Solve the harm of "electric corrosion" of original steel bearings and improve the heat resistance and durability of bearings and greases

AC traction motor with variable voltage and variable frequency control is installed on new electric vehicles in recent years. This AC traction motor has the characteristics of light weight, compact structure and high-speed operation. However, in order to adapt to the characteristics of high-speed rotation of bearings and temperature rise of various parts of traction motor, it is also necessary to solve the "electric corrosion" harm of original steel bearings to improve the heat resistance and durability of bearings and lubricating grease.

Note: electric erosion is a kind of damage caused by local melting when electric current passes through the bearing during rotation and passes through the very thin oil film at the rolling contact part to generate electric sparks.

Fig. 1. Via network of shaft current corrosion

1 Basic requirements for insulated bearings for traction motors

At present, the countermeasures to solve the electric corrosion are to set a bypass circuit to make the current flow to the outside of the bearing, or to insulate the bearing to cut off the current. The specific methods are as follows:

(1) A grounding device is installed on the main shaft or casing of the traction motor so that the current does not pass through the bearing.

(2) Use insulated bearing bracket or insulated bearing.

In terms of price and reliability, insulated bearings are the main ones.

There are three kinds of insulated bearings for traction motors, namely: resin coated bearings; Hybrid ceramic bearings; Ceramic sprayed bearing.

① Resin coated bearing

PPS (polyphenylene sulfide) insulated bearing is widely used in this kind of bearing at present. The outer surface of its outer ring is coated with PPS insulating resin to prevent electric corrosion of the bearing. The advantages are excellent electrical insulation performance and low price. The disadvantage is that this kind of bearing is not suitable for machinery with rapidly rising bearing temperature or occasions with strict requirements for interference change, so it is mostly used in motors with relatively loose conditions.

② Hybrid ceramic bearing

It refers to that only the rolling element parts in the bearing are made of ceramic materials, also known as hybrid ceramic ball bearings. It uses engineering ceramics as the bearing material and has good mechanical and thermal properties, that is, it has sufficient strength, stiffness, hardness, fracture resistance, impact resistance, high temperature resistance, oxidation resistance and a series of better properties than metal materials. Ceramic materials suitable for bearings mainly include silicon nitride (Si3N4), zirconia (ZrO2) Alumina (Al2O3). Among these three kinds of ceramic materials, silicon nitride has become the preferred material for ceramic bearings because of its superior comprehensive performance. Its characteristic is that the failure form is the same as that of bearing steel, and it appears in the form of peeling with warning. However, zirconia and alumina appear in the form of fragmentation failure.

Figure 2. Via network of zirconia silicon nitride hybrid ceramic high temperature bearing

③ Ceramic spray bearing

It is to spray a layer of ceramic insulation coating on the inner ring or outer ring of the bearing. The ceramic insulating coating is prepared by plasma spraying, and then sealed with sealing agent. The bearing with coating on the outer ring is mainly used for medium-sized bearings; Bearings with inner ring coating are mainly used for larger bearings or occasions with higher insulation requirements. The dimensional tolerance and accuracy of the bearing comply with relevant ISO standards and are completely consistent with the prototype. Its performance, size and accuracy are the same as those of ordinary bearings, which can be directly replaced.

In addition to the requirements of general bearings, insulated bearings for traction motors also have requirements for insulation performance. Mainly include:

(1) Insulation resistance stability (insulation capacity caused by changes in ambient temperature and humidity shall not be significantly reduced).

(2) Certain mechanical strength (there shall be no fragmentation when pressing, pulling or falling).

(3) Bearing rotation performance.

(4) Alkali resistance, detergent, kerosene, etc.

(5) Interchangeability with shafts, bearing housings, etc.

From the comprehensive conditions, ceramic bearings are more suitable for the practical application of traction motors.

2 Advantages of combined ceramic bearing for traction motor

(1) In ceramic bearings, silicon nitride ceramic rolling elements are electrical insulators.

In the case of direct current, the ceramic bearing without seal ring or dust cover is an insulator with a resistance of several gigaohms; Under the action of high-frequency current, the ceramic rolling elements of ceramic bearings play an insulating role between the inner ring and the outer ring. Because the capacitance of the hybrid ceramic bearing is very low, the contact of the raceway is very small, the diameter of the rolling element is relatively large, and the dielectric constant of the silicon nitride material is relatively low, its resistance value is above 1 K Ω even in the high-frequency environment of 1 MHz current. Therefore, ceramic bearings are very effective in preventing bearing erosion damage, especially in the case of high-frequency current.

(2) Ceramic bearings have longer grease life than steel bearings.

When the current passes through the steel bearing, the local high temperature caused by the current discharge will cause the additives of the lubricating grease to react with the base oil, burning or carbonizing the base oil, hardening and blackening the lubricating grease, and rapidly decaying the grease. Although the grease of ceramic bearings will deteriorate like ordinary bearings, it will not burn. By comparing the rotation time until burning, the grease life of ceramic bearings is 3 ~ 4 times that of steel bearings.

(3) Ceramic bearings can withstand poor lubrication under static vibration.

When the viscosity of lubricating oil is high and the oil film is easy to form, the service life of ceramic bearings is almost the same as that of steel bearings; When the viscosity of lubricating oil is low, the service life of general bearings is very short. At this time, the rolling element and the inner and outer rings are made of the same material, which is easy to adhere to each other. However, ceramic rolling elements made of different materials lack activity, which is beneficial to the improvement of service life.

More about FRC Ceramic Bearings:

Ceramic bearings are comprised of ceramic rolling elements (balls) enclosed in a ferrous (typically steel) inner and outer race. Since these bearings are constructed from many materials, they often operate in hybrid capacities. We also build ceramic bearings , but they are not as common because they are less durable. These bearings offer several benefits over conventional steel bearings, including enhanced acceleration capability, increased stiffness, and reduced friction. Hybrid ceramic bearings also offer the benefit of not allowing electrical currents to run through the bearing, which allows for prolonged use.

As one of the most professional ceramic bearings manufacturers and suppliers in China, we’re featured by good service and competitive price.  FRC Ceramic Bearings include Ceramic Deep Groove Bearings, Ceramic Thrust Ball Bearings, Ceramic Self Aligning Bearings, Ceramic Angular Contact Bearings, Ceramic Flanged Ball Bearings,Ceramic Ball, and Ceramic Components.Please rest assured to buy high quality ceramic bearings from our factory. Contact us for dimensions and specifications.