what's your most important drive train advice?
 

Hello all. I've been preping a workshop/seminar for WRRF teaching teams' students about drive train, and I thought "why not ask the people on CD about this?" This way I will make sure I don't leave anything important out.


So, if anyone can help out with this, it will be great!

Please answer these two questions:

What's the most important advice you can give to teams about drive train? If you are only allowed to give 10 advices about drive train, what would they be? (please don't limit your reponds to 10 if you have more advices ;) )

How do you come up with a drive train most sutiable to the game? In another word, how do you pick through the many many designs you have from your experience/learned from others and choose the perfect one that you think is best for the game/your robot?

Thanks!

My only piece of advice


Break the darn thing as soon as you get it running. (Not Maliciously)

By doing this, you know where your drive train needs work, by fixing this during the build time, this lowers the risk of your team rushing at competition to fix something.


-----------------------------------------------

Our team uses the drive train from the previous year, but we make additions and changes to improve our weaknesses...

Not much help there, we're riding in our groove when it comes to drivetrains

Well, I don't know if I can come up with 10, but I'll give it a shot.

#1: Simplicity is best. Don't spend too much time building an overly elaborate drivetrain. If it's the greatest drive system ever, but it doesn't work, you're going to be stuck. Whatever you build, make sure it works. A basic drill drive that works is better than a 12-motor drive that doesn't.

#2: Make sure what you build is suitable for the game. Design your drive based on what you need to do in the game, not what would be cool to do. If speed and maneuverability are key, swerve drive might help. If pushing power matters, though, you might want to go with treads or basic 4-wheel drive.

#3: Make sure your drive is suitable for your robot. Like rule #2, if your drive doesn't fit your robot's overall plan, you won't be as successful.

#4: Don't spend too much time on drive. The drive system is the most important piece, but don't wait until the last week of the build season to build the rest of your robot. If you follow Rule #1, this should be a piece of cake.

#5: Plan everything out first. If you're doing something really simple, this doesn't take much, but if you're going to be a little more ambitious, a good plan (in CAD, Inventor, or the like) is a must.

#6: Keep yourself open to all ideas. Some of the best drive systems ever (71's walking bot comes to mind) were totally different from anything ever seen before. Keep Rule #1 in mind, but don't rule out anything just because it is different.

#7: When designing your drive system, remember what resources you have available. If you don't have what you need to build your ultimate drive system, it's not going to happen.

#8: Watch your costs. If you're team is on a small budget, you don't want to break the bank by having all your gears custom made and your housing cut on a waterjet. You can do a lot with stock gears and basic machine shop equipment. One hint: look at automotive parts, like drive shafts and such. Sometimes, you can salvage a very good part from an old car drivetrain.

#9: Watch your weight. Try to minimize the size of your drive; it'll give you more room for other "fun" stuff.

#10: Finally, after viewing all of the above, refer back to Rule #1. It has to be simple to succeed. You don't have to limit yourself, but rather control yourself. Think: "Is what I'm doing possible to accomplish in six weeks, with my current resources and expertise?" If you answer yes, go for it! If you answer no, go back and think about what you can do to solve any feasibility issues. You'll be satisfied with the results, and it will save a lot of time and frustration later on.


Guess I made it to 10 :D. I hope this helps you out. This is, in a nutshell, how we plan out our drive system. If you follow all the "rules," you can't go wrong.

Weight it down
 

When you get your drive train built, you can only see if it really works if you do these three things:

1. Attach weights to it so that it weighs close to 130 pounds (if this is what the maximum weight is next year). We ususlly use cable ties or duct tape to tie down a box of spare bolts or some steel plates to the drive train frame.

If you drive your new drive train around without weighing it down, then you will get a false sense of security that it works just fine. Weighing it down will expose it to what really will happen once you get the rest of your bot finished.

2. Drive it on the playing surface that we will be competing on. I assume that this will be carpet next year, since the past 11 years has been carpet... but FIRST may change this. A 130 pound robot drives differently on carpet than it does on concrete, that's for sure.

3. Like DJ said above... break it. Don't maliciously damage the robot, but put it through at "road and track" test early. Try to drive it like you would during a match in the elimination rounds. Breaking your robot early in the build cycle will make you fix your weak links and give you more experience for what needs to have preventive maintenence.

Andy B.

