Advantage to six wheel drive?

[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

[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.

[Michael Blake]

Originally Posted by Jared341

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.

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? ;-)

[AdamHeard]

Originally Posted by Michael Blake

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...

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.

[TroyCDH]

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

[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.

[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).

[PingPongPerson]

Okay that makes sense. Thanks everyone!
Michael

[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)

[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.

[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.

[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.