Advantage to six wheel drive?

[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

[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

[R.C.]

Originally Posted by lemiant

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

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

[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

[Tom Ore]

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.

[Hawiian Cadder]

Originally Posted by 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

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.