(FW)Nibbler- Horizontal Spinner

[GlenR]

As said before huge thanks for all the info it's a big help.

Out of interest what sort of RPM does the bar spin at on Decimator and how much does the bar weigh (sorry if these are answered elsewhere)? I ask this because it has a great balance between delivering good hits and not pinging itself out the arena.

The entire rotating mass is about 4.5kg, the bar alone is ~3.5. The original motor i had rewound for it was about 7500rpm on 6s, the one it ended up running through the event measured at 5500rpm which was a 12s motor running only on 5s. Turned out well though, will probably keep it around that speed next time and just adjust the gearing / motor wind to suit

[Flag Captured]

The NTM is fine to get you going with the aforementioned mods for reliability as long as the weapon sits on efficient bearings. They have decent enough torque especially the super low Kv ones like that. Check out the SK3 motors too, they are substantially better made and tend to have more power IMHO.

Basically that, there's a 50mm one on Hobbyking that's 4000w on 6s which is very nice indeed. You'll want to get the shaft upgraded from 6mm to 8mm and to a material that isn't cheap steel/plastic-with-some-kind-of-metal-on-it so it's stronger - get some 8mm ti and it should be a pretty powerful thing. And they're not really that much more than an NTM either... I ran one in Overdrive this year and was happy with how it ran in the fights where I actually had a drum haha

[Cameron_S]

Good to know why the large wheels break gear boxes (analogy helped). Unfortunately there seems to be little way of reducing the size of the wheels while keeping it invertible (expect from having two small wheels on top of each other which seems difficult). Might just be a case of have lots of spares and hope it doesn't happen too often.

Thanks for the specs on decimator's bar, faster than I thought.

The design has also been changed to fit the smaller bar and the arms upgraded to 20x20x2mm steel bars. The weapon motor has also changed to this- http://www.hobbyking.co.uk/hobbyking...ize_heli_.html on 7s which I believe is the one Matt is talking about. This seems a worth while upgrade with the increase in peak power. However the increase in kv, puts having a small pulley on the motor going to large one on the bar impracticable. So would it be possible to have a small pulley on the motor going to a large pulley on a shaft near by which also has a small pulley that then goes to the bar? I think an image will be clearer. (The smaller pulleys have 12teeth-the large on on the shaft has 40-the one on the bar will have 36) Should reduce the RPM to about 3500.

Edit:Any way of making images smaller

[maddox10]

Valkiri uses a Z40 around the motor can, and a smooth ali 112mm diameter pulley on the disk.

[overkill]

The technical term for that is a jackshaft. A double reduction will work but has some drawbacks: It will add weight and complexity and takes up valuable space. There is also a potential problem when you have a small pulley close to a larger pulley. The small pulley will have very few teeth fully engaged with the belt (maybe only 4) and those teeth have to transmit all the motor power. this graphic explains it better:



When the blade hits something, there will be a huge amount of force on those few teeth and they may strip off the belt, or they might slip and start to wear down. Either way, the belt will wear out much faster than it should. The way to prevent that is with an idler or 'back-breaker' wheel, which increases the belt wrap around the motor pulley. Its also a good way to adjust the belt tension.

The alternatives to using an idler are to use a wider belt and/or larger pulleys. You can also use a smooth pulley on the weapon shaft to act as a clutch and limit the amount of force on the pulley teeth. I don't really trust this option as its hard to know what the right amount of tension is.

All the above solutions will work but the best or at least most elegant solution is to find a motor with a lower KV and have a single reduction to get a weapon speed of around 3.5K rpm, with a 4:1 reduction ratio and a 7S battery, you will need a motor with around 600 to 650KV.

[Cameron_S]

Thanks Nick for the info about jackshafts (new word you add to my engineering know how). The diagram illustrates perfectly what you mean by having a idler wheel to increase the amount of teeth in contact when the pulleys are close together. With all the draw backs you mentioned, I'll take your advice and try to use the more elegant solution.

So it's back to the pesky weapon motor dilemma. I have quite literally looked at every motor on the HobbyKing website and I can't find something that's high power and low Kv (except from a very expensive Scorpion motor that's a bit out of my price range) of course i've probably missed a perfectly suitable motor. So something going have to be compromise and if the 2 stage reduction is too complex then it's the power. Because of this I've gone back to the original plan to use the NTM50-60 380kv motor. This will most likely to be one of the first parts to be upgraded in the future.

These posts wouldn't be the same without some questions:
Is the anything glaringly obvious that would make this battery unsuitable for the weapon and this be unsuitable for the drive?

Is under volting the drive motors, suggested earlier by Glen, worth the reduced speed to hopefully stop the motors burning out?

As always thanks in advance

[maddox10]

Both batteries are up to the task.
But a 2200 is on the low capacity side. I would go for the 2700mAh compact.

