Belt drive system

[Doctorwho]

Hi, our team is thinking of replacing our usual chain drive with automotive belts for next year's robot. Do you know if they make adapters to convert the andymark AM servo shifter? Thanks for the help.

[Engineer]

Many companies make small belt drive systems and sprockets. Look up Clippard, Goodyear, and Berg for just a few.

[MrForbes]

One of the ideas we were thinking of for next season would be to use a single speed transmission with a gear ratio that allows direct drive to power the center wheels (6wd), then use belts to power the outboard wheels. This would require same size pulleys with a relatively simple 1/2" diameter keyed bore.

[Sean Schuff]

Belts require a great deal of tension (resulting in more friction) to keep from slipping. Our team used belts a couple years ago and found that the weight savings was not worth the hassle. We have since gone back to chain drive because of its simplicity and robust-ness.

That being said, if anyone out there comes up with a simple, effective way to implement belts in a drive system, I would really like to hear about what you came up with.

[s_forbes]

Maybe someone from 125 can shed some light on the subject, they had an amazing drivetrain last year utilizing belts.

[Doctorwho]

Thank you for the help, I found a toothed V-belt section on the McMaster-Carr
site that has the corrent spacing for use with the Andymark AM shifter gearbox our team uses

[Billfred]

Originally Posted by Doctorwho

Thank you for the help, I found a toothed V-belt section on the McMaster-Carr
site that has the corrent spacing for use with the Andymark AM shifter gearbox our team uses

I assume you're using the AM Super Shifter for this? I can't imagine an appropriate pulley with the width of the sprockets found in the AM Servo Shifter. (Then again, I've been surprised before.)

[RyanN]

Our 2003 robot had a belt drive. It still kind of works. Key term "kind of" Sure, after 5 years, the belts do stretch, but the problem with belt drive that I see is simply replacing a belt. On the 2003 robot, it will take at least 20 minutes to replace the belt, and one of the idlers has a broken bolt, disallowing me from just replacing them.

However, on a robot like ours this year, belts would be possible, but deadly. If we were to break a belt, there would be no way to replace the belt. Keep that in mind when designing your drive train.

For every other year's robot, we've bolted the bumpers to our axle bolts. If we were to put bolts on our robot last year, replacing them would have required us removing our bumpers, which was terrible with last year's design.

Basically, the moral I'm trying to get across, make a design that is reliable, but also easy to fix if something break.

Another problem that I can see with belt drives is if you have a design other than center wheel drive with 6WD or a 4WD with direct drive to one wheel, you have to some crazy wraps, which causes a lot of tension on the belts and the axles, something we found out the hard way in 2007 with our robot.

We had a wrap around our output shaft and an idler (I wish I had a decent picture). Our idler axle snapped at the Bayou Regional and we ended up putting something together real quick using some steel bolts.

So my words of encouragement: Make it simple, reliable, and easy to replace. I believe this is a major principle of engineering.

Looking at the picture from 125, their design looks simple, and reliable, but the bumpers and outside rail will make replacing the belts time consuming if they break. Also, the sprockets seem a little small, which places a lot more force on the belt.

We tried direct drive for the first time this year and used #25 chain. We are on the edge of the limit of #25 chain for breaking strength, but if we broke a chain, all we would have to do is get some new chain and put on a new master link. Even though we never had trouble with #25 chain, we're experimenting with bicycle chain, as it is stronger and widely available.

Also, just so I am kind of on topic, I would recommend McMaster or machining your own. Also check with the sponsor that supplied the belts this year.

[kibbs425]

We used belts really effectively this year. You have to take some care when designing the system but it can be done simply and effectively. We got our parts from www.sdp-si.com. They have a great design assistant to know what you need. You should use a timing belt, V belt or friction belts require friction and extra systems. The key to the design is keeping some tension, but no more consideration that if you were using chain.

Most suppliers will have how to choose the size and center distances of your belt. It could be a calculator or a simple formula. Any mechanical design book should have it as well if you start looking around.

