[bojan]

I do not have noticeable problem with slippage. The weight of the tube itself and the pressure of idler on the vertical drive - two strong springs takes care of this - are enough to prevent any such problems.
Also, the use of lubricant on those surfaces is not advised. I think that deliberate contamination with fine but hard sand (like the one used for mirror grinding or sand paper) reduces the possibility of slippage to the minimum (BTW, the so called grit wheel is/was widely used in some designs of printers and pen plotters.. with absolutely no slippage between paper and driving mechanism)
As I started to use the system, I developed couple of procedures I exercise routinely as a precaution because I have noticed that if I slew to and from couple of times between two objects far away from each other the scope does not end exactly in the same position(s) - somebody (Rod, I think) called this micro-slippage .
However, this is is not the major problem as I do slewing only to find the hard-to-find targets and I always start from nearby known object, where I reset the scope position (that means, if my scope is pointing to a bright star for example, I find it in the database and tell the program "you are here now"). Then I GoTo to the object of interest, if I do not know how to find it by other means (star-hopping)
If I move the scope manually for example to the opposite direction in the sky to watch something else, encoders are taking care that the new position is updated with sufficient accuracy for visual observation. Currently, I have them friction-coupled as well, but I am thinking of using timing belt (no slippage here)
I do not use my bartels system for deep sky AP (only for planetary and Moon... for DS I use EQ6), because I (still) do not have field rotator (supported by the software). But I am sure it can be used successfully, with guiding of course. One day.... maybe.