Power drill or SDS that will run at roughly a fixed RPM

[Dave Plowman (News)]

It will burn out.

[tabbypurr]

I'm struggling to believe he doesn't know that already. But it fits the pattern of this thread.

NT

[Dave W]

It won't burn out if it's not loaded, but will if it's having to deliver as much torque as its intended drilling application, especially as there will be negligible cooling from any built-in fan.

I don't understand the purpose of the set-up. The photo has a piece of very open wire netting in a wheeled frame intended to be reciprocated slowly for a long period. What happens?

[Dave Plowman (News)]

Why would you run any motor with no load? ;-)

[James Harris]

I'm not sure what Dave Plowman's term "burn out" means in this context. I could imagine contacts burning out due to arcing. I could imagine a drill overheating and losing efficiency or starting to melt the plastic. But I'm not sure the two are the same.

It is to be reciprocated at something around 100 Hz for shortish periods. The sled in the photo may or may not have a garden sieve insert at different times. There will be a load on the drive motor but probably only a small one, for some definition of "small".

I found a power drill which looks plenty powerful - 1300 watts input - and which is supposed to be able to maintain a constant speed under varying load. It's the only one I've seen with that feature.

I asked Bosch a couple of days ago for some pre-sales info on running it at low speed (as what I was looking for is not in the manual). So far the have not got back to me though I chased them up yesterday. I'll give it 'til Monday.

[Dave Plowman (News)]

The windings in the motor melt. Or just get so hot the insulation breaks down. Either way, it's toast.

[Dave W]

100Hz!!! Good God, that's 6000 per minute. With that weighty sled and earth on top the force required will be enormous. What peak-to-peak amplitude were you thinking of?

[tabbypurr]

He isn't.

NT

[James Harris]

...

...

Haha - that would be "interesting" but No! I meant 100 RPM. A little different!

[newshound]

Well it really depends on the size of the shaft. IME if you start with silver steel shafts (whether imperial or metric) they will be a pretty good fit in imperial or metric ball bearings. Maybe a light press fit. Remember not to put the load through the balls when fitting: use a suitable sleeve which acts on the inner race. It's fine to tap them on with a hammer, most people won't have a suitable press. For your low-ish speed you should get away with a slacker fit. If you want to get into the proper engineering, the SKF web site is a good resource. The quality of fit needed for long life depends on whether you have a rotating load or not. Think about a wheel bearing fitted to a non-rotating stub axle. The load on the inner race is non-rotating, so it can be a looser fit. The load on the outer race is rotating, this needs to be a tighter fit. If it is too slack, the outer race will gradually walk around in its (larger) housing, and this will cause wear eventually. For something like a car drive shaft, it's the other way round: the inner race sees the rotating load, and the outer race the stationary one.

[newshound]

Cheaper to buy a new Belle cement mixer, and throw most of it away. That will give you a continous rated motor and a gearbox at about the right sort of speed.

[Dave Plowman (News)]

If the self contained drill can run at the speed you want, you may be OK. Except that no drill is designed for continuous use.

I thought you were talking about using an external speed controller to slow it down - and unless Bosch sell one for the job, I doubt they'd be much help.

[James Harris]

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OK, thanks for the guidance.

[James Harris]

Cool. Good to hear.

I understand that. What I don't know is what kind of period "continuous use" might be for this application. For the piece of ground I want to clear I am quite willing frequently to stop and let the drill cool while I have a cuppa..!

Do you mean that use of an external speed controller would be more likely to lead to motor burnout than using the drill's own speed control?

[James Harris]

...

Bosch drill £230. Belle Minimix £320 (from a quick search).

Besides, a cement mixer would surely turn much too slowly. I guess I will need around 90 to 120 RPM. At that rate the cement would not leave the inside surface of the drum! As an example, according to one site I found, the Minimix 150 turns at 24 RPM.

By contrast, while a drill could not be used continuously it would have advantages:

• Its speed could be varied - at least until I know what speed I need.

• It could be used for other things.

[newshound]

Chinese copy, £115. It looks as though the shaft of that one drives the drum via rack and pinion, so the shaft speed might actually be a bit too high.

[newshound]

Ten minutes per hour?

[Andy Burns]

Yes, I have one in that style, the drum itself is effectively the final reduction gear.

[James Harris]

That would be a bit limiting.

[James Harris]

Incidentally, one of the things I asked Bosch about their "professional" drill was how long it could run at c. 100 RPM with a light load. Whether they can answer it or not is to be seen.