Some of the things that helped us out over the past few years

1.Start now, get the ball rolling on the different types of drivetrains. Same as what has been said, if you are a new team to first try 4 wheel drive with chains and sprockets, make it robust. Next move on to multi motors, maybe a tank. Finally step up to full gearboxe transmissions and swerve.

If you can have a fully working base by week 2 or 3 then you can drive the heck out of it for 4 weeks and find out the weakness.

Many teams have a 'shelf' of proven designs that they can call upon as soon as the game is announced.

Pay attention to details when you build a gearbox, taking a little extra time to get the proper meshing will save in headaches. We found that using shoulder bearings oir oiled bushings let the drivetrain move freely with little drag. We found that welding all the gears to the shafts as well as yokes for dissimilar metals proved to be indestructable with no failures on the drivetrain for the season and post season. Keep that as an option.

And, ask the forum for help if you need a specific answer on ratios or materials, the answer will probably show up by the end of the day

build it strong
 

After seven competition seasons and countless pre-seasons builds it has taken us years to perfect drivetrain designs.

#1 -- rookies should concentrate on simplicity. two or four wheel designs offer plenty of mobility for most teams. teams that use swerve or omni-directional designs have usually spent lots of time between seaons perfecting the systems. don't try to over complicate a drivetrain. if its your first season i suggest the most simple design that satifies your needs.

#2 -- treads look cool but are often hard to get working properly. four wheels can almost always do as good a job as an entry level tread system. (team like the technokats have years on you newbies.)

#3 -- pins and set screws are a no no. always try to use couplers that are more robust like keyways, of hex shaped shafts.

#4 -- finish it early. if you settle on a simple drivetrain, get it built soon. Have it running by the end of week two or three. the most successful robots in FIRST are usually a product of great drivers. students who have had four weeks of practice time on simple drivetrains can do things that others with less practice and more advanced systems can't match.

good luck! and remember, don't be discouraged by complex systems that look polished and refined........our new drivetrain ideas have been on the drawing board for months now, and many other teams have had them for years!

Anthony Lapp
Team 221 --> 857
Superior Roboworks

From an electrical standpoint...
1. Make it efficient so as to use less current.
2. Make sure the motor leads are tight, solder when you can, which should be everywhere.
3. Dress the leads so that they won't get caught in the drive train or be run over.
4. Make it efficient so it won't overheat.
5. Dress the leads so they won't be bent or pulled when the robot steers to left or right.
6. Make the wiring as short as possible so you can minimize the resistance and therefore the voltage drop.
7. Make it efficient so it doesn't burn up the motor or the speed controller.
8. Test, Test, Test. check the current delivered to the speed controllers under all conditions, especially turning.
9. Make it efficient. (Can you tell I like this one!?)
10. Make it easy to repair or replace the motor, else make the system modular and have replacements.
Good Luck

Just some basic tips...

1. DO NOT USE SET SCREWS!!!! (as ajlapp and wysiswyg said)

2. If you use a chain drive don't foget the chain gurds. you wouldn't want your bot to just sit there and spin in a circle do you.

3. Again if you use chain drive the chain will loosen as it wears in. Use a peice of solid delrin shaft with an off center hole so as the chain loosens all you have to do is loosen the screw on the shaft give it a turn and tighten it down. This will pickup the slack in the chain and keep it tight.

4. If you burn or cut lines or nubs on the rubber wheels for traction make a few extra sets. From what I have seen it may help with traction a bit, but once you start pushing something and your wheels start to spin they will wear down really fast.

5. Don't overcomplicate it. (KISS) You don't want to spend most of you build time on the drive system. (what good is you bot if all it can do is move around on the floor?)

6. My favorite tip of all to remember DON'T USE SET SCREWS!!!

Ooo yeah one more thing don't use the wheels that come supplied with the kit if they are the same ones that they have given for the past two years.

support all axles with bearings from both sides of the shaft.

don't use direct gears driving off the motors

expect the unexpected

WC

Maybe make a test Drive System so its easier to make one during the build

KIS (Keep it Simple)

Prototype then build the real thing

Just my 2cents

Wayne, what do you mean by this one, all of our gearboxes were interfaced to the pinions of the motors and worked flawlessly.

