Thanks for the feedback...I figured that the required increase in motor speed and torque would be a challenge.  This was just a thought which I probably will not execute as the trade-offs are not worth the potential accuracy gains.  Fun to think through the potential options though...

Are you planning on executing your design?

pirvan wrote:

Hazer wrote:

BTW, that is a bad place for your tension pulley. You add tension by pushing it into the other belt, which is teeth-to-teeth.

Beg to differ on that.  Take a look at the timing belt system in your car, it has a tensioning idler pulley.  It's always pressing on the smooth side, not the toothed side.  That's because this pulley, as it presses against the belt needs to be able to slip if necessary.

Hazer wrote:

You would be better off mounting one pulley directly on the motor shaft, and have a motor mount with oval/long mounting holes so you can slide the motor in order to tension the belt.

The motor would be mounted directly to the "driving pulley".  The idea of slotted motor mount to allow for extra tension adjustment is good, I like that.

But the whole point of a spring loaded tensioning idler is to continually keep the belt in tension even if something slips or gives out.  The initial tension adjustment will be done with the adjustment screw on the carriage, and/or by sliding the motor in it's mount as you suggested.

I like the idea of keeping a constant tension on the belt!

Here is something I have been pondering for a while; it uses twin lead screws and one driving belt.  My thought is that it would increase the accuracy due to the screws requiring MANY more steps per mm of travel.  Problem is I would have to put a screw on the carriage as well so both axes have the same precision... Your thoughts?

I was looking at your design, and you might want to reconsider your design.

If you're thinking of using a leadscrew with a tight pitch in order to increase the umber of steps per mm of travel, you'll find that the speed will dramatically affected to the point of being way to slow.  Leads screws are OK for the Z travel, but for the X/Y motion, you have to be careful what you pick.  There's a fine balance between speed and accuracy.

I had a similar design I put together a while back using ballscrews. 
http://www.soliforum.com/topic/2834/pre … m-scratch/
The high precision screws I have move 5mm for every 360° of rotation.  Our belt system moves about 36mm for 360° of rotation.  That's 7 times faster.  So to achieve the same performance with the ballscrew, I would have to drive the motors at 7 times the speed.

You would be in a similar predicament.

There is one way to get around this, create a gearing system that increase you gearing ratio on the output (I'm not sure if that;s the right terminology).  Anyway, you could put a larger gear on the motor shaft, driving a smaller pinion gear on the lead screw.  Your gear ratio would have to be such that it compensates for the difference between the belt and the lead screw. 

The drawback to this is that your torque may not be enough to drive the setup, requiring an upgraded motor.

Got it...maybe a smaller idler with teeth deployed on the interior of the loop biased in one direction...

Hazer's concern is that the tension in the idler could push the belt into the path of the other belt in front of it, and since both of those faces have the teeth, they would clash.

That concern is valid, but only if you allow the deflection to be excessive.  You can also move the inner belt pulleys in slightly during build to create more spacing between the belts (light red outline).

As I mentioned earlier, you don't want a tensioning idler with teeth.  You want it on the smooth side of the belt.

Hazer wrote:

BTW, that is a bad place for your tension pulley. You add tension by pushing it into the other belt, which is teeth-to-teeth.

Beg to differ on that.  Take a look at the timing belt system in your car, it has a tensioning idler pulley.  It's always pressing on the smooth side, not the toothed side.  That's because this pulley, as it presses against the belt needs to be able to slip if necessary.

Hazer wrote:

You would be better off mounting one pulley directly on the motor shaft, and have a motor mount with oval/long mounting holes so you can slide the motor in order to tension the belt.

The motor would be mounted directly to the "driving pulley".  The idea of slotted motor mount to allow for extra tension adjustment is good, I like that.

But the whole point of a spring loaded tensioning idler is to continually keep the belt in tension even if something slips or gives out.  The initial tension adjustment will be done with the adjustment screw on the carriage, and/or by sliding the motor in it's mount as you suggested.

I like the idea of keeping a constant tension on the belt!

Here is something I have been pondering for a while; it uses twin lead screws and one driving belt.  My thought is that it would increase the accuracy due to the screws requiring MANY more steps per mm of travel.  Problem is I would have to put a screw on the carriage as well so both axes have the same precision... Your thoughts?

BTW, that is a bad place for your tension pulley. You add tension by pushing it into the other belt, which is teeth-to-teeth.

Beg to differ on that.  Take a look at the timing belt system in your car, it has a tensioning idler pulley.  It's always pressing on the smooth side, not the toothed side.  That's because this pulley, as it presses against the belt needs to be able to slip if necessary.

The motor would be mounted directly to the "driving pulley".  The idea of slotted motor mount to allow for extra tension adjustment is good, I like that.

But the whole point of a spring loaded tensioning idler is to continually keep the belt in tension even if something slips or gives out.  The initial tension adjustment will be done with the adjustment screw on the carriage, and/or by sliding the motor in it's mount as you suggested.

BTW, that is a bad place for your tension pulley. You add tension by pushing it into the other belt, which is teeth-to-teeth. You would be better off mounting one pulley directly on the motor shaft, and have a motor mount with oval/long mounting holes so you can slide the motor in order to tension the belt.

Regarding the belt, it's one piece.  One end attaches to the side of the carriage block and is fixed, the other endloops through everything and attaches to the other side of the same block, but it's adjustable for tension in a manner similar to what we have right now.

At the other carriage block it will go through, but it will have something like a clamping mechanism that can be released, so you can adjust the "X" carriage so it's square to the "Y" rails.  Basically a simple mechanical adjustment akin to the adjustment that Solidoodle recommends to align the "X" carriage for perfect circles.

I haven't come up with  mechanism design yet, but I'll work on it in the next few days. and post some details.

As for Spectra line, I'm not a big fan of the fishing line type setup.  I have a Printrbot that has that setup and the accuracy is just not there.  I actually modified it to use a rack & pinion system instead.

As for your design, I like it. Granted, its alot of belt for just the Y axis, but it really does simplify the tension and gives you the option of putting bearings into the system. Without any firmware changes. And with belting as cheap as $2 a foot, who cares how long it is?

Perhaps someone with more knowledge on the subject should comment on this.

Never the less, I wasn't trying to achieve a CoreXY or H-Bot implementation, I was only trying to get rid of the drive shaft and use a single, self tensioning belt.