Never Do This, and Other Good Ideas Gone Bad

[1086VEX]

from 2006, 1086 has sworn off making another turret. just not our strong suit. and since 2007 we said no more elevators after making ours that year. but then we decided buying a premade system would still be possible.

[dmitch]

The main problem with the scissor lift is that this year you have to lift 9 and a half feet to get to the top middle peg. To make this scissor lift, you would most likely have to have 2 connected at the joints to eliminate sway. Then when the robot raises this 9 feet, you have an amazingly top-heavy and unwieldy robot.

Also, as our coach likes to point out, more joints=more places for failure.

Archimedes screw is fantastic if implented correctly our team used 1 in 2009 to raise the balls from lunacy from the base of our robot to the shooter. We used a 2.5" piece of PVC that had a garden hose nailed to it in a spiral to provide traction.

[Andrew Schreiber]

On 397 the running joke for NEVER DO THIS was giving the Schreiber brothers access to gas shocks. I would like it noted that no one was ever seriously hurt despite dealing with 1000lbs of force...

NEVER make absolutes in an engineering discussion.

(ignoring the first one)

Never rely on gut feeling, check the numbers or don't build it.

Never trust an engineer to do the numbers right the first time, have a couple people look at the numbers.

"If you need slip rings in FRC you are probably over complicating it" - said by 397's old lead mentor. Totally true for our team.

[theprgramerdude]

Never build a permanently top-heavy robot. In '08 we ended up having to drive backwards because the lifter/ball-hitter was too far back, and we'd flip if we drove forward.

[cire]

2003... Never use #25 chain for your drivetrain. It may seem like you will never hit that several hundred lb limit, but when you throw in missalignment and shock loading, well lets just say we went through a lot of chain that year.

2004 - Never use 4wd in the 4 corners with high friction wheels... as somone else mentioned it is very bouncy when you try to turn - you DO drive straight very well though! We ended up wraping zip ties arround one set of tires which sort of helped..

2005 - Ropes don't belong on robots. We had at least 4 ropes on our robot holding things in place, although we managed to get it down to just the winch by competition, which worked pretty good.

2006 - Trust your numbers calculations (after you have checked them 5 times).. We made a ball shooter, calculated the theoretical speeds using 6" wheels, which fit well within our limits. Somone decided it would be better with 8" wheels - Our shooter ended up shooting too fast and we couldn't use it at the competition.. Another lesson is to make your design adjustable - There was literaly no way to take apart the shooter without taking apart half the robot.

[JackS]

Here is a list of 2791's don'ts with some explanations beneath it.

1. No Set screws
2. No Jags
3. Don't put non graded bolts in drive train.
4. Always account for force of pre-charged pistons
5. Don't put taps where they aren't 200% unavoidable

1. Set screws. We learned the hard way at FLR in '09, tightening the set screws before and after each match. Needless to say, by the time we hit Friday in Atlanta, every last set screw had been replaced with a cotter pin.

2. Jaguars. In '09 we blew at least 4 Jaguars, maybe even more if you count off-season work. Until we are convinced Jags are as tough as Vics, we won't put another one on our bot.

3. Never put any non graded bolts in a drive train. Every last bolt head sheared in '10 and took 3 hours to replace Saturday morning.

4. Never put a piston on your robot until you are sure it can handle the load. Last year every time our piston pre-charged, it bent our entire chassis. 15 lbs. of 8020 later, it still bent our entire chassis.

5. Taps. '10 was our first and maybe only experience with 8020, and we overkilled on the taps. It took forever to fix anything on our chassis, and by the end of the season, the screws had been put in and taken back out so many times the taps were loose.

[GaryVoshol]

Originally Posted by EricH

This can lead to penalties (the 84" cylinder, for example--a gravity-lowered lift that falls over while trying the top peg could easily be over 84" from the floor, causing a cylinder violation)

Nope - the cylinder falls over with your robot.

Originally Posted by <R07>

(note: these limits are defined in reference to the ROBOT, not the FIELD).

[114Klaatu0x72]

During Overdrive, our team decided on using a pivoting arm with one or two 80lb constant force springs, along with an underpowered gearbox, Chinese cast iron gears and "pwm cables" aka painted string, and a broken anti-back drive mechanism. It worked about once, and then the gears sheared off half their teeth, the anti-back drive pins fell out, and a cim burnt out in the manor of seconds. We switched out all of the motors with pneumatics between two competitions, and some of our problems went away. Later that year it knocked me out twice from a blows to the head, one time during a presentation to a middle school, as well as being just terrifying to be around in general. All of those problems pretty much stemmed from a few simple issues: a lack of prototyping, a love of cheap parts, and miscalculations in math. If we spent another week prototyping and a few hundred more dollars, and double-triple-well-beyond-the-point-of-redundancy checked our math, we could have done great that year. So, pretty much anytime you think something will work, despite how sure you are of it, try it and make sure it works.
Also, large amounts of force absolutely suck to work with, try to avoid designs that use them, as they tend to fail in some way after a sometimes very short while.

