Is there any "rule of thumb" to know how much would a 6mm GT2 belt creep/stretch when tensioned?

Hi guys, I hope this is not a very stupid question but I would like to know what difference in length one could expect after crocheting the GT2 belt through the corexy XY stage and tensioning it via whatever mechanism? Say I can manage to have 1500mm of belt per motor. I will pull one end and insert it in a toothed printed part. How much would the belt elongate during this procedure? I don't have the belts yet so I can't test (if you wondered).

Edit: on the other hand, how many gripper teeth is enough for a stable system?

Thanks.

Edited 3 time(s). Last edit at 06/13/2016 05:03PM by realthor.

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RepRap Lander concept on Concept Forge
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Don't know about the stretching, but read one of those engineers manuals somewhere that said that spring loaded tensioners were no good of the belt was running back and forwards. They should only be used on belts that turn in one direction apparently.

Um, if the belt turns in one direction you can't use a spring tensioner because it will run into the pulleys.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

you can't use a spring tensioner if the belt turns in one direction, like the belt on auto engine.

Please advise the auto engineer designers

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the_digital_dentist
Um, if the belt turns in one direction you can't use a spring tensioner because it will run into the pulleys.

You can use your force measuring thumb to do this?

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the_digital_dentist
Um, if the belt turns in one direction you can't use a spring tensioner because it will run into the pulleys.

Not one of those little spring clips, I meant a spring loaded idler pulley.

I didn't mention a car motor either.

Spring loaded idlers are best applied on the low tension side of the belt loop. If the belt reverses direction, the idler alternately operates on the high and low tension sides of the loop. Since it doesn't work well on the high tension side, it's best to use only on belts that don't reverse direction, and only on the low tension side of those belt loops.

It's similar to the derailleur on a bicycle. The derailleur is always on the low tension side of the chain loop, and couldn't do its job on the high tension side.

Edited 1 time(s). Last edit at 06/14/2016 12:36AM by the_digital_dentist.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

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the_digital_dentist
Spring loaded idlers are best applied on the low tension side of the belt loop. If the belt reverses direction, the idler alternately operates on the high and low tension sides of the loop. Since it doesn't work well on the high tension side, it's best to use only on belts that don't reverse direction, and only on the low tension side of those belt loops.

It's similar to the derailleur on a bicycle. The derailleur is always on the low tension side of the chain loop, and couldn't do its job on the high tension side.

That's actually roughly what I meant.

Please excuse me if it didn't come across clearly.

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cozmicray
you can't use a spring tensioner if the belt turns in one direction, like the belt on auto engine.

Please advise the auto engineer designers

[attachment 79727 Auto_TB.jpg]

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the_digital_dentist
Um, if the belt turns in one direction you can't use a spring tensioner because it will run into the pulleys.

Er, actually as an ex automotive engineer, this is something I do know about and you both right and wrong. Let me explain.

With a camshaft timing belt (which is what all the pretty pictures show), a sprung loaded idler can be used but only to set the initially belt tension, then it's locked in place. So basically, when you change a cam belt, you loosen the bolt which is holding the idler in position, then you can back off the idler against it's spring and lock it in that position. Then you fit a new belt and line up the timing marks. Once that is done, you undo the bolt which is holding the idler back and let the spring push the idler gear against the belt to set the initial tension. Then you lock the idler in place so the spring is no longer doing anything.

The reason it has to be that way is because of something called torsional vibration. (actually that might not be the correct term but it'll do). If you imagine a very simple cam shaft with a single lobe operating on the stem of a single valve. The valve is held shut by a spring. A very strong spring in fact. That is because, if we have a 4 stroke engine doing 6,000 rpm, the valves open and close 3,000 times a minute. That's 50 times a second and we don't want the valves to bounce off their seats when they close, hence a very strong spring. So when we turn the camshaft, initially the lobe pushes down on the valve stem against the spring until it reaches the point where the top of the lobe is pushing the valve as far open as it will go. If we continue to turn the shaft, the valve stem starts to go down the back side of the cam lobe, driven by the spring. So at this point the valve spring is now trying to turn the camshaft faster than it wants to go. For something like 180 degrees, the cam lobe is more or less a circle so no torsional force. For the next 90 degrees or so, the cam is trying to push the valve open against the torsional force exerted by the spring. Then the lobe reaches its peak and for the final 90 degrees, the valve spring is trying to push the cam lobe round and the torsional force completely reverses direction. If the belt wasn't constrained by fixed pulleys, it would try to accelerate and decelerate very quickly depending on the direction of torsional forces being applied at any one time.

Of course, that is a very simple explanation because in reality a 4 cylinder engine will have at least 1 inlet and 1 exhaust valve per cylinder meaning at least 8 lobes to our camshaft. Or we could have a V12 with 4 valves per cylinder but in that case it is likely to have 4 camshafts ( one for inlet valves and one for the exhaust valves on each bank of the vee). The same principle of torsional vibration applies though, but the magnitude and frequency of that vibration will be be different depending on valve/camshaft configuration. Either way, you couldn't rely on a spring to maintain the belt tension.

So (at least in automotive engineering) a spring loaded idler can only be used to set the initial belt tension. After that, the idler must be locked in place. I'll leave you guys to debate whether or not that is relevant to 3d printers (I know what I think).