Know how much power, what and how many motors, you are going to be running with the system and figure out a max torque that you could experience. Based on that torque and the pulley you use you can figure out the force in the belt. This will tell you the belt width and what type of braid you should have, kevlar, steel, glass, etc. Make sure you design in a safety factor, you want your drive train to work all the time right, 2 should be plenty maybe even 1.5. Your experience will allow you to choose what is best for you.

We did this and figured on a 3/8 timing belt. This was for 2 cims and 1 fisher price on each side. We had to replace the belt once during build testing because our cast urethane fell of and got mingled in with the pulley and caused a rip. After AZ, Boston regionals, Battlecry, Beantown Blitz, Mayhem at Merrimack, and several demonstrations on various surfaces (Brick, dirt, not carpet) we finally have to change out the belt on our drivetrain.

So with some care and consideration when initially designing you can choose what you want and not have to worry about chain, grease, noise and most importantly, 4-5 lbs of weight (depending on the chain you use)

If you want to know more I think I have an excel spreadsheet somewhere of what we did and I can send it to you. I'll have to find it though.

[Doctorwho]

Originally Posted by Billfred

I assume you're using the AM Super Shifter for this? I can't imagine an appropriate pulley with the width of the sprockets found in the AM Servo Shifter. (Then again, I've been surprised before.)

Our team uses the AM Servo Shifter for the gearbox. McMaster-Carr carries a cogged V-belt that is designed for use in motor scooters but has the correct spacing and tooth size for use with the Servo Shifter. The back wheel would be mounted onto a slide that would work like the back tire on a bike. Loosen the bolt that secures the back wheel to move the slide forward to loosen the belt for drive train work or belt replacement, sliding the slide backwards re-tightens
the belt.

[Brandon Holley]

Originally Posted by s_forbes

Maybe someone from 125 can shed some light on the subject, they had an amazing drivetrain last year utilizing belts.

Guys, thanks for pointing us out as an outstanding belt drive. I personally, along with a lot of the other team members spent a considerable amount of time hashing out the details of our belt system.

We used 3/8" pitch belts from sdp-si.com, along with the pulleys they sell. We chose to use the kevlar reinforced timing belt because it gave us a considerable amount more breaking force than the fiberglass reinforced ones. The picture above shows the main part of our drive "pods". We used a single speed gearbox with 3 motors, 2 CIM 1 Fish. The center wheel shaft extended itself into the gearbox where it was integrated via spur gears into the system.

We used hex on the wheel shafts because it makes the system very robust and easy to assemble (no keyways to lose). The belts come in very specific lengths, so we designed our center to center wheel distance to be a round number (16" exactly). The belt sizes are whole numbers plus the pitch diameter (to give you the "circumference" of the pulley system, otherwise known as the belt. We ordered the belt as the *exact* size that it would be. After assembling the entire drive system, I measured the slack in the belt and went and machined down a chunk of delrin to the size needed to pick up the slack (which wasn't that much). The delrin slid right over our standoffs used to attach the two drive plates together. Throw in an e-clip on both sides and voila, you have a tensioner.

The reasoning behind this system, as opposed to an adjustable system you may see in a chain drive is that kevlar, for the most part, does not stretch....at least not anywhere near where you might see a chain stretch to. The tension put on the belts when the drivetrain was first assembled is the same as it is now.


As for changing the belts....our bumpers came off with 4 pins you could remove by hand. A few bolts after that you had the sideplate off, and could replace anything you needed in the side...wheel, tread, pulley or belt....although we never needed to do that.

I really like belts, and aside from the weight savings, it was just a cool little addition to a drivetrain. If anyone has any questions, or wants to see so me CAD let me know, I am the guy to ask.


-Brando

[aztech75]

Originally Posted by Doctorwho

Hi, our team is thinking of replacing our usual chain drive with automotive belts for next year's robot. Do you know if they make adapters to convert the andymark AM servo shifter? Thanks for the help.

We have done it every year since 2005... it works great.

I suggest using Brecoflex Belts and pully's.

[Doctorwho]

Thanks for the advice Brandon.