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JamesCH95 07-09-2012 16:13

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184575)
You have to assume that an exact c-c will run at a reduced strength.

For a given toothcount, beltwidth, etc... combination the strength is proportional to tension in a bell curve shape; too little or too much tension is a strength loss.

With exact c-c you are assuming you're merely high enough on the peak, it would be foolish to believe you both picked the EXACT best c-c value and had zero error machining.

The lack of a tensioner is certainly attractive with less work to fab, assemble and maintain. The cost of this however is reduced strength compared to a properly tensioned setup.

The question to answer is which weighs and costs more (in a holistic sense): wider belts and pulleys, or tensioning mechanism?

Brian Selle 07-09-2012 16:39

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184575)
The lack of a tensioner is certainly attractive with less work to fab, assemble and maintain. The cost of this however is reduced strength compared to a properly tensioned setup.

This. Earlier in the summer I emailed Gates about doing C-C drives and while their engineer said it could be made to work they recommended tensioners to dial things in and maximize performance. One of the biggest things I learned was that while stretch is almost negligible (0.00529" for a 700mm belt), the manufacturing tolerance is not (+/-0.024" for a 700mm belt). For this size belt it means +/-0.012" adjustment in center distance.

pfreivald 07-09-2012 17:58

Re: Timing belt in drive success
 
We loved our octocanum drive last year. Loved. It. It was, however, too heavy, too expensive, and too hard to work on. This fall is dedicated to fixing all three. My answers are about last year's system (which was only possible to us because of Gates. They rule.)

Quote:

Originally Posted by AdamHeard (Post 1183704)
-Was it 1:1?

Nope. Direct drive 6" mecanum (from BB p.80 9:1's) to 4" Andymark performance.

Quote:

Originally Posted by AdamHeard (Post 1183704)
-Toothcount on pulley

24 on the mecanum, 48 on the performance.

Quote:

Originally Posted by AdamHeard (Post 1183704)
-Profile and Pitch

Gates GT2, 5mm.

Quote:

Originally Posted by AdamHeard (Post 1183704)
-Belt Width

Uh... 1 cm?

Quote:

Originally Posted by AdamHeard (Post 1183704)
-Wheel Diameter and Type (roughtop, colson, etc...)

"Glidey" AM HD Mecanum (6") on the primary drive, "Pushy" AM Performance (4") on the secondary drive.

Quote:

Originally Posted by AdamHeard (Post 1183704)
-Gear ratio before the pulley (high and low gear)

9:1 (Banebots, direct drive). After the smaller wheel and larger sprocket we were at 15:1 on our roughtop AM performance wheels. We were not 100% happy with the pushy wheels on the bridge, though they were awesome on carpet.

Quote:

Originally Posted by AdamHeard (Post 1183704)
-tensioners?

Nope.

Quote:

Originally Posted by AdamHeard (Post 1183704)
-If exact c-c, did you add/subtract from this number? Also, what type of machine was the part fabricated on.

This past year we used only COTS parts with no modification aside from drilling bore holes and making keyways in the sprockets.

Sorry... Quoted first, saw the Google Docs part last, and am too lazy (and/or busy) to post there. :)

Adrian Clark 07-09-2012 18:09

Re: Timing belt in drive success
 
Quote:

Originally Posted by DampRobot (Post 1184538)
Actually, I would guess not. I was doing some calculations for bevel gear max HP and torque, and my results were within rounding error of the published values. Perhaps Gates divides by a safety factor, but I would suspect not.

Also, keep in mind that the amount of torque that the pullys will start slipping at is highly dependent on the tension on the belts. Perhaps all these stories of belts slipping is less because they exceeded their rated HP and torque, but because they were in a center to center design and couldn't be properly tensioned.

Gates does use safety factors, they call it service factors, but I don't know if their service factors are incorporated into their charts. I would suspect that the numbers in the charts are not generated with any application in mind, they're just a very black-and-white representation of torque and speed capabilities of gates products possibly rounded up or divided by a safety factor.

