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

Martyn Goodwin

Well Known and Active Forum User
Non-VOC Member
Hi Martyn - I have gone through 2 Burman gearbox sprockets , just as you describe ( I have done 40,000 miles on the Comet) - both times have left me at opposite ends of the country (Ireland) .Fortunately it hasn't happened on any of our longer trips abroad! . Anyway, I am preparing the Comet for this years international , and I was wondering if there is any way to check the sprocket for "rocking" on the splines without having to strip out the primary side ? The most recent sprocket was a very tight fit , and has about 2,000 miles on it. I cant see any way to avoid taking the primary side off, but its worth asking.
Hi Pete, Problem as you have found is that the drive sprocket PR50-22A is behind the inner primary case T4/2. If you are very agile and flexible (and that is NOT me) you may be able to reach in to that area with a long flat blade screw driver to see if there is any discernible side to side wobble. Come to think of it, if you drained the fuel and oil , using suitable cushioning you could lay the bike on its side to make it a bit easier to gain access - I have NOT tried to do this - just a thought.

I now carry as part of my standard touring spares a new drive sprocket and a new driving gear lock washer. In the interest of preventative maintenance I fit a brand new lock washer every time I replace the sprocket - cheap insurance.
 

Matty

Well Known and Active Forum User
VOC Member
Hi
This still seems to be a problem for some, but I have never had a problem with the sprocket splines on my Comet in the 61 years I have owned it.
I replaced the worn sprocket about 30 years ago and have done a lot of work on the gearbox since for other reasons but still have this second sprocket on the box after about 50,000 miles and the splines still looked good the last time I had the gearbox apart around 2 years ago.
I wrote my findings on this post about a year ago and hope that all is still well because it is a lot of work to take the primary driveside of the bike apart just to find out if any slack has developed.
Will ride up to Liverpool from Essex for the Classic TT/VMCC Manx rally in August for about the 40th time and hope it all holds together as usual !!!
Happy New Year
Matty
 

oexing

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VOC Member
Someone asked about use of Loctite on the sprocket fit, but no experience on this so far? I am a big supporter of Loctite but cannot tell if this application may be great. But then, in case of a loose fit I´d try this anyway - after a good degrease and 638 on all faces . Either this works - great - or it does not . You will find out after some mileage and this "bodge" would not do any harm while trying. If Loctite shows to be suitable you will some day want to have a look into the gearbox eventually. Now you will have to deal with high strength Loctite. So before using the Loctite method the question is: Can you get a good puller onto the sprocket for extracting it ?? I cannot tell, not my type of bike. Just thinking . . .
But then, a torch for heating the sprocket to 300 degrees C would do and the sprocket would be scrap anyway due to poor fit.

Vic
 
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ray vinmad

VOC Drawings Manager
VOC Member
A spline is basically a multiple key set up to overcome higher loads. For that to work & transfer the greater loads, all the castellations must accurately match the slots & due to that necessary accuracy is only likely to occur on a `new' set up without any wear. With high reciprocating loads, if only two spline flanks are making proper contact it will eventually fail enough for another flank to start making contact in one direction & a different flank helping with the loads in the opposite direction and so on until they are all sloppy. Unless the general flank fits are all good to start with, a tight nut will not do much once fretting starts. Tightening/over tightening the nut will only help maintain a `good' fit. The nut is there to create end friction on the items unless it squeezes so hard it distorts the splines!
Probably Loctite is the only answer for serious mileages.
Having said that many modern bikes have the sprocket retained by a circlip and is a sloppy fit to start with.

Ray
 

Martyn Goodwin

Well Known and Active Forum User
Non-VOC Member
Thanks Martyn..When you type the part No you gave into the search box it comes up. When I looked at the parts diagram for BA boxes the output gears were shown but not listed in blue, where stock levels and price are found, which is why I assumed they were not in stock..
Almost 2 years and 15,000 miles later I have just discovered the burman gearbox drive sprocket was loose AGAIN ! But when I got into it things are much worse than before. This time the nut was secure (I needed to use my impact driver to remove it) but again the sprocket was sloppy and unlike before, the splines on the output shaft - being part of PR50-15BA are damaged. See photo.

No stock in the Spares Co but I am hopeful that Draganfly have a used one in stock. I am waiting to hear back from them.

The engaging dogs on the driven sprocket , like last time, are also damaged but good fortune is I have one of those in my box of spares.

I wonder if there is a way I can reclaim / repair the damage to PR50-15BA? Has anyone made this repair with success?


Damaged PR50-15BA.jpg
 

erik

Well Known and Active Forum User
VOC Member
May be I`m wrong ,but I think the load of the sprocket is not mainly transfered by the splines ,it is transfered by the pressure of the nut to the sprocket and over the inner ring of the bearing to the shoulder of the 4th gear shaft idler.Sometimes it could happen that the nut can only screwed down to the end of the threads and not to the face of the sprocket.The sprocket is only fixed by the splines and they will wear.What do you think of this ?Erik
 

Nulli Secundus

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VOC Member
VOC Forum Moderator
That is exactly what I was thinking Erik.

I had that problem with Talon replacement sprockets for my Rudge race bike. Original sprockets were thicker at the spline area, so the best I could do with limited facilities was fit a washer between the nut and sprocket.
 

bmetcalf

Well Known and Active Forum User
VOC Member
A prof in Mechanical Engineering told us in class that a car flywheel is attached to the crankshaft flange not by the bolts in shear, but by the friction produced by the bolts' tension. Otherwise, the precision of the drillings required for equal shear loads on the bolts would be uneconomical. That concept makes sense here, too.
 

Simon Dinsdale

VOC Machine Registrar
VOC Member
VOC Forum Moderator
That is exactly what I was thinking Erik.

I had that problem with Talon replacement sprockets for my Rudge race bike. Original sprockets were thicker at the spline area, so the best I could do with limited facilities was fit a washer between the nut and sprocket.
I was thinking the same as well. Is the nut becoming threadbound before the full pressure of the nut side face is on the sprocket so the nut appears to be fully tight? If you do put a washer in there, then a hardened steel one would be better than mild steel otherwise the washer may creap over time.

Simon
 

robin

Well Known and Active Forum User
VOC Member
Nobody has mentioned PR50-23BA the sprocket spacer. This is a mild steel item which compresses and then allows end float of the sprocket leading to sprocket chatter. Result-apparently a loose nut. PR50-23 BA should be 15/32" wide.
Robin.
 
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