JPG wrote:Depends upon who you ask!
No question a double bearing quill is better. Question is when is that slight improvement needed? Also where does the second bearing go?
All a second bearing in the Mark V does is minimize the wobble effect of the splined shaft in the nylon coupling.
Two opposing design issues involved here.
The prime objective is to minimize output shaft runout(wobble).
Another important objective is to allow the shaft to extend and be driven by the drive sleeve. In a perfect(or $$$$) world the splined shaft will be perfectly straight, have no play in the splines/coupler junction and ditto for the coupler/drive sleeve junction. Oh also no play in the bearing(s).
Alas our world is neither perfect nor are we suffocating from all That $$.
If we be making a milling machine(gross side thrust) the added bearing(s) need to be near the output end of the shaft and any slop at the other end becomes less relevant. A large turning on a lathe is the same scenario.
However our bearings MUST have some clearance or friction becomes a problem. That is obvious from the little top hat in the single bearing Mark V/5 that centers the splined shaft in the quill body so as to make insertion of the far end of the shaft into the drive sleeve coupler easier.
So where to put the second bearing? Too far from the first bearing at the output end and shaft straightness becomes more critical. The further from the first bearing does indeed decrease runout.
It is a bunch of tradeoffs both from a goal standpoint and a manufacturability standpoint.
The fact that there have been design iterations illustrates it ain't easy/simple!
There is no BEST answer. Only better from different perspectives.
You are correct as usual. It is called a stack up in engineering terms. All the tolerances of all the components must add up to have the result you want on the output of the device. That does not necessarily mean that if you want a one thou run out tolerance on the output spindle that all the components of the spindle (bearings, bearing seats, concentricity, etc.) are all one thou tolerance. It depends on the + or - deviation allowed for the part. So the deviation allowance must be considered in the end item. Stack up.
As JPG says it is complicated but that is why we pay engineers the big bucks.