ft.lbf/s so easier to work in ft/s

final velocity = 1.388 ft/s
but it's accelerating so you need average velocity to make this work...
av. velocity = 1.388/2 = 0.694 ft/s
acceleration = 1.388 /4 = 0.347 ft/s/s
mass = 220lbs/32.2 = 6.83 slg

.347 x 6.83 x 0.694 / 550 = 0.003 hp

Check that by doing it another way...

ft.lbf = half mv squared
0.5 * 6.83 * 1.388 * 1.388 = 6.579 ft.lbf
divide by 4 seconds = 1.644 ft.lbf/s
divide by 550 for hp = 0.003 hp

Hardly any force is required

f=ma 6.83 slg * 0.347 ft/s/s = 2.37 lbf

A horse would have little trouble providing this acceleration

[energyforce]

lamicron let us know what you come up with. Your motor should have no problems turning that heavy beast of a gantry.

Post your calculations with explanations and we can check them for you. Sketches help too. For instance draw your whole x axis setup with the motor, gear reducer, shafts, rack and pinion whatever, showing all diameters, weights, acceleration, velocities etc and get one of us to calculate it. If something is missingf from the equation somoene will speak up and say oh we need to know whatever. This will help boost your confidence as you go along and build your machine. Its a great way to catch mistakes and gather opinions. I mean look at the mistake I made by using final velocity instead of average velocity.

I wish I posted up all my calculations on cnczone and had the guys here to criticize the hell outta them before I spent all that money on pulleys, belts, gearboxes, motors etc. It all worked out at the end so whew i guess. My machine moves 5 times faster than it needs to and i can even hold onto my gantry and go for a ride.

Erik

[lamicron]

Thanks for the help, I know you people are ready to help,
Here the first pics of my "monster" .

[Robin Hewitt]

Originally Posted by energyforce I mean look at the mistake I made by using final velocity instead of average velocity.

Hi Erik

On reflection I think I'm wrong and you are right, it all depends on how you look a it.

I've done the sum for a motor that will acheve the 1000 ipm then run out of steam.

You did the sum for a motor that will deliver 2.37 lbf at 1000 ipm which is what you would need for a linear acceleration.

I think your .006 hp is the better answer

Apologies for that, I'll leave you to do the maths in future

Robin

[Robin Hewitt]

Originally Posted by lamicron Here the first pics of my "monster" .

It must be wonderful to have that kind of space to spare

[energyforce]

Robin im glad i met you on this board here. Hopefully we can check eachothers calculations. Do you go as far as calculating the inertia ratio of the servos? Apparently from a robotics point of view (especially with systems that are used for critical applications) it is required or highly recommended. I did this with my table. I didnt know the inertia of my servo so I did some research and used an educated guess. I know the inertia ratio has to be between 3 and 5? How about tuning the servos? Do you go as far as using an oscilloscope to tune your servos? I know they are expensive and no one has one in their garage haha. I know for these home built machines using youre ears to tune your servo is probably good enoug eh?