I don't know how much 6 mm wide belt will stretch, but for the same tension, 9 mm wide belt will stretch less. It is a minor change in design to use 9 mm vs 6 mm belt, and cost difference is minimal, so I'm going to use 9 mm wide belt. When numbers aren't available, I tend to err on the side of overbuilding and it has worked well for me so far.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I know very few designs that use the kind of spring that d_d reffered at the top of the thread and those are pretty much known to be wrong designs. I am talking about designing a lock-in-place tensioner, either at the steppers mounting plate or at the printhead.

Because there is no easy way to tension a belt to the proper value by the thumb-test cozmicray was talking about, I thought that there might be a proper tension at which the 6mm gt2 belt would elongate X mm. That is easier to measure than using other methods: just mark the belt at some point and tension until that point is now 2-3-6-etc mm away from the first point.

I know that it will be different between the various base material/reinfocing fiber but let's be honest, most of us will be using chinese black type gt2 belts or white steel reinforced ones. A knowledgeable guy with a "trained" thumb or other measuring device could possibly do a test and report the numbers for all us the rest to avoid under or over-tensioning, thus introducing yet another variable where the performance of our repraps will degrade.

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RepRap Lander concept on Concept Forge
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Right-On Deckingman
Yes right on --- power transmission belt for dragster blower (extreme power transfer)
has idler to set tension not continuously (4 sec) run.
Other devices run off belt (water pump, air pump, oil pump etc.) are fixed (not spring tensioned)


My Father went over to Turin, Italy to work on Fiat 124 spyder
The double overhead cam (DOHC) version was the first mass manufactured DOHC to utilize reinforced rubber timing belts,

Also belts for IBM selectronic typewriter

Do you know of any work by the timing belt manufacturers for back and forth applications in CNC, 3D printing etc.

I think for 3D printer the tension device should fix initial tension of the belt, then just let it run
Tweeked every once in a while, to take out stretch and other slack

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Deckingman
Er, actually as an ex automotive engineer, this is something I do know about and you both right and wrong.

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the_digital_dentist
I don't know how much 6 mm wide belt will stretch, but for the same tension, 9 mm wide belt will stretch less. It is a minor change in design to use 9 mm vs 6 mm belt, and cost difference is minimal, so I'm going to use 9 mm wide belt. When numbers aren't available, I tend to err on the side of overbuilding and it has worked well for me so far.

That's what I've done, but the belt I've ordered was described as 10mm.

For long belts in a corexy system it seemed the thing to do.

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cozmicray


Do you know of any work by the timing belt manufacturers for back and forth applications in CNC, 3D printing etc.

I think for 3D printer the tension device should fix initial tension of the belt, then just let it run
Tweeked every once in a while, to take out stretch and other slack

No (about timing belt manufacturers for CNC, 3d printing etc), and my automotive knowledge dates from the 60s through to around the year 2,000 - lost track of developments since then. I can tell you all about 3 way catalytic converters and emission control strategies, why you should use dwell angle rather than setting the points gap with feeler gauges, how to balance the airflows on a Ferrari V12 with 6 twin choke carburettors, how to adjust the timing on a 12 cylinder distributor with a 3 lobe cam and 4 sets of points and all sorts of other stuff, but I don't suppose any of that is much use either.

I agree with the set the initial tension thing, then lock it. The whole point of timing belts is that they shouldn't stretch. How often does you garage check the timing belt tension when you get your car serviced? (answer is never - they just change them after 40,000 miles or so because they can perish over time, but they never adjust the tension once it is initially set). V belts as used to drive the alternator, water pump etc are a little different as they can wear.

Spring tensioners seem to be wrong way to go, whilst there are no valve springs pushing on the belts like in the engine, a sudden direction change is likely to compress the spring and result in inaccuracy. The same applies to the spring clips (inside of a clothes peg type things) a rapid direction change compresses the spring before the carriage moves. I've seen printed tensioners which sit on the belt, deflect the spring and are then locked which seems like a better idea, but with corexy it's tension relative to the other belt that counts so...

I was thinking about using some digital luggage scales attached to a pulley with an adjustment screw to measure and set the tension. I've got the scales they measure 10g increments whether they're very accurate I don't know but when I connect two scales to each other and pull lightly they both read the same, so they're consistent. For a corexy two identical devices would allow the two belts to be set to the same tension haven't got very far with it as there's no nice way to mount them so I can see the displays.

The steel reinforced belts I currently use will not stretch much the reinforcing wires are high tensile steel. I have to use tin snips to cut them.

Edited 2 time(s). Last edit at 07/07/2016 12:12PM by DjDemonD.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

I have a music tuning app on my phone. Just ping the belt (like a guitar) and measure the frequency. Easy to get the 2 belts the same tension if the lengths are the same (they should be down the Y axis).

I tried that and didn't get a reading from the app, which one do you use? Maybe my belts are not tight enough.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

CarlTune. I just tested it on my Mendle 90 machine and one belt is 103.52 Hz and the other 67.87 Hz. Yes quite a big difference which you can hear. Actually unless you were tone deaf it would be pretty easy to do it just by ear.