4-Point Contact or 2-Point Contact?

2024-04-29

When it comes to linear guides, there are two main principles for how the balls or rollers contact the raceways. It all comes down to how many contact surfaces exist between the ball and the raceways in the carriage and the rail.

In systems with two ball tracks, you usually get 4-point contact—two contact surfaces in the carriage and two in the rail, all pressing against the ball. It’s generally a cost-effective design approach, but it comes with a few drawbacks:

• Noticeably higher friction
• Under side loads, they can actually lock up due to the increased friction
• The balls often fall out if you remove the carriage from the rail (some 2-point contact versions do this too)
• If there are ball retainers to keep the balls in place, they're often made of thin wire requiring a groove in the rail, which can be hard to keep clean—possibly leading to failure
• More frequent lubrication is needed due to greater sliding between contact surfaces

In contrast, systems with four ball tracks tend to have 2-point contact—one contact surface in the carriage and one in the rail per ball. This design helps minimize many of the issues above:

• Lower friction thanks to less sliding
• In many cases, the balls stay in place even when the carriage is removed (especially in O-configurations)
• Better rail profiles that are easier to keep clean
• Less maintenance
• Usually a bit more expensive, but the total cost of ownership is better in the long run

And here’s a video on the topic with Carry Lindholm from Bosch Rexroth: