Live Axle vs Dead Axle

[apalrd]

I see a lot of confusion in this thread between live axles with west coast drives, and dead axles with sheet metal. Neither type of drivetrain requires a specific type of axle.


Just to clarify:
-Live axle drivetrains - The wheel is attached to an axle which transmits power to the wheel. This axle can be driven by a chain, belt, or gear. Nothing mandates that the live axle be cantilevered, there can be a bearing on both sides (our 2012 robot's third wheel was live, driven via a chain on the inboard side of the chassis rail).
-Dead axle drivetrains - The wheel is riding on the axle via bearings. The axle is fixed, in some cases a stressed chassis member, and does not spin. Nothing requires that the dead axle be non-cantilevered also, I have seen cantilevered dead axles in west-coast setups.

-West-coast drive - A drivetrain design and construction method pioneered by teams 254 and 60 almost 10 years ago that is usually characterized by a welded aluminum box tube frame, 6 or 8 wheels in a drop-center configuration, and cantilevered axles (often live).
-Sheet metal drive - A drivetrain designed and fabricated out of folded sheet metal. Due to sheet metal fabrication methods and strengths, these usually use axles supported on both ends (no requirement for dead or live axle, though).

There's also some comments about tensioning being easier on one or the other. This is simply not true - One can design any drivetrain with slotted axle holes to tension chains by sliding axles, it's not unique to west coast drives and it's not really any harder to fabricate for any type of drive. The same goes with speed, torque, gearing, etc. - Any axle, live or dead, can have any type of reduction to it. We've run live axles with chain reductions from the gearbox, and live axles direct-drive from the gearbox. It's possible to do the same with dead axles and chain, or dead axles and gears.

There are really no advantages/disadvantages to each, since the live or dead axle in itself is not frame-specific. There are many advantages in frame design to use one or the other in certain frame designs, but this isn't a function of the axle choice, it's a function of the frame choice.

There are also some totally false claims in this thread. Specifically:

Live axle usually means faster, less torque, and less backlash. Dead axle usually means slower, more torque, and more backlash.

This quote has no bearing in fact. The axle makes no difference for any of these. This is dependent on gearing selected, fabrication quality, and chain tension. If anything, I've seen more backlash in keyed live axle setups as the keyway opens up under shock load.

I think one advantage of a live axle is they usually make a more rigid chassis . A sheet metal dead axle is usually a lighter setup.

The axle makes no difference. You can argue west coast vs sheet metal, but that's not live vs dead axle. You can live or dead axle both west coast (although it might not be called 'west coast' depending on who you ask) and sheet metal designs. In fact, it's significantly easier to use a dead axle as a stressed chassis member, which would increase the rigidity of a dead axle setup in the right application.



The power transmission methodology is a tool to be used when designing the drivetrain. There are really no benefits to dead or live axles in general, although one can be highly favored over the other in a specific chassis setup. Design the chassis as a whole integrated design. Don't just pick the best individual elements, since there are many cases where they won't work well together. Also, you should design around what machine tools you can use, and what fabrication methods are easy or hard on those tools. For us, we use sheet metal because we have a brake and shear in our shop, and access to CNC waterjet and plasma machines, while the small precise components of a west-coast drive would have to be made by hand on our manual mill, which we just recently got a DRO for. This is the driving factor for us to use sheet metal - If we had access to a CNC mill instead of the waterjet and plasma, we would almost certainly design everything around milled box tube.

We use all dead axles with chain now. We are very aware that a west coast design makes it easier to change a wheel, and designed and built a west coast drive in our 2011 season, but we continue to design using dead axles in our sheet metal setup because of weight, strength, and ease of manufacturing.


In conclusion, you (OP) are asking the wrong question. The proper question should be on the design of the entire frame - And to properly answer, you (and us, if you ask us) need to know your manufacturing resources. There is no single right answer.

[Clem1640]

Originally Posted by nathan_hui

It really depends on the final gearing that you want. Live axle usually means faster, less torque, and less backlash. Dead axle usually means slower, more torque, and more backlash.

I'm not sure I agree with this analysis. The choice between live or dead axle is part of the overall design process and there are good design reasons to choose either, but by itself it does not influence speed or torque.

Which is best for a drive-train is situational. It very much depends upon the design objectives.

[BJC]

Edit: Andrew put it more elequently than I

[IndySam]

Very well said Andrew. There is not necessarily a correlation between chassis construction and axle type.

We have used live axles the last four years. These included a tube chassis, wooden chassis, and sheet metal chassis in 4 wheel drive, mecanum drives, 6 wheel drop center chain driven and all gear driven drive. The one thing they had in common was cantilevered axles.

It's easy to fall into the mindset of "if you use this type of chassis construction you must use this type of axle." Nothing could be further from the truth. Use what best suits the construction methods you have available and how you choose to play the given game.

[nathan_hui]

I stand corrected. Yes, I did confuse live axle/dead axle with direct drive/indirect drive.

That said, there was an identical thread on this subject (now that I do some research for myself) on CD almost 7 years ago.

http://www.chiefdelphi.com/forums/ar...p/t-47117.html

[FrankJ]

The last couple of years we have run mechanum wheels. We use the andymark nano tough boxes with the long shaft that allow bearings on both sides of the wheels. The back wheels are direct, live axle drive. The front wheels are dead axle chain driven to get the gear boxes out of the way of the intake system. So we are running a live-dead axle setup. We have no issues with either system although the live axle is slightly less complex & lighter by the weight of the chain & sprockets. With a very rigid chain path, we have had no issues with chain tension of the lack thereof.

