The idler gears are causing the woodruff key failures.
Posted July 05, 2003 - 01:59 PM
Yamaha of Japan has a new design for the idler gears that allow them to slip at a lower torque so that during a kickback condition the flywheel will not be mechanically locked on the starter.
Yamaha USA either does not know about it, or they are playing dumb. Check out the posts at http://lists.off-roa.../listinfo/wr450 for proof.
Posted July 05, 2003 - 02:36 PM
Posted July 05, 2003 - 04:16 PM
I bet if anyone removed his idler gears from the front of the starter, they will not have another woodruff key failure.
Posted July 06, 2003 - 09:06 AM
It is not the gears at the front of the starter which resist the clockwise motion of the crankshft in the event of a backfire it is the starter clutch gear situated on the crankshaft directly behind the flywheel.
Regarding this backfire theory, when the woodruff key went on mine at 11km the bike backfired when it failed to start after a stall but IMHO this backfire occured post key shearing as the timing slipped. On strip down the sheared key was not a good fit in the flywheel or the crank.
I carried out the lapping, loctite mod and manufactured a better fitting key. Have done 130km since without any problems. Checked nut torque today - didn't budge.
Why no backfire to cause problem now?
Posted July 06, 2003 - 05:33 PM
Posted July 20, 2003 - 04:37 PM
I tested the torque slip and sticktion of gears by holding the larger gear
in a bench vice with a 2mm thick peace of copper rapped around it. I grabbed
the smaller gear the same way with some large set of hand held vice grips
and attached a set of fish scales 150mm from the center of the gears. I then
pulled on the fish scales and noted how many Kgs it took to move the gears
and calculated the Nm.
The first time I tested the old gears it took maybe 50Nm to move them.
Testing them several time straight after that repeated 40Nm. When I tested
the new gears they slipped every time at 27Nm. It seemed to me that the old
gears had probably never slipped since they were made as they a lot of
sticktion when I first moved them. With the new gears I marked them before I
installed them so I new they had slipped recently before I tested them.
I understand sticktion is an effect that happens with friction between
movement parts and is always there to some degree. It takes more force to
first overcome the friction than it takes to keep things moving. Also the
amount of sticktion varies a lot with materials and time between
movements..... but it sound like yours are seized together.
Posted July 20, 2003 - 06:58 PM