iko blocks seem too stiff.

[bgolash]

Hi Group

I`m in the process of mounting my iko rails to my Y axis.
This is a 4 X 4 tube which has been blanchard ground. I`m
concerned that it is too difficult to move the sliding blocks.
I realize that the two rails must be parallel and coplanar.
It seems these ebay purchases have a considerable preload
designed into the sliding blocks. There is a good amount of
resistance even when one block is pushed. I mounted plywood
to both blocks on each rail and bolted them down. It is very
stiff now. I`m not certain a 600 OZ*IN stepper turning a
.200 pitch ball screw will have enough power to push the
load. Does anyone have experience with the iko that have
preload. I`m wondering if its normal to be so stiff.

LWHG 25 IS THE BASE NUMBER.
T2 MEANS MEDIUM PRELOAD
P IS PRECISION ACCURACY.

[jcc3inc]

Sir,

With your setup as described, you will have a max available drive force of 1179# available to drive the table. Assuming an efficiency of .8, this becomes 943#. You should have no problem driving the ways. As you probably realize, the stepper's torque decreases with increased stepping speed and is a function of power supply voltage AND the type stepper driver you use!

Regards,
Jack C.

[turmite]

bgolash,

How much weight do you have on you y axis? Depending on the amount of preload you will not be able to move an unloaded axis very easily. That is what I have discovered with some bearings I bought. These things are big enough to mount a car to, and with no wieght on them they are much more difficult to move. I can stand on one of them and it is like glass! Add some weight and see what happens...assuming you do have everything parallel.
Mike

[mwp]

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.

[mwp]

Whoops, I meant to say that a set of two non-preloaded guides would only have about 4.6 lbs of actuation force when unloaded. (or 2X the intrinsic resistance).