Live vs. Dead axles
 

Which do you prefer on your robots?

Re: Live vs. Dead axles
 

604 has used dead axles simply because we like to quickly switch out our wheels during competition, so we press the bearings into our wheels and screw on our #35 sprockets. This allows us to slide the wheels onto the dead axle, tension the chain up, and we're good to go.


What came first: the student or the mentor?

Re: Live vs. Dead axles
 

Dead axles. It's a bit easier to do (I think) because you can put one piece on per wheel instead of four (sprocket, wheel, and two keys).

Re: Live vs. Dead axles
 

You can do it in two ;)

Re: Live vs. Dead axles
 

Having seen both in action (Ockham went from dead to live when we switched the tires out), I really prefer dead. Fewer moving parts, less rotating mass.

Re: Live vs. Dead axles
 

Dead, they are simpler... this year we just used 3/4 or 1/2 bolts with locknuts for axles... it worked great ;)

Re: Live vs. Dead axles
 

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

Re: Live vs. Dead axles
 

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

Re: Live vs. Dead axles
 

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.

Re: Live vs. Dead axles
 

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.

Re: Live vs. Dead axles
 

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.

Re: Live vs. Dead axles
 

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

Re: Live vs. Dead axles
 

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.

Re: Live vs. Dead axles
 

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.

Re: Live vs. Dead axles
 

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.

Re: Live vs. Dead axles
 

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

Re: Live vs. Dead axles
 

.. for ease of maintenance silly :)

Re: Live vs. Dead axles
 

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.

Re: Live vs. Dead axles
 

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.


What came first: the student or the mentor?

Re: Live vs. Dead axles
 

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. ;)

Re: Live vs. Dead axles
 

We use dead axles for pretty much the same reasons. Plus, old habits die hard...

Re: Live vs. Dead axles
 

I think we've used both, but I've only worked on dead. I can see the benefits of live, but I'm with JVN in this, I like dead better than the idea of live. The efficiency, durability, and the fact that we already have the parts is worth loss of quick change capability. Tristan's idea is interesting though... I really need to register Inventor.

Re: Live vs. Dead axles
 

I can't stress enough how important it is for robot parts to swap in and out in a matter of seconds or minutes. I can't say we would be National Champions now, but I know we could have come alot closer if our CIM motors were an a more easy to access position and we had full spares on hand.

I like the idea of the modular wheel that sits on a permentantly mounted hub. This solution seems to give you the best of both worlds (but may add weight. again, a trad off.)

Re: Live vs. Dead axles
 

If you have the resources to do it, I really can't see much of a reason not to use a live axle setup with hex broached wheels and sprockets. It makes everything so much easier. In the time it would take to remove one or two wheels from a dead axle setup, you could replace every wheel on a live axle setup. Using hex shafts to transmit power to your wheels also eliminates any concern of keys slipping, etc.

Re: Live vs. Dead axles
 

Another reason for dead axle--resources, or lack thereof. And for the live axles, you still have to pull the axle partway out (or all the way, thereby making it hard) of the frame, if you have a closed-frame robot (frame outside the wheels). So it may also depend on your frame design which is easier to maintain. If you have your wheels on the outside of your frame, it's easier to maintain them, but they take a beating and must be live axled. Inside, it doesn't matter.

Re: Live vs. Dead axles
 

This was the first year sense I have been involved with 116 that we have used dead axles, and if we had swapped our front wheels to the AM Trick Wheels for improved turning, the front axles would have become live (we had our front axle's with a double bearing set up).
I prefer live axles for ease of maitenance and for ease of sensor application. 116 has, for the past several years, used shaft encoders (or attempted to) to help our autonomous navigation. Because of the dead axle situation this year, we couldn't simply add them to our drive axles this year, and they only spot we could with our simple set up would have been on the output shaft of our gearbox (but because of a lack of space and time we couldn't).
Id also have to say that having an open frame over your wheels is probably a much bigger factor in the ease of replacing your wheels, but a live axle system is alot easier, imho.

Re: Live vs. Dead axles
 

I voted live axle on this one.

The live axle design we have used the last 2 years is very simple, low cost, requires little machining resource, and has been maintenence free (not a single failure through 6 competitions last year and 2 so far this year)

I think in reality the real answer is whatever you can do best is what is best. From reading the various benefits of each system in this thread, both can be great for you if done properly. I can also attest that either can be a nightmare if done improperly or if problems arise.

Some reasons that our decision to use live axles is easy are cost and machining. We do not have the machining resources to make good wheels for dead axles, and cannot afford the off the shelf products offered by some vendors. Our wheels, bearings, shafts and sprockets were under $175 for a six wheel drive setup.

So analyze your budget and machining processes and go with whatever you can do best. (good design helps alot as well :) )

Edit** Here is a link to a picture of how our live axle is set up. There is one long key going through the wheel and sprockets for each shaft. http://www.joemenassa.com/Images/ROB.../IMG_2877.html


Rob

Re: Live vs. Dead axles
 

Dead axles are simple, lightweight, structural, and lend themselves to standardization of: bearings, sprockets, axle shafts, pillow blocks, axle spacers, etc. We used the same general construction for wheels, pick-up roller, and launcher.

I would like to see some examples posted of live axle designs that have been refined and are working well for those teams that use them.

Jay

Re: Live vs. Dead axles
 

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