Anyone blown up a Rack/pinion? Belt Drives?

[JerryFlyGuy]

I'm looking at building a gantry style router/mill [still in the design stage right now]. I plan to use some fairly large steppers to drive the unit [ 1 on each end of the gantry] I set out trying to get as close to the same resolution from this set up as from a 3/4-10 screw. However to do this, I came out with a fairly high reduction ratio on my gear down. I planned on using a syncro drive belt to gear down the stepper.

Now the Questions:

1- Is it better to do your gearing thru only 1 belt or 2 or more belts? ie; If I do use two belts I could go thru two 3:1 step downs and come out w/ a 9:1 ratio within a fairly compact area, to do this in 1 step would involve a fairly large foot print [ if memory serve's me.. it was like 50" c-c on the pulleys and 1 pulley was over 20" dia. ... actually I think that was only a 5:1] Will combined belt stretch over two belts be a big resolution problem?

2- So say we get our two belt reduction set up, Next you want to spec as small a pinion gear as you can. I settled on a 1.0" PD gear. So quick numbers..

1160oz-in stepper X 9 [ reduction]=10440 /16=652.5#-in torque. Now we look back at our gear spec's and it tells me at 25 RPM that they are good for only 200#-in torque [ thats another memory number but its close] so.. we're over 3 times the rated load. Is this going to blow up on me? I noticed a note down below the spec's [ Boston Gear online Cataloge] that states you can go up to 3 times this "calculated" load. That puts me pretty close to the rating torque. Of course the life of the gear is in total relation to how hard you abuse it, if your doing huge accel's and decel's with a heavy gantry your probably apt to have more problems than someone who has set their unit up w/ lower ratings.

Of course we could increase the pinion size and get that rating up pretty easily however when we increase the size our resolution goes down by a factor [ Pi ] with this setup we have 0.0017" /step on a 200 step unit. This is fairly fine, however for my work its probably just right, the system is going to be sloppyer than this so.. its in the range. If we moved from 1.0" up to 1.25" PD on the gear that resolution jumps to 0.00218" /step, not quite doubled in 0.25" dia.

Anyone overdriven their rack pinion setup? How far? Ever had it blow up? If so under what conditions?

JerryFlyGuy

[NC Cams]

You REALLY need to know what "rated load" means for any machine component.

EXAMPLE: a bearing has a dynamic load rating of 100lbs (purely a number for illustration). This does not mean that you can put a 100 lb static load on it and it will run for the projected fatigue lift. Chances are, it will fail in a very short period as "rated load" is NOT what you can load it to but a labratory method of CALCULATING life using a controlled, industry standardized (not necessarily real world) method.

Now, if you start at a 10 lb load on our hypothetical bearing and ramp up to 90 or so, you'ldd see much more life that may or may net match the rated life a bit closer. The explanation is too invovled to go into in a message board. You can find out more by looking up "bearing L10 life calculation" in various bearing handbooks.

The point being made is this: rated load usually means something to the guy who MADE the part and something totally different to the guy who's going to USE the part and it does NOT necessarily relate to real live loading conditions/potential.

Do your research carefully. Run at a lower percentage of the rated load for anything and you'll see much better fatibue life. Otherwise, be prepared to feed the system parts on a regular and nauseating basis.....

[JerryFlyGuy]

Originally Posted by NC Cams You REALLY need to know what "rated load" means for any machine component.

NC, Well you've definatly brought some good info to light, I went back to the cataloge and looked at the spec.
What it states is the following:
Title: "Approx. Horsepower and Torque [ Lbs. Ins.] Ratings for Class 1 Service [ Service factor =1.0]"

I went and looked up the service factor ratings agian.. a 1.0 is:
"Moderate Shock- not more than 15 mins in 2 hrs.
Uniform-not more than 10 hrs per day"

This is a hobby machine so this would seem to apply.
A 1.25 Service factor is:
"Moderate Shock-not more than 10 hrs per day
Uniform- more than 10 hrs per day"

Down below it says..." Ratings are based on strength calculation. Basic static strength rating, or for hand operation of above gears is approximately 3 times the 100RPM rating."

The rating for my Gear [12 tooth, 12 DP-Steel] is:
25 RPM-0.08 hp & 202lbs. ins.
100 RPM -.31 hp & 196 Lbs. Ins.

I guess if worst came to worst, I could double up the gears on the same shaft and use 2 rack's per stepper. anyone ever done this? It might create more problems because of the larger overhang. Come to think of it, this is probably more load than I should be putting on 1 over hung gear through a 1/2" shaft, I'll have to add a bearing at the bottom of it.

Does that help maybe in opening the spectrum for advice?
Jerry

[turmite]

Jerry I don't quite understand your question. In your first post I though you were talking about the rack and pinion but after reading your second post I think you are talking about the timing belts. Which is it?

Mike

ps just for clarification, I run a machine with belts, gearing and rack and pinion.

[JerryFlyGuy]

Originally Posted by turmite Jerry I don't quite understand your question. In your first post I though you were talking about the rack and pinion but after reading your second post I think you are talking about the timing belts. Which is it? Mike ps just for clarification, I run a machine with belts, gearing and rack and pinion.

