WR450 Serial Numbers, Flywheel torque, key failure
Posted February 12, 2003 - 05:18 PM
Posted February 12, 2003 - 05:20 PM
Posted February 13, 2003 - 01:03 AM
Posted February 13, 2003 - 03:58 AM
Posted February 13, 2003 - 04:25 AM
Your are correct. I mis-spoke, I should have said the force will be greater using a lube. Thanks
Posted February 13, 2003 - 04:42 AM
any reports of this happening to a yz?
Posted February 13, 2003 - 04:44 AM
...force will be greater using a lube.
That's what she said! Ahem..uh, sorry.
Posted February 13, 2003 - 05:13 AM
Posted February 13, 2003 - 06:29 AM
Torque is the most often used means to determine the amount of stretch a bolt or shaft undergoes to obtain the optimum holding force for whatever mechanism it is used for - because its easy. However, for certain kinds of critical equipment, torque is not used, measurement of bolt stretch is used because it is more accurate - torque readings can vary in accuracy depending on thread lubrication, corrosion, machined thread finish, washer types, etc. It may be that over-torqueing of the shaft nut is causing some flywheel taper fit distortion (I think Dutch is right). Yamaha would certainly recommend the key being replaced because it could have been stress-damaged by a possible poor-fitting flywheel. Lapping, or at least spot-checking would likely be necessary for any bike that had spun a flywheel because of the potential for shaft-fit damage. The poster who lapped his flywheel had a spin-failure as I recall (let me know if I'm wrong).
The real test of reliability (in my mind) is whether any of the retorqued bikes have experienced any failures.
Posted February 13, 2003 - 07:38 AM
Significant under torque on tubes/nuts or a flywheel/shaft is a failure mode for sure. Overt torque to a moderate level on either system should not be a problem, neither material low carbon thin tubing, or much thicker more robust masses such as the tapered shaft and the flywheel mating surface are not going to distort or stretch so much as to create any other problems with the flywheel/shaft relationship. Over torque to a very high level, and threads can slip/strip, the shaft could stretch causing misalignment of the tapered surfaces, though I feel this would require very high torque values. If threads on the nut or shaft are messed up enough two things can happen. If you are using a torque wrench (clicker type) you can under torque it if the bolt to shaft interference is greater that your torque value. If interference is less, the torque wrench will drive if home to the desired value, no more.
If you are using an air driven wrench any interference greater than your torque value you will under torque. Clicker wrenches and air wrenches set to the correct value cannot over torque regardless of how easily the nut/shaft threads can turn. It’s true that as you wrench up to torque you are pulling hopefully mating surfaces together. At some point the surfaces will not get any tighter as you continually increase torque, you will strip threads and or stretch the straight portion of the shaft. I plan to lap mine to fit the tapers if it isn’t a good surface to surface fit. I will torque to 47 minimum, and if I can find information on the shaft and nut that show they could be torqued higher with out damaging the threads I will torque higher by about 25 % of the difference.
Posted February 13, 2003 - 09:03 AM
For a taper-fit to hold, very little torque is needed. As an amateur machinist, I know that inserting a morse-taper tool-holder into a sleeve - and having it hold a drill chuck tightly (for example) only requires hand pressure. So, if the taper is correct on both the crankshaft and flywheel, and there are no burrs or other anomalies to prevent proper contact, the flywheel should "bond" to the shaft with very little torque on the shaft nut.(probable reasons for higher torque is to handle thermal expansion and the effects of sudden speed and rotational changes found in an engine) If you were to see a chart on bolt stretch vs. torque for a small diameter bolt such as 1/2 inch you would be amazed at how much design stretch in built into fasteners. It is not inconceivable that a shaft nut tightened to higher torque (as someone mentioned, via an air gun) causes the excessive stretch on a portion of the shaft that would cause improper fit. Imagine the tapered shaft as a progressive series of ever-larger diameter "bolts". The larger diameter area would resist stretch more than the smaller diameter - and thus the possible distortion of the taper. This may not be what has occurred, but it is a real possiblity to explore - by keeping track of re-torqued bike success or failure. The distortion may have even occured in other earlier models to some degree, but the combination of incorrect torque, changed flywheel design, and backfire/starter issues may have brought a formerly inconsequential effect to this problem level (for some bikes).
But then again, it may all be flatulence. Hey, I'm in the middle of doing my income taxes and I don't know what's going on anymore!
Posted February 14, 2003 - 09:27 AM
Once the two tapered surfaces are line to line it's very difficult to distort those surfaces any more since in our case the masses that we have in the shaft and flywheel will force the threads to distort and/or the unsupported shaft to stretch. Therefore the threaded shaft which is smaller than the smallest taper diameter will stretch especially since it has nothing constraining it, or as mentioned the threads will distort. Therefore some extra torque is ok as long as it does not weaken the threads or unsupported shaft.
Posted February 15, 2003 - 05:25 AM
There is one other aspect to reusing fasteners that is interesting in this application. Bolt breakage due to "stretch fatigue" is not the only failure mode.
A few years ago there was an investigation into a helicopter crash. When the NTSB looked at the rotor, they found that some fasteners had backed out, the rotor came apart and somebody got hurt. The root cause of the crash was identified as something like "reuse of safety critical fasteners".
Seems that when the bolts / nuts are fastened and torqued for the first time, the mating surfaces sort of bed into each other. And the back off torque (force required to disassemble) is usually about 95% of the original torquing value. When they torqued the bolts / nuts through a second and third cycle, back off torque dropped to 80% then 65%, but the 5th cycle the backoff torque was under 50% of the original torque value. There was a continual deformation of mating surfaces.
This is something we all need to be aware of - motorcycles are a lot like aircraft, one small detail gone wrong can kill you.
Posted February 15, 2003 - 07:13 AM
Posted February 15, 2003 - 07:11 PM