Building a small trolling boat.

I can't agree. The unit would fly apart at that speed(44k rpm). Nothing on that machine is revving faster than 11000 rpm.

 

I hold in my hand a Mirka 125mm cutting disc for steel. Printed to the surface are two limiting numbers, 12.250 rpm and 80 m/s, the latter being the perimeter max speed. 80 m/s on a full size disc calculates to ~12223 rpm. I don't see a reason for these limits, if the spindle was only rotating ~3000 rpm. A standard turbo charger rotates over 100.000 rpm and the structure is not that different.



cheers
Lurvio

 

I saw the same thing Lurvio, just scaned one, see my edit above post #102.

- click pic to enlarge​

Cheers,
Angel

 

As can be seen above I digged up some angle grinder disks and those of ø 125 mm (the size for Lurvio's machine) all say the same as Lurvio posted above and is shown on the one I've scaned and posted. So the 11.000 rpm on Lurvio's machine is for the disk.

For info about the inside gearing and motor revs I've counted it on an old Makita GA5000 angle grinder for max ø 125 mm disks on which the type plate reads ‘‘850 W -- 10.000 rpm’’ of which we now kow that's disk rpm. I counted 40 motor revs for 13 disk revs. That was the first match on which they were both at the same time exact back in the start position of the count. So the bevel gearing ratio is 40:13 which is 3.077:1 So that motor spins 30.769 rpm.

I've made a compilation of 'angle-grinder-bevel-gear-pics' that show the bevel gearing works in the direction Lurvio says and I just have counted on the Makita....

- click pic to enlarge​

My first impression was the Makita has a 3:1 bevel gear ratio but that was a bit off. Same could go for Lurvio's 4:1 bevel gear from which we can calculate motor revs and from that the motor torque. Exact bevel gear ratio will give exact figures of this . . . .

New calcs soon . . . . :idea:

Cheers,
Angel

P.S. - Just found a exploded view of the Makita GA5000.

- click pic to enlarge​

 

That makes it far worse . . .

I was surprised the outcome of torque loads on the bevel gear for operation in grinder use were so close to those for paddling at ease with the highest calculated sprocket gearing so that only peak torque loads were a real problem there.

But as we know now... In the previous calculations I've misinterpreted the disk revs for motor revs in the specs. So motor torque calculations for grinder use were off by a factor of the bevel gear ratio.

So now I'm really concerned about the lifespan of the bevel gear . . .

Here's the old post but with corrected figures and some adjustments in the text . . . . .

Cyclists have a low power output but due to their low revs they still produce quite a bit of torque. So it's easy to destroy the bevel gear by over torque . . :idea:

From post #51 and post #5 (in another thread) I found you have either this or this angle grinder used as a donor for the bevel gear. Both are 900 W and 11.000 revs/min for the disk. From the posts (with missing pics ) I saw it has a 4:1 bevel gearing.

---------------------------------------------

So what we have is this angle grinder:



Motor 900 Watt -- 44,000 rev/min -- bevel gear ratio 4:1 -- disk 11,000 rev/min -- € 19.99

From that I've made the following torque calculations with some different sprocket gearing:

---------------------------------------------

Original angle grinder design loads:

Motor torque: 900 Watt / ((2 pi / 60 sec) x 44,000 rev/min)) = 0.195 Nm (motor side of the bevel gear)

Torque on disk: bevel gear ratio 4 x 0.195 Nm = 0.78 Nm (disk side of the bevel gear)

---------------------------------------------

Cyclist at ease: 100 Watt -- 80 rev/min *

Cyclist at ease torque: 100 Watt / ((2 pi / 60 sec) x 80 rev/min)) = 11.94 Nm *

* on the crankshaft​

---------------------------------------------

Chain sprockets: 33 T on crankshaft x 14 T on disk side of bevel gear.

Torque on bevel gear: 11.94 Nm x (14 teeth / 33 teeth) = 5.06 Nm (disk side of the bevel gear)

Torque to prop: 5.06 Nm / bevel gear ratio 4 = 1.27 Nm (original motor side of the bevel gear)

Prop revs: 80 rev/min x (33 teeth / 14 teeth) x bevel gear ratio 4 = 754.3 rev/min

---------------------------------------------

Chain sprockets: 44 T on crankshaft x 14 T on disk side of bevel gear.

Torque on bevel gear: 11.94 Nm x (14 teeth / 44 teeth) = 3.80 Nm (disk side of the bevel gear)

Torque to prop: 3.80 Nm / bevel gear ratio 4 = 0.95 Nm (original motor side of the bevel gear)

Prop revs: 80 rev/min x (44 teeth / 14 teeth) x bevel gear ratio 4 = 1005.7 rev/min

---------------------------------------------

Chain sprockets: 48 T on crankshaft x 14 T on disk side of bevel gear.