See picture of swerve 'Modular' drivetrains that we had great success with. Modular is the word that I think Al mentioned. Make the drivetrains swapable in the even of a failure

The plans for this gearbox are in the WHitePapers

Whoops here is the picture

1. Use the simplest system that meets your basic needs and is consistent with your team resources (money, design, manufacturing). I'm reiterating everyone else's advice because this is soooo important and many teams forget this.

2. Expect to put in more time on your drive system than you originally estimate. If you cannot move around, you cannot use those wiz-bangy fancy devices that you have mounted on top of the drive system.

3. Stay focussed on the drive system until it actually works. This includes breaking, redesigning, and fixing.

4. Don't forget that you have to wire it and program it and put together an operator interface too. If you have a complicated drive system, you will need to invest some more time and resources in programming (and variables).

5. The more complicated the drive system, the more training time will be required to drive it effectively. If you get 10% performance out of a spectacular system, you may not be as effective as someone getting 100% performance out of a simpler system.

6. Sometimes differential back wheels and front-casters work. But they have lots of hidden disadvantages, especially if there is a hill in your future.

7. Consider forced air cooling of your drive motors using the fans in the kit.

8. You have limited resources. The more resources you devote to your drive system, the fewer resources are available for other devices.

9. Make it robust. If you plan to attend more than one event, your drive system is going to take a beating.

10. Buy extra drive motors (if possible). The drill motors only go for about $25 each. You don't want to have your entire competition go down the tubes for the lack of a $20 part. Buy extra 30 amp breakers.

11. Make your drive system easy to get to and easy to replace/repair. If you have to disassemble your robot to tighten that one loose screw, you might miss matches.

1. List all possible types of drive systems (rear differential-caster front, rear differential-Ackerman front, tank drive (treads or chain driven tandem wheels), swerve drive, etc.

2. Qualitatively determine what the attributes of each system are (speed, maneuverability, power, complexity, etc.) and assign a value.

3. Look at the game and determine what strategies are available.

4. Assign attributes to the strategies (maneuverability, speed, etc).

5. Pick strategy and assign weights to the drive system attributes based on that particular strategy.

6. Add up the value*weight for each drive system. Make a decision based on which one floats to the top.

Andrew, Team 356

Advice - in general
 

I'm surprised no one mentioned this.

BUILD SPARE PARTS!

Anyone with machine shop experience will know that setting up the machine often takes longer than actual "cutting metal" time. So, while you're set-up for an operation, build a couple of spares.

-=- Terence

Matt-

what I mean is don't just drop a sprocket onto a motor arbor shaft and expect it to carry the entire weight of the machine. I can't tell you how many teams we see which have a drill arbor with a sprocket "bit" directly driving a wheel sprocket. The immense drain on the motor either causes a fuse to blow or a wobble in the sprocket destroys the drill gearboxes.

Better would be to have the drill motor axle supported on both sides of a sprocket. This drives a series of two or more sprockets which are equally well supported and that builds up the advantage for the wheels.

Personally we prefer sprockets to gears at team 25 since, in the event of a seizure, the chain is the weakest part of the system and it jumps before teeth get ground off.
You can also easily replace a chain without tearing out a whole gear box by using a snugging device somewhere in the system.

We have not have one fail us in three years. And our results speak for themselves.

WC:cool:

The main thing I try to stress to rookie teams is to design the drive train so that you can fairly easily modify your speed (whether it be through a chain drive where you can change the sprockets or wheels where you can change the diameter). I ran a workshop a few weeks ago on Mobility; you can download it from the Resources page on the web site for the Canadian Regional (go the first table on 'Workshops and Seminars' and the Mobility workshop is available in PowerPoint and Acrobat format). The robot I used as a case study, though, isn't really great; I built it in three days (you'll notice the shaft from the motor should be better supported).

Guys, Guys, Guys...

You all are forgetting something... Just make shure it runs first! THAN get all the bugs out...:D

Short and simple: adding more motors for power is pointless if your wheels slip. Only gear it down to a low enough velocity such that your wheels just barely never slip (i.e. they are providing maximum force without slipping at stall torque). Or... go to tank tread as many teams do, where you rarely (if ever) have to worry about slipping. On the other hand your options for mobility is decreased with a tank tread.

Too much traction can be a bad thing. It leads to motors burning and smoking. Not to mention as the person above me said too much traction actually leads to increased current draw as the robot turns. Plus tank treads if not built properly will pop off when you turn.:)

1. If you use chain, be ready to use tensioners. A slack chain is one that's just begging to fail.

2. Lube! Getting your bot slick and oiled can be like adding RAM to a computer, it's cheap and gives you a huge boost in performance. If it takes 1/4 throttle just to overcome the static friction in your drive and get moving, something is in need of lubing or redesign.

