Friday, 19 July 2013

Converting a 2.3 kW Generator Alternator to Belt Drive.....


As part of my mini CHP project - more on that in a future post when it's a little further advanced - I bought an old petrol-driven generator set on eBay for £51.

It was only the alternator and panel etc I really wanted, but the old genset had a few other things I could use, such as the frame and fuel tank.  Everything seemed to be working fine before I started taking it apart.

However, when I stripped the generator off the engine, I discovered that there's no front bearing assembly within the generator.  In its original configuration, it plugged directly onto the engine shaft and therefore simply used the crank main bearings for its front support.


the re-assembled 4.5 hp petrol engine after stripping off the genny - the
engine's going up for sale on eBay shortly

the generator....



Makes sense for an in-line mounting configuration I suppose, but it isn't any use for my application since I want to set it up alongside another (diesel) engine and run it with a belt drive.

So I designed a conversion to the generator, to include a new front bearing arrangement.

I made an adaptor plate / bearing housing on the CNC out of some thick aluminium flat bar, and machined the location diameter for the generator concentric with the new bearing.   The housing machining included the four recessed holes for fixing it to the face of the generator end cap.  The base of the adaptor plate is also tapped to mount a bottom rail for assembly in its new location.

I then set about making an adaptor drive shaft to plug into the generator.  I first measured up the old engine shaft to obtain the key dimensions, and checked a few old genset service manuals to establish details of the drive taper.  It's actually a 2.1/4" in 1 foot taper which gives an included cone angle of 10.6196553 degrees (all nice round numbers ! ).

I machined one end of the shaft to length, and parallel at 20 mm diameter, and then using the lathe top slide I set it up to roughly 5 degrees and cut a taper at the end.   When it was around 15 mm in length, I measured the diameter at the small end and the horizontal length as best as I could with the vernier.  After a little trial and error, measuring each time as accurately as I could and then tapping the top slide around to increase or decrease the angle, I managed to get the cone angle as close to 10.62 degrees as is possible with my machine set-up.

I used aluminium 6082-T6 round bar  for the shaft - it's not an ideal shaft material because aluminium alloys are prone to fatigue when they're subjected to constant stress reversals, such as when the shaft is rotating under the radial load from the belt tension, but I did a few drive calcs and the stresses seemed so low that it's not likely to be a problem.

I bought a 6205 2RS bearing on eBay - usually for my projects, I buy the cheapest Chinese bearings available, but this generator deserved a genuine SKF which was only £4.30 delivered, a real bargain.



the genuine article installed in the housing....

So I finished the lathe operations on the shaft, tapped both ends at M8 (the plug-in end for the generator is pulled tight against the drive cone on assembly via a central bolt through the rotor coil assembly), and then machined a keyway on the 25 mm shaft end to suit a 8x8 mm drive key.



machining the keyway - the last operation
on the adaptor driveshaft

So then it was just a case of putting it all together.  The bearing was already a decent tight fit in the machined housing but I added some bearing retainer fluid for good measure when I pressed it in.


the kit for final assembly....

The shaft was installed first and the central M8 bolt partially tightened, to hold it in position.  The housing was then fitted over the shaft, and driven home using a piece of steel tube on the inner bearing ring - this connection was deliberately made to be a tight fit.

Some threadlocking fluid on the central M8 bolt and the M12 mounting capscrews and, hey presto, a generator now converted for a belt drive set-up.


ready for the pulley....

I'll add to this post when the pulley arrives and I've machined the bore and keyway to suit - I found a new-but-old-stock 150mm diameter SPA v-pulley on eBay for just a tenner delivered, but it hasn't arrived yet.

I've also ordered a used multi-step pulley with the largest of the pulley profiles also at 150 mm.  This will be mounted to the new engine shaft, for a 1:1 drive at an engine speed of 3,000 rpm.


Update 21-July-13

The generator pulley arrived in the post yesterday, and I machined the shaft bore and keyway.  I also found an old 10mm square sump plug removal key at this morning's car boot sale (20p) and machined a drive key from it.


drive key fitted....note the two tapped holes for the endcap

Then I assembled the pulley to the shaft with the key and fitted an endcap I'd made for axial location of the pulley.  More thread locking fluid and then everything was fully tightened. 

I also made a bottom mounting rail from a piece of aluminium angle and fitted that too.


pulley and bottom mounting rail assembled....

So it's basically ready to be connected-up electrically at the rear end and then fitted in the new engine frame.  When I've modified the engine crank pulley, I'll first carry out a trial fit to determine the centre distance and then buy a suitable SPA drive belt....



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