Rob said:
I doubt there's much friction moving powder up a hole, verses smashing the sh*t out of limestone. I'd guess a lot of the power for long holes is the momentum in knocking large drill bits axially forwards and backwards. However even if it's twice as slow and inefficient, i can get 6x75mm holes from one battery, generally drilled badly at full reach, so i'm guessing at least one 250mm hole per battery should be fine.
Yes I'm sure you are right. Overcoming friction to get the powder UP the hole will absorb a trivial amount of the available power.
But I think the greatest demand will be turning the drill bit against the friction with the hole, where the ?lubricant? will be the powder.
Taking the spec of the Uneo: 0.5J/impact, max drill size in concrete 10mm, 280/900rpm, max torque 14/30Nm. The impact rate was not quoted, but the Uneo MAXX which seems similar does 0 to 5000 impacts/min.
I don?t know how the impact rate is changed ? perhaps it varies according to the force applied to the drill? ? but giving it the benefit of the doubt and taking 0.5J/impact with 5000 impacts/min that?s 2500J/min which is about 42J/s or 42W.
It is possible, in a rather more complicated way, to come up with an estimate for the power required to turn the bit against resistance when the drill is developing maximum torque.
The torque figure of 30Nm means that the drill is doing the equivalent of applying a force of 30N at the end of a 1metre lever to turn the bit in the hole. So at 5mm from the centre of rotation (where the 10mm drill is rubbing round the inside of the hole) the drill will be applying a force 200 times bigger, or 6000N.
Energy converted is force times distance moved (by the point of action of the force, in the direction of the force) so as the 10mm bit is turned once the work done is the force times the circumference of the bit. This will be 6000N * Pi * 0.01m or about 190J per rev.
When the machine is required to develop this maximum torque it will not be able to do its rated rpm, so on the lower speed setting of 280rpm (which I imagine pairs with the higher torque figure) the drill motor will not be developing the obvious 4.7rps & 190 J/rev or about 900W, but even if its speed is down at 1 rps (and no one would be so cruel as to persist with that, surely!) the power required to keep it turning will be well above that to deliver the maximum impact rate.
Cordless drill manufacturers don?t seem to give overall power figures, unlike mains derived drills. AC and DC motors have different operating characteristics. But a medium size corded impact drill might have a 600W motor, so I suggest the above estimates are reasonable.