Slewing bearing's development
Slewing bearing has been developed as a new machine part in recent 30 years. It is first used in excavator and crane, and then applied to other machines.
Slewing bearing is almost the same as oversize ball and roller bearing. Figure 1-1 shows double-row ball Slewing bearing's structure. Figure 1-2 shows how Slewing bearing is used in Caterpillar Crane. It connects the upper and lower parts of the machine, bearing the weight and working load of the upper part, and makes the upper slew relatively.
Slewing bearing is widely used in different industries. It is suitable for hoisting machine (truck crane, tower crane, etc.), construction machinery (excavator, loading and unloading machine), transporting machine, material processing machine, Metallurgy Machinery, food processing machine, military product (tank, s anti-aircraft gun, radar, missile launcher, etc.), medical instrument, research equipment, etc.

1. outer ring (upper); 2. outer ring (lower); 3. inner ring (internal gear); 4. roller (ball) 5. Seal

Slewing bearing, like common bearing, has roller and rolling ring with raceway. But, compared with common roller bearing, it has many special features. They can be summarized in the following points:
The size of Slewing bearing is great; its diameter is generally between 0.4m~10m, some even is up to 40m.
Slewing bearing usually bears load from different parts. It doesn't only bear the axial force, radial force, but also tipping moment simultaneously. Therefore, a set of Slewing bearing often completes the functions of many sets of common roller bearing.
The speed of Slewing bearing is fairly low, usually below 10 r/min. Moreover, on most occasions, Slewing bearing doesn slew continually,but only slew back and force in a certain angle like the so called Swing Bearing.
Slewing bearing differs greatly from roller bearing in process techniques, material and heat treatment.
Generally, Slewing bearing has gear ring as driving device and Seal Device to pretend dust.
Unlike the common bearing fixed on the central pivot in the bearing box, Slewing bearing has big size and is attached to upper and lower plates by bolts.
In the past, central pest Slewing bearing such as Slewing bearing with rotary column (figure 1-3), Slewing bearing with fixed column (figure 1-4), and turntable Slewing bearing is often used in cranes and excavators, etc.

Figure 1-4 Slewing bearing with Fixed Column
(a) Raceway stay still, the roller rotates along the raceway,
(b) Raceway slews so as to drive the roller to revolve.
1.upper and lower places 2. central pivot 3.roller
Slewing bearing ring has two types, wheel Slewing bearing (figure 1-5) and roller Slewing bearing (figure 1-6)

Figure 1-5 Wheel Slewing bearing 1. dial, 2. wheel, 3. central pivot 4. rail 5. anti-wheel

Figure 1-6 Roller Slewing bearing
(a)Taper-roller Type (b) Roller Type (c) Anti-roller Taper roller Type(d) Anti-roller Roller Type
1. dial, 2. slewing rail, 3. pivot 4. fixed rail 5. rod 6. roller 7. anti-wheel
At present time, these devices has been taken place by an-friction Slewing bearing. Compared to the previous Slewing bearing, the latter has many advantages:
As it is light and flexible, it can run againt little resistance.
It features compact in design, small in size, short in height.
it is easy to maintain and has long service life.
Composed of gear ring, seal, bolt, etc, it is easy to assemble. It can be used in specialized production easilly.
Without central pivot, other machine parts can be installed in the vacancy within the Slewing bearing.
Figure 1-7(a) is a Slewing bearing with rotary column used on portal slewing crane. A Roller Slewing bearing and a Spherical roller Slewing bearing are used in the lower plate of the device to bear the axial force and radial force [figure 1-7 (b)] or a centriletal force bearing [figue 1-7 (c)]can be used to bear axial forece and radial force at the same time. The upper plate often uses several horizontal wheel which is equivalent to a enormous radial bearing to bear the horizontal force of the upper plate. Figure 1-8 is a example of useing slewing bearing ring as the slewing part in order to lower the machine's center of gravity and reduce cost significantly.

In the early 1960s, slewing bearing rings began to be applied in Excavators, and Cranes in China. However, Hefei Mining Machine Manufacturing applied Double-Row Slewing bearing with 1600mm diameter of the outer ring in W1-06 and W3-06 Excavators. Beijing Crane Works used Double-Row Ball Slewing bearing in their 16 ton Tire Crane. Afterward, most of new products of various Slewing Cranes, Excavators, Load and Unload Machine use slewing bearing rings. At present time, slewing bearing ring (hereinafter referred to as Slewing bearing) with 16m diameter of outer ring has been produced in China and is used in Load and Unload machines. Since later 1960s, Cross Roller Ball Slewing bearing has been widely used in China. In recent years, national manufacturers begin to design and produce 3-roller Ball and 3- Taper-roller Slewing bearing. In the past, national manufacturing factories that were able to mass-produce main engine produce Slewing bearings by themselves and had black cast produced by Wheel Tire Factories. While those factories that can not produce main engine in great number buy Slewing bearing from outside suppliers. Now, Luoyang Slewing bearing Co., Ltd, Xuzhou Slewing bearing Co., Ltd, and Maanshan Slewing bearing Co., Ltd, are specializing in production of rotary support.
Foreign Slewing bearing manufacturers are specializing in design and production of rotary support. Each of them makes Slewing bearings of their own size and design series. The main companies include ROTHE ERDE and FAG in Germany, RKS in France, TAPEREX in Britain, NSK and KOYO in Japan, and companies and factories in America and Russia. ROTHE ERDE is the most famous among them. It produces 54000 sets of Slewing bearings with diameter of 0.35~ 40m yearly. Its produces has great varieties and specifications. SKF is a quite large industry corporation in Europe, and also the earliest and most advanced bearing manufacturer in the world. It has branch offices in Britain, France, Germany, and Italy and a modern laboratory centers in Netherlands. Its head office is located in Sweden. It founded its French Branch KS in 1932 having 330 staff and workers. Its main products include Crossed Roller Ball Slewing bearing, Single-Row Four Point Contact Ball Slewing bearing and Double-Row Roller Slewing bearing. It can produce 1600 sets of Slewing bearings with diameter of 0.5~5m yearly. British TAPEREX Corporation has a 30 year history and 110 workers. It only produces Crossed Taper-roller Slewing bearing. Every year, it produces 2500 sets of Slewing bearing with diameter of 0.5~3m, 60% of them are fitting for the main engine made in Britain, others are exported to America, Middle East, etc. Its complicatedly processed products are of roller type and can run with little resistance.