Deathly Hallows - FW - Alligrater

[Deathly Hallows]

Next project in the pipeline a featherweight.

Early stages of design - but rough concept as follows:

MY1016 drive motors for plenty of grunt
1/2" thick HDPE wedge body with perhaps an aluminium floor (for weight saving)

I have been struggling with a weapon concept for a while, current plan is..
Weapon a circular saw blade
Weapon drive will be a brushless motor

I have a few ideas to make the bot a bit more interesting.
For the Alligrater concept the HDPE will be green.
I intend to add serrated bars on the ramp too.

The motors and HDPE are heavy so I am a bit worried about the weight. Calculations rough at the moment but should iron out the basic design in the next week.

[Ocracoke]

HDPE isn't particularly heavy in of itself. The Honey Badger 2.2 uses 10mm HDPE which, all told, probably adds up to only 1kg, maybe less. If you want it green though, why not look at RG1000, which is a recycled variant of UHMWPE.

Looking forwards to seeing this one

[Maxamuslead]

MY1016 drive motors for plenty of grunt

You know that those motors what about 2.5kg each right ? drill motors are the norm for drive motors or a brushless system. Not saying a scooter drive system is not impossible but that the only robot I know of using one is a RAM bot

[Deathly Hallows]

I know they're heavy I hope I can get it to work. Saw a pair for£30 and couldn't resist!
But I have some drill motors handy too if needed.

Rough calculations:
Drives x2 = 4kg (I think the ones I found weigh 1.9kg each)
6S LiPo = 1kg (depending on size - I estimate 4000mAh should suffice but have budgeted for larger)
Wheels = 1kg (these are heavy tough ones quite wide, could save weight here)
Brushless motor = 0.31kg (Turnigy XK series inrunner)
RX & ESC x2 = 0.2kg (Turnigy/FLYSKY RX, FeatherTwo, Hobbyking 80A)
Armour/chassis = 6kg
Bearings, weapon disc and misc. = 1kg

I'd like a self right mechanism in there too but perhaps that would push it over the limit.
It's a bit of an experiment, and may need adapted. If it's anything like Evil L'Weevil it'll be a under my estimates. EKW came in a full 100g less than my calculations.

Once the motors arrive, I'll weight them, finalise the dimensions of the chassis and update my calculations. The wedge slope needs to be a low enough angle (35' would be good). I am concerned I have over cooked it, worst case I use this pair of motors for an axe weapon system on another concept I have.

[Maxamuslead]

ah so you got the 250w ones, have you designed a gear reduction for it ?

[Deathly Hallows]

have you designed a gear reduction for it ?

I am currently thinking about a chain drive, if I can fit a larger sprocket on the motor's drive shaft, I can get a 4 or 5:1 ratio.

http://www.gearsandsprockets.co.uk seem to have lots of options. Double grub screw fix to the wheels axle, counter sunk for extra bite.

I'd fit some kind of tensioning arm/idler to hold it in place.

The big question on the drive for me at the moment - is how to fix the drive shaft to the wheels (from Screwfix). If I want the axle supported at both sides I need a round shaft at both ends, but to fix in the wheel a hex would be best as I was thinking about the thermal insertion method. I don't have a lathe to mill down a hex bar at both ends. Do you know if anyone supplies something like this or have an alternative idea?

[Maxamuslead]

any reason why you can't use a dead shaft ? that way you can attach the sprockets directly to the wheels?

Or if you want a live shaft why not a plate which is attached to the shaft and bolt the wheel to that?

[Deathly Hallows]

any reason why you can't use a dead shaft ? that way you can attach the sprockets directly to the wheels?

Or if you want a live shaft why not a plate which is attached to the shaft and bolt the wheel to that?

I am learning stuff every day! I love this hobby.

So a 'dead shaft' would mean the shaft is fixed (not rotating) the sprocket is fixed to the wheel so when the sprocket turns the wheel does too. At these low speeds for short duration then no bearings needed I would assume, just a dab of grease or PTFE. Though I could insert something to reduce the bore to allow a narrower gauge of rod/axle.

