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Author Topic: How big a shaft for driving a 20kW alternator  (Read 18007 times)
jonesy
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« on: November 05, 2017, 10:01:21 AM »

A friend asked me to look at the electrical end of a home made hydro generator.
It was an installation in the mid 80's and appeared to work for a good while, but no longer works. The guy that designed/installed it cannot be contacted.
The alternator is 20kW/3 phase 230V and came as a lump requiring 1500rpm on a 20cm V pulley. This is driven by a 140cm pulley direct off the horizontal water wheel. The water wheel is best described as a series of spoons ~20x10cm, most of which are whole.
It was a flour mill, and the top rotating stone was originally removed. The rotating shaft terminates with a rectangular spigot approx 50mm x 30mm, and sticks up about 40mm. It's not clear how this was then attached and continued up to the 140cm pulley as there is a gap of about 20cm, but there are plenty of unused parts that almost fit lying around. The only broken and bent bits are M14 x 15cm bolts from a 30cm flange, that form what was most likely a flexible coupling to the spigot. The spigot is mainly clean and in good condition, and may have had something (badly) welded to it.
Any (fag packet) way to determine a likely maximum power that can be transmitted through a spigot of this size? It would be a shame to restore it to find that you can only run a kettle. A new water wheel and shaft will most likely be required. The flow is readily controlled, and is pretty consistent year round, so how likely will 50Hz be maintained?

I have no photos of the mechanical end.  I'll need to go back and take some.
« Last Edit: November 05, 2017, 10:09:28 AM by jonesy » Logged

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Philip R
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« Reply #1 on: November 05, 2017, 02:14:11 PM »

20kW @ 1500 rpm equates to 127.3 newton meters. Approx 13 KG meters or approx 95 ft lbs.
The shaft diameter need not be large. Think of the cross sections of the half shafts of a car transmission. Less than an inch in diameter, and the torsional loads they endure at harsh accelerations!
Philip R
 
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renewablejohn
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« Reply #2 on: November 05, 2017, 03:32:09 PM »

This is just not on. Get us all excited about a new hydro project and then no pictures.
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jonesy
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« Reply #3 on: November 05, 2017, 04:41:52 PM »

This is just not on. Get us all excited about a new hydro project and then no pictures.
I can't quite believe that I didn't take any photos of the boring mechanical bits.  Grin Got lots of the control panels etc. There was a lovely museum piece from Compaq which turned out to be a UPS.
Thanks for the figures. Something must have gone wrong to shear/bend the 4 off M14 bolts on the flexible? coupling. I'd been thinking it could have been a large electrical load applied, but could it have been caused by the pelton jamming on a log and stored energy in the larger pulley (more like a large cart wheel) doing the damage?
I did try to spin up the alternator shaft on a cordless drill and it turned fine, just not fast enough to excite.
Would I be correct in saying the ratio is 1400/200 ie 7:1, so the large pulley spins about 3.6 times per second.
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jonesy
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« Reply #4 on: November 14, 2017, 05:20:09 PM »

I've been sent a picture. I'll try to get some more.
At the bottom is the drive from the water wheel, terminated in a rectangular spigot. The large pulley is above and stops at the flange with the bolts. This pulley is held up about 100mm, and needs to drop down to align with the alternator pulley. The spigot will end up supporting this.
Any ideas how to join the two in a flexible manner.


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offthegridandy
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« Reply #5 on: November 14, 2017, 05:31:58 PM »

Is there any way you could bodge summat from a tractor PTO shaft? 
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knighty
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« Reply #6 on: November 14, 2017, 05:58:36 PM »

there's loads of flexible couplings around you can buy off the shelf which you could have machined/adapted to fit that

most are pretty much a bit of rubber between two steel plates

Google image search for flexible couplings to give you an idea

or... are you aiming more for home made/cheap? - pretty much all rear wheel drive cars/vans have a flexible rubber part between the gearbox and propshaft - it would be pretty easy to adapt one of those

*

the square shaft - does it have a hole through it ?  it looks like it does but not 100% sure by picture - is that shaft removable ?

it looks pretty rough, it would be much much better if you could remove that and have it machined square again

the round bit below it... is that some sort of bearing/bush housing or does that turn too ?
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Nickel2
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« Reply #7 on: November 14, 2017, 09:15:12 PM »

If the generator and the turbine each have their own full set of supporting bearings, perhaps a Granada/Transit/BMW prop-shaft coupling would do the job. They are suitably rubbery and would tolerate a reasonable amount of mis-alignment without problems.
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« Reply #8 on: November 15, 2017, 08:06:37 AM »

Use a Layrub coupling. Just google "layrub coupling" does exactly what you want. Lots of different sizes and data on max torque etc.

http://www.davidcornwallis.com/downloads.php

Click on layrub couplings to get datasheet of sizes etc.

