Slewing Ring Technical
A slewing ring allows relative rotation of two machine parts whilst handling a combination of radial, axial and moment loads, all in a single sturdy, sealed and easily fitted package. Design and fabrication times are reduced, assembly and repair of parts is simplified. These factors soon outweigh the initial costs of the slewing ring.
Some slewing rings are ungeared, but the addition of an internal or external spur gear facilitates movement and allows use of either hydraulic or electric motive power. In some cases worm gears or even chain drives are used.
Although slewing rings are often thought of as being quite simple bearings, there are many factors that need to be considered so that the best and most economic solution is chosen. Using “state of the art” computer software NBC is able to assist designers from project conception stage.
In some applications like offshore vessel use, a degree of certification is required (e.g. Lloyds). We can offer full certification on all slewing rings delivered, as long as this is specified at the time of manufacture.
The following steels are commonly used in slewing ring production
C45 or 50Mn Standard industrial grade slew rings
42Cr2 and 46Cr2 European manufacturers / higher gear strength than C45
42CrMo4 Optimum gear strength/ Offshore applications
Each type has it’s specific benefits depending on the application. The steels can be used in the normalized or quenched and tempered states.
Other materials such as aluminium, stainless steel, tool steels and bearing steels can also be used.
Rolling elements are usually made from 100Cr6 steel with a surface hardness of 63 Hrc.
The load capacity of a slewing ring is dependant on a number of factors including
- Types and condition of materials used
- Size, style and number of rolling elements
- The contact parameters of the rolling elements
On a typical load curve, axial loads are plotted along the X-axis, and moment loads are plotted up the Y-axis. Any combination of axial, radial and moment loads can be combined to create an “equivalent axial and radial load”. This calculation is best left to the experts! In general terms, the further under the load curve you "equivalent loads" plot, the longer service life your unit will offer. Knowledge of the load cycle enables us to estimate the ultimate service life of the unit.
This specific load curve shows the maximum dynamic capacity, as well as a lesser capcity curve that already incorporates a safety factory of 25%. Not all manufacturers curves are calculated in the same way and thus cross brand comparison of load curves is not always completely valid.
Bolts and bolt capacity curves
Another important consideration in proper selection is the size, quantity and quality of bolts specified. The bolt limit curve is usually drawn on the bearing load curve - illustrated above by the blue line.
High tensile bolts of Grade 8.8 and 10.9 are most commonly used, and care should be taken the examine any manufacturer's load curves to see which grade of bolt a specific manufacturer has used in their load charts.
Below the recommended tightening torques for ISO bolts. The figures assume an average friction coefficient and show the the tightening torque for assembly with a torque spanner of hydraulic torque wrench. Allowable error 10%.
It is important to tighten the bolts in the correct order to ensure the bearings is allowed to settle onto it's frame properly. A chart indicating the correct sequence is available for download here. IT IS ALSO IMPERATIVE THAT THE MAINTENANCE SCHEDULE OF THE EQUIPMENT INCLUDES A CHECK OF BOLT TIGHTNESS.
Consult us in the case of suspended loads at all times.
Welding of most industrial slewing rings is strictly forbidden and will negate any manufacturer's warranty.
General purpose industrial slewing rings are generally factory filled with a lithium or lithium-calcium EP2 grease, suitable for operation between -20C to +70C, and with a minimum base oil viscosity of 150 c/s (mm2/s) @40C. Grease containing any hard additives (e.g. molybdenum-disulphide or MOS2 are strictly forbidden!)
It is recommended that all slewing rings are checked and lubricated on-site before use. Any bearing that has been stored for more than 6 months should be lubricated before use.
Relubrication intervals depend on the environment. As it is unlikely you will ever exceed the "grease life", the main function of the lubricant is to keep dirt and moisture out of the bearing.
In general terms it is recommended that relubrication is effected every 100 hours of operation with grease being slowly added through all available grease ports, and while the bearing is turning. This will ensure lubricant is spread evenly through the raceway and not form a pressure bulge under the lip seal (thus "popping" the seal). Pressurised grease guns are not recommended.
Special lubricants can be offered for corrosive or wet environments, high temperature or high radiation areas.
A variety of protective coatings can be offered including:-
Zinc plating and passivating (usually yellow)
Painting (primer or to any RAL colour specified)
Other specialist (e.g. MIL-SPEC) coatings can be arranged.