3. Keep it clean. Dust, carpet, rubber, blood etc. will all get picked up and find it's way into your drive. Compressed air and a tooth brush every few matches will keep it from building up.

-Andy A.

Worms?
 

Consider using worm gears because the worm gear can turn the gear but the gear cannot turn the worm gear, this allows from some serious "stayin put" power.

Planetary gear systems are great if u have a limited amount of space and need some serious reduction or serious durability.

I would also suggest that everyone on the team read this:

http://www.howstuffworks.com/gear.htm

This will give everyone a nice understanding of the basics of gears and allow them to talk inteligently to the judges on the subject.

Excessive use of pneumatics is a NO NO!, if you are going to employ pneumatics into a drive train, be sure to keep the range of movement to a minimun (for instance short strokes). Example... a gearbox, though a very risky undertaking in itself for a rookie, or even some veteren teams... use short stroke cylenders to and little room for error.
Also, don't forget wear and tear, especially treds. Though a tred system may be perfect on AUTOCAD, after a few uses, stretching of the material could jepardise the whole design!
Aside from all that, the key to a long lasting prescision drive train might not be in the calculations and the tolerances.. But a rugged chassis. If u have a sturdy chassis, it greatly improves the performance of the robot... and affects of age.

Broken Chain
 

If your using chain between your drive motor/transmission and a sprocket on the drive shaft, consider using 3/8" (#35) pitch chain. Too many teams (especially rookie teams) wind up running around in circles due to broken 1/4" (#25) drive chains. Try to minimize the lengths between components to keep the weight down.


FirstCadLibrary

Note: the "anti-back-drive" property of worms depends on a bunch of factors (number of threads on the worm, friction in the system, lead angle on the worm). It is not a guarantee if you just put a worm/worm gear in your drive system that you will be impervious to back drive.

Worms also have an added inefficiency due to sliding contact between the worm and worm gear. This is a function of the contact friction between the two. Use lubrication. And be prepared to lose a little of your "available work" to friction (which is not as big a problem with spur gears).

Worms and worm gears experience significant thrust loads! The thrust load in our drive system was sufficient to push the snap rings holding the gears into the proper position out of the groove. The worm would also grab the key in the shaft and shove the shaft around. We ended up putting in spacers to keep everything in place.

Make sure you properly support all shafts. You might want to use angular contact bearings on each end of both your worm shaft and your worm gear shaft in order to accommodate both radial loads and thrust loads.

We designed our first worm and gear system last year and are still crawling up the learning curve. I think it is probably worth the effort. But, there are a few more quirks than we expected.

Maybe they'll give us air motors some year.

For those of you who have used pneumatics, how many things can you drive at once? Do you experience "sag" in your devices as the match proceeds?

Can you charge your air tanks to 200 psi before a match? Or just to 60 psi? This seems to be an extra source of potential energy and therefore a good thing.

Andrew, Team 356

i designed a pneumatic drive but never actually built it.. i started to but then my injuries got in the way(not injuries from the pneumatics).. from my calculations the engine i designed would have 1 horsepower and be able to run constantly at around 10 feet per second... not bad, eh? too bad i dont have a machine shop to use. its too time consuming to precisely cut steel with a hacksaw ;) or aluminum for that matter.

Max Tank Pressure
 

If I remember correctly, the rules stated that you may pre-charge your system but only with the existing pump. The pumps have a pop-off valve that release at around 100 psi.

There hasn't been any rule that excludes using other pneumatic cylinders as additional storage tanks however. The tradeoff is the additional weight.

The only components that you are allowed to hook up in the pneumatics system are the components they give you. You cannot put anything custom-built in the line under current rules.

I believe that FIRST allows a stored pressure of 120 psi and a working pressure of 60 psi. The pressure switch stops at 120, and the regulator restricts the flow to 60.

When working with gears always use an odd gear ratio. ie. 40:11 instead of 40:10 This always promotes even wear and tear on the gears.

Drivetrain
 

When working with any kind of motor it's always good to keep the magic smoke inside the case. We have always used the Drill Motor for drive and think it is a quite reliable unit when geared properly.

