If you have already dealt with the point and linear contact of rolling bearings, you will recognise a few things. As with rolling bearings, the rolling elements of linear guide systems have either a ball or roller shape. The contact with a raceway (rolling contact) is different for balls and rollers due to their round or elongated shape. With rolling contact, as with rolling bearings, a distinction is made in linear motion between point, surface and line contact.
Point contact
Point contact is characterised by a small contact surface between balls as rolling elements and a flat or strongly convexly curved surface, whereby this contact surface has a circular structure. It therefore occurs in linear guide systems which have no profiling. This applies, for example, to ball bushings or ball sleeves, where the balls roll on a large shaft diameter. The advantages of point contact are minimized friction and a comparatively low lubricant requirement. However, the logical consequence of the small rolling contact is a high surface pressure compared to surface and line contact with the same load. For this reason, applications with point contact can only accommodate relatively small loads with comparatively low rigidity. Point contact is the conventional type of rolling contact for balls as rolling elements.
Surface contact
In abstract terms, surface contact is a modified, large point contact and represents an alternative to this. With the aim of increasing the contact surface, the raceways of certain linear guide systems, for example linear guides, are manufactured with a specific radius. The ratio of the ball diameter to the radius of the raceway is called osculation. In other words, raceways with a defined osculation are used instead of flat raceways so that the ball makes flat contact with the raceway. The osculation usually used results in a contact surface that is 13 times larger compared to point contact, which means that 13 times more load can be applied to each ball. In addition to the higher load capacity, further advantages compared to point contact are the higher rigidity of the balls and the reduction in surface pressure. Last but not least, the improved force distribution with surface contact ensures a longer service life for the rolling elements and the linear guide system itself.
Line contact
The name says it all: unlike point and surface contact, the contact surface of line contact is linear. Line contact occurs in all linear guide systems with cylindrical rolling elements, for example in roller guides, recirculating roller bearing units and flat cage guide systems. Line contact is characterised by its ability to absorb higher loads than point and surface contact. It is also characterised by greater rigidity, in this case you can imagine the rolling element as a stiff spring: The roller guides have a significantly higher rigidity than the ball guides. At the same time, the load is distributed over a larger area with line contact, which is why the surface pressure is lower than with point or surface contact for the same load.
Roller profiles
The profile of the rollers can be described as cylindrical. Although the rollers are crowned to a very small extent, the radius is so large that this is hardly significant. Because roller guides – unlike roller bearings – do not have a fixed cage that prevents the rollers from tilting, this crowned shape is used here to minimise the effect of edge pressure.
In addition to rigidity, higher load ratings are the decisive reason for choosing a linear guide system with cylindrical rolling elements. The disadvantage of these rolling elements compared to balls is a lower permissible travel speed and significantly lower permissible mounting tolerances of the linear guide systems.
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