Axe mechanism

[jamesb]

Axe mechanism

I would reccommend having a belt or chain in there, we found on the Rad fan motor axe mentioned by James Cooper that the shaft was a simple push-fit, so with the impacts directly onto the gear welded to the motor, it seperated the shaft from the armature. Some shock protection is needed. Also, dont weld the case of the motor, it melts the glue on the magnets and allows the magnets to move inside the motor case. I wanted a fusee drive in this robot but was out-voted, so Id recommend it here.

Lastly, watch your gear reduction, we used approx 6.6:1 but this was way to low and was basically a cost driven solution. Invest the extra few quid and do it properly. If using the mondeo deisel fan motor aim and asuming a shaft length of 330mm and an arm weight of 1.5kg, I would suggest 9:1 or even 12:1, if for no other reson than it makes self righting less of a battery smoker.

Also, the fan motor worked quite well at 12v, but was insane at 24v (until the motor was damaged by welding) so if you get the gear ratio right there should be no need to have silly voltages, and of course as the voltage goes up, the gear reduction can go up to, so 9:1 at 12v can easily be canged to.... say... 12:1 or 14:1 at 24v. all of which helps self righting.

Hope this helps.

[grant_ploughbot]

Axe mechanism

I remember quite well the conversations between you and james when decided on what to use. You were saying 6:1 was perfect and james thought around 12-15:1. After long discusions and some talking to john ried, the outcome was 6.6:1 would be the best and most efficient for the job.

Worked well in the end, just the quality of the motor wasnt much to be desired. Might look at making another along the same line, but with a better quality motor.

[jamesb]

Axe mechanism

hummm.......... you seem to have proved me right all along then... see, I do know what I am talking about.

Increased ratio suggestion is purely to help self righting, as the one we built was very high current draw on self righting. But, 6:1 (or 6.6:1) to be exact is theoretically the best (doing the maths) as I said in the beginning.

I would stil reccommend the fussee drive though, and to be honest for the £5 the modeo motor costs, its worth a try, just dont weld it. Welding motors usually breaks them, it isnt worth risking.

[storm]

Axe mechanism

Welding things onto motors = bad idea

[grant_ploughbot]

Axe mechanism

we used approx 6.6:1 but this was way to low and was basically a cost driven solution.

nah well. welding wasnt a good idea

[nick]

Axe mechanism

Deviating a lot from the theoretical peak does hurt the energy of fixed ratio systems quite a lot. More so than with a torque shaping mechanism. With such a mechanism, you may want to let the motor rev quite a bit high, but for reasons other than limiting current.

[razerdave]

Axe mechanism

Whats a Fusee drive ?

[nick]

Axe mechanism

Like the arrangement on beta, using snail cams.

http://www.johnhmreid.dsl.pipex.com/beta/progress/leafchain.jpghttp://www.johnhmreid.dsl.pipex.com/.../leafchain.jpg

Its only one option, there are others. In a heavyweight a clutch is practically mandatory especially if you use anything resembling an ETEK to power it. You can probably get away without one in a feather with some motors if the hammer shaft is sufficiently flexible. In that case, drivetrain mass will have to suffer so the gears dont get damaged.

[razerdave]

Axe mechanism

I didnt realise about clutches for HWs. Ill be sure to note that when I get back to building it.

I know that the Fan motor produces about 600W and 4800rpm at 24v, but Im trying to find the torque of the motor (I can probably work out the torque from the power and rpm, but I cant remember which way around it goes right now), so that I can work out what speed and with how many Kj my axe will hit with

[kinforester]

Axe mechanism

I reckon the stall torque would be about 4.8 Nm if max power is 600 watts and no load speed is 4800rpm

But then Ive been wrong before.

GORD

[blazerbotics]

Axe mechanism

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

[razerdave]

Axe mechanism

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]

Axe mechanism

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

[razerdave]

Axe mechanism

Same here funnily enough :P

[nick]

Axe mechanism

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.