HardWired II

[mattsdragons]

One thing I have picked up on is if you can use your base just to keep the wires etc inside then you can put the weight to good use. Try and make it so the base isn't a structural point, especially on a machine that's not invertable.

Also, if you know the amount of force in a full backlash you can determine the amount of metal needed. After a few talks with people I concluded its less to do with the thickness and more to do with the fact that someone will hit your weapon side on, then can your bulkheads take it?

[overkill]

Actually, you don't always save weight by going to a lower battery pack! There are two reasons, both to do with those inflammable numbers . First, motor power in watts is determined by one of Ohm's laws; power = volts x amps. That means that if you needed a 4,000W motor, one that ran at 5S (18.5V) would draw 216A, while a motor designed for 10S (37V) would only draw 108A. In both cases, the motors have the same output and require the same amount of energy to run for three minutes.

Next, batteries store energy and its measured in watt-hours. The watt-hour capacity is calculated by voltage x amp-hours (like that last formula, with time included). That means an 18.5V, 4AH pack has the same watt-hour capacity as a 37V, 2AH pack. If the packs were both from the same manufacturer, they would probably weigh almost the same.

Putting those facts together, it means the lower voltage 4KW motor needs the same weight of batteries as the high voltage motor to run for 3 minutes - there is no weight saving in the battery itself. There are some other factors that affect the total weight and cost; the lower voltage system is drawing twice the current and requires thicker, heavier cables, connectors and removable links. Its also harder and more expensive to make a high current ESC compared to a high voltage ESC. This is why large RC plane motors are mostly low KV and high voltage - its just easier and cheaper for the manufacturers to do it that way.

The only real reason to stay with low voltage is if you want to power the drive motors off the same pack as the weapon motor. That has the simplest wiring but runs into the high current issue above. To get around that, you can run completely separate packs for the drive and weapon, or run a split voltage system where each drive motor runs off one pack while the weapon runs off both packs and twice the voltage.

Bottom line: Ohm was a mean bastard out to ruin our fun!

[overkill]

One thing I have picked up on is if you can use your base just to keep the wires etc inside then you can put the weight to good use. Try and make it so the base isn't a structural point, especially on a machine that's not invertable.

Also, if you know the amount of force in a full backlash you can determine the amount of metal needed. After a few talks with people I concluded its less to do with the thickness and more to do with the fact that someone will hit your weapon side on, then can your bulkheads take it?

Very good points! I think the best vertical disk spinners over here have quite thin bases made of mild steel. All their structural strength is in their vertical weapon arms and external armour.

[Flag Captured]

That is a worry, but the bulkheads will be bolted to each other pretty solidly and it'll have a 25-30mm dead shaft so will be rather tough! I am concerned about the bulkheads bending which is why I wanted solid ones as opposed to machined down ones... but it all depends, really. I'd like a front Hardox scoop too but HDPE back for the weight, may be my best bet to bolt it all together as best I can but I don't know whether HDPE would be best for something as structural as that... That may be an area I need to review!

Ahh, that makes a lot more sense with the batteries too, thanks Nick! I knew that higher voltage meant higher efficiency, but that explains a lot!

I would quite like to run the entire system on 6s, that would make things very easy but I'd worry of course about the capacity... What I might do is build it to be solid enough and then see how much capacity of LiPo I'd be able to fit in... I've looked on Hobbyking and there's a 8000mah 6S Turnigy nanotech battery that ends up at about 1.1kg which is a lot, though I'm sure I'd be able to fit something like that in at a push...

The Scorpion 50-something and 12s is still doable I reckon but I'm not sure how cost friendly that'd be, though I can always upgrade if I need to I suppose!

[overkill]

Just a crazy thought; how about running two of those SK3 motors? Much cheaper than an equivalent Scorpion motor, as long as you can squeeze in an extra ESC as well.

[Flag Captured]

That could work! I do have worries about dual brushless (timing etc) though on a belt system I reckon any problems would be lessened! Might be something to look into at the very least...

[Flag Captured]

Doing a bit more design work on what will eventually become Archangel (when I finally stop being lazy/poor and build the thing)

Main thing I'm looking at at the minute though are the bulkheads - what would people suggest for these? Initially I had pocketed Alumec in mind, but because I may have underestimated the cost of materials I'm going for 6082 to keep just the general expense down.

Long story short, what sort of thickness should I go for, and what should I do in terms of machining them down? I had 20mm solid bulkheads in mind, but that will be insanely heavy I'd imagine. The main worry is them not taking the force of the bar in case of a 'perfect' hit... They'll be supported at the back by way of an equally thick panel bolting to them both, I'll get a diagram of what I mean if needs be, but it'll probably look a lot closer to Electric Boogaloo's design for reference.

[overkill]

Any particular reason for using aluminium rather than steel? You can likely SAVE weight and definitely money with the right design

[Flag Captured]

Well, nothing other than looking at other people's designs, really - was a little worried that steel would be too heavy too!

There was the whole thing with the design being based around the bulkheads too, making that as strong as possible and then just adding a bit of HDPE round that to shield electronics. If possible I wanted it to bolt together too so I could replace bits if needs be instead of having to angle grind it apart and replace a bit!

Truth be told, I was actually debating 25mm HDPE bulkheads but I don't think they'd be a solid enough option for a spinner of this design at least. Could be wrong though...

[overkill]

Agreed, HDPE just wouldn't be stiff enough to support the weapon! Its hard to argue against the versatility of a screwed together frame, although welding is vastly stronger. Perhaps you can combine both methods: it would make sense to weld the front armour together as you don't want opponents peeling it apart at the seams. The weapon arms could be made of welded steel subsections that bolted on to the front armour and the base using either steel weld nuts or aluminium nutstrip. On the other hand, I have been up since 2AM and may not be making any sense

If you do go with aluminium, 7075 from an off-cuts store would be fairly economical and you are not paying for all the fancy mould-making properties of Alumec. That 6082 aluminium is weldable, so rather than buying thick plate and machining half of it away, you could design a thinner walled structure with buttresses to make it stiffer. The only part that really needs to be thick IMHO is the axle support.