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.