Load capacity in linear guides

2024-08-07

There are generally three types of loads that are referenced in the linear motion world. And let’s be honest – a lot of users don’t really know the difference between them. So we thought we’d try to break it down.

The three key values we usually talk about are:

  • Maximum load or maximum force
  • Dynamic load rating
  • Static load rating

Let’s start with maximum load or maximum force (Fmax)

Depending on the product type, this is essentially the point where the product breaks. It could be a weak link somewhere – like the balls cracking, the housing deforming, or even the carriage body failing. Sometimes it’s the mounting bolts that give in. In other words, it’s when something in the system physically fails.

Static load rating (C₀) is one that’s often misunderstood. Many assume it’s the max load a bearing can handle while stationary – but that’s not quite right. It’s all about the ball and its raceway.

When a ball is loaded, it flattens slightly – becoming somewhat egg-shaped. Once the load is removed, the ball springs back to its original shape. But if the load exceeds the static load rating (C₀), you get plastic deformation – meaning a tiny dent or flat spot remains on the ball. If that dent exceeds 0.0001 x the ball’s diameter, the industry standard says the bearing is considered damaged.

Dynamic load rating (C) is another one that’s commonly misunderstood. People often think it’s the max load a bearing can handle while in motion. But actually, it's just a number used to calculate a theoretical service life.

If you load a guide at its dynamic load rating (C), then you can expect 90% of:

  • Linear guides to last 100,000 meters
  • Ball screws to last 1,000,000 revolutions
  • Ball bearings to last 1,000,000 revolutions

Since most applications involve much lower loads, you usually get much longer service lives.

Unfortunately, it's not easy to compare different brands based on their load ratings. That’s because every manufacturer uses different test setups, methods, and safety factors – and those safety margins are usually kept secret. So direct brand-to-brand comparison is tricky.

What you can do, however, is use the ratings to compare different sizes within the same brand.

Check out this video on the topic featuring Carry Lindholm from Bosch Rexroth.