[OWilliamson]

Thank you all for your replies, it really helped clear up a few misconceptions I had.

Our team has access to a CNC, water jetting, and breaks so I think we could try to build any drivetrain we wanted. At our next meeting I will bring this all up and see what we decide to do, thanks again everybody

[TheMadCADer]

Originally Posted by OWilliamson

Thank you all for your replies, it really helped clear up a few misconceptions I had.

Our team has access to a CNC, water jetting, and breaks so I think we could try to build any drivetrain we wanted. At our next meeting I will bring this all up and see what we decide to do, thanks again everybody

Just be sure not to get your minds set on one particular system before the season starts. That sort of plan can easily lead to a robot that doesn't perform well because the drivetrain doesn't fit well with the game.

Try looking at some of the old games, and ask yourself what sort of drivetrain would/did work best with that game. Then look at how that would affect your decision to use live or dead axles and why (though this will still depend mostly on what machining you can do and what you already know how to do). This is a good way to learn how to use both power transmission methods. If you have the time, maybe mock up a few designs in CAD, or even build one if you have the resources.

[apalrd]

There's absolutely nothing wrong with setting your minds on a type of drivetrain construction. You can decide pre-season you would rather build out of sheet metal or welded box tube or plate or whatever else you want, and get your process in order (e.g. methods for manufacturing, communication with sponsors as necessary, etc.).

I would also argue that it's better to have a full design set on your design shelf pre-season rather than just ideas. Even with game changes, modifying something that is already done is easy, and no game in the past few years (except Lunacy which we won't talk about) which requires more than optimization of ground clearance, long/wide/square aspect ratio, number of wheels, and especially gear ratio.

We calculate out the optimum gear ratio from COTS parts every year, we talk about long/wide (or this year square), ground clearance requirements, if we should taper the ends, etc. but it's not hard to built fundamentally the same thing (same wheel axle module, same wheel or wheel size, etc.) with changes in specifics. In fact, once you've nailed the design elements like that, it's trivial to design a wide version of a long chassis, or a square version, or an 8wd vs 6wd, or anything like that. It's still the same chassis design, adapted to the game.

The years that we did less game element/chassis integration and more design-shelf utilization, we performed better because we finished earlier and had more testing time. Sure, maybe we could have optimized a bit more out of it, but finishing a few days earlier is way more important.

[DampRobot]

First, I'd like to say that this is shaping up as a really good thread. I haven't seen this level of discussion of design on CD in a while.

I'd like to add one more thing about how frame design effects the live vs. dead axle choice (and visa versa).

It's often in a team's best interest to protect their chain runs behind a frame member. Of course, there are exceptions, but it's generally worth it to put a chain run away from where game pieces, other robots, and debris can get caught up in it.

In a dead axle setup, the chain necessarily needs to be adjacent to a wheel. In many cases, it's even on the outside of the wheel. If teams want to have a frame member between the field and their chain run, it needs to go on the outside of the wheel. Because it's "right there," often teams make the decision to use that frame member to hold the other end of their dead shaft, and to use that shaft as a frame stiffener (as others have mentioned).

With live shafts, it becomes possible to separate where the wheel and the chain are. Sure, you can make them adjacent, but if you do, it's usually easier to go with a dead shaft setup (because it becomes desirable to have an outside frame member, and so on). With the live shaft, you can put the wheels on the outside of the frame, and keep the chain protected on the inside. If you've got that, you've got a drive with many of the aspects of a WCD.

Of course, as Andrew eloquently pointed out, you can do live sheetmetal drives, and WCD like drivetrains with dead axles (I guess I define a WCD kind of narrowly). And, there's nothing keeping you from direct gearing a wheel with a sheetmetal or dead axle setup. Just ask 971.

Perhaps I'm just saying that drivetrains tend to converge on either a WCD setup (sliding bearing blocks, cantilevered hex axles, aluminium box construction, center wheel direct driven off the gearbox) or a sheetmetal drive design (two side sheets with the wheels and chain runs in between, dead axles). Which one you chose really depends on what manufacturing resources you have. And, of course, there's nothing keeping you from modifying one of the more typical approaches into something that fits your team better.

[Peter Matteson]

We were a staunch defender of dead axles up until 2009 when 1/2" hex bearing became available because the keys on our shafts were always the weakest point of the system.

Once the AM hex bearings came out we switched over because we could use hex shafts which allowed us greater flexibility in our designs while still maintaining a good torque transfer.

Our designs have taken great advantage of the use of Hex now that we have that ability.

[Brandon Holley]

I was a firm backer of dead axle setups until our team really embraced belt driven drive systems. We swapped over to live axle for these setups because the assembly of the systems became so simple with belts.

We have a gearbox (usually 2 speed) with an output hex shaft plugged right into a drive wheel, and then on that same output shaft we would mount 2 timing pulleys with hex broached into them. Simply place your belts/pulleys/wheels onto your hex shafts for the other wheels and the whole system is together in a matter of seconds.

Maintenance also becomes a breeze, however we've never had to swap a a part out on one of our belt driven drive systems. This is due mainly to the robustness of Colson wheels and the rock solid Gates timing belt setup.

-Brando