Turmite, Well I am actually planning on running both also. I can't direct drive my steppers as the movement per step would be huge, so I want to gear it down a little, however I'm not interested in paying $450 or more per planetary gear box so that leaves me with a syncro belt style reduction. So first of all I'm wondering if its better to do my reduction via 1 large belt/ pulley set up or two smaller ones. I could do it with just 1 set of pully's per stepper but the large pulley would be over 20" in DIA. and the c-c distance of the pulleys would be 50"+. That would give me a 5:1 reduction. The second way would be to do two smaller sets of pulleys [ still only on 1 stepper]. Two 3:1 pulleys works out to be like 7" c-c on the pulleys and the large pully is 3" in DIA. However on the down side... you have two belts for every stepper.

The other question I asked was to do w/ the rack and pinion side of things. I think thats been fairly well covered.. So I won't go through it all again.. unless there are any specific questions I could answer for you?
How big of a unit are you using? How large of steppers? What size gearing did you use?

Jerry

[turmite]

Jerry I am using an old retrofitted Digital Tool Router. It was designed to cover a 4X8 sheet of plywood with two different heads mounted 8" apart.

This thing is and has always been a dependable machine mechanically, well other than all those little drive axles in the transmissions I had to rebuild.

I will try to find the transmission sheet and scan it for you but if memory serves me correctly, it uses "close" to the following.

Motor mounted to trans box and coupled to driveshaft that holds a 1.24 to1.5 diam pulley. This is belted to a second shaft that has a 3" that is mounted to another shaft that has a 4.5 or 5" on the end of it. that is belted to the pinion which is about 1" in diam.

Wow that is confusing, but I think that is close.

I ran this machine for 3 years with 300oz steppers with full steps and I could rapid at 400ipm with no problem. I upgraded in the 4th year and went to 450oz using half/step and I did a rapid on 1000ipm with the service tech riding the granty. It took it about 15" to get to speed and about that much to ramp down.

I now use Mach2 with Gecko drives running microstepping and I would never go back. If you have not built or bought the machine yet, do yourself a favor and start from the beginning with microstepping. You will not be sorry.

Mike

[JerryFlyGuy]

Mike, let me see if I have this correct. On the stepper you have a 1.25 [ or 1.5"] pulley. This runs a belt to a 3" pulley (~ 2:1 reduction). There is a common shaft thru this pulley [ 3"] which has another pulley on it which is 4.5-5" this goes to a fourth pulley which runs on a shaft that is commons with the pinion? Do you know what size the last pulley is? Do you have any idea of how much your gantry weigh's? Pictures?

Thanks for the offer of information, this helps me greatly to compare what others have done to what I am building and their capabilities to estimate mine.

Jerry

[turmite]

Hi Jeff,

Forget what I posted earlier. I had them wrong. I could not find the print so I tool the end caps of my transmission box and here is what I found, again only estimates on size, but they will be real close.

Motor couple on to shaft with first pulley 1 1/2" This is belted to a second pully that is about 2 7/8" that is on a common shaft with another pulley that is 1 1/2". This pulley is belted to a 5" which is on a shaft with the pinion gear. Diam of the pinion is close to 1 5/8" diam.

I hope this helps. I forgot to measure the number of tpi on the rack as that would have helped you figure out the teeth on the pulleys. I'll keep looking for the print and will post it if I find it.

Mike

[JerryFlyGuy]

Originally Posted by turmite Hi Jeff, Motor couple on to shaft with first pulley 1 1/2" This is belted to a second pully that is about 2 7/8" that is on a common shaft with another pulley that is 1 1/2". This pulley is belted to a 5" which is on a shaft with the pinion gear. Diam of the pinion is close to 1 5/8" diam.

Mike, ok thats a great help! It looks like you have around a 10:1 reduction and your 1 5/8" gear will most likely be a 1.50PD gear. I looked it up and a 12DP-18 tooth [1.50pd] gear is rated for 379 lbs-in of torque. With a 450oz stepper you'll have (450 x 10)=4500/16=281 lbs-in torque. Thats way under the estimated rating for that gear. You mentioned that you have to rebuild some drive axles in the past? What happened to them? Do you know what size they were? also any guess's on how heavy your gantry would be?

You've been a great help in offering all this info! I'm making headway into what I need to design with all that you've passed on!

Jerry

[turmite]

Hi Jerry,

This machine is a moving gantry that has a piece of alum tube on each side. One side is the transmission and the other only holds the vee-bearings, and another pinion which is driven by a shaft running the length of the x axis that connects the two sides. The shafts were .375 in diam and the pulleys had been drilled and pinned with a .125 diam split pin instead of using 3 set screws at 120 degrees. Each shaft in the machine that had been drilled for that pin broke. When I replaced them I used the set screw holes in the pulleys, milled a small flat for them to set on and have never had to worry about them. Trust me, when one of those shafts break, it becomes very evident in a hurry. My problem back then was that I didn't know what lost steps were.

I hope this helps a little more.

Mike