Torque on bevel gear: 11.94 Nm x (14 teeth / 48 teeth) = 3.48 Nm (disk side of the bevel gear)

Torque to prop: 3.48 Nm / bevel gear ratio 4 = 0.87 Nm (original motor side of the bevel gear)

Prop revs: 80 rev/min x (48 teeth / 14 teeth) x bevel gear ratio 4 = 1097.1 rev/min

---------------------------------------------

As you see 100 W @ 80 rev/min paddler at ease output on the highest calculated sprocket gearing (wich gives the lowest torque) gives 4.5 times more torque on the bevel gear as the original 900 W motor.

And it's easy to produce peaks of 2 to 3 times more torque as these calculations are based on.

So if want to safe the bevel gear the advice is to use the highest possible sprocket gearing and go very at ease . .

If the highest possible sprocket gearing is too heavy to paddle then you could reduce the prop diameter but that might also reduce the prop's efficiency. The other option is to replace the bevel gear if it ever worns out by a heavier one, after all those angle grinders are not that expensive . . - But if it's worn out I recommend calculations before replacement . . . :idea:

It's a good thing there is slip between the prop and the water. If this kind of bevel gear was used on a bicycle shaft drive then it could easily be wrecked by just a few take offs. On a boat the max torque is limited by what the prop can transfer to the water, so you might be safe here . . . - Although I doubt the last part now, but hope you're lucky . . . :!:

Transmission losses are not included in the calculations . . . .

Good luck !
Angel

 

Quite high rev's.. wow! No wonder the cheapo's usually break in 10 min..

 

Hello

Thanks Angélique for the calculations. I am not that concerned about the gears for a couple of reasons. First off, the revs these things are meant to withstand require a safety factor as does the usage where the disc can get stuck from full revs etc. Also my thumb feeling (having taken the gear apart) says the gears will be plenty strong for my use. And lastly, it's only 20 € and fifteen minutes with a grinder to get me a new one.

cheers
Lurvio

 

I am amazed! But I am convinced. Sorry for doubting. 44k rpms it is.

 

Enjoy the boat as it is now and time will tell if the manufacturer's safety margin will save the bevel gear as it is used now. If lifespan turns out to be acceptable then there isn't a problem. And in case of a breakdown you now know a possible cause which could help to find a solution.

Just for your own personal safety margin always have a paddle aboard so you can help yourself in case of a mechanical failure.

If it breaks down it could be in the middle of a big lake as Murphy says . . . . :idea:

Maybe Saimaa would be such a place . . .



And... a paddle also might become in handy on shore, defence only of course . . . . .

​

Good luck !
Angel

 

Not that important, but did you count the teeth . . ? ?

That would give you the exact bevel gear ratio much easier than the 40:13 revs I counted on the Makita which was still in one piece . . . but not operating at the time . .

Cheers,
Angel

 

Angelique I don't remember if I actually counted the teeth, but I'm 95% certain the ratio is 4:1.

cheers
L

 

Hello

Just got around to take a couple of pics of the pedal setup.






There is no screws on the front stem connection, but the whole setup feels sturdy enough without. I may need to weld a bracket for a chain tensioner if I'm not lucky with the bigger sprocket matching the chain lenght. As it is now, the chain slips off the bottom sprocket too easily. There is also 3-4mm misalignment that doesn't help.


cheers
Lurvio

 

But why not put a few screws in ? It would make it even more sturdy for almost no effort and weight . . . :idea:

Remember to put the tensioner in the non pulling part of the chain in forward propulsion. Otherwise the traction in the chain would straighten the chain against the force of the tensioner and the other side going to the bottom sprocket would still be loose. So put the tensioner in the fore part of the chain and now you have this problem for backwards propulsion which isn't a big problem as you can go easy there.

Maybe this can be adjusted. Those chainring bolts are available in different lengths and there are spacers for them. Google: ‘‘chainring bolts’’ and ‘‘chainring bolts spacers’’ and they'll pop-up... Depending on which side the misalignment is it might also help to mount the chainring on the other side of the crank mounting lips.

Only misalignment that can't be adjusted this way is when the chainring has to be exactly there where now the mounting lips of the crank are. This can be adjusted by a little center offset* of the crankshaft or a crankshaft of different length. Cheap (not so good ) bicycle crankshafts are about € 10 around here . . .

* A little center offset, if needed, was a job for an alignment mockup, alas it's a lot of work to change that now. But maybe it's just a spacer job to get it right, which is the best way anyway.

Good luck !
Angel

 

- click pic to enlarge​

BTW, is that hose the only coupling between the bevel gear and the drive shaft . . ?

If so, than that's a nice safety feature against over torque. Just not tighten the hose clamps too much.

Good luck !
Angel

P.S.

Maybe make a mockup* and see if you can set it at max 1 Nm with a torque wrench as it was original 0.195 Nm on that end.

* without the bevel gear, just use the hose in some different lengths and clamp tensions on axes of the same diameter and surface finish as on the bevel gear shaft and the drive shaft.