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bushmechanic November 28th, 2013 10:35 PM

Definative main bearing thread
Johns thread is full of good info but i noticed the last few pages have some people wondering what to do.


the ideas here are not just my opinion, there are a great many good minds who have worked out what causes the failures and even better have tried a number of different solutions. the majority of these solutions are scattered through KTM talks RFS section.

what causes the oem roller bearings to fail ?

1) crank spread under its own weight is the main problem

2) misalignment of the bearing bore, seems rare, search lineaweavers posts

3) insufficient crank axial play (end float) with crank spread

what is crank spread?

when the crank spins, its own weight causes it to spread outward opposite the pin
when you add in the combustion forces they contribute further to the spread.

sorry for the crappy pic there is a better one i will add later

in this condition while running the axial play of the crank reduces until the forces deforming the crank outward are matched by the crank resisting deformation (a function of its stiffness)

if you don't have enough end float or stiffness for the weight of the crank and the rpm its doing your end float (axial play) will become zero

with zero axial play the NJ series roller bearings will experience side loads as they are now subjected to some of the force causing crank spread

bushmechanic November 28th, 2013 10:44 PM

NJ206 bearing design
first a pic of the failure

you can see there was about 1.5mm of the inner race unused and that the rollers were butted hard up against the lip of the inner race

schematic of the NJ bearing .. see the lip

an NTN bearing beside an SKF bearing note the SKF bearing has failed and the ends of the rollers on the SKF bearing have brocken off to the extent it almost looks like the same radius as the ntn

an SKF bearing note the sharp corners of the rollers compared to an NTN bearing

every single failure pic of these bearings shows the rollers were hard up against the inner race lip.

the NTN bearing has both nicer end radii and the lip itself has a bigger relief so that the contact with the rollers is closer to the centre

bushmechanic November 28th, 2013 10:50 PM

axial play (end float)
there are 2 bearings one each side so when you measure the axial play of the crank you are measuring the distance between the rollers and the lip.

(you have 2 NJ series lipped locating bearing inner races one either side of the crank)

I ran 1mm endfloat

this is my ignition side Nj206 (ntn) after about 30 hours in the 700 build

bushmechanic November 28th, 2013 11:11 PM

side loads
when the crank spreads and rams the rollers into lip on the bearing inner race they are subjected to side loading

this is OK up to a point but if its too much they will stop rotating and start to skid on the inner race ......

bushmechanic November 28th, 2013 11:15 PM

what do you do about it?

1) first thing you can do is make sure the endfloat is big enough opinions differ but I suggest 0.5 - 0.6mm minimum for all the stock engines but i used 1mm on my 700

2) after a failure or during a rebuild check the crank isn't spread too much; correct the alignment or add pinch (opposite of spread) and..

3) weld the pin.. viewtopic.php?t=12083&f=5

4) use NTN mainbearings instead of skf ones

5) make the crank lighter reducing the outward loads due to the cranks own mass

6) use a completely different style of main bearing

bushmechanic November 28th, 2013 11:23 PM

different setups
the first one that was popular was the use of a KTM crate engine deep groove ball bearing on the drive side and an NJ 206 on the ignition side.

many jap bikes use this idea but they fasten the ball bearing to the cases and also to the crank so the crank cannot move.

they also have about 2mm room between the rollers on the roller bearing and the lip or.. no lip at all so the rollers never contact the lip in these bikes

in the husaberg the ball bearing is just a stupdly tight fit in the cases and on the crank but the crank is not actually locked in position and can move.. you wouldn't know about it though till the roller bearing failed.

it is also very difficult to measure the space between the ends of the rollers and the nj roller bearings lip... you still need the same amount of room for the crank to expand because the axial loading is shared by the 2 bearings and the NJ bearing still gets squashed into the lip

it has worked for some people but i have seen it fail in 20 hours

bushmechanic November 28th, 2013 11:29 PM

bigger drive side main
weed removed the counterbalancer and ran a 20mm wide nj2206 c4 "rollway" roller on the drive side with an nj206 on the ignition side

i tried it also with good results, need to make a spacer to replace the counterbalancer

bushmechanic November 28th, 2013 11:38 PM

Other setups ACs
for a long time the accepted solution in the rfs engine was to use 2 Koyo or Nachi 6206 C3 ball bearings with a slip fit on the crank, not many people had any issues with these setups up to 570cc

there is also the possibility of using preloaded angular contact ball bearings like ducatis

as far as i know this has not been tried in a husaberg...

the issue is that the side loads normally contributing to crank spread are absorbed by the cases and IMHO the LHS case is not strong enough.

if you made a steel sleeve to better distribute the side load and reduced the weight of the crank to reduce the side load itself it would be worth trying simply because it prevents crank spread occurring at all. the code for the ACs is 7206B in abec7 watch the contact angle.. they are made in 2 different angles

some people have had good success with them in the rfs engines, others have broken their cases

bushmechanic November 28th, 2013 11:41 PM

Re: Definative main bearing thread
the ultimate setup IMHO is that tried by JBS and is a set of 22206 spherical rollers with pressure oil feed mounted in steel sleeves in the cases.

spherical rollers are cool because they can do this

I have one on the drive side with an nj206 on the ignition side and 0.8mm axial play..

bushmechanic November 28th, 2013 11:47 PM

why the 550 ?
the majority of failures in Australia were the 550 engine

the crank is heavier than the 628 crank and also has a smaller pin, the engine revs more as well

so compared to the 628 you have more forces spreading the crank, less stiffness to limit the spread, typically less endfloat and SKF rollers with sharp radii

Orangeberg worked out the NTNs work better and stands by them as a fix.

this is also the consensus among the RFS engine builders, in fact the NTN bearings were used in the RFS engines as an OEM part for some years so they get to see the difference on a larger scale

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