G: Gearbox (Single - Burman) Burman Drive Sprocket Loose

oexing

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Sorry, no, I cannot see a way to feed all that torque into the sprocket just by torquing up the nut without having a good condition spline there. Just imagine, the gearbox plus the primary drive multiply all torque produced from the crank to about ten times into the rear chain at the sprocket spline. No way to hope for the effect of torquing the nut. Same with the clutch nut plus its spline. There is only one way to achieve a lasting solution by restoring a PERFECT spline fit with definitely minimal play in it. Imagine, the rear chain pull onto the sprocket tries to dislocate it offcenter and only the spline fit can keep it centered. Any play in there produces wear by rubbing motions at all contact faces.
So really only rescue to the actual Burman question is to get a new quality sprocket , get the gear shaft hard laser welded and machine it to sprocket dimensions as close as ever possible. The gear shaft will be less worn at the rearward face on the spline, so when machining it after welding plan to first clean up these faces and have only the forward faces welded up by laser thick enough to restore the lot to perfect fit into the sprocket.
In case no good new sprockets available, I´d probably get a new soft universal chain sprocket and have the spline wire EDM cut - except you can use a standard broach. Broaching would be less costly but wire EDM is allright, only six slots to cut, so no big deal. Just care for a good finish after wire cutting, roughing and finishing cuts, any rough finish will wear fast and slop returns in the assembly.
Basically laser welding in the internal splines of the sprocket is another way, not so nice to do and best to be avoided . Again, machining will be best by wire EDM as the weld will be necesssarily hard. But I´d rather go for stock chain sprockets, get the spline wire cut, case hardened - and the last step is to machine the gear shaft for perfect fit. You can find carbide milling cutters for 65 HRC material and milling is best done face milling for evenness of faces , all set up in a dividing head. Below photos of laser welded splines and ground , same place for sprocket on B-Rapides.

Vic

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Matty

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All very interesting but I think the basic problem is that many of the spare sprockets in the past were probably manufactured from poor and possibly cheaper material than the originals.
As pointed out the drive sprockets on many modern bikes are only held on with a circlip and are quite sloppy on the splines and seem to be fine though it does not look on the face of it a very good piece of engineering.
I have changed chains and sprockets on many modern bikes and not noticed any significant problems with badly damaged splines.
As I have said in the forum before, I have only once changed the front sprocket on my Comet at around 50,000 miles and am still using the "new one" at 106,000 miles.
I am however now a bit worried and may have to go through the trauma of inspecting mine by removing the primary drive casing etc. though the last time I rebuilt the gearbox around 6,000 mile ago it seemed OK and I did the nut up VERY tight on the bench with no Loctite.
The nut which holds the clutch on is subject to similar forces and I have in the past had to replace the clutch cage where the splines were reduced to about half the width - though the mainshaft splines looked fine - probably because the shaft was made of superior steel.
I am only an electronics Engineer and not a metallurgist or mechanical engineer so my views are really only subjective without a great deal of real science applied.
I hate to say this, but have heard that some people have welded the sprocket onto the shaft which I should think would have Phil Irving turning in his grave.
 

oexing

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I just returned to the first page of the thread and discovered that I posted about the same ideas one year ago like in that one above, sorry for repeating me. But I still stand to my propositions. As told by someone else above even extremely torquing up the sprocket nut will not fight any loosening of the assembly, in fact, the nut did NOT loosen but the shims, washers and sprocket material were worn due to rubbing action by micro motions as soon as the splines get minimal play. Some owners here have got very high mileages out of their bikes at no troubles, some had more than once to replace components. I say this is dependend on what perfect fit within the spline joint you had to start from. With perfect fit there will not be much wear for very acceptable mileage, but as soon as some play has developed you can observe progressive wear in due time and securing the nut will be no help at all with the underlying defect.
One can test the material of the sprocket, it is only the hardness that is critical, so try with the edge of a file to get a feel about heat treatment. If too soft you could have the sprocket case hardened once more, best in inert athmosphere or vacuum to keep it clean - but only if the part is in perfect condition, else it is a hopeless case. Then you better get a new spare or if unobtainium or junk production, have one remanufactured from stock sprockets and have the gear shaft lasered and milled to fit. TIG welding will heat up the shaft too much so only laser will do safely. Seems the Burman type is only very narrow spline length , my guess 5/16 or 3/8 ?? Modern bikes got a more substantial dimension possibly, so more durable ?
Anway, not much to add now, you will have to see what is available today.

