# Another RC Mower - Axle Rigidity Ideas?



## Fly Quietly (May 6, 2020)

Hello Canadian Hobby Metal Workers,

I've had an ongoing project over the past years building up an RC mower/yard rover. Video (a while ago and some refinements - mainly to the mower attachment - have been done since this YouTube post, but you get the idea):  




While it works pretty well, I am continuing to sort out a few issues and hoping members of this forum can give me a few ideas. The biggest problem I've been having is with occasional chain derailment. Running lightly loaded, it's fine. Running with a load (I have a dump box that sits on top which is not shown in the video and have commonly loaded it up with 150lbs+ of material) it does fine going in a straight line. But turning when heavily loaded, especially in loose dirt, commonly leads to drive-chain derailment. This always seems to happen right when the machine is located somewhere difficult for me to reach (I am a wheelchair user so have some mobility impairment). This means a huge PITA to sort out - although I am getting pretty good at it, when I can get to the damn thing!

I had some early concerns that my problem was related to chain size as was somewhat limited in selection due to available chain-ring and wheel specifications. However, the combination of heavy load and turning which leads to derailment suggests to me the issue is more one of frame and/or axle rigidity. It is really difficult to actually see where the problem is. I've managed to consistently duplicate the derailment by pushing up against a fence post with one side or the other of the front blade/accessory mount (either side) and it sure doesn't take much flex to see the chain start coming off the ring (~1/16" gets it up on the teeth). However, seeing where, exactly, the flex may be coming from is a bit more difficult - like I can't figure it out.

My current plan is to start by triangulating the bottom of my frame with some 3/16 x 1.25 inch flatbar, mainly as this is the easiest thing to do. But also looking at the axles, wondering what I might be able to do here. They are currently made of 5/8" readi-rod (upgraded to Grade 8 after I bent the original Home Depot material in a little "incident" which involved the ROVer tumbling ~100' down the bank at the back of my property - all kinds of fun getting it back up the hill with a come along). I MIGHT be able to bore out the axle receivers to accept some 3/4" stock, either more threaded rod or round stock with the ends threaded. Also thinking there may be some possibility of adding a center support for the axles.

I guess I'm posting this here just looking for some alternate ideas:
- Should I be looking at some higher grade materials for my axles - i.e. 4130 steel - thinking may not gain much as Young's Modulus is pretty similar for all steels, correct?
- Opinions regarding going to a 5/8" round rod with ends only threaded rather than readi-rod, versus moving to 3/4" round or threaded rod (keeping in mind boring out the axle receivers is going to be a problem in my garage/shop)?
- Other ideas?

Photo attached of the underside of the machine for reference (you can see where I am planning on adding cross bracing - paint removed).

Thanks for any input/advice/smart-ass remarks you may be able to offer!


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## eotrfish (May 6, 2020)

I want one  and a yard big enough to use it.  Well done and welcome aboard!

Is it possible to move the chain idlers and lengthen the chains in order to get more chain engagement?  Something more than 240 degrees of engagement would be ideal.

The Redi-rod could be part of the problem also.  Redi-rod is not known for accurate dimensions.  How is the fit through the frame holes?  I notice that the front axle has lock nuts inside the frame while the rear does not.  Is that letting the rear axle shift in the frame holes or sideways enough to let the chain ride up on the sprocket teeth?

Hope you get if fixed - it is a super cool machine.


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## CalgaryPT (May 6, 2020)

Wonderful job. Really great work. Welcome aboard.

WRT the idler wheels, two things come to mind: one is the chain engagement as @eotrfish mentions (always an issue on these things due to the close proximity of large and small drive components...but that would be a ton of rework. There was a thread on one of the RC or Robot forums (of course I can't find it now) about this. I think the thought was to check the runout on the wheel axle vs. the idilers. The theory was if one has significantly more runout than the other, this can lead to your issue...especially with narrow drive chain. Either tighten both up WRT runout, or loosen both to the same degree (can't tell if your design permits this); this allows the idler to move slightly with the chain sideways as the tires experience sideways thrust (fencepost etc.). Looking at your gear ratio there's a lot of torque there...I don't doubt for a min. you've got some frame flex happening. There used to be these idler gears you could buy that had some lateral movement built in; they literally moved sideways a bit to prevent narrow gauge chain coming off. But I haven't seen them in years. They were popular in the early 90s when BattleBots/RobotWars was first popular.

Can you get a small trail cam under there to watch the deed as it happens?

Beautiful work BTW.


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## Fly Quietly (May 6, 2020)

Thank you for your replies and compliments - although my sense is if you could see better the quality of my welding you may have a different opinion...

