Slew ring bearings are special types of bearings which are generally used in heavy machinery. The difference between these type of bearings and normal bearings is that, they have higher pitch value diameter, the way they are mounted on the structures, working nature which is generally “static”, higher load carrying capacity, and larger number of rolling elements.
Slew bearings are generally selected on the basis of the static load, which is calculated using the forces for axial and radial loads coming on it as well as the tilting moment. For correct selection of the slew bearing, certain parameters such as speed, load, and operating temperature are required. There are certain assumptions which are to be considered:
1. The bearing is made from high quality material and is free from manufacturing defects.
2. Bearing is selected as per the load calculations.
3. Installation of the bearing is proper.
4. Grease used is of good quality and it reaches to every corner of the bearing.
5. Bearing is sealed using high quality seal.
6. Operating temperature of the bearing is within range.
7. Proper care for maintenance is ensured.
The conditions mentioned above, if fulfilled, ensure maximum bearing life. But, one must consider the fact that the life depends more on practical conditions and not on theoretical values.
Slew ring bearing, just like the other ball and roller bearings are subject to wear and tear due to the continuous moment of the rolling elements (either balls or rollers) inside them. The bearings which can sustain the calculated hours of life operation are subject to wear which is natural. As compared to the conventional roller bearings, slew ring bearings work in relatively harsh environments where they are subjected to heavy to very heavy loads in restricted working span. Uneven load distribution, deformation in the races, proper lubrication for the rolling elements, all these parameters affect the working life of the bearings. Apart from natural wear, these conditions, if not taken care of, can considerably reduce the life of the slewing bearing.
When the slew bearing is rotating, the rolling elements are rolling in cyclic pattern over the inner raceways. Since the rotational speed is very low (mostly below one RPM), the phenomenon of gradual pitting and chipping takes place. Pitting is caused by the heavy loads on roller elements. The durability of the slew bearings who have undergone improper induction heat treatment are more prone to failure in early stages of the operating cycle. If this occurs, the bearing must be replaced or repaired immediately, as by not doing so, pitting will be more and that might result into jamming of rolling elements inside the bearing. This may also cause the ring to crack in extreme conditions, if it is not taken care of.
The causes for such failures is mainly due to the excessive over loading of the bearing. Another reason may be due to the unusual gaps between rolling elements, which may cause slackness in the bearing. This type of breakdown maybe due to the improper foundation structure on which the bearing is mounted. Similarly, if the layer which is hardened is small, the bearing will lead to catastrophic failure. The layer thickness should be properly selected on the basis of the stress which the bearing is subjected to.
In case of some slew bearings, fatigue may occur due to improper internal lubrication. In this, the surface of the raceway undergoes spalling, which results into premature wear, finally resulting into pitting. Also, if the races are not properly lubricated, there is a risk of them being corroded. This accelerates the premature wear which was already caused due to spalling of the surface.
Finally to conclude, axial play in the bearings, especially larger diameter bearings should be taken into consideration. This is generally an important factor in case of bearings with rollers as their rolling elements, which can lead to failure. The surfaces which are induction heat treated, need to undergo strict inspection procedures to ensure there are no micro cracks in them. These micro cracks can then develop into larger cracks and the bearing fails instantaneously.
The failures mentioned in these articles are generally observed failures and may not necessarily be limited to these failures only.
We, at Kavitsu Bearings, provide full refurbishment services to slew bearings up to 3000 mm diameter. Our state of the art facility ensures that the repaired bearing gives almost the same life, if not more, as compared to a new bearing. All this at fraction of cost for the new bearing. Please visit us at Kavitsu bearing repair if you need to get your bearing repaired. The only manufacturer in India to provide such services.