[Tristan Lall]

Originally Posted by 114Klaatu0x72

During Overdrive, our team decided on using a pivoting arm with one or two 80lb constant force springs, along with an underpowered gearbox, Chinese cast iron gears and "pwm cables" aka painted string, and a broken anti-back drive mechanism. It worked about once, and then the gears sheared off half their teeth, the anti-back drive pins fell out, and a cim burnt out in the manor of seconds.

I'm curious about the gear specs and the load you put on those gears; would you mind explaining further? (Besides, I also want to know the supplier of the gears, so that I can consider avoiding them!)

Originally Posted by cire

2003... Never use #25 chain for your drivetrain. It may seem like you will never hit that several hundred lb limit, but when you throw in missalignment and shock loading, well lets just say we went through a lot of chain that year.

188 broke about 13 strands of ANSI #25 in 3 events that year. Of course, we didn't exactly have the best setup—small drive sprockets on very powerful transmissions,* rigid and convoluted chain guides instead of idlers, and a bit of misalignment.

On the other hand, on a different drivetrain, ANSI #25 chain can be a perfectly reasonable choice—this year, for example, with all the high-speed driving, teams can probably expect substantially less chain load, especially if they've got room for big sprockets on their big wheels.

*Actually, as far as I know, they were the most powerful transmissions on any FIRST robot to that point; each transmission had a CIM, a Bosch drill motor and a Fisher-Price motor.

[114Klaatu0x72]

I don't have the exact gear specs, but I beleive the gears which broke were something attune to:
*80 and 16 tooth
*face width of a quarter inch
*14.5 or 20 degree presure angle
*pitch of either 16 or 20
*Made of cast iron, cheap cast iron.
There was maybe about 20 to 40 ft-lb of torque going through them, but its hard to say exactly because really nothing is left of that robot (cad, docs, notes, pictures) excpt for a stripped out frame and a single, half bald gear on our wall of shame. As for where we bought them, all I remember was that they came from some Chinese distributor and had BOSTON embosed in them. Unless you are constantly looking up cheap items and can read Mandarin, you probably won't come accross them.

[jwfoss]

Based on my years on Aces High (FRC176) and Division by Zero (FRC229) heres my list of things to avoid in season:

Never Agains:
1. Sissor Lifts
2. Tank Treads
3. Vaccuum Devices

Avoid at nearly all costs:
4. Cast Iron Worm Gears
5. Gravity Fed Hoppers
6. Gravity Dependent Lifts/Arms

And until done completely and perfected in an "offseason"
7. Crab Drive/4 Wheel Steering/Holonomic Drive

[thefro526]

Originally Posted by GaryVoshol

Nope - the cylinder falls over with your robot.

In previous years, this wasn't always the case.

[IKE]

As many have noted "Never" is a really strong word. Most posters got the intentions of this thread which were to talk about some ideas that should ahve worked, but had major issues in implementation.

[Al Skierkiewicz]

Although the games lately haven't brought out shopping cart wheels on the front of a robot, don't use them. They are especially bad when the game has an uneven playing surface. The robot goes where the wheels tell it to go so you can't climb a ramp with shopping cart wheels.
Don't ever believe someone who says "of course that's legal, we have been using them for years." That is why at least one team will be down hearted when I have them remove their Globe motors this year.

[boomergeek]

Originally Posted by JackS

Here is a list of 2791's don'ts with some explanations beneath it.

...
2. No Jags
...


2. Jaguars. In '09 we blew at least 4 Jaguars, maybe even more if you count off-season work. Until we are convinced Jags are as tough as Vics, we won't put another one on our bot.

No Jags... without using the CAN interface to limit the rate of voltage change allowed.

http://www.chiefdelphi.com/forums/sh...k#post9 47848


Team 241 had 3 Jaguars short out in 2010 and one in 2009:
All resulted in the circuit breaker on the associated Jaguar breaker trip continuously that made the rest of the robot uncontrollable- (caused variable lag time of one to three seconds on all other functions on the robot).
We guessed that the failure mode was caused by voltage spikes caused by attempts at wide voltage swings too quickly.

One failure occurred at BAE unveiling event ; (less than 5 minutes total use prior to failure)
One at the Granite State Regional ; (less than 10 total minutes use)
One at the Connecticut Regional ; (less than 40 minutes total use)

The first two were on our drivetrain (CIMs).

The last on was on our ball magnet roller (Fisher-Price).

At Manchester, we added a CAN control to limit the rate of voltage change so that the Jaguar would not be swung from +12V to -12V too quickly (and vice versa).
Since we make that change, no more the Jaguars blew on the drive train- Because we had 2 blow out of 40 minutes total use (10 minutes times 4 CIMs) and we have not had any more blow after 240 minutes total use- we think the workaround helped.

But due to an oversight, our CAN workaround to baby the Jaguars did not get put on the Jaguar driving the fisher-price until after we blew another Jaguar.
After we put the CAN workaround on the roller, we have not had another Jaguar blow. (16 minutes use without a failure).

---update---

In over 8 hours of off-season use, not a single Jaguar has blown since we put in the CAN workaround.