Did I understand correctly? I don't have an engineering background so I could be totally off mark

Thanks for your input - very much appreciated.

[Maxamuslead]

Bang on, I dont have a engineering background either but have always had a health interest.

I would look into using some bronze bushings at the very lease as it would help reduce friction prevent as much wear on the wheel hub.

[Deathly Hallows]

Since the last post I have been trying to source parts and get the layout sorted in my head. Unfortunately my son is revising for his exams and he is hogging the laptop, so not had the time I need to work in CAD.

Oilite bushes purchased they will work fine.
I bought a 25H chain seems a perfect fit for the existing MY1016 sprockets. But I am out of my depth when it comes to the names and codes for sprockets and chains. I was looking on Gears and Sprockets for a suitable one to mount on my wheels. I thought I may find a cheap one on eBay but the mini dirt bike and mini atv ones look too big. The outer diameter of the wheel is around 150mm and I would like something nearer 90mm. This may limit the ratio I can achieve without more complexity than I want, >5:1 would be nice. 2750 -> 550 would still be very fast. Would need to configure the controller for logarithmic to make it controllable at low speed.

Is 04B compatible with 25H or do I need to look elsewhere? Suggestions appreciated
I'll need an idler sprocket on a slide too, or at least i think i will so I can adjust the tension/allow easy removal replacement.

Cheers

[Maxamuslead]

04b IIRC is a 6mm chain and 25h works with fine (i can vouch for t8f and 05b 8mm chain). i take it a 13 tooth sprocket ( think that what stock on those motors) so that will require a 65 tooth sprocket (if your going for single stage) which will require a 126mm platewheel. https://www.gearsandsprockets.co.uk/...lex-sprockets/

or you could use a two stage system which will make it more compact

I fine THIS website helps with designing a gear ratio

[Deathly Hallows]

Took some pictures of the sprocket.

IMG_20180501_195327.jpgIMG_20180501_195347.jpg

So it's a 10T and as you see from the chain picture it fits nicely no issues despite low diameter.

I will need to order the wheel sprocket soon as it is key to the layout. Once I know how it will fit together, I can work out the lowest angle for the wedge that I can get away with.

[Maxamuslead]

One note about those motors. Epoxy the magnets.

On the gearing design it on cad (fusion sketch up whatever) and see how compact you can get it.

[Deathly Hallows]

One note about those motors. Epoxy the magnets.

I was told I'd need to glue them in. Any particular product I should look for - something special a thermal adhesive epoxy or good old super glue?

I found a suitable sprocket on Robotshop, 48T and 0.25" so should work fine. 10:48 will give me 2750/4.8 = 573 rpm
Still rather nippy but it will be manageable with a tweak to the TX settings I think. It's important I get the drive sorted quickly so I can work on the other systems. If it's a rolling wedge at least I can push stuff, and at 30kph (if my rough maths is right) I could push quite quickly.

Looking at this sprocket it will go around the fixed axle no problem, fixing it to the heel is another question. Needs to be dead center, probably need to make a tool to ensure I fix it right. Was thinking I would bolt it through the wheel hub. No appropriate mounting holes so I'd make those, and a plate or spacers to keep the plate off the wheel.

I think a belt tensioner will be essential. Found a few options mostly for dirt bikes.

Also need to do the calculation for the battery.

Can anyone tell me how much time is there between rounds typically? Do you get time to top up charge or should I plan for a massive 6S battery or a couple of smaller ones?

[Ocracoke]

Can anyone tell me how much time is there between rounds typically? Do you get time to top up charge or should I plan for a massive 6S battery or a couple of smaller ones?

Depends on the event. Extreme Robots tends to be about an hour or so between FW melees. If Robots Live continues its 1 per show thing, it will be roughly the same. I can have 2x 2S LiFe batteries charged comfortably in that time but I tend to take 2 sets of batteries (4 in total) so I can swap one set out for the other and have the original on charge.