The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: UKlittleguns on 06.05. 2020 15:19

Title: Dynamo cog fixing
Post by: UKlittleguns on 06.05. 2020 15:19
Hi Everybody,

Sent my dynamo away for a refurbishment but didn't look at it too closely before doing so.  Excellent job and no problems but when it arrived back the small taper fit cog wheel had been left loose with a new tab washer underneath the nut.  The nut is castellated and the shaft is drilled for a split pin.  Does this nut need both methods of locking?  Seems a bit over kill. 
Title: Re: Dynamo cog fixing
Post by: RDfella on 06.05. 2020 19:40
Was never a fan of split pins and castellated nuts. Aston used to use them on their big-ends. Means you either have to leave the nut loose or overtighten it to get the holes to line up - unless you're extra fussy in which case you file washers until everything lines up. I almost never replace split pins in castellated nut scenarios.
Title: Re: Dynamo cog fixing
Post by: muskrat on 06.05. 2020 20:50
G'day UK.
I don't recall the shaft having a split pin hole and castellated nut. May be a PO modified it. The nut would have been left loose so you could time the mag to engine then tighten.
Cheers
Title: Re: Dynamo cog fixing
Post by: Rex on 06.05. 2020 21:29
I think he's talking about the dynamo Musky.
Drags supply the tab washer but I don't recall seeing castellated nuts too.
Title: Re: Dynamo cog fixing
Post by: trevinoz on 06.05. 2020 22:32
They did originally have slotted nuts and pins but usually the threads are hammered and the hole is closed up or the thread is shortened due to aforesaid hammering.
I just use a drop of Loctite on a plain nut or use the slotted nut and Loctite if it is still on the generator.
Title: Re: Dynamo cog fixing
Post by: beezermacc on 06.05. 2020 22:36
Some armature shafts have a drilling for a split pin. The castellated nut used on these early shafts was shallow so you could screw it on beyond the split pin hole then fit a split pin. Some armature shafts have grooves for a woodruff key and some don't. Some of these inconsistencies are because the shafts were used across a number of models. On A10's everybody uses a plain shallow nut 3/8 BSF and a tab washer. (Trev was typing at the same time as me)
Title: Re: Dynamo cog fixing
Post by: Slymo on 07.05. 2020 04:47
That 10mm chain is a delicate wee thing and so I split pinned my castellated nut especially as I had turned the taper into the sprocket. I'd feel awfully silly if it undid and let go somewhere out back of beyond.
Title: Re: Dynamo cog fixing
Post by: Swarfcut on 07.05. 2020 08:39
By my reckoning this nut is a standard right hand thread. The dynamo is driven anti- clockwise, so a natural tendency for the drive cog to rotate a/c in relation to the shaft. A keyed shaft is fine, but an unkeyed sprocket is relying on the taper to hold it in place. Once the taper slips, nut and tab washer will undo.

 So belt and braces, adding a split pin which costs zilch  will save a whole load of  hassle and expense.

 Next question....How can you be sure the nut is adequately tightened if the taper is plain?  Usual way is to assemble the bearing plate and drive cog onto the armature as the first stage of assembly. At least the armature can be held firmly.  Doing this with a ready built unit  is more awkward, relying on the taper to hold the shaft still.

 Add a split pin, sleep easy.

 Swarfy.
Title: Re: Dynamo cog fixing
Post by: muskrat on 07.05. 2020 09:11
Doh!
Why did I have mag on my mind?
Cheers
Title: Re: Dynamo cog fixing
Post by: UKlittleguns on 07.05. 2020 12:45
Hi Everybody.

Thanks for all that.  Agree with everything said.  Now have belt and braces with tab washer and split pin.  Had to machine the thickness of the tab washer off the back of the nut to get everything to sit right (nut fully on shaft and holes for pin lined up when tight).  Suggests that Lucas only ever intended one or the other but I had both so why not.  It is a problem holding the dynamo shaft fixed.  The last thing I wanted to do was pull the dynamo apart just to grab the armature.  So I put the cog on the taper, a socket over the top and gave it one sharp rap.  The cog locked onto the shaft in a very satisfying manner.  I held the cog by wrapping the old drive chain around it and them grabbing the chain with a pair of mole grips.  Worked a treat for the final tightening.  Again, thanks to all *smile* 
Title: Re: Dynamo cog fixing
Post by: UKlittleguns on 07.05. 2020 15:27
Hi Everybody,

Just as an add on.  When looking for some other info found the cog fixings shown in the Haynes manual on page 39 "Fig 1.6 Crankcase, inner and outer covers - component parts".  Shows item 22 castellated nut and item 23 tab washer.  Not a split pin in sight! 
Title: Re: Dynamo cog fixing
Post by: RDfella on 07.05. 2020 16:53
With small tapers it often helps to lap them in with a little valve grinding paste. After all, a taper is merely another way of getting a press fit (in this case achieved by tightening a nut). Remember many things (eg the cam lobes on the camshafts of my project) are located solely by interference and if the taper is shallow enough (I forget the critical angle) once pressed together they cannot be parted again.
Title: Re: Dynamo cog fixing
Post by: Speedy on 16.05. 2020 09:03
Hello  all,
What type of camshaft do you have where the cam lobes are press fitted on to the shaft?. It can not be a A10 or A7 cam as you not be able to get to the 2 centre lobes,unless the shaft is split in the middle.
I would guess that the tapper would be about 5 degrees. And on a single cylinder machine. When it comes to relapping things in I find that grinding paste  even the fine paste is to course. So I use tooth paste it will give you an almost mirror finish on the valve heads and seats giving a nice tight gas seal. On the oil pump to stop wet sumping To seat your ball bearing  give it a sharp tap first and then use the tooth paste to finish grind it in. Best not to use the same ball bearing that you intend to run in the machine on reassembly.
Hope this helps all of you on a couple of topics.
Title: Re: Dynamo cog fixing
Post by: RDfella on 16.05. 2020 14:11
"What type of camshaft do you have where the cam lobes are press fitted on to the shaft?"
Those in the attached pic of an earlier project - both cams and gears are pressed onto the shafts. Shafts & cams are own design and manufacture, gears were bought in. Apologies for poor quality of the pic.