- Oct 24, 2007
- Reaction score
- Central Texas
Kimmo, et. al.Hello,Terry
Could you explane why torque goes through the sleeve? I´d thought the flats would take the torque.
Thanks for the interest and comments.
What happened in my tests is that one half of the joint was held fixed in a vise while the other half was twisted until something broke. What broke in all my cases was that the facing flats of two halves opened up and applied enough pressure to the sleeves to break them. If the sleeves had been thick enough, one of the shafts in the joint would have sheared off instead which would have been the failure mode of a solid crankshaft.
Keep in mind the Loctite isn't gluing the flats together. Its purpose is to fill in any gaps inside the sleeve (all test parts were machined for a one thou fit-up except at their ends) so the Loctite's compressive strength can eliminate any relative movement between the parts inside the sleeve after curing. It's other purpose is to keep the sleeve from spinning over the joint, and its more than adequate sheer strength keeps this from happening.
As a thought experiment, if you ignore the need for spun bearing protection, and visualize an essential zero gap fit-up, the Loctite can actually be left out. Then the net loss in strength by these joints is to reduce the shear strength of the shaft since the diameter of the shaft was reduced to make room for the sleeve, and then the cross-section of what was left was milled in half to give the two flats.
The shaft diameter starts out at 5/8" and tests with the 1/16" wall thickness sleeves showed the joint can still handle up to 90 ft-lbs which is stratospheric for a model engine. The three inner bearings will hold the three sleeves in place and limit their radial motion during operation, and so flex to deteriorate the cured Loctite (which is designed for much sloppier fit-ups) should be minimal. The advantage of using splices at these three points is that the Loctite can cure and fill in any gaps while the crank is laying dead straight in its alignment fixture. - Terry