pic: FRC294 Prototype base almost done

As far as traction I was mostly referring to the width of the wheels. In my mind I was comparing them with wide tank treads or a 3" inch wide natural rubber wheel that we had custom wrapped with 60 durometer natural rubber a couple of years back. I realize that the normal force will not change because the weight of the robot is limited so widening the wheels only spreads out the normal force on a wider surface area. I haven’t really experimented with this so I don’t know what the trade off is. I know that there would be a point of dimenishing return as far as increasing the tread contact area to normal force ratio. I just know what I have observed which is I have seen tank tread and wide wheeled robots winning the pushing battle against the narrower wheeled robots. My imppresion was that the wheels on your frame were of the narrower variety. This is purely subjective. I would be interested in knowing the width of said wheels.

I would also love to do some testing with comparing different traction device and finding out what the return is on increasing the traction surface area realtive to the normal force. If my team does this experiment we will post the results of our findings.

As far as the different gear ratios being traction limited I agree that a slower ratio would definately give you more accurate control. I would love to see the spreadsheet of speeds when you get back home and have a chance to post them.

I agree with you EricH that the four speed in this type of shifting is easier to design then a three speed. I would love to see someone design and build a squential shifting three speed ball shifter I think that would be sweet. If I ever get the time to actually sit down and dore more inventor work I will make one and post the pics. One of the down sides of having a full time job and other demands on my time other than robotics.

I imagine that is more an effect of the wide wheels wearing down slower than thin wheels; since many tank tracks and wheel treads have a ‘cleat-like’ pattern, tread wear probably significantly reduces traction. Plus, if the tank treaded robot is running something like Brecoflex Red Linatex (sp?) treads with a CoF of 1.6, I am not suprised that they beat robots running traction wheels with a CoF of 1.3-1.4.

Its actually already been done.

Actually the custom wrapped rubber wheels never wore down because they did not have a tread pattern in them and the tank treads that we have used are solid with no tread either. So that is not really a problem. In my opnion the tread pattern doesn’t really help but I have nothing to substantiate it and this thread is not rellay the place to discuss it. Also I was referring mostly to the surface contact area difference between the differnt wheels and treads not really to the coefficent of friction.

Coefficent of friction is not the end all be all comparisson to tell if someone has more traction it is a factor inthe equation which includes in addition the normal force per square inch and the square inches in contact with the ground. However you do want to maximize the coefficent of friction if you are looking for the best traction you can get.

Sorry if I’m contributing to pulling the thread away from the original intent which is to discuss 294’s prototype drive system. I don’t really want to do that and I don’t really like when it happens.

Back on topic!

I really like the design of the drive sytem, using the pins on the corners really makes a lot of sense.

Another question what bearings are you using on the drive wheels?

I have found that a standard bronze bearing can wear down an aluminum shaft over time. Just something to think about. We had really good luck with rulon and peek bearings which are high perfomance plastic bearings. They work well because they are designed to be compatible with softer shaft materials. The rulon bearings are a reinforced PTFE bearing and I know that you can get them and the peek bearings at mc-master carr.

You could probably save some weight by hollowing out the axles some if you wanted. We have run aluminum tube axles and have great sucess with them. There is minimal shear on them and no unspported length so it hasn’t been an issue.

correct… for the wheels, the drive chain is #35 & yes we will be placing idler sprockets to decrease noise/improve efficiency

What size wheels are you running they look about 4"?

6" wheels with approx. 1" tread width

What shifting method are you using. Two dog shifters, a ball shifter, or what? "?

We are using a design based on The KillerBees FRC33 4-speed pneumatic shifter… there are 2 sliding geared axles between 3 fixed geared axles…
The shifting is done by 2 pnuematic cylinders…
http://www.bcr294.org/images/Tensioner/4-speedTopview.gif
http://www.bcr294.org/images/Tensioner/4-speedIsoview.gif

gear ratios are:
1.00:1
1.78:1
2.78:1
4.94:1

What speeds (FPS) are you estimating from the different gear ratios?

Adam Heard, I think in a later post answers that question…

I have to ask do you really need four gears? I mean you don’t have a huge amount of traction so having a really torquey gear ratio could mean that you don’t have enough traction to actually realize that troque and your wheels would just slip out. And how fast do you really need to go the field is only 50 something feet long so 12-15 FPS means so can get across the field pretty quick. Don’t get me wrong a four speed transmission is really cool and I respect the engineering and the maching that goes into the effort.

This off-season project was done more for “the fun and challenge” in preparation for 2008… We developed some new machining methods… learned what will work and what needs to be corrected in V2.0… By putting this out on CD we also get feedback from other teams who can speak from experience… invaluble to the process…

I would also be interested in the total weight of the system and the weight of the transmissions assembly.

The side modules with wheels were if I remember correctly are 7ish pounds each… the transmission assembly I don’t know… but I will find out soon and post that when we have them off for a bench break-in period…

Also some one was interested in the tensioner assemblies for the drive chain…

http://www.bcr294.org/images/Tensioner/TensionerAssm.gif
http://www.bcr294.org/images/Tensioner/TensionerExploded.gif
http://www.bcr294.org/images/Tensioner/TensionerInPlace.gif

The axle block slides up and down in the cage and is held/drawn into tension by the overhead thumbscrews…

Hope this answers some of those questions…

Thanks for reminding me about the speeds John, I forgot about them :).

Well, we think an output sprocket of 12-16 is appropriate depending on the game situation. Which gives us 5 reasonable sets of speeds. Now, keep in mind, not all 4 speeds of each set are the ones that would be useful for a game; We already know 4-speeds are not the most practical and we already know that really high speeds can not always be used. Also, these speeds are with estimated inefficiency. I think I tended to overestimate the inefficiencies, so I don’t know if they are above or below.

12; 3.4, 6.0, 9.2, 16.4
13; 3.6, 6.5, 10.0, 17.7
14; 3.9, 6.9, 10.8, 19.0
15; 4.2, 7.4, 11.6, 20.3
16; 4.5, 7.9, 12.3, 21.6 (that’s the field length in 3.3 seconds from a standstill. That’s with only two small CIMs as well, it’s faster with the Big CIM and/or FP added)

Okay, once again, I know the high speeds there are high… really high. But, that is just the available range. The lowest gear is well below traction limited in each, so it didn’t hurt to bump up the speed much. (3.4 vs. 4.5 fps in low is such a small difference).

Like Uncle Waldo said; this was more for an all around fun/learning experience. I got a lot of feedback on my CADing (apparently things don’t allways work like in inventor…:smiley: ), and they learned a whole bunch of machining and mechanical things. Also, it’s being handed off the programmers shortly to give it a nice gyro and encoder based closed loop control system with an automated shift scheduler. Now, considering this was cheaper than the AM Gen3s, and they can still just bolt right on; I think it was very worthwhile to use the 4-speeds on the prototype.

EDIT: and yes, those are spinners that ride on ball bearings in the last picture… The students asked for them, how could I say no?