Axe mechanism

[Guest]

Axe mechanism

Ah, the master have spoken...

[killercarrot]

Axe mechanism

We ran a motor similar to a WW motor on 2meat hammers electric axe, and at 24v it provided just the right sort of whacking speed, about 2 hits a second.
We coupled the output shaft to a crank, and con rod to an accelerator copied from dominator. It worked fine in tests, but eventually tore half the teeth off the gearbox. I have tried replacing the gearbox with soemthing a little stronger, but have given up, and am converting it to pneumatics.

[talulah]

Axe mechanism

John,

However, on an electric axe, the only way of getting a good level of enery into the axe is by having a very heavy head on the end of a long shaft. The motor is producing energy at a constant rate, so the longer the swing time, the more energy will be in the axe head at the end.

The weight of the axe head doesnt have any bearing on the energy in the axe on impact. The problem part of your comment is that the motor is producing energy at a constant rate. The power (rate of energy) the motor is taking from the battery is Volts x Current - voltage is fixed but the current varies hugely. At start, it is very high (motor is stalled), and drops off as the speed of the axe increases.

On my axe weapon webpage (http://homepages.which.net/~paul.hills/AxeWeapons/AxeWeapons.htmlhttp://homepages.which.net/~paul.hil...xeWeapons.html) I have a spreadsheet for performing the physics of an electric axe: http://homepages.which.net/~paul.hills/AxeWeapons/MotorDriven.xlshttp://homepages.which.net/~paul.hil...otorDriven.xls. In this spreadsheet, you can alter the moment of inertia of the axe (which is the rotational mass), and see what energy you get after a certain angle (I chose 180 degrees in which case you may have to extend the table a bit). If you alter the moment of inertia, you find that the energy after a certain angle stays pretty much constant. The slight variation is because this calculation is using an iterative sampling approach rather than the pure maths which is rather hard!

Thats the theory that I worked out anyway. If you have any practical experimental results (and I know you have a lot of experience with these things) that disagree I would be very interested - it would mean my theory has a mistake somewhere!

[fluppet]

Axe mechanism

If I can have an intuitive stab at what Johns saying (to see if Ive got the gist):

With a pneumatic axe, the amount of energy imparted to the weapon before impact is independent of its weight (and presumably depends on the energy of the air expanding in the cylinder), no matter the speed/weight trade-off.

With an electric axe, the more weight is present for a given sweep, the longer the motor will have to impart energy to it - and therefore the more energy the axe will have on impact.

i.e. a pneumatic ram imparts a given amount of energy regardless of how long it takes to expand, whereas an electric motor imparts energy (potentially) proportional to the time its running - with provisos about efficiency, variable gearing, etc.

Did that help, or have I just muddied the waters? (Its one of those things which could make sense, but could easily *not* make sense, too).

--
Fluppet

[terrorhurtz]

Axe mechanism

Youve got it spot on Andrew.

Paul, the energy delivered by an electric axe is pretty much proportional to the weight of the axe head, assuming that you optimise the gearing for a particular axe weight. You need to include a gearing factor in your spreadsheet, which you can adjust to get the maximum energy output. Judging by how flat the speed line is, it would produce much more energy with a gear reduction.

The motor is steadily churning out energy (OK, maybe not at an absolutely constant rate, although with the snail cam on beta we manage to keep the motor pretty close to max power output). The longer you can keep it feeding energy into the axe, the more it will have at the end. The way to make it take longer is to increase the weight of the axe head or increase the length of the arm.

Voltage x Current of course gives you the electrical power input to the motor, not mechanical power output. As the revs rise (and the current decreases), the efficiency rises. Hence it produces over 75% of maximum power within the centre 50% of the rev range. Which is nice for those of us making electric axes, as it means you dont have to get the gearing spot-on.

[terrorhurtz]

Axe mechanism

Sorry, energy delivered is not purely proportional to head weight, but it is highly dependant on it.

If you have a 10,000 Watt peak output motor attached to a very light axe, so that it only took only 1 millisecond to fire, then there is no way that it can deliver more than 10 Joules of energy.

