Axe mechanism

[blazerbotics]

i come up with the same as gordon, about 4.8Nm.

[razerdave]

Ok, cool. Is trying to figure out how fast the axe comes down and with how much force.

If Energy = (MxP^2xs^2)1/3 and Sweep time = (2xS^2xM/P)^(1/3), I require s (sweep distance) to work it out. This is measured in Radians, right ? My axe has to travel about 200 degrees.

Say I use this radiator fan motor (No Load RPM is 4800rpm)to power my axe through a 12:1 ratio on 24v (belt or chain, I havent deciced, but I think belts would lose to much energy), the axe arm is 330mm long, and the arm weighs in at 1kg and the head weighs in at 700g.
Motor Torque: 4.8Nm

What will I end up with in terms of energy of the blow and speed of the blow ?

[grant_ploughbot]

well 200 degrees is 3.49Radians
Im in college atm so i dont hasve time to wo4k out the whole equation

[razerdave]

Same here funnily enough :P

[nick]

For calculation purposes, all angles are measured in radians and all distances in meters. Lets also call the effective head mass 1kg and forget about the shaft.
If we assume peak power is 600watts at 2400rpm (Im really not enormously sure it is)
For fixed ratio drive you chalk up at least an effecitve 10% of the available power as lost over a varable system as it wont be running at peak power all the time, you should decrease this further to reflect additonal losses expected in the drivetrain.

So say power is 540 watts at 251 radians per second
Energy = (1.0 * 5402* (0.33*3.49)2 )-3 = 73 Joules

For the variable ratio system the ideal raito at angle x=radians_per_second*(1/3 * shaft_length2 M P-1 x-1)1/3
For a fixed ratio system a ratio equal to that of the ratio at the halfway point in the ideal variable ratio system works well.

ideal ratio at 1.75 radians=251*(1/3 * 0.332 * 1.0 * 540-1 *1.75-1)1/3 = 8.5:1
So at 12:1 that energy figure would be a little lower, the sweep angle makes a difference.
You can get an idea of the sweep time by doing 73/540 but in reality it will take rather longer.
I havent checked these figures, so you might want to.

73 joules is not a lot :S

[Mouldy]

Gord- Are you using the same fan motor in your hammer that you used in Arggh?

James and Grant...dont know what You mean by cost effective...I bought you one of the fan motors you destroyed so you owe me lol

[nick]

At 73 Joules a conventional electric feather will already be beginning to jump. For many opponents it is a damaging level of energy. Feather hammers are handicapped in two ways compared to their heavier counterparts. For a given weight they cannot make as high a energy to weight as a heavy without generating more power to weight, as there will be less time in which to generate it. This *is* somewhat possible for motors and gearboxes which can increase in power performance as they get smaller, but is more of a challenge for available batteries which put more sharp limits on performance.

With pneumatics so much power can be made available, unless you are stuck with low performance components it is not in my mind even an issue, pneumatics scale down remarkably well. Given the need and motivation it is possible to construct pneumatic parts in a heavy capable of reaching hundreds of kilowatts of power. The second disadvantage is that gravity is a less significant effect the smaller you are, so jumping becomes a problem even with a reduced energy. It is easy for feather to lift other feathers, they dont have to move things as far so the stresses are less.

There are a couple of ways to generate more energy, the simplest is often to increase stroke or shaft length. Neither are totally problem free. It isnt worth increasing the head mass much so you need to concentrate on power. Secondly more volts, it works best to really punish the motors and the batteries. If you havent thought of overvolting your probably just not trying.

If you use a toothed belt or chain drive, plausibly you might be able to handle shock energy elastically in the tensioners, and avoid a clutch - if you are careful. The tensioners have to be sprung low friction rollers for a timing belt or sprockets for chain.
The effect on the overall energy shouldnt be too great. If you can cut your own sprockets it might be possible for you to make a torque shaping linkage. However with only one ratio per position, you cant get much of a retract jolt, the same effect is there for elliptic gears except you wont have the shock absorption either. This kind of torque shaping mechanism can be used to reduce jumping to a degree. Shaping mechanisms work efficiently with sprung energy storage, though too much use of these mechanisms can cause trouble in self-righting (and possibly retract). The former might be less of an issue in a feather.

Nick.

Dave - Yes I am using the same motors as Aaargh! but at a higher voltage. Ive designed it to take upto 32 C cells,whether i use that many will just be down to testing.

GORD

To tension the drive chain for the axe, cant a use a piece of self lubricatin plastic like George fransis does for Chaos 2s drive chain ? saves the weights on spring loaded ones