Re: Live Axle vs Dead Axle
 

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:
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.

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.

Re: Live Axle vs Dead Axle
 

Edit: Andrew put it more elequently than I

Re: Live Axle vs Dead Axle
 

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.

Re: Live Axle vs Dead Axle
 

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

Re: Live Axle vs Dead Axle
 

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.

Re: Live Axle vs Dead Axle
 

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 :)

Re: Live Axle vs Dead Axle
 

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.

Re: Live Axle vs Dead Axle
 

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.

Re: Live Axle vs Dead Axle
 

I'd like to put some numbers to emphasize some of what Andrew and others are saying about torsional rigidity and frame decisions helping drive the live axle vs. dead axle design choice. They have already stated why torsional rigidity is important to consider in chassis design, especially skid steer design.

I'm going to use the term torsion constant, J, to compare the torsional strength of different cross-sectional shapes commonly used in FRC frames. This term defines the angular deflection of a beam about its axis when a torsional stress is applied. In general, open cross-sections like angle and c-channel pieces have less torsional rigidity with respect to closed cross sections like square or rectangular tube. This is easy to discover on your own: you can easily twist a piece of KoP c-channel with your own two hands. If you try to do the same to 1" x 1" x 1/8th" square aluminum tube, you will likely hurt your hands before you observe noticeable angular deflection.

Based on these two sources (http://www.colemetals.com/channels.htm, https://engineering.purdue.edu/~ahva...sion_Guide.pdf), I am able to compare the torsion constants of the different cross-sections given similar sizes and the same material.

High Strength Steel (HSS) 2.5" x 1.5" x 0.25" wall rectangular tube has J = 1.14 in.^4
HSS 3" x 1.498" x 0.258" thick c-channel has J = 0.04 in.^4
HSS 2.5" x 2" x 0.25" thick angle has J = 0.023 in.^4

It is impossible (or at least impossibly difficult) to bend sheet metal pieces into a closed cross-sectional shape. This is why most sheet metal chassis designs use c-channel and angle shapes quite often. In comparison, a chassis designed with rectangular/square tube as a main structural element has closed cross-sections. This grants inherent torsional rigidity. Based on the numbers above, a rectangular tube cross-section has about 29 times as much torsional rigidity as a piece of c-channel of comparable size. Wouldn't it be cool if there was a simple way to make sheet metal chassis more rigid in torsion without much added material?

That's where dead axles can come in handy! A dead axle can be implemented in such a way that it turns the cross-sectional shape of the 2 c-channel side rails into an I-beam-with-extra-flanges cross-section over a short distance. This solution also happens to achieve this result exactly where the wheels are supporting the weight of the robot and making it move. Dead axles can be designed to add structural strength to the chassis in places where it is needed most. It can be more difficult to do this with live axles because torque transmission, dynamic loading of the shaft, and shaft retention must all be accounted for as well. If anyone can satisfy all of these design requirements and still provide a torsional member in their live axle, then kudos. But I know for a fact that my team would be taking a huge risk in doing so.

This is why it can often be advantageous to use dead axles in a sheet metal chassis. My team does not have access to the necessary resources to properly cut out and bend dozens of sheet metal parts for a sheet metal chassis. We do have a few small CNC mills, a large (relative to our CNC machines) bridgeport 3-axis mill, and a nice lathe in our machine shop in addition to a mentor with more than 20 years experience welding aluminum FRC robots (how many teams have that???). Based on our resources, it makes sense to use welded rectangular/square tube aluminum for our frame.

Since we do not need added torsional rigidity for our frame members, live axles through the frame members become an attractive option. Luckily, we have the proper machinery to pull off a live axle design (nice lathe + mills). Live axles allow us to more easily cantilever the wheels because the sprockets driven from the gearbox can be run on the inner side of the frame member and transmit the power to the wheels on the outside through the live axles. Cantilevering wheels can be preferable because it allows for easy access to the drive system for wheel, axle, and chain/sprocket issues. Driving cantilevered wheels via chain/sprocket with a dead axle setup is more difficult because the outside of the rail is also a convenient placement for our vertical bumper supports, which would get in the way of the chain.

Live axles also allow us to direct drive the dropped-center wheels of our skid steer drive straight from the gearbox. Even if every link of chain on the robot broke, our drive train would still be in business. In my opinion this single feature heavily tips the scale in favor of live axles because reliability is my number one priority in a drive system. It simply has to work. However, this is just an opinion and others may have a just as valid opinion that dead axles are the way to go because in a sheet metal chassis they provide a method for making EXTREMELY light weight and effective designs.

I am not saying that live axles necessarily correspond to square/rectangular tube chassis construction or that dead axles necessarily correspond to sheet metal designs. I am providing evidence that supports common trends and design decisions in FRC chassis with respect to frame and axle setup. It is up to individual teams to decide what style of chassis/axle setup is best for them based on their resources and expertise. Ironically enough, for far more teams than realize it the clear answer is that the KOP drive train or KBoS is optimal selection.

Re: Live Axle vs Dead Axle
 

We also use dead axles, primarily because they are much easier with the machining resources we have.

Since so many wheels come with correctly sized center holes for bearings, it is a very simple operation to poke a hole in your frame (be it "West Coast" or twin-beam) and slide a bolt through. Most of those same wheels come with ready-made sprocket mounting locations.

Generally speaking, live axles result in more machining requirements: hex stock requires a fastening method at the ends of the shaft, and also usually requires broaching certain elements. If hex is not used, then keys and keyed shaft is used, and you still need some type of end-fastening like e/c-clips or drilling and tapping the end of the shaft. Then you have to fabricate either a gear, keyed sprocket, or hex sprocket power transmission system.

We try to spend as little time as possible on our drive train, and the dead-axle setup is much faster for us to create. We've used it every year since '08.

Re: Live Axle vs Dead Axle
 

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.

Re: Live Axle vs Dead Axle
 

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.

Re: Live Axle vs Dead Axle
 

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

Re: Live Axle vs Dead Axle
 

That was team 45 the Technokats.