In this day and age where productivity means higher speeds, higher operational temperatures and reduced maintenance, bearings are expected to exceed the bounds of former possibilities. New and advanced applications are setting higher demands on bearings, especially under extreme operating conditions, where there is the risk of smearing, boundary lubrication, sudden load variations, low loads or high operational temperatures.
To withstand these types of severe operating conditions SKF bearings can be furnished with a low friction ceramic coating on the contact surfaces inside the bearing. This coating, trademarked NoWear, was developed by SKF for rolling bearings and is covered by an SKF patent.
NoWear bearings make sense whenever bearings are failing prematurely due to severe operating conditions. NoWear bearings can withstand longer periods of insufficient lubrication, sudden variations in load and rapid speed changes, vibrations and oscillations.
NoWear bearings open up new possibilities to existing applications operating under severe conditions, without introducing major design changes and enable freedom in new designs. They have already been proven in a wide range of extreme applications, including paper machines, marine and offshore applications, fans, compressors, hydraulic pumps and motors.
Most SKF ball and roller bearings can be obtained in a NoWear execution as described below and shown in table 1. Other executions might be available on request
NoWear bearings of L5DA execution
NoWear bearings of the L5DA execution are the most commonly used bearings. These NoWear bearings are equipped with coated rolling elements (fig 1) and are recommended for applications where the bearing load is low to medium, or where vibrations and oscillations are present.
NoWear bearings of L7DA execution
NoWear bearings of the L7DA execution have the coating applied to the rolling elements and the inner ring raceway(s) (fig 2). This execution is recommended for applications where abrasive wear resistance is crucial or load is heavy.
Application of NoWear bearings
For highly demanding applications where NoWear bearings are to be used, normally several operating parameters must be considered and weighted against each other. As a result, NoWear coated bearings should be selected in close cooperation with the SKF application engineering service.
The following recommendations are intended to illustrate the types of application that can benefit from the NoWear executions.
When cylindrical roller, needle roller, spherical roller or toroidal roller bearings are subjected to low loads in combination with high speeds, L5DA NoWear bearings are recommended in cases where the calculated bearing life cannot be reached.
With the L5DA execution of the NoWear bearings extended relubrication intervals are possible without negatively influencing bearing service life. However, if relubrication intervals are maintained, operating speeds can be increased.
Bearings subjected to oscillations or external vibrations can fail prematurely due to insufficient lubrication. Under these conditions, SKF recommends the L5DA execution. However, in extreme cases, the L7DA execution may be preferred.
If operating conditions cause low operating viscosity (? < 1) and no appropriate lubricants are available, NoWear bearings are an excellent way to extend bearing life and to achieve operational reliability. The L5DA execution of the NoWear bearings is normally recommended. However, for unusual lubrication conditions, for example when the bearings are to be lubricated by the process media, the more advanced L7DA execution should be used.
For additional information about the NoWear bearings please consult the SKF application engineering service.
NoWear service life improvement
NoWear bearings are suitable in situations where lubricating film is insufficient for undisturbed bearing operation, i.e. when ? is below 1. To calculate life of a NoWear bearing, the traditional bearing life calculation should be used but with ? = 1.
Improvement in service life by switching to NoWear under low load and high-speed conditions depends on the application, but experience shows a multifold improvement should be expected. However, calculating service life under these conditions is difficult.
For grease lubricated bearings running at speeds near or above bearing speed rating, or at high temperatures shortening grease life, selecting NoWear execution prolong relubrication intervals up to 15 times depending on application conditions.
When extension of service life would be desirable in applications with heavy loads and marginal lubrication, NoWear can be a solution. The NoWear coating cannot protect the bearing against spalling as a result of constant heavy loads. At heavy loads the maximum shear stresses are located below the coating within the bearing steel, which still has the properties of normal steel. For such applications, contact the SKF application engineering service.
Dimensions, tolerances, clearance
Dimensions, tolerances and internal clearance of NoWear bearings are the same as for standard bearings.
Load carrying capacity
The basic dynamic and static load ratings for NoWear bearings are the same as for standard bearings.
NoWear coating material
A physical vapour deposition process applies the low friction, ceramic coating. Bearing surfaces coated in this way have all the resilience of the underlying material, but with the hardness, low friction coefficient and wear resistance of the NoWear coating. During operation there will be some micro level transfer of the coating material to the counter surface. Such transfer plus the inertness of the material reduce friction and improve resistance against wear and smearing, even for bearings where only the rolling elements are coated.
The essential properties of the NoWear coating are listed in table 2.
NoWear bearing lubrication
As default, the same lubrication guidelines are valid for NoWear bearings as for standard bearings (see section "Lubrication"). However, NoWear bearings will operate with high reliability, even when adequate surface separation cannot be achieved (? < 1), by preventing direct metal-to-metal contact between rolling elements and rings. It should be noted that it might be possible to reduce EP and AW additives in the lubricant with the NoWear coating, as the coating already acts as a powerful additive.
NoWear bearings are not intended for vacuum or other completely dry running applications.