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Re: Is the little captive ball at the end of a Thorens TD125 MkI spindle supposed to roll? As in...

Many TTs have been designed such that the veritical support of the axial (rotation) bearing is provided by simply placing a hardened ball of some kind in the bottom of the well in which in the shaft sits. My old R-O-K K-33-H had a steel ball you just droped down the hole, put in some oil, and then inserted the TT shaft/axle. There are some intrinsic problems with this.

First, the TT shaft has a rotation axis defined by its bearings. Generally this is well defined, because you don't want the TT to wander or wobble as it rotates. Unless the well in which tht ball sits is made to very high tolerance or has some very clever self-centering mechanism, it is likely that the ball's center will not lie exactly along the shaft's rotation axis. This means that the high point of the ball will not rest on the bottom of the shaft where the axis passes through it, but slightly off center. This means that the point of contact will not stay fixed on the end of the shaft, but will slide around in a small circle. I would assume that even if the ball could roll, forces would be such that the ball would stay fixed and the bottom of the shaft would slip on the ball as the contact point describes a small circle, but the ball could roll if it weren't fixed in place. How bad this is depends on the smoothness of that sliding contact, but I would think the ideal way thing would be to avoid it and have the shaft rotation axis pass exactly through the ball center.

One way to achieve what I say above is to attach the ball (or portion there-of to the bottom of the shaft and carefully machine the shaft or make things so that the ball is centered very accurately on the shaft. Then the the shat is supported by a flat plate at the bottom of the shaft well, and that part is much simpler. I have come up with some other designs for thrust bearings should I ever get ther urge to build a TT, but I think I would stay away from a simple ball dropped down a well for the reasons above. How bad this actually is in practice is another matter. I have some magnetic thrust bearing designs as well, but I haven't fully explored the eddy current questions.

Joe


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