QCB® slewing ring technical
"An elegant solution"
Slewing rings allow 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, and assembly and repair of parts is simplified. These factors soon outweigh the initial costs of the slewing ring.
While some slewing rings are ungeared, 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. We check raceway stresses and can offer comparative life calculations for different designs. Gearing and bolt strength is also checked, and our engineering staff issue a complete and comprehensive design report.
QCB® slewing rings are subject to strigent quality checks througout the production process. The selected factories used are audited by our technical team annnually - and in some cases by our larger OEM clients as well.
Each QCB® slewing ring is numbered and we can trace this back to the orginal factory QC records. A comprehensive database of the final inspection results is held at NBC.
Proof of our commitment to quality is the fact that our satisfied clients come back to us time and time again.
Note: In some applications (e.g. offshore vessel use) a degree of additional certification is sometimes required (e.g. DNV or Lloyds). We can offer this certification as long as this is specified at the time of manufacture - but there are additional costs involved.
The following steels are commonly used in slewing ring production
C45 or 50Mn Standard industrial grade slew rings
42Cr2 European manufacturers / higher gear strength than C45
42CrMo4 V Optimum gear strength/ Offshore applications
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. Ceramics can be used.
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 set of load curves shows the maximum dynamic capacity (in blue) as well as the bolt limit curve (in red) for a range of slewing ring bearings.
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.
NBC OFFERS A RANGE OF SPECIAL BOLTS THAT ARE CORROSION RESISTANT FOR OFFSHORE WORK.
ASK FOR DETAILS
General purpose industrial slewing rings are generally assembled with a small quantity of 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.
It is IMPERATIVE that 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.
Greases containing any hard additives (e.g. molybdenum-disulphide or MOS2) are strictly forbidden!
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.
ASK ABOUT OUR SPECIAL LUBRICANTS AND GREASEMAX AUTOMATIC LUBRICATION SYSTEMS
IDEAL FOR SLEWING RING APPLICATIONS
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.