Providing rotary motion between two large, heavily loaded structures has been an issue confronting mechanical engineers for years. One of the first solutions was a wooden bearing which led to various combinations of wood and metal designs. Bearing design evolved to a configuration known as the king post which essentially is a large vertical shaft with radial or tapered roller bearings mounted to each end in order to absorb combination loads in the radial and thrust directions as well as any overturning or tilting loads.
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.
Four-point contact ball
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.
The angle between the aforementioned contact points and the horizontal centreline of the ball is referred to as the contact angle. Slewing ring designers select a contact angle based on the application. An application such as a gun mount, for example, with significant radial loads due to the recoil from firing, will commonly have a low contact angle, usually in the 35° to 45° range. A crane, with significant thrust and overturning moment loads, generally will have higher contact angles, of between 50° and 60°.
Four-point contact ball slewing rings are among the most lenient of configurations when mounted to inadequate mounting structures. Out-of-flatness and lack of rigidity, within reasonable amounts, generally are more readily accepted by four-point contact styles. Since the slewing ring is designed with ‘point’ contact, the points or contact angle move to accommodate an inadequacy. While not an ideal situation from a slewing ring manufacturer’s standpoint, such forgiveness can be an advantage to the crane manufacturer with respect to cost and the mounting structure.
Cross roller
Slewing rings of the cross roller type depend on cylindrical rollers to transmit combination loading. Rather than a Gothic arch in a four point contact ball style, a cross roller has a ‘V-way’ machined into each race. Rollers are typically inclined 45° to the bearing centreline in alternating fashion (first roller inclined to the left, second one perpendicular to the first and inclined to the right, etc.). In this arrangement, referred to as a one-to-one configuration, half the rollers transmit forces in one direction and half in the other. Applications with thrust loads primarily in one direction call for rollers orientated in a two-to-one or three-to-one configuration to increase thrust capacity.
For a given size, a cross roller slewing ring has slightly less static capacity than a four-point contact ball configuration. The roller design, however, provides improved dynamic capacity over the four-point contact ball style. With the roller’s ‘line’ contact versus the ball’s point contact, the loading is spread over a larger area of the raceway. Since a lower number of rolling elements are loaded and passing over a specific point, the number of load cycles is reduced per revolution.
Cross roller slewing rings require a flatter and more rigid mounting structure than their four-point contact ball counterpart. Due to the line contact and inability of the contact angle to move appreciably, a cross roller style is less forgiving to mounting structure inadequacies.
Three-row roller
Slewing rings with three rows of rollers usually comprise three races and three sets of rollers. The non-geared race is generally composed of two races that are separated so that the slewing ring can be assembled. The geared race is commonly a single race configuration. Two rows of cylindrical rollers with a horizontal centreline of rotation handle the thrust and overturning moment loads. One row of cylindrical rollers, perpendicular to the thrust and moment load rows, transmit the radial loads.
With this roller arrangement, a three-row roller slewing ring has more static and dynamic capacity than the other two configurations. Because the rollers are perpendicular to each other, it offers higher stiffness as well.
Due to the extreme capacities of a three-row roller slewing ring, it is vital to have a mounting structure with adequate stiffness to assure uniform load distribution. Additionally, this style of bearing is the least accommodating to out-of-flatness inherent within a given mounting structure.
Common features
Virtually all slewing rings are made from a high carbon steel which is usually through-hardened to a level dictated by the application. After initial machining, the slewing ring raceways are induction hardened to obtain the strength to handle the high compressive loads between the rolling elements and raceways. After hardening, the raceways are precision ground or machined to achieve the proper geometry, thereby maximising the load carrying capability.
Slewing rings commonly have a gear machined into either the inner or the outer race. In order to increase wear resistance and strength, gearing can be induction hardened.
The rolling elements in slewing rings are through hardened to a level slightly higher than the raceways. In most designs, spacers made from high strength nylon are used to separate the rolling elements. Depending on the application, separators made from metal are also used.
Most slewing rings are equipped with lip seals to keep out contaminants and to keep fresh lubrication inside. Special seal configurations and materials are often needed where the slewing ring is subjected to harsh environmental conditions.
Which style is most suitable?
The most suitable slewing ring style for a given application is based on several factors. Naturally, the applied loads are a major factor in designing or selecting a slewing ring for a crane. Duty cycles are another characteristic to consider. The designer needs to determine, for example, whether a crane is going to make lifts only intermittently, or frequently as with with yard and scrap handling cranes.
Once loading and operational data have been identified, it is common to establish dimensional preferences and limitations of the crane manufacturer. Based on loading data, for example, the slewing ring manufacturer may decide that a 5m diameter, four-point contact ball style meets the performance criteria. Perhaps from a cost or design standpoint, the crane manufacturer may not be able to accommodate a 5m slewing ring, preferring to limit the diameter to 3m. At this point, the slewing ring designer considers alternative designs, such as a three-row roller bearing, to achieve the necessary capacity yet meet the dimensional criteria of the crane manufacturer.
Once the bearing style has been identified along with the approximate diameter, preferred mounting configurations, typically relating to mounting bolts and access, are usually discussed. Likewise, gear location (inner or outer race) along with available torque and number of drives is another design characteristic covered. Finally, environmental issues are considered. Special grades of steel are commonly used in offshore applications where shock loads in cold temperatures are typical. Seals are also important in particularly harsh environmental situations.
The design process for the slewing ring manufacturer is complicated. While three slewing ring configurations (four-point contact ball, cross roller and three-row roller) were discussed, there are many other styles used in cranes, although they are less common. Performance criteria, dimensional limitations and other factors require the slewing ring manufacturer to design alternative styles such as two-row angular contact ball, combination single-row ball and roller, two-row cross roller, two-row eight-point contact ball and multi-row roller configurations.
Whether you have a 5t truck crane or a 200t crawler crane, rest assured a significant amount of engineering time was invested to ensure that the most technically and economically efficient slewing ring design was used.
