Lubrication of linear guides

Posted by Emelie Neckman, 2024-08-20

In any kind of linear guide, you have surfaces—often metal—moving against each other. If they touch, you get friction. The roughness of these surfaces plays a big role in how much friction you get. Even ground surfaces have imperfections (called asperities), so nothing’s ever truly friction-free.

And all friction leads to heat and wear. That wear creates metal particles, which make things even worse. To extend the lifespan, you need something between those surfaces to reduce friction. That’s where the lubrication film comes in.

By applying oil between surfaces, you create a film that lifts the metal parts slightly apart—preventing those imperfections from catching on each other, and virtually eliminating wear.

At least three factors affect how well this works:

• Surface texture
• Oil viscosity
• Speed between the surfaces

Surface texture is critical. It has to be smooth enough to allow a proper oil film to form, and wide enough to maintain it. You can compare it to aquaplaning on tires. It’s easier with slick tires and even more so with wide ones. Here though, aquaplaning is what we want—but on an oil film!

Viscosity refers to how thick the oil is. Some are watery thin, others more like liquid honey. Aquaplaning works best when the surface speed is high enough—for instance, bald tires on water. But if you're on slushy snow, you don’t need much speed to start skimming. It’s the same idea here.

Speed—the faster the movement, the thicker the oil film becomes, simply because the oil doesn’t have time to squeeze out. When speed is high enough, the surfaces stop touching completely—and wear stops altogether.

If increasing speed isn’t an option, you can switch to a thicker oil.

In a ball bearing, friction is lowest when the lubrication film is just thick enough to keep the surfaces apart. Go too fast, though, and friction increases again because excess oil can’t get out fast enough. It’s like driving into a big puddle—resistance goes up.

What about grease?

Grease is just oil mixed with a thickener (soap) that helps the base oil stay in the bearing. But it’s still the oil’s viscosity that does the actual work of forming the lubrication film.

Bottom line: use oil or grease with the right base oil and viscosity for your product.

Here’s a video on the topic featuring Carry Lindholm from Bosch Rexroth.