6m X axis Rack and pinion or belt drive?

[D-man]

My experience, with belt drive on that size you will suffer belt stretch over time resulting in continuous adjustment.

R&P how it's done is you have the pinion in a aping loaded mechanism. From what I've read the correct tension on R&P is that you put a piece of paper between so it only fold and not tears.

[JAZZCNC]

At this length and your requirements I would go with belt drive if just cutting foam but done in a slightly different way to what your probably thinking.? Check these out. . .Bell-Everman Dual Carriage Servo Belt.wmv - YouTube
ServoBelt: amazing belt drive - YouTube

It's one belt on top of another with rollers and pinion, it's easy enough to duplicate. I've played with it a little before for long machine but found R&P was better suited to router with a heavy gantry but for light weight gantry cutting foam then it would work fine I'm sure.

If you do a search for Mike Everman beltdrive you'll find more about it.

[charlieuk]

At this length and your requirements I would go with belt drive if just cutting foam but done in a slightly different way to what your probably thinking.? Check these out. . .Bell-Everman Dual Carriage Servo Belt.wmv - YouTube
ServoBelt: amazing belt drive - YouTube

It's one belt on top of another with rollers and pinion, it's easy enough to duplicate. I've played with it a little before for long machine but found R&P was better suited to router with a heavy gantry but for light weight gantry cutting foam then it would work fine I'm sure.

If you do a search for Mike Everman beltdrive you'll find more about it.

That does look very interesting however my fear would be it would be prone to getting clogged up with foam dust under the belt. I would also like it to be substantial enough to use for cutting wood which i assume the belt should be all be it you would have to cut slower.

ignoring cost and setting up what are the disadvantages of rack and pinion on something this long?

many thanks charlie

[JAZZCNC]

That does look very interesting however my fear would be it would be prone to getting clogged up with foam dust under the belt. I would also like it to be substantial enough to use for cutting wood which i assume the belt should be all be it you would have to cut slower.

Well that depends where you put the belt and how you shield it plus you'll have the same same problem with R&P regards keeping foam out of the Rack. Belt drive will easily cut wood and it's also much better regards efficiency and accuracy, much less backlash than R&P.

ignoring cost and setting up what are the disadvantages of rack and pinion on something this long?

Not too many really other than it's less efficient and accurate than belt drive. In all honesty I'd personally use R&P has it's more proven and will easily cover your needs but you need to be aware of what's involved with R&P to get best from it.
One of things you need to be aware of with R&P is that it's less efficient with much more friction so will need more powerful motors/drives unless gantry is light weight.
Regards the gantry you say your having a cantilever design and would like possibility to cut wood.? Well unless you substantially build the gantry then you will struggle to cut wood has it will deflect at the outer ends. This will definitely take you into Nema 34 size motors which require larger drives with high voltage so bigger PSU to get the best from.

This leads to the next issue with R&P (And Belt drive.!!).? . . Resolution, Torque and Direct drive. The pinion size determines Resolution and even the smallest pinion size still gives a resolution that is too large to be useful. IE: If you use common Mod1 rack the smallest pinion is 12Teeth and 12mm DP so the affective pitch would be 12xPi= 37.7mm so your pitch would be 37mm which is quite large but nearly usable.!! . . .But it gets worse.??

12T pinion dimensions are too small and won't fit on the motor shafts so in reality you'll end up around 24-25T but now the DP is also 25 so 25xPi=78.5mm affective pitch.!! . . . . Massive and useless for anything other than linear speed.
Also on top of this you have friction to over come and R&P is quite inefficient at around 50-60%, compared to 80-90% for belt and 90-95% for ballscrews so this is why larger motors are required but with direct drive they would need to be larger still.
So to help reduce the motor size/load along with increasing resolution then you need to have a ratio applied, what ratio will depend on several factors.? Resolution and feeds required, Weight to move, motors used, sticktion to overcome.!! etc
Often 3:1 ratio is used which at 25DP is still around 25mm pitch which will give high feed rates around 20Mtr/min and given an average micro stepping of 800Ms give theoretical resolution of 0.03mm, say theoretical because you won't actually get that but even if it was double still ok.
You mostly need the ratio to give the torque and get the Resolution down to a level that the motors work best with.? One of the problems with Low resolution is the speed, the motors struggle and are very unhappy when low feed rates are required has they are hardly spinning so can give very rough action. The ratio needs to be a balance of providing the best torque and resolution while still keeping the motors spinning in speed range they like at lower feed rates.

So has you see it's not just simple has R&P or Belt drive, which by the way you still need a ratio for belt drive has most of the above applies with the exception it's more efficient so less Sticktion to overcome and can often use smaller Nema23 motors.

Hope this helps rather than blows your mind. . .Lol

[Montegoman]

Hi

I just wondered if anyone else has used a tractor type "pinion" on a belt rack?
The small belt is double sided and is quite wide but the belt rack needs to be even wider, this allows for clamping it to the frame.
The belt rack can be any length you like.
I have found that there is no backlash as the tractor is totally engaged with the toothed pulley and the belt rack at the same time.
This type of setup will move a lot of weight, quite quickly if you want it to.
Try to design the frame and the rack so that the belt rack is facing downwards, this helps to keep it clear of waste materials.
However that is not easy to do.

I hope this helps to someone.
I can forward the suppliers name and address if required.

regards
Montegoman