The service factors gates provides are used by multiplying the factor by the horsepower and using the product to determine sizes. It's hard to fit an FRC robot into a service factor category, since the loads are so diverse and the runtimes so short. But the lesson to be learned from the service factor chart is how important it is to tailor the belt drive to the application and not just the HP. The gates service factor chart ranges from 1 all the way to 2.5 (2.5 is constant duty), with direction-changing applications having the highest service factor. All that being said, it's pretty clear that application is extremely important when designing a best drive and for that reason the gates charts are more of an indicator than something to be followed exactly.

tl;dr belt drive failure comes from poor execution, wrong pulley size, and not having the experience and engineering intuition necessary to do it right.

roystur44 09-09-2012 15:44

Re: Timing belt in drive success
 
[quote=JamesCH95;1184543]My limited experience with belt drives tells me that exact C-to-C is fantastic.

C to C will work for a lot of short run cases but when running belts in a drive train we would hesitate using permanent center to center distance without some adjustable tensioning device. You need to think of the assembly and maintenance of the system. Things like how a team would change out a belt needs to go into the design. Is the frame stiff enough to hold the center distance? Can you access the belts for inspection? How could a team tension the belt if the numbers didn't work out exact?

Adrian hit the mark. Proper tension of a belt is needed to tune your drive train for maximum power

AdamHeard 19-10-2012 02:01

Re: Timing belt in drive success
 
Only a few hours into drive practice so far, but we are happy with the performance.

Reposting specs for reference;

8wd, 7.5" - 15.5" - 7.5" wheel spacing with 3.5" colsons. Outer raised .3"
24T 9mm wide gt2 Pulleys/belt
Andymark stock gear ratio (12:40 initial, 28:35 and 15:48)

Exact c-c+.003 was applied. For the short runs this works really well, and was almost hard to install. For the longer runs it could be tighter, and we might add more c-c in future.

No ratcheting issues so far. Robot competes in 9 days, and again 14 days later. After that we'll know for sure how we feel about this setup.

Doug G 19-10-2012 02:55

Re: Timing belt in drive success
 
We re-did our drive system with gates HTD belts (15mm wide, 5 mm pitch) for Cal games and never had a problem. We are running AM Shifters (4:1) directly to our center 4" wheel and out to the front and back wheels with 34 tooth aluminum sprockets from sdp/si (faced the hubs off and broached the bore to hex). Used live .5" steel hex axles. Tensioning is done on the axles with bearing blocks within the frame. We were nervous about how they would do, especially when our bot is designed to carry another robot up and down the bridge. While it never happened at Cal Games, we performed the maneuver 20 or so times in practice. Belts never skipped... never. We even ran the bot (fully loaded to 250 lbs) into the wall a few times in low gear to see if we could force them to skip. Motors/breakers overheated before any skipping.

We're sold... we plan to use Gates belts on our 2013 drive system, it also saves about 3 lbs. The only thing we still need to perfect is the correct amount of tension. One side of our drive is over-tensioned we think because it doesn't drive straight any more and we were too worried about letting up on the tension when at Cal Games. Now is the time to experiment a bit.

Brandon Holley 19-10-2012 08:58

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1190972)
Only a few hours into drive practice so far, but we are happy with the performance.

Reposting specs for reference;

8wd, 7.5" - 15.5" - 7.5" wheel spacing with 3.5" colsons. Outer raised .3"
24T 9mm wide gt2 Pulleys/belt
Andymark stock gear ratio (12:40 initial, 28:35 and 15:48)

Exact c-c+.003 was applied. For the short runs this works really well, and was almost hard to install. For the longer runs it could be tighter, and we might add more c-c in future.

No ratcheting issues so far. Robot competes in 9 days, and again 14 days later. After that we'll know for sure how we feel about this setup.


Can we see some pics/videos?? Glad to see you guys are happy with exact c-c.

-Brando

Akash Rastogi 19-10-2012 10:15

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1190972)
Exact c-c+.003 was applied. For the short runs this works really well, and was almost hard to install. For the longer runs it could be tighter, and we might add more c-c in future.

Adam, why was the +.003 necessary in this case? Just to ensure tight tension/account for machining tolerances? How much can you add before the belt is over-tensioned? Just wondering.

roystur44 19-10-2012 11:22

Re: Timing belt in drive success
 
Just an FYI. Equal tension is critical on the drive train belts. If any of the 4 belts are over tensioned stress is introduced to the belt. We have had a couple of belts break through practice wear and tear and over tensioning. When designing the drive train make sure you can easily replace the belts on the go and they all have the same tension.

sdcantrell56 19-10-2012 11:27

Re: Timing belt in drive success
 
Quote:

Originally Posted by roystur44 (Post 1191001)
Just an FYI. Equal tension is critical on the drive train belts. If any of the 4 belts are over tensioned stress is introduced to the belt. We have had a couple of belts break through practice wear and tear and over tensioning. When designing the drive train make sure you can easily replace the belts on the go and they all have the same tension.

This is one great reason why teams should look at running properly spaced belts without tensioners. This is of course only feasible if acceptable tolerances can be held.

AdamHeard 19-10-2012 13:30

Re: Timing belt in drive success
 
1 Attachment(s)
Playing around with an idea, the attached picture is a quick infographic of sorts detailing the pertinent dimensions, factors in the drivetrain. I'm trying to find a good compromise of minimal drawing, maximum information conveyed. It'd be cool to develop a psuedostandard for teams to compare/post drivetrains. Any comments?

Quote:

Originally Posted by roystur44 (Post 1191001)
Just an FYI. Equal tension is critical on the drive train belts. If any of the 4 belts are over tensioned stress is introduced to the belt. We have had a couple of belts break through practice wear and tear and over tensioning. When designing the drive train make sure you can easily replace the belts on the go and they all have the same tension.

I don't think that will affect us the way we have it as an 8wd. Attached Picture shows it decently. For season we'd likely add more than .003 to the longer run to get more equal tension.

Quote:

Originally Posted by Brandon Holley (Post 1190982)
Can we see some pics/videos?? Glad to see you guys are happy with exact c-c.

-Brando

Sure! Probably not quite what you're looking for, but it's a video.

Brandon Holley 19-10-2012 13:39

Re: Timing belt in drive success
 
Quote:

Originally Posted by Akash Rastogi (Post 1190995)
Adam, why was the +.003 necessary in this case? Just to ensure tight tension/account for machining tolerances? How much can you add before the belt is over-tensioned? Just wondering.

We added +.002 to all our c-c spacings for our drive this past year. We did this to kind of artificially bias ourselves to the + side of our tolerance. On our plates, 3 of the 4 bearing holes for axles were machined in 1 step, with the last one being machined in a separate op. The +.002 helped ensure we didn't fall below the necessary c-c to keep proper tension on the belts.

We had 0 issues with our belts all season.

-Brando

waddell 22-10-2012 11:47

Re: Timing belt in drive success
 
These may be a bit too close to the edge of disaster but has anyone successfully used the 9mm width polycarbonate pulleys that SDP/SI?

In the 5mm pitch, they are only available in the HTD profile.

Was considering doing some testing with an 8wd. They tend to be less expensive than their aluminum counterparts but still a considerable expense for test purposes.


--Pat

DampRobot 24-10-2012 01:48

Re: Timing belt in drive success
 
Has anyone using belts for their drivetrain experienced belts that are too short to be assemble? If .003" is acceptable and assembleable, is there an upper limit for "add-on?" Is .003" it? In any case, how can adding a few mils get around the tolerance problem (+/- .012" center distance per a previous poster)? Does gates simply advertise a "tolerance" that is much larger than their usual deviations from nominal length?

I'm just a little surprised by the fact that anyone has had any success at all with exact c-c drivetrains. The setup just reeks of improper constraint to me, which typically causes many assembly headaches and binding in mechanical transmissions.


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