28-06-2011 21:50

Hawiian Cadder

Re: pic: West coast drive design

1/16 tubing is maybe a little thin, depending on the challenge and how heavy the robot is. The only teams i have seen using 1/16 tube also had 8 wheels, i don't know if the fact that 8 wheel drives tend to rock less violently had anything to do with it.


I am assuming that those are 4 inch wheels? If so how many stages are in your gearbox and what are the speeds.

looks pretty good. how much does it weigh?

28-06-2011 21:52

Chris is me

Re: pic: West coast drive design

Are you speaking with experience?

28-06-2011 22:01

Hawiian Cadder

Re: pic: West coast drive design

I was too busy at nationals to collect any real information so I had our scouting team (we had no chance of picking or being picked, so they had not much to do) collect information from 250 ish teams (the number there with custom skid steer tank drive (did not ask about any other drive-train, did not ask about kit-frames), this data included:

which wheels were dropped, and by how much (or if omnis were used)
wheel diam
speed / speeds
motors in drive-train
width of track

One or more of them (dont know which) wrote addition info on material thickness. I don't know if it was sheet-metal or tubes, but there was a definite correlation between more wheels, and thinner materiel. What I don't know is if this was because more wheels weigh more and so they were trying to save weight, or if they actually looked at any FEA or prototypes and decided that with less weight on each wheel the frame would experience less stress and thus could be thinner, teams were more likely to use thicker materiel if they were using larger wheels.


edit: info also included distance between wheels

28-06-2011 22:12

Danny Blau

Re: pic: West coast drive design

Is there any chance that your team is willing to share that info?

Have you thought of correlating it with scouting data you could ready start to draw some powerful conclusions as to what are the most competitive setups.

If you would like any help feel free to let me know.
Danny

28-06-2011 22:29

Hawiian Cadder

Re: pic: West coast drive design

right now the info is not typed, its in about 100 3 by 5 cards on my cork-board. I am writing a paper about the development of our multiple drive-train frame, I will add this data to that paper. unfortunately we never scout at nationals, so all I have is the mechanical data.


edit: sorry for thread-jack.

28-06-2011 22:43

Danny Blau

Re: pic: West coast drive design

One way to get the scouting data would be to use 1114's world database, they compile all the match scores and from that they are able to calculate OPR (Offensive Power Ranking). The nice thing about OPR is that its very impartial and non-bias compared to other scouting methods.

Currently 1114 has all the tabulated data from all of the regionals available (which would be a good place to start) and normally they don't analysis the data from champs but maybe we can work on that!

Thanks for sharing the data

Danny

28-06-2011 22:48

AdamHeard

Re: pic: West coast drive design

1/8 wall tubing isn't that much stronger than 1/16, and at a massive weight penalty.

We predominately use 1/16" wall, and haven't suffered any frame failures the past few seasons on any of our robots (including practice and prototypes), and a lot get really beat up.

I know you're just trying to help, but it can mislead new people when you make such bold blanket statements with such confidence.

Bending is the primary failure mode for frames in FRC, and the bending is directly related to the area moment of inertia. If you do some research on on how this is calculated, you'll see the shape and cross section of the tubing is what grants strength, not the thickness.

28-06-2011 23:21

Hawiian Cadder

Re: pic: West coast drive design

Sorry if i have miss-lead anyone, it is not my intention.

as for the failure mode. i would expect that the failure mode for 1/8 inch tube would be bending, however i would expect the failure for 1/16 tube to be bending in or getting crushed. especially if the robot takes a hard hit from the side. i think 1/16 is a LOT easier to parallelogram too.

28-06-2011 23:24

42!

Re: pic: West coast drive design

The weight is about 28 pounds right now according to Inventor, Yes the wheel's are four inch, They have the same dimensions as the Andymark one's with a custom lightening pattern. The gearboxes have all the same internal's as a standard Andymark Toughbox, so the reduction is 12.75:1. this makes the final speed pretty slow (around 4 fps) so I'll probably half the ratio.

28-06-2011 23:51

AdamHeard

Re: pic: West coast drive design

Crushing from impact (and to a lesser extent parallelogramming) are really a result of poor design in this era of mandatory bumpers.

We've never crushed in our 1/16" wall with some brutal impacts while using bumpers. Without bumpers, the exposed corners often get smashed badly, but just the corner and it's purely a cosmetic issue. If we ran without bumpers, we'd beef it up. However, we know we're using bumpers and take advantage of it.

29-06-2011 00:10

Hawiian Cadder

Re: pic: West coast drive design

I was thinking more about the bearing for the middle wheel and gearbox.
if you are using flange bearings (which you have to with walls that thing) and take a hard side hit, have you ever had problems with the flange punching into the tube wall?

adding a bearing block, with holes rather than slots, could help to minimize the risk of this happening.

29-06-2011 00:38

42!

Re: pic: West coast drive design

Excellent point, I hadn't considered that. And yes the bearing are flanged 1/2 inch hex bearings from Andymark, on a different note the gearboxes as they are shown in this have no protective covering around the gears, I've seen a couple teams do this (notably 254) and it saves some weight on the gearbox, Does anyone know any of the advantages or disadvantages this has?

29-06-2011 01:05

Chris is me

Re: pic: West coast drive design

Can we not make claims about robot strength or material thickness based purely on guesswork please? Seriously, if you have no experience with the material, there's no shame in just not posting.

29-06-2011 01:10

AdamHeard

Re: pic: West coast drive design

He's got a valid point, it just took him a few posts to get around to it.

Yes, we avoid 1/16" wall on our rails for drive that have bearing blocks, etc...

We're pretty sure it'd work, but for a base that's already around 30 lbs with motors, we take the few extra pounds that just two rails of 1/8" represent.

29-06-2011 01:24

Hawiian Cadder

Re: pic: West coast drive design

I was never suggesting making the cross-members 1/8, but 1/16 tends to bend around where you attach other stuff to it that will experience a lot of force. My fault, I should be more clear.

If I don't have experience with what is being discussed in a thread I don't post anything. We used 1/16 and 1/8 tube this year on various parts of our robot, and the 1/8 turned out to be overkill on the members that did not have axles or the tower bolted on. In the members with axles or the tower bolts, we should have used more washers to prevent the tubes bending. The 1/16 was about perfect where we used it.