Live vs. Dead axles

[Paul Copioli]

Live, live, live. Dead is not simpler when chains are involved. 217 has used both. In 2000, 2001, and 2004 we used dead. In 2002, 2003, 2005, and 2006 we used live. After 2002 or 2003 I would have said dead, because we poorly implemented the live axles. In 2005, we saw the light and live is the way to go for us. The main advantage: you do not have to touch the chain to change wheels because the sprocket is not attached to the wheel.

We use hex shaft (3/4" this year, but maybe smaller next year) with a hex hole in the wheel and sprockets. Wheels are off in no time flat and the chain doesn't get touched.

Paul

[Ben Piecuch]

I prefer a design that uses live axles on the drivetrain. They are more robust, simpler to maintain, and can be an overall lighter design.

However, I've never actually built a live axle drive system, simple because we've never used a wheel/hub combo that can use a live axle. The kit skyways, IFI, and A-M wheels all require a dead axle. Finding a keyed wheel hub, or a wheel with a hex shaped hole (as Paul uses) is difficult. For those teams that can't machine their own wheels, it's almost impossible to find hex-keyed wheels.

I've always marveled at the simplicity of the "West Coast Drive" used by 254, 968, 22, 980, etc... the list goes on. While that's a cantilevered live axle system, you can add an outboard bearing with a little more framework.

For higher torque applications, such as arm joints, I prefer a dead axle. Transmitting all the torque through a keyway can become a weak point. By mounting your gear directly to the arm or pivot, you transfer the torque through the bolt pattern, which can usually support much higher loads than the key (or double key...) Assembly is obviously a bit more challenging, but I'd rather build it once at the school, than to constantly repair it at a competition.

Good thread! I hope a bunch of teams start to post their live axle designs, so that us less fortunate ones can learn how to design and build our own.

BEN

[Tom Bottiglieri]

As Ben and Paul stated, I prefer live axle for a drive train system. While installation and initial design of a dead axle system using common off the shelf FIRST parts (AM, IFI, Skyway) may be easier, incorporating a live axle system will prove much easier to swap out parts in competition (a lesson we DEFINITELY learned on Einstein this year.)

I must also comment on the West Coast drive train. I always knew it was nice, but watching 968 swap wheels in less than a minute (nothing was broken, mind you... they just wanted new tread) convinced me this is by far the best design FIRST has ever seen.

You may also consider a combination of these two systems, such as team 25's drive train. While their wheels sit on dead axles, there is no chain to play with. A big spur gear is attached directly to the wheel hub, and a line of gears transmit power to all 6 wheels. They can swap off wheels and not have to worry about configuring live axles in the design.

[Not2B]

Mmm... both.

I like live shafts, because it's easier to get things that don't fit together to go together. Drill a hole, cut a keyway, and you're done. Easy to swap parts.

However, we go with dead shaft most of the time. Why? Because we design for low-accuracy and low-repeatablity. If our frame members get out of alignment, a life shaft with bearings has proven to be more difficult to dead with. Same if the shaft bends. With dead, the frame can warp, the shaft can bend, but as long as the chain (or timing pulley) can still compensate - you are still driving. Somewhat.

We kind of look at each application to pick a live or dead shaft. How easy is it to put together, how much abuse will it take, how well can we make it.

[Tytus Gerrish]

Dead axle's have always been easy as long as you have good wheels and sprockets. they work Beautifully. swampthing in 2005 had billet aluminum wheels with monster bearings and water cut sprockets. it was very ,fast reliable, never broke. and it is still running today. this year we went with lighter wheels and had nothing but problems. putting torque through 6 wheels using sprockets that are cantilevered off the wheel tend to want to tear themselves apart so they have to be pretty strong.

[JVN]

Dead axles. Why would I transmit torque through something I don't have to?

[MattB703]

We always use dead axles, but we use Brecoflex belts and pulleys. We are able to slide the dead axle out and drop the whole tread system out of the machine. Because we don't use any chain, (spur gear drive right to the wheel/pulley), we don't run into some of the problems that have been discussed above.

[Cory]

Live, for the reasons outlined by Paul above.

We've done live axles the last few years, and this year moved to a 7/16" hex on the wheel shafts, to engage both the wheel and sprocket. It makes changing the wheels a breeze. Pop off one snap-ring, and the wheel is off. No fussing with chain.

[John Neun]

We have used both extensively on 6-wheel and 4-wheel arrangements. I like dead axles much more. The torque transmittal issues and simplicity issues I think are close to a wash. With the bearings in the wheels, the machine is much less susceptible to being incapacitated from battle damage. We had our sidewall knocked in about 2 inches in Hartford, but were fully functional. If the bearings had been in the bulkheads, the drive configuration would have been destroyed. Dead axles can be lighter but most importantly, they are far more robust.

[Paul Copioli]

Dead axles. Why would I transmit torque through something I don't have to?

.. for ease of maintenance silly

[JVN]

Originally Posted by Paul Copioli

.. for ease of maintenance silly

Design is all about tradeoffs.
I just happen to TOTALLY disagree with the tradeoff you're making; but that is pretty much par for the course.

[Cuog]

i prefer the use of dead axles, i have had to deal with too many thrown and eploding keys

[Ken Patton]

We usually have used dead axles. One year when we cantilevered our front wheels, we went live.

Using dead axles allows you to use the axle as a structural member. I see that as one strong advantage. It does not take us very long to do a wheel swap (but I admit its longer than some of you live axle guys are describing).

As somebody already mentioned, there are some nice wheels out there (IFI's, AM's, etc) that are suited to the dead axle approach.

Ken

[Tristan Lall]

Though I haven't tried this specifically, you could always get most of the benefits of both systems by using a dead axle, and a live, quick-change hub. That would permit you to remove the wheel from the hub, but keep the chains attached. It seems easy enough to do with a couple of dowel pins for transmitting torque, rather than the usual bolted-together wheels and hubs, or single-piece wheel-and-hub units; you just need a way to constrain the components axially.

Incidentally, Woburn robots have typically used dead axles, but that was often because it was more convenient to use cheap bearings/bushings in the frame, and better bushings/bearings (or even oiled wood bores!) on the shafting.

[Gabe]

Originally Posted by Tristan Lall

It seems easy enough to do with a couple of dowel pins for transmitting torque, rather than the usual bolted-together wheels and hubs, or single-piece wheel-and-hub units; you just need a way to constrain the components axially.

Like a car! I hadn't thought of it that way. A car has a live axle which ends with a faceplate that has 4-6 lug bolts. You mount your tire by sliding the tire rim onto the faceplate, and tightening the lug nuts. Instead of dowel pins, a faceplate with bolts sticking out would handle your axial and radial movement when you tighten the wheel down with nylon-insert nuts.


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