[Nuthead]

Hello all,

I'm in the early stages of planning my upcoming build, which will also be my first which has led me to alot of research lately. One thing I cannot come to terms with so far is my decision with my motors connection to the screw. On one hand the direct connection via a coupling seems to certainly be the easier of my two choices as well as more cost effective. It seems like the most straight forward and obvious choice but I'm nervous about the motors cantilevering off the assembly in my shop and getting hit over time. So that brings me to the idea of using a timing belt pulley system which is marginally more money but helps with my concerns about the exposed cantilevered motors that is, if I opt to go this route I would be inverting there positions back within the assembly. There also seems to be a benefit of being able to change the gearing down/up, later as long as I size the motors with extra headroom to do so. So with those thoughts in mind, that has me leaning towards the pulley setup but I still have a reservation to it, especially keeping in mind this is my first experience. My reservation is this, assuming I properly maintain correct belt tension, will there still be some loss of precision by nature of a belt system as compared to that of a direct drive to the point of affecting my repeadibility? I've seen other machines with pulleys, so I know its been done before but for whatever reason that has not eased my mind. Also what about wear? I guess at the end of the day I just want to know more before I go pluging forward. So any insight is much appreciated.


Thanks

[ger21]

Use belt profiles that have minimal backlash, like GT2, and you won't have any issues at all. They should last a very, very long time in this application.

[JerryBurks]

You may actually have the space for direct connection, depending on your design. You usually need to space the linear bearings of an axis somewhat apart for rigidity to prevent torque loads on the bearings (typically 6"-12"). That means the actual stroke is that much shorter than the length of the linear rail. It also means the screw can be shorter than the rail and if designed properly the combined length of the screw and motor with coupling may fit "into" the machine instead of the motor hanging out. Example: I have 48" rails for x-axis and have the linear bearings spaced 12" apart, leaving 36" theoretical max. stroke (practical somewhat less). This 12" reserve is sufficient for the end bearings, the coupler and 4" long dual shaft NEMA34 motor with 20mm synchronizing belt to the other side screw. It all fits under the rail.

[Devastator]

I have timing belts on all of my axes and have never had a backlash problem. I believe they are the reason I can run my machine with low cost drivers without dampers because the belts add damping. And I don't have to worry about perfectly aligned couplers/shafts or axial loading the stepper bearings.

[wizard]

You have nothing to worry about when it comes to belt drive. I've worked on machines keeping production tolerance of plus and minus a couple of microns that ran all day with 6 second cycle times and much larger machines that where far slower and no where near as accurate. Use good quality timing belts designed for this usage and you won't have problems.

One word of caution there is an art to setting the proper tension on a belt. If you are using servos in a high gain environment you can get oscillations due to the belts and the tuning of the drive. Obviously drive tuning is a factor here but the belt tension often needs to be just so, not too tight and not too loose. As for the pulleys, the ratios selected are often for inertia matching and sometimes to get the resolution at the screw you want.

In most cases I see belt drives as a positive thing in a design. It makes access to the leads crew easier for one and as you note folds the motor out of the way. You don't need to fold the motor out of the way in every design but accessibility is a big positive in my mind. The worst thing here with belt drives is the potential for belt breakage, if that can be a problem ( for example where you need a brake) then a belt drive might not be a good idea.

[SteveWill]

I have not used the belt drive. I built my first machine this year. I was worry the stepper sticking out would be a problem, that has not been a problem. If it was me and with the feed back You have gotten here. I would try the belt, but I would make the design so it would be easy to go to direct drive if do do not like the belt system.

[blau_schuh]

I used GT2 belts on one of my machines, and in the current one I am building. Never had an issue...

My first machine used dinky little (~50 oz-in cont. ?) DC servo motors and using a pulleys let me run a heavy gantry on really small motors. Speeds took a hit but I still could get 150ipm out of it on rapids.

I think the added expense is worth it.

[Nuthead]

So after contemplating this a bit I am leaning towards the belt drive route. Obviously some of you mentioned a type gt2 but I've also seen other post about xl. What should I be looking for while considering the different types? Both the gt2 and xl have similar profiles generally speaking. So I'm guessing that's the recipe, not to fine while also not being to course with moderate teeth count.... Open to more info

[blau_schuh]

I think gt2, xl, or HTD would fit the bill as the pitches are close (3mm to 5mm), I'd stay away from "L"... i actually think I've got an XL or HTD on one of my axis on my new machine.

I think you want something with a kevlar or fiberglass strip in it to keep it from stretching.

I bought all my stuff from Small Mechanical Components: Precision Gears, Gear Assemblies, Timing Belts, Timing Belt Pulleys and Couplings - SDP/SI (also has a handy calculator to figure out pulley pitches/belt lengths)

Also if you can't find an exact pulley w/bore size to match your ballscrew/motor shaft diameters, I would think about stepping up to an all aluminum pulley and drilling/reaming to get the right diameter. More money, but you get a bit more "meat" on the pulley to work with.

[ger21]

Both the gt2 and xl have similar profiles generally speaking. So I'm guessing that's the recipe, not to fine while also not being to course with moderate teeth count.... Open to more info

GT2 has much less backlash than XL, due to it's different tooth profile. It also comes in different pitches. You probably want to use either 3mm or 5mm pitch.