Vic
 

Martyn Goodwin

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Thanks for all the info and guidance. I have ordered a replacement PR50-15BA (thou second hand) driving gear from Draganfly and I already have a new sprocket PR50-22A on hand along with a new lock washer.

The photo below is of the existing disaster and having rechecked - twice - I can confirm that the securing nut was not thread bound but was firmly secured and tight against he face of the sprocket. Extent of damage to the gear also confirms this. FYI you can just see the end of the spacer PR50-23BA protruding from the seal.

Checking the NEW sprocket, the spline length is 3/8 inch, that is also the clamp thickness so as the ruler shows the nut does not get anywhere near the end of the thread.

So now its a case of strip the box and wait for bits to arrive. When they do I will be VERY carefully checking for fit.

M

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Martyn Goodwin

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I think I MAY have found a contributing factor.

I have a brand new PR50-17-1BA driving gear lock washer AND a brand new PR50-22A gear box sprocket both from the Spares Co.

The OD of the lock washer is 2.6165" with a thickness of 0.0350 and it appears to be lazer cut.

The ID of the sprocket central depression appears to be 2.6370" but that is at the outside edge of the central depression (see photo). Problem is that depression has round shoulders at it base so the flat face for the washer and nut to clamp against is only 2.4910" in diameter.

What this means is when assembled, before the nut is secured, the face of the washer is well clear of the face of the sprocket and consequently as the nut is tightened against the washer, the washer acts like a spring absorbing a lot or the hoped for clamping force.

I do not know how significant this is - anyone have any thoughts?

My first thought was reducing the OD of the lock washer to 2.5" to eliminate the spring effect.

On reflection a spacer between the face of the sprocket and the lock washer may be a better solution. Has anyone encountered this before? Should there be a spacer there anyway and if yes I wonder what the part number is??

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Matty

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I have just had a look an old driving gear in my stock, of maybe useful bits and the splines on it look virtually perfect though I have not used it because the top gear dogs are a little tapered so I would lose a little of the 3.5 /4.00 mm top dog engagement.
So why are the splines so damaged on the one in the Photo? Is this the result of bad material or incorrect assembly in the past?
Afraid I do not have a new or old sprocket to check the tab washer problem which of course could be fixed by either reducing the diameter of the outer of the tab washer or making a simple spacer to go behind it.
Matty
 

Cyborg

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I would be wondering about the hardness of that shaft. If shafts like these can survive beyond the useful life of the motorcycle, they must be somewhat harder that yours. The lock plate is just slipped over the splines, rotated in the groove and bolted in place. Sprocket is not rigidly fixed on the shaft.
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oexing

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Rigid fixing the sprocket by torquing that nut will not have ANY consequence to the useful life of the joint, as the power going through this joint is higher by factors than your torquing as confirmed above in no. 35 by Martin , he found a tight nut but the splines were worn even so. So I wonder how owners who had no troubles had assembled their gear components, good fit I guess but another factor possibly a good lubrication onto matimg faces like good chain grease or even better MoS2 paste ??? The modern Japanese designs rely on decent hardened components and a wider spline for spreading loads to acceptable conditions. So yes, doing up that nut will defintely NOT help, just conceal a flaw for a moment. Material is typically case hardened steel - universally accepted standard since decades so I would not think this to be a topic anywhere. Testing with a file would help though.

Vic
 

Cyborg

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Not sure about the width of the splines on the Burman sprocket, but I measured a 750 Honda sprocket and the contact area of the splines is slightly less than 12 mm in width.
 

oexing

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Now you could add up all side contact faces of the slots to find a total for Honda and Burman or Vincent that has to bear all loads from chain pull. The Japanese splines look a lot deeper than the very flat type of the Burman - or Vincent as well. I suspect to see very high specific loads in kg per square millimeter or in imperial units if you like, in that old design, that takes all the wear and will be difficult for lubricants to prevent fretting. Still I think all you can do is to care for a perfect fit in the spline plus EP grease or MoS2 paste when assembling. Any extra high torque on the nut is wishful thinking. That is why there are designs with push fit and circlip , no nut and thread as not helpful at all.

Vic
 
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