Axe mechanism

[jamesb]

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.

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]

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.

Welding things onto motors = bad idea

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]

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.

Whats a Fusee drive ?

[nick]

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]

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]

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