After last year's competition we spent part of the summer developing a new drive transmission design for our robot ED. The drive features a 1:4 ratio between low and high gears, 250rpm and 1000rpm peak power output speeds, up to 65 ft-lbs of torqe, uses the Bosch and Chiaphua motors found in the 2002 kit of parts, and an 8-millisecond response time to shift gears. The prinicpal feature is that the design can be constructed with hand tools, a lathe, and a mill. No CNC, wire EDM, castings, or carbon-carbon componsite construction capabilities are required (hopefully putting it within the reach of most teams)! We play-tested Version 1.0 of the drive system at the Maryland State Fair competition at the end of the summer, and refined Version 2.0 during the fall.

Rather than keep it as a secret, we are making the design available to any teams that want it. It is posted in the White Papers Section. Download it, build it, improve on it, or just look at the pretty pictures - it's all up to you. All we ask is that you let us know about any feedback, and if you do improve on the design, please post your improvements so the entire FIRST community may benefit and improve their capabilities. In the mean time, we are off thinking about Version 3.0 for the 2003 season...

-dave

Besides agreeing with all of your other comments I would just like to add one thing if your going to have a Good Drive train its not just the Quality of the drive train. Its the quality and know how of who works on it. We all had some part in working on the drive train on fluffy but their were a few special people on our team who know those gear boxes like the back of their hand every, screw, every gear, every sound that those drive trains make they knew them. And in our case Fluffy our robot well it was very strong and robust but at both regional and nationals we had a few problems with the drive train. Those few select people I was referring to well they put their blood sweat and tears into to those drive trains. They rebuilt our drive train in under 3 hours changed treads in 5 min, and as a freshman last year I worked with many of these people and though they don’t brag and probably don’t even think about it but I hope that they realize they are really what brings our team and Robot together under pressure. That in my view is the most important part of any good drive train. And that is the only other thing besides design that really makes a drive train come together.

I was confused by this so I asked an expert...

"If the large tooth number is evenly divisible by the small tooth number, then the same teeth will always contact each other. So if one tooth gets
slightly damaged, it will continuously wear on the same mating teeth and wear out sooner. If the numbers are not evenly divisible, then the damaged tooth will mate with every other tooth after a certain number of revolutions. Thus the wear will be spread out across the entire gear. So, it does not really have to be odd and even - just not evenly divisible. However, in our applications, this is irrelevant because we do not expect the gear mates to last for many years of constant use. A much bigger consideration is to properly design the gears for size and materials based on the loads required to be carried.

Raul"

Is this what the criteria for the odd ratio is meant to achive?

Yeah. I guess I should have explained this better. But thats the whole gist it.

ya i know.. custom stuff were: crank shaft, connecting rods, cams. actuators would be controlled by the cams hitting into some limit switches.. not a bad design.. the cam shaft would be rotated by one of those little motors we got last year and depending on how fast u rotate the cam shaft, the bot goes forward or reverse, fast or slow.. wouldnt have been IMPOSSIBLE like it was, had i had a machine shop to use ;)

Can you define "rear differential", "Ackerman front" and "swerve drive" for us?

Here is all of the info on Ackerman Steering (That you could ever want)


http://www.pbracing.com/handling/ack...principle.html

Swerve - Is pretty much the same as 4 wheel steering which could be independent, front & back, or all 4 linked to steer together. The main benefit is incredible maneuverability. Think of driving straight toward something then 'swerving' to the side to avoid it, in this case think up to straight sideways

Rear-Differential (in my mind at least) allows each rear wheel to spin at a different rate opposite of a locked rear axle, by changing the speed of the rear wheels the bot can steer

Link not working.

Try it now, for some reason the capital A in Ackerman was causing a problem

Check this out. Similar to Anthony Lapp's (team 857?) drive from last year

http://www.howstuffworks.com/gadget122.htm

NEVER use PLASTIC pulleys and 1 in timing belts.We toasted 10 belts and all the pulleys at our regional Sprocket and chain is the only way except the transmission povided this year is a no brainer..............use it Insure you fabricate a plastic cover for it to keep Foreign objects out of the mesh and lube the gear with lube from one of your old drill motors.Its special lube designed for high tooth contact loading............and PLEEZZZZZZZEEEEEEEEEE go here .www.nuts4first.net/library/Koff02drive-sys.ppt
Jim H :yikes: