Advantage to six wheel drive?

[Jared Russell]

Originally Posted by Hawiian Cadder

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).

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).

[Molten]

Originally Posted by Jared341

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.

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

[akoscielski3]

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)

[Andrew Schreiber]

Originally Posted by Michael Blake

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...

Generally the sprocket for the live axle would be smaller.

[thefro526]

Originally Posted by Molten

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

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%?

[Molten]

Originally Posted by thefro526

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%?

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.

[Michael Blake]

Originally Posted by PAR_WIG1350

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.

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...

[JamesCH95]

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.

[lemiant]

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

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

[Hawiian Cadder]

Originally Posted by Jared341

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).

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.

[Jared Russell]

Originally Posted by Michael Blake

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

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.

[lemiant]

Originally Posted by Hawiian Cadder

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.

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.

[JamesCH95]

Originally Posted by Hawiian Cadder

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.

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.

[JamesCH95]

Originally Posted by lemiant

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

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.

[lemiant]

Originally Posted by JamesCH95

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.

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