Axe mechanism

[psychostorm]

Axe mechanism

Thanks for the advice on motors but the reason I chose the gold for the example was because I could get it easily in Britain.

36V isnt an option. I have 24V 1.2AH Drill batteries, of which I dont know is gonna be enough anyway... Unless you fancy bending the voltage rule for 48V theres not much of an opportunity...

Length, will be easy what with it being essentially half a cone. It cant be longer than it is high.

[nick]

Axe mechanism

BTW. Im getting ~19Joules for the swing with just one set of 1.2Ah (Nicads?) at 24V, thats about a 17MPH hit. It is very nearly as much at 18:1. This is neglecting motor inertia and friction.

[kinforester]

Axe mechanism

Hi guys,

With an electric axe you should be relying on the weight of the head more than the speed its traveling at, thats not to say speed isnt important !

If youre after a good motor,go to your local scrapyard and look under the bonnet of a peugeot 405 ,the radiator fan motor is as good as a EV Warrior, and cheaper.

Progress on my feather hammer is slow but sure,i hope to have something driveable for RAF Fairford.

Pic on profile


GORD

[ewan]

Axe mechanism

koooooool... looks evil

[psychostorm]

Axe mechanism

Cool. Reminds me of Beta.

Actually it was torque I put above speed as I understand the sums better...
lol

Ill consider the radiator motor. Since Bosch made the EVs, (to my recollection) any similar motor should be reasonably the same. same logic as - It it aint broke, dont fix it.

Hopefully I would like to fight your hammer bot with my Axe bot but I would warn you Im planning around weapons on a vertical axis.

[james...venom]

Axe mechanism

that looks great gordon. we produced an axe for a customer last year. it performed really well and was powered by a diesil mondeo fan motor. Ye electric axes dont need the speed, its more about the torque of the motor and the weight of the head to create the damage. whereas on punmatics its the opposite, a nice light axe head is best. thinking of making another one some time. Wayne cummings was designing a pumatic axe and the design concept was really good, unlike an axe out there.

[terrorhurtz]

Axe mechanism

if you look at the formula for the energy for a hammer/axe:
Energy = (M P2 s2)1/3
you will see that the power of the motor and the length of the sweep have a bigger effect than the mass of the hammer. Doubling the mass of the head will only increase the energy by 26%. Doubling the power the motor kicks out will increase the energy by 56%.

So to crudely simplify things, if you have to choose between a heavier hammer head and more batteries, go for more batteries.

I am ignoring the 9/8 in the above formula, as it only makes four percent difference.

[kinforester]

Axe mechanism

Then just remember to nail it to the floor !


Gord

[nick]

Axe mechanism

In a similar vein to what John said, the equations make a kind of specific prediction, that if you have already decided that maximum length of shaft length you can allow, then for maximum energy it is worth spending roughly twice as much weight on power generation as on head mass. At the same time an amount of weight roughly proportional to the amount of energy you expect to achieve must be spent on the drivetrain. Power generation would include the motor and its associated batteries and the switch which drives it, but nothing else except perhaps the first stage gearing and the wiring.

Batteries are available with relatively good granularity, ie. in theory you can add cells one at a time. Head mass and shaft length have unbeatable granularity. Motors are typically not available in such a huge variety of sizes, the equation makes no prediction on the optimal balance of mass between the motor and the batteries, but that is not hard to work out. Granularity is important, because it forces you out of the optimal region of weight expenditure. In effect it is hard to find a motor of just the right size, with just the right windings. If you are limited to packs of 24volts and you are stuck with a motor which makes hardly any power at that voltage your out of luck with that combination. The balance between the motor, the switch and the batteries is a sub-problem to be optimized, in some systems you can fail to get good performance by failing on any sub-problem. The sub-problem for the power-generation system is simply maximise power to weight.

The case with pneumatics is rather similar, except the amount of power available for a given effort is usually very high, potentially so high that jumping has already become a more important factor thing than pure energy transfer, particularly for a feather. In this case to reduce the jumping, you can spend more weight on power generation while reducing the head-mass, you are losing potential energy transfer ability but it is a useful thing to do.

To get a further appreciation of why compromise is important the energy equations also predict that if you have chosen a shaft cross section, it is not energy optimal to have any head mass at all, but to split available mass roughly equally between shaft and power generation. Clearly there have been things left out of this analysis, It further predicts that having a constant shaft dimension is not optimal, and the shaft should taper....If you follow all these temptations you are not left with even much to hit with.

The robot as a whole is a compromise between a great many factors including how well all the pieces fit together, and how hard they are to armour amongst other things, so many that even a much more complicated analysis which bears this in mind can only give reasonable suggestions. Hints, perhaps even insight, but not complete designs.

[razerdave]

Axe mechanism

How much output torque does a Radiator Fan motor have, and what voltage is required for it ? Hells Angel is running on 24v, 3Ah.

I wanna get the best out of my axe, and so I need to know if itll do what I need it to do.

I have to work out the gear ratio so that it can self right my robot (12kg, 59cm in length, top panel is about 39cm due to shape)

The axe length is undecided, because it has to swing over the scoop at the front (meaning it has to travel over 180 degrees, like Behemoths or Shunts)

[big_nipper]

Axe mechanism

The first version of our Axe robot Blue only had about 8kg of lift on the axe head but this was plenty to self right.

[razerdave]

Axe mechanism

ok, and from what I hear, the radiator fan motor should be able to muster that at least, but I need to find out how much whack power I can get out of it at 24v, and at what gear ratio.

[nick]

Axe mechanism

Assuming the motor is very similar to the EV warrior, see John Reids post on 10 November, 2004 - 12:18 pm earlier in this thread.

[nick]

Axe mechanism

20Nm at the output stage is probably just enough to self-right your robot, and you should have much more than that available.

[kinforester]

Axe mechanism

David , The stall torque of fan motors I spoke of, is about 1Nm per 6 volts applied.
I cant remember what the no load speeds were ,I think they maybe around 200rpm per volt.

There nominal voltage is 12v, but Ive had them running very happly at 28.8 volts in Aaargh!.

GORD