We fitted them some years ago to replace ball bushings on a laser cutter that was plauged by dust and grit from the cutting process getting into the ball bushing.

The drylin ones cured all this.

Somewhere I have some linear tables, with rails and bushings, not A4 but the rails could be extended.

http://www.stevenson-engineers.co.uk/files/slide1.jpg

http://www.stevenson-engineers.co.uk/files/slide2.jpg

http://www.stevenson-engineers.co.uk/files/slide3.jpg