Not sure on pricing.

Roger
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jonesy
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« Reply #9 on: November 17, 2017, 04:02:25 PM »

Thank you all for the suggestions. As we haven't even got the water wheel turning, I think we'll need quick and dirty to see what other problems there are. The belt tension is made by steel ropes and tensioners back to the walls. Wear suggests that one belt was used, but both pulleys have 3 grooves. There are 2 belts ready to use.
To answer some of the questions.
There isn't a hole in the drive peg. The peg seems to be machined from the larger circular part - no obvious welding. Might be preferable to weld a sleeve over it, once we've worked out what type of metal it is.
The water wheel must be fixed at the base with some sort of thrust bearing. So the shaft could come out, but I'm guessing we'd need remove the lower mill stone, then hoist the shaft/water wheel clear of the water.
The generator has its own bearings and rotates freely.
There is one picture showing parts that have been made and not used. The OD is ~30cm. It looks like these parts relied on the mass of the large pulley and gravity to keep them together. There are no broken or worn parts.








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Tinbum
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« Reply #10 on: November 17, 2017, 04:39:08 PM »

Is it possible that the slotted new part with the 4 bolt holes goes on the bottom shaft and then you need a large thick rubber washer that sits between that and the top shaft. The two are then bolted together with the 4 bolts that are hanging down. Huh

The rubber washer could be drilled with 8 holes so there is no fixed rigid connection. Huh
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« Reply #11 on: November 17, 2017, 08:17:24 PM »

This is off a mercedes propshaft you just need to find one the right size.


* s-l1600.jpg (319.95 KB, 1600x1200 - viewed 344 times.)
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knighty
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« Reply #12 on: November 18, 2017, 12:33:17 AM »

the flexible joint is the easy bit, fixing something to that square pin is the tricky bit

are you sure the square pin and the round bit below it are one piece ?

the shaft would need some sort of bushing or bearing at the top of it ?

unless, there's a bushing there we just can't see in the photos ?

or, it relies on the bushing above, which makes the joint a lot more important and also means you don't want a flexible joint, it would need to be rigid



check for a bush/bearing below the square peg

and also check to see if the square peg and the round part are cast - because that'll make a big difference as to how you fix to it

(quick google/youtube search will show you hot to test for cast better than I can explain it)
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jonesy
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« Reply #13 on: November 18, 2017, 09:27:59 AM »

Is it possible that the slotted new part with the 4 bolt holes goes on the bottom shaft and then you need a large thick rubber washer that sits between that and the top shaft. The two are then bolted together with the 4 bolts that are hanging down. Huh

The rubber washer could be drilled with 8 holes so there is no fixed rigid connection. Huh
There is a large piece of lorry tyre lying about, drilled as you suggest. The only bit I can't get is once the slotted piece is in place, there is a large part of shaft sticking up. This would require a stack of rubber washers to reach the upper flange, but webs have been welded on. There is a much heavier piece, no photo, that is basically flat (~30mm thick) with a slot and bolt holes which would work with a single rubber piece.

There is probably a bush/bearing under the peg. Plenty of grease has worked its way down to the wheel from the bearing. Looking at the remains of another mill stone (there were 3 stones originally) it looks like the peg used to be longer, with something that looks like a pickaxe fitted over the peg. I'm guessing this rotated the upper stone, so was height adjustable in some way to change the gap between the stones. So there must have been some sort of bearing originally.

You make an important point knighty, and the penny only dropped last night. The large pulley is standing on the mill stone, and is bolted in a few places. The alternator is similar. You might be able to see the ropes holding the large pulley go via a bearing. I'm beginning the think the large pulley, belt and alternator (tensioned off the opposite wall) all work together to ensure alignment onto the main shaft.

That joint off a mercedes propshaft looks good.

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Tinbum
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« Reply #14 on: November 18, 2017, 09:48:13 AM »


There is a large piece of lorry tyre lying about, drilled as you suggest. The only bit I can't get is once the slotted piece is in place, there is a large part of shaft sticking up. This would require a stack of rubber washers to reach the upper flange, but webs have been welded on. There is a much heavier piece, no photo, that is basically flat (~30mm thick) with a slot and bolt holes which would work with a single rubber piece.


I imagined the webs on the slotted piece being on the underside. ie the flat part with the slot being the top. The circular tube being to locate the part on the lower  shaft.
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