Whereas, if you connected it to a very heavy axe, so that it took 0.5 secs to fire, then it has the potential to deliver up to 5000 Joules.

[razerdave]

Axe mechanism

Im building a Rexs Robot Challenge style FW for my IVA project at college, and I want an axe robot rather than a flipper, and I am unsure of a mechanism that uses both rams. Any clues ? I can use a motor, but Id need extra batteries for that

[ewan]

Axe mechanism

A motor would be easier, as you cant get much space and pressure in a fizzy drinks bottle (meaning less potential shots from the rams than Co2). Ive got to admit a Co2 axe would look better though. It would be quite easy to couple two bicycle rams together and use the same rack & pinion meathod as used on terrorhurtz, although the rack and pinion would probably require machining a new rod (the moving bit) for the ram(s).

[knightrous]

Axe mechanism

You could use a crank lever system powered by a motor. Similar to what Mortis has.

[jamesb]

Axe mechanism

for electric featherweight axes, e-mail me on james@jamesbaker.f9.co.uk and Ill send you a pic and info.

the design was entirly from my own knowledge of degree level engineering, and nothing else.

Having an astronomical respect for my knowledge, my friend then asked everyman and his dog for advice, including John Reid, and the solution he came up with using Johns figures and many computer programs was.... exacly the same as my solution.... exactly. So Id say that sticking to basic pricipals is by far the easiest way to build an effectice axe and if you do not have the maths background e-mail me and Ill send you info about mine

cheers
james

[Guest]

Axe mechanism

Ive had a look at Pauls equations and Ive fed in all the stats for my feather axe robot. I had to do this on paper as I dont have Excel. I ended with an answer that I was expecting, but when I then tried to modify the inputs for the gear reduction which my robot has the answer ended up at less then a 3rd of the energy. Anybody care to help explain this?
The robots weapon is a 1kg hatchet driven from an E.V. Warrior at 24V with a 4:1 reduction through chains and sprockets. The energy calculated before reduction was 1.4kj and after reduction was 0.4kj. A photo of the half completed robot can be found here http://www.robowars.org/forum/album_pic.php?pic_id=346http://www.robowars.org/forum/album_pic.php?pic_id=346

[terrorhurtz]

Axe mechanism

An EV Warrior powered by two 24V 3Ah Nicad sets would have a peak power output of around 600 Watts.

Say the axe sweep lasted 0.2 secs, then the maximum possible energy would be 120 Joules.

To get a crude approximation to the sweep time, use the formula: t=(2 * s^2 * m / P)^0.3333
where s is the sweep distance of the axe, m is the mass of the axe head and P is the power of the motor. Assuming a 300mm long shaft, then this gives 0.14 secs and 86 Joules.

If you doubled the length of the axe shaft to 600mm and doubled the axe head to 2kg, then you could get it up to 0.29 secs and 170 Joules. With this setup, you would want a gear ratio of around 20 to 1.

To get a very crude idea of the gear ratio required, use the time of the swing to get the average rotational speed of the axe. With the current setup 0.14 secs for half a revolution = 3.5 revs/sec = 210 rpm.

The no load speed of an EV on 24V is 5000 rpm, so the speed at which peak power is produced is 2500 rpm. So the gear ratio should be around 10 to 1.

[terrorhurtz]

Axe mechanism

Youll be glad to hear that you are only losing around 10% in energy by running at 4 to 1 rather than 10 to 1. You will however be drawing a lot more current from the batteries.

[bugs]

Axe mechanism

Ive made a fether with an axe. Its just a thwack bot, using the stinger-like system. I still have to get it a bit more reliable though
See my profile for a picture.
It uses 2 drill motors for each wheel. I still have to replace the actual axe, but its more of a show-bot anyway.

[terrorhurtz]

Axe mechanism

a better way to express the sweep time formula would be:

t = (2 * s^2 * m / P)^(1/3)

x^(1/3) is the cube root of x
s^2 = s squared