[overkill]

You could use a lower KV motor and a higher voltage pack to get the RPM you want. For instance this 530KV motor: http://www.hobbyking.co.uk/hobbyking...ter_Motor.htmlan 8S battery and this ESC: http://www.hobbyking.co.uk/hobbyking...PM_Sensor.html will get the job done. I prefer to use two 4S packs in series as they are easier to find and easier to charge on two smaller chargers.

[Cameron_S]

Just when i thought I've finally figured out what motor to use. Must of missed that one, but it ticks all the boxes. So thanks for the spot and ESC you've mentioned would certainly do the job.

I see where you're coming from with preferring two smaller packs over one larger one. This would also mean only one type of pack is needed for the whole machine. So the current plan is to use 3 of these (1 for drive and 2 in series for the weapon).

[Cameron_S]

The build log might actual become just that! My HobbyKing, RangleBox and DirectPlastic orders have just arrived. No problems with any of them as they all arrived quickly and well packaged.

Looking quite cramped should I be worried?

In case the image isn't too clear there are:
12Guage Red Wire (2m)
12 Guage orange wire (black was out of stock )
3-zippy 2650Mah 4s 40c lipo battery (scaring me before they have even been used!)
2-Tz85A ESC
1-spektrum dx6i
1-OrangeRx GR300 3ch (The 620 was on back order so had too look for a replacement. I would only be using 3 channels so think this would work- Correct me if i'm wrong)
15-Male and female XT60 (definitely have spares)
1-Imax b6 charger (Instead of buying a 'proper' power supply for it would a small 12v car/motorbike charger work?)
50-m6 50mm socket head bolts
50-20mm m6 barrel nuts
2-cordless drill motor and gearbox
1-100x25cm 20mm HDPE (sides)
1-100x25cm 10mm HDPE (top/bottom)

Before starting to cut the chassis/Armour i'm going to wait for the wheels to arrive as I fear they won't fit. Probably being a bit paranoid as they fit in CAD design.

EDIT: Forgot to mention I didn't order the brushless motor or ESC. This is because I've yet to fully decide on a motor and want to get a working rambot before adding a weapon.

[Cameron_S]

Some progress at last. So after converting a drill press adapter thing into a ghetto press, I went about heating the hubs. The first attempt was with a hot air gun, however this struggled to get the hub much above 100c.

So it was blow torch time which worked far better. One thing worth mentioning is that having a bolt with some washers the same size as the hole in the wheel makes lining it up much easier.The photo is a bit clearer.

A bike tire from a road bike was used to increase the grip. It was attached with screws through the side into the wheel. The end result isn't too pretty but should work- I hope.

So far only the base and top have been cut out of the HDPE sheet. The mess it makes is incredible and the piece was only 10mm. Hate to think how bad the 20mm sheet will be like.

[razerdave]

Progress seems good Cameron, one thing I did notice is that GR reciever which is a Ground based one. I know they have odd failsafing issues so I'd run a few checks with servos to make sure all 3 channels go to neutral before you put it in something dangerous.

[razerdave]

Also, bike tyre usually gets screwed in through the top, the heads sit within the tread on most peoples.

[Cameron_S]

With the champs approaching, I've had a bit of extra motivation to try and finish this in time. So the progress so far:

Firstly, You were right about the receiver, it didn't even want to bind yet alone failsafe, so I bought a lemon rx instead. The reason for the the screws going into the side was because the tire was from a road bike which meant the tread wasn't deep enough to hide the screws' head.

The 20mm parts then got cut out making even more mess. I then went about cutting the arms (not very well). Then holes for the bolts and barrels nuts were drilled, most of the barrel nuts lined up well, but a couple didn't so the holes had to be widened.
sides.jpgside barrel.jpg

So with the body mostly done, I went about soldering the wiring. However, in my haste I completely forgot about the led being needed so that needs to be added. The motor mounts were then cut and drilled as were the corresponding holes in to lid and base. After the motor mounts were cut and fitted, it was time for a quick test drive which was interesting considering it was my first time driving it and the patio was very bumpy.

Nibbler 2#.jpgNIbbler.jpg
With it now moving, it's time to buy the spinner parts. Luckily since the last time I searched for Hardox, there's an online ebay seller. So the current plan is to use a 400*50*15mm hardox 500 bar (2.35kg) with this at a 1:4 reduction to have a bar RPM of about 3500 (2Kj). The only concern I have with this motor choice, is that is it going to be run at 8s which is quite a bit under the 12s max. Should this affect the outputted power too much?

[overkill]

Without delving too far into Ohm's laws about power, when you double a motor's voltage the power increases by 4 and when you halve the voltage, power decreases to a quarter. Since your motor is running at .6667 of it's rated voltage, the power will be .444 of 5,500 watts or 2,444 watts. That is a large reduction but still a very respectable amount of power.