Stiff THK Bearings?

[bigz1]

I have bought 2 matching THK rails of ebay with 4 carriages. I was supprised to note that the carriages exhibited varying degrees of force in order to move them . One even runs a bit clunky. Is this normal?

[NC Cams]

What's normal???

Depends it it was disassembled.

Depends if it was disassembled/reassembled properly.

Depends on the grease (some greases at room temp in preloaded bearings feel like you're running over marbles - this can be due to the thickening agent which can conjeal and yield ruff/clunky motion).

Depends if the reassembly/assembly was botched and they brinnelled the bearings or ball paths in raceways.

Some of the above are normal and accptable, some are not. Can't tell which form which from the available input.

Ahhh, the fun of E-bay shopping....
Depends if the carriages are preloade or not, are they???

[bigz1]

All 4 carriages are the HSR25 type( although 2 have silver bodies and the other 2 are black). I have tried runnning the bearing on the both rails with the same result. One of the black carriages runs very smooth with .75lbs of force while the other runs rough and requires 4.75lbs(This is very rough using kitchen scales). The other silver bodied carriages run smooth but one requires 2.5lbs and the other 4lbs.

From a visual inspection all seem to have used the same grease. I thought this type was already preloaded and is their any easy way to fix the friction problem, excluding brinneling?

I only intend using the best 2 anyway but was just curious.

I appreciate the help thanks.

[NC Cams]

Be careful how you use the word "STIFF" when you talk bearings.

Stiffness (as in deflection resistance) is NOT the same as operating friction.

A highly preloaded carriage will have much more stiffness and usually it has more rolling resistance.

Sort of like press fit. A slip fit will have slop and allow easy movement - a press fit will be RIGID and movement will be much more difficult.

You can probably have the "stiff" (your words) carriages reloaded professionally with slightly smaller balls to make them easier to move -they'll be less rigid (my word) and more subject to deflect under load.

It all depends what you're looking for/to do....

[bigz1]

Many Thanks.

I can appreciate using the right definitions. I work in an industry(wood working) where when talking about timber virtually everything is refered to as 'grain'. But everyone has a different defintion for it!!!