Slewing Ring



Inner and Outer Ring

The bearings are comprised of two rings, each of which contains a precision raceway that is induction hardened on the surface to a specified depth. Each ring is made from a medium carbon steel forging. Silverthin™ SK and ST Series bearings have raceways constructed in a four-point contact configuration, with 45° contact angles as standard. Other raceway configurations are available, such as crossed roller and eight-point contact. These other options are often considered in cases where very high loads or special stiffness requirements are needed.

Rolling Elements

Precision rolling elements (specifically hardened chrome alloy steel balls for Silverthin™ SK and ST Series bearings) facilitate relative ring rotation between the inner and outer rings. Rollers as rolling elements are also often used.

Spacers, Spacer Balls & Separators

Spacers, typically made from a plastic material, are commonly used to separate balls to prevent them from rubbing directly against each other. Sometimes spacer balls are utilized instead of spacers. In this case these are balls that are slightly smaller than the load carrying balls, placed between each load carrying ball. Occasionally, application parameters warrant the use of a separator.

Taper Pin

Secures load plug to the inner or outer ring.

Mounting Holes

Mounting holes are generally spaced around the faces of the inner and outer ring along a uniform bolt circle and equal spacing. These holes can be thru holes, tapped holes, blind tapped holes, counter-bored holes, etc. Sometimes bolt circle or spacing requirements need to be customized to meet the demands of a specific application, in which case custom options are available.

In the case of either the standard part numbers listed in this catalog, or other custom versions provided by Silverthin™, the equipment designer, manufacturer, or user is responsible to determine that the mounting design is adequate.


Seals are provided on each side of the bearing, with their primary functions being grease retention and prevention of some contamination such as dust and debris from entering the bearing. The seals are typically made from an extruded Buna-N rubber strip, and in smaller sizes are a metallic non-contact shield.

Load Plug

Due to the nature of the race hardening process, there is a gap in the race hardness of each race. In that location for the non-geared ring, or the outer ring for ungeared bearings, a hole is drilled to permit the loading of balls into the final slewing ring assembly. This hole is plugged and secured with a taper pin.

Note that for the ring where a load plug is not present, an ‘S’ is stamped on the face of that ring at the location of the unhardened gap in the race (also known as the soft spot). Removal of the load plug voids the warranty.

Taper Pin

Secures load plug to the inner or outer ring.


Slewing ring bearings can be supplied as ungeared, or with gears on either the ID of the inner ring or the OD of the outer ring. Gears are typically a standard stub involute spur gear with backlash provisions and AGMA Q8 quality minimum. Details for each bearing can be found in the enclosed dimensional tables, and drawings are available from Silverthin™ Engineering. Custom gear configurations are also available.

Grease Fitting

At least one grease fitting is included in one of the rings. The quantity may increase with the diameter of the bearing. For bearings with gears, the grease fitting(s) are located on either the ID or OD of the ungeared ring. For ungeared bearings, Silverthin™ ST and SK Series slewing rings come equipped to accommodate grease fittings on either the inner or outer ring. Custom quantities, locations and configurations for grease fittings are available.

Part Numbers

Part numbers and serial numbers are etched on one of the faces of the bearing.