Short timing belt - fixed motor or adjustable?

[blades741]

Hello Engineering community,

I'm in the process of doing some major modifications to my X-Carve CNC router. I've already converted it to ACME screw drive, and am about to upgrade once again to linear rails.

My question is this: When designing for my timing belt that drives the screws, is it necessary to design-in adjustability for the center-to-center distance if it's very short? The dimensions in question are a 15mm wide GT3 timing belt, 70 tooth, running on 20t pulleys which come out to a 75.0mm center distance. My previous design has been running these belts for several months now, and have never required adjustment. I measured the actual assembled distance with the belts nicely tightened, and it measured exactly 75mm at all 3 locations (2 Y-axis motors and 1 X-axis motor).

In the realm of machine design, with this short of a timing belt, is it common practice to design in adjustability? Or fixed distance? Since I need to make a new set of machine plates, I'd like to know if I should design a fixed distance or keep the slots (currently +/- 3mm from 75mm centers).

Thanks for any input!

 

[dgallup]

I would think you need some adjustability simply to install the belts.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[3DDave]

Even if one assumes the belt and pulley are perfect, the stiffness is such that the machining of the mounting will have too much variability to provide sufficient tension - not too much and not too little. Being very short means that everything is much stiffer, so the need for precision/adjustment is higher. Sometimes an adjustable intermediate tension pulley can be used.

It also makes it a lot easier to get the belt on if the pulleys have flanges.

 

[blades741]

Not really, initial installation is easy to accomplish by merely installing the bolts after the belt is in place or by installing 2 bolts loose and tipping the motor. Either way, installation is not part of the concern.

 

[blades741]

Repeatability and accuracy have not been a problem thus far on previous plates I've had laser cut. But you do bring up a good point about the ability to do precision adjustment. I was mostly wondering what the 'norm' is for such an application, in the realm of machine design.

 

[Compositepro]

Generally, an adjustable idler or spring loaded tensioning pulley is used on the slack side of the belt. If your drive is bidirectional the idler pulley should be screw adjustable to take all slack out of the belt. Otherwise you will have positional hysteresis when changing directions. Without any adjustments you are relying on the springiness of the belt and machine frame to make this work.

 

[blades741]

Yes, in this case it's bidirectional, and I understand what you're saying. In this case, while not as technically "elegant" as an adjustable idler pulley, I think the adjustment slots allowing motor movement for tensioning will be more feasible than the extra components required by an idler arrangement.

Thank you for the input everyone. I will consider my question answered. Sticking with my original slot configuration.

- Bill

 

[itsmoked]

My router has very short belts AND slots in the motor mounts.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

You implied in the first post that you had no adjustablity. Slots for mounting screws are one common, but crude, method of adjustability.

 

[blades741]

@Compositepro No, I currently have adjustability (see pic attached to first post), but my question was related to my replacement plates I'm going to have made. I was questioning the need to have adjustability, based on using such short belts that, quit honestly, will probably never stretch to the point of actually requiring tightening. Also based on my assembled and "adjusted" position of the motors, which come out to exactly 75mm. Based on the measurements, it appears that I could have used a fixed-position setup.

I was more interested in what the industry 'norm' is, however you addressed that in your previous reply when you mentioned the tensioning pulley, and Keith also mentioned that his router has very short belts and slots. That tells me I should, likewise, retain some means of adjustability.

@Keith, what type of router is that?

 

[itsmoked]

I do feel for you as slotting something is a royal PITA as compared to just drilling holes.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[blades741]

Actually it's not a big deal, as these are going to be laser cut, so basically the only effort is in the design. It's more about the quality of the final assembly. I felt that if I didn't have to slot the motor mounts (tensioning idlers are not an option), that the belts would be tensioned identically, or at least within the manufacturing repeatability of the belts themselves.

 

[Compositepro]

Do provide slots. Since you are laser cutting the best option is to use one tight round hole as a pivot point and have the other three slots as arcs around this point. That is much easier to adjust to the correct tension. Laser cutting will not provide super precision holes.