Advantage to six wheel drive?
 

Hi Chief Delphiers
I know there have been a lot of posts about six wheel/eight wheel drives over the last year or two, but I couldn't find the answer to my question: Why would anyone use six wheel drive (especially a WCD) unless there are bumps on the field? It seems to me that four wheel drive accomplishes the same thing that six wheel would.
Thanks,
Michael Groom

Re: Advantage to six wheel drive?
 

Turning ability.

In any skid-steer drive with more than two wheels, a phenomenon known as turning scrub arises. Wheels are dragged sideways as the robot rotates, creating significant frictional resistance to turning. This resistance can be large enough to completely prevent the robot from turning at all. Typically, this point is reached if the wheelbase is significantly longer than it is wide, as is the case in a typical, long orientation four wheel drive.

There are a number of ways to reduce turning scrub within a four wheel drive, none ideal. Casters, slick wheels, omniwheels, and unpowered wheels reduce tractive force, and shift the robot's pivot point. Shifting the robot's center of gravity to one end or the other can make it unstable. Wide orientation drives don't work with every robot design. Shifting wheels closer to the center of the robot makes the robot tip-prone.

Drop center 6 wheel drive takes full advantage of a robot's 38"x28" footprint, does not sacrifice traction, and reduces turning scrub. Since only four wheels ever contact the ground at once, the wheelbase at any given time is short and wide. But the presence of 6 wheels, spanning the length of the robot, gives the robot great stability.

For more on the phenomenon of turning scrub, I highly recommend this white paper: http://www.chiefdelphi.com/media/papers/1443

Re: Advantage to six wheel drive?
 

I agree with the 6-wheel WCD advantage...

Is there any advantage in using live-axle vs. dead-axle in a WCD setup?

THANKS!

Re: Advantage to six wheel drive?
 

Live axles vs. Dead axles is more a matter of personal choice. There are definite pros and cons to both, and great teams have used both.

Live axles are generally easier to work with. With a hex shaft system, wheels and other components can be switched out extremely quickly, and independently, so you don't have to remove a sprocket to change the wheel. They are generally easier to build in a cantilevered setup. Live axles can be built inside of structural members, and transfer torque, allowing sprockets to be mounted further away from the wheels. This also makes live axles suitable for direct drive. Live axles also make the attachment of encoders easy.

Dead axles are less expensive and time consuming to build initially, since you don't have to broach parts, buy hubs, or deal with keyways. Axles often take the simple form of shoulder screws, further reducing their price. Dead axles can also be squeezed into tight places, since they don't need bearings at the interface between axle and structure. In fact, dead axles can double as standoffs, and serve key structural roles! See the Revolution swerve module for an example of this. Finally, some argue that dead axles allow faster acceleration, since the motor does not have to spin the mass of the axle.

Re: Advantage to six wheel drive?
 

THANK YOU, Joe G... that was _two_ excellent explanations... it's _appreciated_... ;-)

Re: Advantage to six wheel drive?
 

seriously?

Re: Advantage to six wheel drive?
 

But the added mass of a sprocket doesn't matter? I'll leave it as an exercise for the reader to find out how wide the shaft must be to have the same moment of inertia as a 22 tooth aluminum sprocket.

Re: Advantage to six wheel drive?
 

Another advantage to dead axles is that they can be tubes, while live axles in the .5 inch range would be very difficult to make hollow. I think a lot of people think that live axles allow for better reliability, because the forces acting on a dead axle are always in the same orientation, they are more likely to fail due to fatigue, or bend (depending on the application).

Re: Advantage to six wheel drive?
 

Okay that makes sense. Thanks everyone!
Michael

Re: Advantage to six wheel drive?
 

To be fair, there's generally a sprocket on a live axle too....

Re: Advantage to six wheel drive?
 

BUT, isn't the same sprocket you mention required for either approach?

Live-axle Mass = sprocket, shaft, wheel
Dead-axle Mass = sprocket, wheel

In fact, though _both_ use bearings... the live-axle would have at least _two_ friction points... and the dead-axle would have _one_ friction point...

Re: Advantage to six wheel drive?
 

Live axles also often use hubs for the wheels and sprockets, if not, they probably use heavier sprockets since now a hub must be incorporated into its design. I'm not saying that this has any significant effect on the acceleration, I'm just pointing it out.


But some designs allow for dead axles to rotate and might even facilitate it for the reason you mentioned.

Re: Advantage to six wheel drive?
 

Most of these reasons are so minor as to not matter. I like dead axles, because all I need is a bolt, no hex shaft, broaching, bearings, hubs or set screws. It is much easier to assemble and repair (unless you guys have some really efficient live axle set-ups I hacmven't seen.)

Re: Advantage to six wheel drive?
 

I like this thread, there are some entertaining responses. :)

Generally I prefer dead axle systems, since they are usually faster and easier to build (for those of us with less tooling) and leave more time to develop interesting robot mechanisms that aren't drive trains. The standard six wheel drive built from a kitbot is also great for the same reason.

Re: Advantage to six wheel drive?
 

In the entire history of FRC, I would be surprised if there has been a single match that was won or lost because one robot didn't have to "overcome" the inertial of a shaft where another one did. We are talking absolutely puny theoretical advantages here.

(But I have seen many, many matches won or lost because a robot did not have a reliable or well designed power transmission solution.)

Re: Advantage to six wheel drive?
 

Try as I might, I cannot understand the logic here. An axle bears the same load whether it is live or dead. I have never seen a drive axle in FRC bend due to fatigue (violent shock loads are almost always the culprit, and live vs. dead makes no difference here). If your axle is fatiguing, it is woefully undersized (and/or a poor choice of material).

Re: Advantage to six wheel drive?
 

There are so many sayings/ideas that would disagree. The straw that broke the camels back. The butterfly effect. Calculus in general. I'm not saying that such a match has occurred for sure. Just saying you shouldn't be surprised if it did. If FIRST is around long enough, there will be a match where this happens at some point. Never underestimate the impact small changes can make on the big picture. They tend to make a difference at some point.

Jason

Re: Advantage to six wheel drive?
 

Think of 4 wheel drive as using a push lawn mower. You have to push down on the handle and lift the front wheelsvto turn it because otherwise it has a hard time turning. insert the middle wheel and the it will have a pivot point making it easier to turn :)

My team has never used live axles before, dead axles have always been the way to go for us. (except when we used maccanums)

Re: Advantage to six wheel drive?
 

Generally the sprocket for the live axle would be smaller.

Re: Advantage to six wheel drive?
 

In the grand scheme of things, if one match in the history of FRC has been lost due to the inertia of an Axle, I would say that's nothing to worry about. One match out of a few thousand equates to what, less than .1%?

Re: Advantage to six wheel drive?
 

That 1 match could have completely changed things though. It might have prevented a team from making it to championships that would have otherwise won it. Once again, my point is that dismissing a variable is a dangerous game. Perhaps it isn't worth basing your design off of, but it shouldn't be completely ignored either.

Jason

To anyone that doesn't know me personally: I have a habit of trying to consider all variables whenever possible. Sometimes this makes my work better then average, other times it makes it worse. I'll fully admit sometimes your better off ignoring something, it just becomes a betting game. Either way, I'm not much of a betting man.

Re: Advantage to six wheel drive?
 

So, a better representation would be:

Live-axle Mass = sprocket, hub(s), shaft, wheel, mounting screws
Dead-axle Mass = sprocket, wheel, mounting screws

BOTH approaches use bearings... the Live-axle would have at least _two_ friction points... and the Dead-axle would have only _one_ friction point...

We shouldn't underestimate the friction/drag created at each bearing-point on a Live-axel setup... if you insert a shaft into one secured flange-bearing and spin with your hand and then take the same shaft and insert into two secured flange-bearings and spin with your hand, you'll notice it takes more effort to spin the shaft... now times that by six wheels...

ALSO, the mechanics of the Live-axle setup causes more friction/resistance at the bearings because of the angular pressures caused by the shaft pivoting inside the bearings as the pressures on the attached wheel (at far end of the shaft) change...

I KNOW these are _minute_ differences (or, are they?)... but, it's fun to think this through...

Re: Advantage to six wheel drive?
 

I've done a little math (pardon my mixed units):

150lb robot traveling@ 15ft/s has 711 Joules of energy

Six 1-foot axles of 1/2 in OD made from steel rotating @ 14.32 hz (15ft/s w/ 4inOD wheels) has 0.148 Joules of rotational energy.

If those same axles are moving with the robot @ 15ft/s they have 18.8 Joules of energy.

What difference does that make?

Rough numbers here:

Assume 500W power train (i.e. 4 CIMS with some inefficiency and non-peak power output slapped on)
Assume perfect traction

It will take 1.4220s to output a total of 711J, i.e. get a dead axle robot to 15ft/s

It will take 1.4222s to ouput a total of 711.148J, i.e. get a live axle robot to 15ft/s

The amount of rotational energy in the axles at full-speed is utterly trivial. If you need that extra 0.2ms to get to full speed... well... good luck. You could shave 0.03 lbs of static mass from your robot and break even.

I may have used rough numbers, but we're talking multiple orders of magnitude of triviality.

Bottom line: live or dead axle, it does not matter.

Re: Advantage to six wheel drive?
 

It does not matter... as far as rotational inertia. I'm interested as to the effect of a large first stage gear like in the CIMple, gearbox

Re: Advantage to six wheel drive?
 

because a live axle is always spinning, every time there is a shock load, or the robot sits for a long period of time, the axle is in a different orientation. however ALL the shock loads on most dead axle setups occur with the axle in the same orientation. So after a while the axle may start to bend in that direction. I haven't noticed this problem with our .5 inch dead axle robot> But on the cart was made from the kit-frame, the axles are severely deformed and bent. this was also a problem on our 2009 prototype. In a year like logomotion, this may not be an issue, in a year like breakaway, where every time a robot came off the bump the axle took the shock in the same way, I think it could definitely contribute to some bending.

