I have been reading the various 6WD threads and I have some questions.
Regarding lowering the center wheel when driven directly from the motor/gearbox. How can you make it adjustable up and down or do you make the outer wheels up and down adjustable? Which method is easier to maintain and more effective over time?
Many people talk about 1/2" hex axles. Wouldn’t 3/8" hex be just as effective and strong mechanically, easier to machine (if you have a smaller lathe) and cheaper to purchase a broach for (2/3rds the price)?
It all comes down to how you are mounting your wheels, and how they are driven. There are so many different ways to manufacture a 6WD, that it’s hard to enumerate through all the possibilities. How do you plan on building the frame? (Kit frame, extrusion, parallel plates, sheet metal, 2x1 or 1x1 tubing?) Just keep in mind that it is generally going to be easier (but not always) to move a wheel that is driven by belt or chain than it will be to move something that’s direct-driven by a gearbox shaft.
Also keep in mind that once you have found a drop you like, you probably will never need to change it (unless your frame gets warped somehow). So unless you are designing specifically for a rapid-prototyping system, I wouldn’t get hung up on being able to easily adjust wheel drop.
I defer to someone else on the effectiveness of 3/8" hex axles in FRC drivetrains, but of course, thicker axles are going to be stronger regardless of profile.
As far as 3/8 hex axles you would run into an issue because there are no hex bearing that have a 3/8 profile that are strong, the ones AM sells state that they can not take a lot of load
Typically, you don’t. Most 6WD drop drivetrains are in a fixed location. A few are flexible–the center goes up and down and is locked in. I think 1323 put out a concept for that. 330 used pneumatic center tires and could adjust the pressure for what we wanted.
That’s a design decision you have to make. It’s a little bit smaller, so the same amount of torque results in more shear stress. The more stress you have, the more likely to fail a part is. 1/2" provides a bigger safety factor than 3/8" in this application. The question is, after doing the analysis, are you comfortable with the safety factor in the 3/8"? Or, to put it another way, does the reduction in cost offset the reduction in safety factor in your mind?
Almost all 6WD’s I’ve seen used in FRC had fixed wheel locations. I don’t really think there’s any advantage to making the wheel heights adjustable in most games or on most game fields. On occasion, some teams will make the 4 outer wheels adjustable along the length of the drive train to allow for chain tensioning, but again the height from the floor does not change.
From what I’ve seen, 1/2" Hex is just a bit more common to find parts for. AM makes 1/2 Hex bearings and 1/2" hex wheels that have already been proven in competition.
25 has been using their 6WD since 2002/2003 and all 6 wheels touch the floor. It works fantastically for them.
1/2" axles are almost a must in cantilevered robots. In our past offseasons we have bent 1/2" axles in experimental testing which has led me to believe that 3/8" cantilevered axles during competition will simply make us have a bad day.
We’ve used 3/8" axles when both ends of the axle are supported by the frame with the wheel somewhere in the middle.
We have a fixed drop, been running .125" drop on ~4" wheeels has served us well.
Universally saying X size is required isn’t the greatest thing to do.
We had a prototype cantilever drive with 3/8" axles perform and last just fine, we however do 1/2" axles with 7/16" hex on our comp for geometry reasons.
Rather than being universal, I meant to imply my own conclusions from my own experience. I feel that it is simply confusing to someone with a question when people respond with (in a Brian from Family Guy voice) “it depends, and here are some vague anecdotes of reason from which you cannot possible derive a logical conclusion”.
Additionally, I was trying to provide context as well. Very different conclusions are drawn from the statements ‘1/2" cantilevered axles didn’t hold up in our experiments’ versus ‘3/8" cantilevered axles are fine for our prototypes’. Indeed, the original post didn’t have anything about the context of competition or extreme environments, however it is implied by the fact that there’s a good chance a third party reader who arrives at this post via search will take the information as fact during the build season (regardless of how illogical that can be, does anyone really have time to ‘experiment’ during the build season?).
I should have further explained my point I suppose. Saying 1/2" shafts will work is just as dangerous as saying 3/8" shafts will work. The diameter of your wheel, how far it’s cantilevered, how many wheels you have, what you’re driving over, etc… will all effect the size you really need.
I’m just as guilty of any of doing this, I just hate the thought of a kid browsing chief and seeing, “1/2” axles! they work, we’re doing it!", then spend a whole season busting axles. The other end of the spectrum is true as well, but less damaging, I’d dislike to see a team waste a half pound for no net gain.
Before we can really quote shaft sizes, we need more information about their specific shaft setup and design.
I have to agree with Eric, do the math for your situation. I usually then have someone else check the math and then add some fudge factors in (erring on the side of caution) but YMMV.
Regarding lowering the center wheel when driven directly from the motor/gearbox. How can you make it adjustable up and down or do you make the outer wheels up and down adjustable? Which method is easier to maintain and more effective over time?
Our Universal Chassis system uses change blocks to allow for adjustable center wheel drop. The axle mount holes in each block are drilled off-center creating different amounts of drop. Universal Chassis
Since the center wheel typically interacts with your gearbox it is easier in my opinon to move the outer wheels.
This option also allows you to indepoendently adjust the front and rear wheel position. In the past we have raised the front wheel only…leaving the center and rear on plane.
Teams that don’t build a little adjustment into this system can run into other problems as teams found out at Champs. When the subsurface floor is neither flat nor smooth, some adjustment should made to the drop wheels in order to get the desired performance. At Champs, the plastic underlayment sagged in the middle. Since the underlayment was a four foot square, robots in the 38" dimension sometimes had difficulty in turns when in the center of a square. Also, if you cannot quickly change wheels, adjustment may allow you to compensate for tread wear on the dropped wheels.
