Shapeoko 3 belt tensioning technique(s)

Assembly questions and answers for the Shapeoko 3 CNC Mill
TomO
Posts: 171
Joined: Mon Dec 29, 2014 1:19 am

Re: Shapeoko 3 belt tensioning technique(s)

Post by TomO » Tue May 19, 2015 1:29 am

Are there any concerns of having the belts too tight? Any simple test like you should be able to put _ _ _ between the belt and rail?


Sent from my iPhone using Tapatalk

CastIrony
Posts: 650
Joined: Mon Jul 21, 2014 10:21 pm
Location: Portland, OR

Re: Shapeoko 3 belt tensioning technique(s)

Post by CastIrony » Tue May 19, 2015 1:36 am

The belts being sold by Inventables (the ones from the Shapeoko 2, and AFAIK similar to the ones in the SO3) have the following strength specs:
124 lb / 56kg Breaking Strength
6.25 lb / 2.8kg Working Tension
Which means, as I'm reading it, that you'd have to apply 20x the intended tension before you'd be in danger of breaking the belt. I don't know if there's a failure state before that, maybe stretching the belt so much that the teeth don't mesh with the gears anymore, but I think you'd notice if something like that happened.
Shapeoko 3 #192 - Complete! Needs limit switches.

TomO
Posts: 171
Joined: Mon Dec 29, 2014 1:19 am

Re: Shapeoko 3 belt tensioning technique(s)

Post by TomO » Wed May 20, 2015 11:28 am

My concern is mainly with the force put on the motor by an over tensioned belt. Anyone have issues with this in the past?


Sent from my iPhone using Tapatalk

WillAdams
Posts: 8542
Joined: Mon Apr 09, 2012 6:11 pm
Location: Pennsylvania --- south of the Turnpike, East of US-15
Contact:

Re: Shapeoko 3 belt tensioning technique(s)

Post by WillAdams » Wed May 20, 2015 12:28 pm

AIUI, good quality stepper motors are rated for industrial usage --- our lightweight machines and intermittent / occasional usage barely registers in comparison.

That said, excessive tension is a concern, which is why I'd prefer not to just torque down on a mechanical connector w/ the combined mechanical advantage of a lever (the Allen wrench) and screw.

Presumably excessive tension would result in uneven wear on the bearings?
Shapeoko 3XL #0006 w/ Carbide Compact Router w/0.125″ and ¼″ Carbide 3D precision collets
Nomad 883 Pro #596 (bamboo)

Hans
Posts: 290
Joined: Tue Dec 25, 2012 5:19 am
Location: Connecticut, USA

Re: Shapeoko 3 belt tensioning technique(s)

Post by Hans » Wed May 20, 2015 2:19 pm

While it's easy to exceed the rated overhung load on the motors, it's a lot like the rating for the belt in that the actual failure load is many times higher. With a conservative estimate that the distance from the front bearing to the belt is equal to the distance between bearings, using the 2.81 kN dynamic rating for SKF's .25" bore bearings, taking C/P = 3 and then downrating an additional 50%, you're still at about 50 lbs acceptable load. I think stepper motor manufacturers rate them lower so that the bearings don't go bad in heavy service, with years of dust ingress, shock loads, etc. Heck, the motor shaft will probably fail before the bearings: I get 30 lbs max working load designing for 20 ksi shaft stress and 1" overhang. Note that this is all for the S3 NEMA 23 motors with .25" shafts, and working load for the little NEMA 17s would be closer to 12 lbs (6 lbs tension) by the same method.

In short, I think it's a safe bet to overload everything 2X or so because our application is so light-duty. 10-15 lbs belt tension gives 20-30 lbs overhung load on the motors, so I say go for it. Now if only I could get that much tension on my S1...
WillAdams wrote:I'd prefer not to just torque down on a mechanical connector w/ the combined mechanical advantage of a lever (the Allen wrench) and screw.
You're not going to put thousands of pounds on the belt even though the mechanical advantage would allow it, because the belt isn't stiff enough to resist with that kind of force. If I'm reading the Gates belt tensile properties doc correctly (linked on the belts & pulleys wiki page) then 600mm long, 6mm wide, 2M GT2 belts like the S3 uses should take ~.17" of stretch from just-barely-taut to 12.5 lbs tension. In reality the stretch-vs.-tension relationship breaks down near zero tension, but if there's a consistent way to get to just-barely-taut tension then we could specify a number of turns on the screw to achieve recommended tension.
Shapeoko #1,123

cvoinescu
Posts: 4442
Joined: Thu Jul 19, 2012 6:50 pm
Location: Camberley, UK
Contact:

Re: Shapeoko 3 belt tensioning technique(s)

Post by cvoinescu » Wed May 20, 2015 3:43 pm

FYI, it's actually better, because NEMA 23 motors from Wantai Motor have 8 mm diameter shafts internally, so their bearings are even larger. They might be 608, but I don't remember and I can't be bothered to open up a motor now. (Their NEMA 17 motors come with 625 bearings, same form factor as the V-wheels.)
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

Post Reply