Hi, I just did a few calculations using some data for similar (as in drop-in replacement) linear guides. Maybe this will help shed some light on your problem.
According to the Thomson catalog the actuation force is given by a frictional resistance+intrinsic resistance+intertia forces+viscous lubricant drag
At this point, only the first two are of concern.
Frictional resistance is given by the coefficient of friction (typically .003 max) times the guide internal forces. Since the guides are unloaded in this case, Thomson suggests using 2 times the preload as the guide internal force. Assuming the preload is .10 c and c is around 5100 lbf, this gives a preload of ~510lbf for long length 25mm guides. Multiplying out gives a frictional resistance of ~3.0 lbf, per guide.
The value for intrinsic resistance is given to be 10N or 2.3 lbf for a long length 25mm block, each.
So, overall you should expect 2 X 3lbf+2 X 2.3lbf=~10.5 lbf resistance for 2 long length guides (this sounds like what you are using but I'm not totally sure from your post). If you are expecting a smooth, frictionless "ball bearing" feel, this is a lot of force! In contrast, 2 of the same non preloaded guides would have nearly zero actuation force in the unloaded condition.
Nevertheless, I think that moderate preload is a good thing, since it increases the stiffness of your machine to a degree (stiffness vs preload charts are in the Thomson catalog as well). Also, the additional force accounts for a reasonably small fraction of your available axis actuation force.
Good luck.  |