Re: Advantage to six wheel drive?
 

Regardless of axle setup, it is strongly advised to have two support points (bearings) to fully support the "beam". Whether they are in the wheel (notice all AM wheels take two coaxial ball bearings) or in the frame (even cantilevered WCDs have two bearings per axle)...doesn't matter. You don't want to put radial loads onto a single ball bearing.

Re: Advantage to six wheel drive?
 

But if we're comparing WCD to dead axle set ups then the fact that the shaft is unsupported more than accounts for any problems caused by multiple shocks in the same place. Bottom line, your axle shouldn't ever be bending, don't rely on an equal distribution of bends to keep your axles straight.

Re: Advantage to six wheel drive?
 

In some ways a dead axle would be better in the situation you described. Sure it could deform more, but the wheel wouldn't start to wobble because the shaft was bent. If the live axle was bent in one direction, then got bent back to straight, that's a fully-reversed fatigue cycle beyond the shaft's yield strength, that is BAD NEWS. The shaft is now compromised structurally, perhaps to a large degree, and will bend more easily the next time it's hit.

A dead axle bending in the same direction more and more gets stronger and stronger, until it fails of course, but these impacts are non-reversed fatigue cycles, which are better than fully-reversed fatigue cycles.

Re: Advantage to six wheel drive?
 

You can do the math too! I found all of the equations I needed from Wikipedia and Google does a great job handling units. Give it a shot.

Re: Advantage to six wheel drive?
 

Haha, I tried, but without knowing the dimension of the gear itself, I'm pretty much out of luck

Re: Advantage to six wheel drive?
 

Thats also pretty easy to find. Open a cad model or look at the martin gears website. They have a pdf of physical dimensions for 20DP gears.

-RC

Re: Advantage to six wheel drive?
 

GOOD CATCH, Jared! Rookie mistake on my part...

So, I'll restate my thesis:

Live-axle Mass = sprocket, hub(s), shaft, wheel, mounting screws
Dead-axle Mass = sprocket, wheel, mounting screws

BOTH approaches use at least two bearings... the Live-axle would have at least _two_ friction points at each end of the shaft... and the Dead-axle would also have _two_ friction points built into the center of the wheel from the two separate bearings residing in the center wheel hub-area...

The mechanics of the Live-axle setup causes more friction/resistance at the bearings because of the angular pressures caused by the shaft pivoting inside the bearings as the pressures on the attached wheel (at far end of the shaft) change... times that resistance by six wheels...

I KNOW these are _minute_ differences (or, are they?)... but, it's fun to think this through...

HOW am I doing so far? Not bad for a former insurance-salesman, right? ;-)

Re: Advantage to six wheel drive?
 

This is quite a bit off actually. You'd still have two bearings in a wheel in a dead axle system.

For a dead axle system, your axle is held by two plates, often inches apart, and any tolerance issues will make that shaft crooked and lead to extra friction on whatever is driving it.

The double bearing issue you mention for live axle has always been a nonissue for us, our bearing blocks are one piece for the bearings and use the same hole. The load from the cantilever is higher, that is true, but it's still well within the spec of the bearing. We slammed down HARD in 2010 and didn't have a single issue anywhere, and our drive was still nice and low friction at the end of the season. I'd argue that few teams had lower friction drivetrains than us.

If anyone wants to see our live axle drivetrain, check out my uploads. Our previous 5 robots are there, all using variations of the same system.

Re: Advantage to six wheel drive?
 

Terminology Help needed???

The discussion here live axle vs. dead axle yet both options still powered wheels? I am sure I am wrong but, to me the term dead axle, lead me to believe a non-powered wheel. A simple bolt (axle) through the frame rails, with a wheel. No sprocket, chain or belt.

For example the rear wheels on a front drive car are a dead axle.

So here in FIRST does live axle = wheel fixed to the axle (power to axle), axle rotates in frame bearings--and dead axle = axle fixed to frame, yet wheel is powered?

I guess if that is true, live or dead axles could be, or not be, powered?

Thanks Troy

Re: Advantage to six wheel drive?
 

With a dead axle, the axle does not rotate. The wheel has bearings in it that ride on the shaft and the drive sprocket is attached to the wheel.

With a live axle, the axle and wheel are connected together. The axle rotates on bearings and the drive sprocket is attached to the axle.

With the WCD setup, the wheel is cantilevered. It is very easy to maintain since you don't have to remove the drive sprocket and chain to remove the wheel.

Re: Advantage to six wheel drive?
 

In a live axle system, the wheel is attached to the axle, and whatever form of power transmission you have is attached to the axle.

In a dead axle system, the wheel is attached directly to whatever is powering it, and the axle does not necessarily have to spin with the wheel. Examples of both would be:

Live axle
http://www.chiefdelphi.com/media/photos/36934

Dead axle (from our 2011 robot)


There is a sprocket bolted directly to the wheel on the other side, and the flat head shoulder bolt does not spin.