Yeah, the main purpose of the lower idler is to increase chain wrap as much as I could and position was determined by frame design (and you're correct, changing would be a job). The upper idler does move and is designed to adjust chain tension. May play with that. Good observation on the lock-nut situation. There are outer lock nuts on both axles, but only inner ones on the front. Didn't think I had room for them due to battery clearance on the back, but now thinking if I shave down one side of a couple nuts, may be able to squeeze something in there and would be a reasonably easy thing to try. Going to see if I can do that and add some cross bracing and see what happens. Will update.

Also yes on the variance in redi-rod tolerances. Honestly can't remember how tight the axles were going in and kind of afraid to take them out to find out. Would like to ream/bore or somehow otherwise improve/increase the tolerance on the receivers and maybe move up to a 3/4" axle with just the ends turned down to 5/8" thread to meet bearing specs, but thinking increasing bore size while not messing up the alignment may be a problem (unless I can find a 2' long x 3/4" reamer at reasonable cost!) 

And yes, regarding torque - it's a bit scary. This thing will pull your limbs off if it can get the traction!


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## francist (May 6, 2020)

Fly Quietly said:


> 3/4" axle with just the ends turned down to 5/8" thread


Even just going to a solid rod axle instead of all-thread should gain you some rigidity. With a 5/8” threaded rod you really only have 1/2” of solid material because the thread is continuous for the whole length of the axle. Thread peaks don’t help stiffness any, the root diameter will be the driving factor.

If a 5/8” threaded rod has a root diameter of about 1/2” that’s roughly 0.8 square inches in section. A 5/8” rod with threaded ends only has a section of 1.2 square inches through the unthreaded portion. Almost double the cross section area compared to the 5/8” all-thread.
edit: Actually it’s only 50 percent more, not double.

Still may not be enough for what you want, but you do stay in size without having to bore larger holes.

-frank


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## Fly Quietly (May 8, 2020)

Well, if any of the issue was frame flex, pretty sure that's taken care of (photo). Even managed to squeeze in a pair of locking nuts on the rear axle with some artistic angle grinding. Now if I can just catch my wife in a good enough mood to help me flip it back over again, I'll get a trial run in this weekend!

Taking the rear axle out... wow, there is still enough room for a LOT of improvement if these basic fixes don't do the trick. There is a ton of play between axle and receivers, the wheel bearings are crap and moving up to a round-bar axle is still on the table. Pretty sure I'm going to get this solved now or later. Likely will wait for fall if I really want to get carried away on new axles etc. - the yard is growing and I need this thing up and running - plus purchased a used lathe last fall and trying to find the time to learn threading. Looking at the chain wrap around my front wheel sprocket, I'm amazed the machine works as well as it does. Never mind 240 degrees of wrap, don't think I even have 180 degrees. I vaguely remember chain wrap being a concern in the initial build, but was pretty constrained by motor configuration. That's the problem building things from scrap. Ideally, I wouldn't be using a chain drive at all but would use 4 direct drive motors with each side slaved together. Oh well, next time...


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## CalgaryPT (May 8, 2020)

I'll be interested if you slave them together. I'm playing with the idea of a four wheel drive model using individual optical encoders and independent suspension. Now, I have zero practical purpose for such a machine (like you do), but that's never stopped me before.

If this Covid19 apocalypse doesn't end soon I'm going to get so bored I might go back to building another pulse jet engine. No way we're zoned for that


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## YYCHM (May 8, 2020)

CalgaryPT said:


> If this Covid19 apocalypse doesn't end soon I'm going to get so bored I might go back to building another pulse jet engine. No way we're zoned for that



I want a front row seat when you fire her up please  The one we had in JHS shop, shook the entire shop.  I'm sure the entire school felt/heard that thing when we ran her.


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## Chicken lights (May 9, 2020)

CalgaryPT said:


> I'll be interested if you slave them together. I'm playing with the idea of a four wheel drive model using individual optical encoders and independent suspension. Now, I have zero practical purpose for such a machine (like you do), but that's never stopped me before.
> 
> If this Covid19 apocalypse doesn't end soon I'm going to get so bored I might go back to building another pulse jet engine. No way we're zoned for that


New thread please on said pulse jet engine. Zoning can go fly a kite


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## PeterT (May 9, 2020)

CalgaryPT said:


> If this Covid19 apocalypse doesn't end soon I'm going to get so bored I might go back to building another pulse jet engine.



Consider combining your passions!


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## Fly Quietly (May 11, 2020)

PeterT said:


> Consider combining your passions!



Does jet exhaust kill Coronavirus?


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## PeterT (May 11, 2020)

Hmmm... sounds like one of those home remedies like bleach ingestion. But exhaust should be good for scorched earth weed control.

Re Fly Quietly handle, I have to ask, aviation related interests?


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