In today’s session of “Mech Minutes”, we continue our deep dive into the world of linear guides with a look at the types of contact inside the bearings.
Let’s start by reviewing the two main contact types in linear guides. The contact between the ball and rail in a linear guide can be classified as either 2 point contact (commonly called circular arc) or 4 point contact (or gothic arch). When using 2 point contact, 4 raceways, or ball paths, are required to handle loading in all directions.
In contrast, gothic arch 4 point contact only requires 2 raceways. This allows for a more compact linear guide and can be less expensive to manufacture. To understand the downside of gothic arch, we need to look closer at how the balls actually roll in the raceway. As the ball rolls, its surface speed depends on the distance from the axis of rotation. The larger the diameter, the larger the linear speed per rotation. When looking at the contact area, the diameter near the center of the ball (d1) is much larger than the diameter further out (d2) This is very similar to trying to roll a cup in a straight line. It always wants to turn towards the smaller end.
To roll in a straight line, one end must slip or slide. This is called differential slip. In the circular arc contact, the difference in diameters across the contact area is very small. This causes less differential slip and allows for smoother motion with less friction. Another factor affecting the performance of linear guides is the contact angle of the rolling element and the bearing raceway. The contact angle is measured from a horizontal axis. Most linear guides use a 45-degree contact angle on every raceway.
This allows the linear guide to handle loading in all of the primary directions equally (radial, reverse radial, and lateral). Some linear guides are designed with different contact angles to handle loading in a specific direction. For example, as the contact angle for the top raceways increase, the radial load capacity increases as well. However, the load rating for lateral loads would decrease at the same time.
Watch below for the full video!