Never Do This, and Other Good Ideas Gone Bad

[IndySam]

In 2005 we used casters on the front of our robot. When we would go to the loading stations if they got on the raised part of the loading station we would get stuck.

I have seen many instances of caster problems on robot so "no casters is a fixed rule for us.

#1 No casters
#2 No scissor lifts.
#3 No elevators (I have seen many instances of very effective elevators but not from us)
#4 If balls are the game piece they must always be under the influence of at least one moving part.

[Brandon Holley]

Lots of good ones out here already, but my personal favorite:

NO SET SCREWS

I will never ever trust a set screw alone to retain something to a shaft on a FIRST robot.

-Brando

[artdutra04]

Originally Posted by Brandon Holley

Lots of good ones out here already, but my personal favorite:

NO SET SCREWS

I will never ever trust a set screw alone to retain something to a shaft on a FIRST robot.

-Brando

Retain, or transmit torque?

Relying on set screws to transmit torque on an FRC robot is almost always a bad idea (except for things like potentiometers), but we've used them dozens of times to hold something in place (laterally along shaft) without any problems. That being said, I prefer to design around them, and either use Delrin spacers, stepped shafts, tapped shaft ends, E-clips, or roll-pins to avoid them. If we need to use them, we almost always drill for two set screw holes 90 degrees apart, use Loctite threadlocker, and only use the cone-tip set screws.

[IKE]

Originally Posted by Brandon Holley

Lots of good ones out here already, but my personal favorite:

NO SET SCREWS

I will never ever trust a set screw alone to retain something to a shaft on a FIRST robot.

-Brando

What about trusting Set Screws on FRC Mini-Bots? The Mini-bot FTC kit was full of set screw based components.

[JesseK]

1.) Do not gear a drive train for >10 fps if it has less than 3 CIMs' worth of power. It does ok in a straight line, but even with a 6WD drop center it had some difficulty turning 145lbs.
2.) Do not use 80/20 for a triple-compound elevator that has 20lb worth of manipulator and game piece on the top stage. The stresses on the bottom stage will increase the friction in the 80/20 linear slides too much.
3.) Do not make a 'naturally articulating' drive train. The drive train needs to be rigid or controlled in all aspects at all times. Our 'naturally articulated' drive train pods worked 'fine' in 2010, yet they didn't quite perform as expected. By the time we were testing it, we didn't have time to change to a new design.
4.) Do not design a multi-configuration 'automode' that is configured via the little switches on a Classmate PC. The little buttons are just too durn small.
5.) Elevators that have multiple pre-programmed positions + override/tuning are great in theory, yet are very difficult to master from a controls perspective. We're going to do a multi-position elevator for the driver again, yet with a very nice twist to add some autonomy.

[JesseK]

Originally Posted by IKE

What about trusting Set Screws on FRC Mini-Bots? The Mini-bot FTC kit was full of set screw based components.

They will require some tightening eventually. On the kids' 9:1 double shooter last year in FTC, we had to tighten the set screws about once every 3 matches.

[Brandon Holley]

Originally Posted by artdutra04

Retain, or transmit torque?

Relying on set screws to transmit torque on an FRC robot is almost always a bad idea (except for things like potentiometers), but we've used them dozens of times to hold something in place (laterally along shaft) without any problems. That being said, I prefer to design around them, and either use Delrin spacers, stepped shafts, tapped shaft ends, E-clips, or roll-pins to avoid them. If we need to use them, we almost always drill for two set screw holes 90 degrees apart, use Loctite threadlocker, and only use the cone-tip set screws.

If I have no other choice I will give in to using a set screw for retaining only. I much rather use spacers, e-clips, pins, you name it before I'd choose a set screw though. Obviously transmitting torque is a massive no-no.

-Brando

[Brandon Holley]

Originally Posted by IKE

What about trusting Set Screws on FRC Mini-Bots? The Mini-bot FTC kit was full of set screw based components.

I really dislike the way the FTC kit handles transmitting power through the shafts with setscrews, amongst many other things in the kit. However, given the minibot rules and such, I'm sure many teams are going to be pigeon-holed into using the set screw solution. I'm sure it will work well for most teams, and some loctite here and there may help avoid any complications...still doesn't mean I have to like it.

-Brando

[Racer26]

1075's no-no's are:

NO SET SCREWS (too many things have slipped in the past)
NO POTENTIOMETERS (code runs away and busts them, alleviated by using a 10turn pot, or encoder.)
NO BEVEL/MITER GEARS (this stems from our rookie year where we used one cantilevered on the end of a 4" shaft, which shredded, and cost us the 2003 Canadian Regional because we couldn't play in the finals, because the gears were toast)

[Jon236]

From the Controls Team:

NO potentiometers - use multi-turn or continuous turn pots or encoders
NO limit switches engaging fast moving arms or other robot parts - use magnetic reed switches instead

[Chris is me]

Using set screws for retainment is IMO just as bad, when you could be using snap rings.

[FRC4ME]

Do not use window motors for lever arms.

339 tried this both in 2007 and 2008 with the same results: we ended up switching from a motor to a pneumatic cylinder on the day before ship.

[Sean Raia]

Never rely on pneumatics alone as a fast, pushing force. Our team learned this lesson the hard way in '09.
Never forget to leave room for electronics and then try creating space the night before ship.

Also, our team has never had much success with belt systems.

[Tristan Lall]

Originally Posted by Brandon Holley

I really dislike the way the FTC kit handles transmitting power through the shafts with setscrews, amongst many other things in the kit. However, given the minibot rules and such, I'm sure many teams are going to be pigeon-holed into using the set screw solution.

Another stopgap measure (if you can't take advantage of any alternatives, and need to use the setscrews) is to carefully centerpunch and drill a small indentation in the shaft, for the setscrew to sit in. (Alternatively, you can file a flat on the shaft.) When combined with some purple or blue Loctite (on the screw threads only; not on the shaft!), this will generally make an acceptable joint. It's still going to be limited, but it's just about the best you can do.

(Occasionally, wrapping something around the hub to keep the setscrew from falling out when it loosens can be marginally useful. It won't transmit power with a screw loose, but at least you won't have to hunt for the screw.)

[Madison]

Originally Posted by Tristan Lall

Another stopgap measure (if you can't take advantage of any alternatives, and need to use the setscrews) is to carefully centerpunch and drill a small indentation in the shaft, for the setscrew to sit in. (Alternatively, you can file a flat on the shaft.) When combined with some purple or blue Loctite (on the screw threads only; not on the shaft!), this will generally make an acceptable joint. It's still going to be limited, but it's just about the best you can do.

(Occasionally, wrapping something around the hub to keep the setscrew from falling out when it loosens can be marginally useful. It won't transmit power with a screw loose, but at least you won't have to hunt for the screw.)

For what it's worth, the Tetrix-brand shafts have a flat along their entire length.