Fast forwarding to today’s modern crane, most use a large diameter bearing commonly referred to as a slewing ring as a means of rotating the main structure. Slewing rings comprise an inner and outer race, one of which generally has an integral gear. Typical slewing rings range in size from approximately 300mm to 6m in diameter although some have been manufactured to diameters exceeding 20m. A slewing ring rotates by means of balls or rollers contained within the races. There are several advantages to the slewing ring configuration over the king post style for use on cranes:
• Slewing rings need less axial space than a king post arrangement does.
• Open space in the centre of a slewing ring provides a means of accommodating crane components and simplifies the machine design.
• An integral gear is often machined into one of the slewing ring races rather than a separate bullgear as needed with the king post arrangement.
Slewing rings can be manufactured in a variety of styles with several different types of ball, roller and combination designs available. Cranes today, however, typically use slewing rings manufactured to one of the following three configurations.
This configuration is arguably the most common style of slewing ring for cranes as well as other applications rotating slowly and transmitting significant forces. A single row of balls rotate within deep grooves located in both the inner and outer races. This groove or raceway in each race is precision ground or machined to the shape of a Gothic arch. A Gothic arch, basically two radii within each race, allows the ball to contact each race in two spots. Two contact points on the inner and two contact points on the outer race equates to a four-point contact slewing ring.