SKF INSOCOAT electrically insulated rolling bearings

SKF insocoat electrically insulated rolling bearing are widely used in many kinds of industry, Motors, generators and related equipment are in danger when current passes through the bearings. This not only damages the contact surfaces of the rolling elements and raceways in the bearing (electrical corrosion), but also quickly degrades the grease. Another additional risk comes from high-frequency currents in motors and generators, which can form a loop through the parasitic capacitance inherent in the device. Increased risk of damage if frequency converter is used.

SKF insocoat Electrically insulated rolling bearings:

— Designed to prevent current from flowing through the bearing

— The outer surface of their inner or outer ring is coated with an insulated alumina layer, and a sophisticated plasma spray process is applied to obtain an excellent quality finish.

— A very cost-effective solution compared to other insulation methods

Reliability is an essential requirement for modern bearings with longer intervals between maintenance.
For example, they must be robust enough to operate in electrical equipment with modern AC technology.

The passage of electric current through rolling bearings can lead to damage in a relatively short period of time. Small craters caused by electric current passage which have been tolerated occasionally in the past are no longer acceptable. SKF, as a consequence, has developed the INSOCOAT bearing range (fig. 1). These bearings have special coated outer rings with an electrically-insulated layer for a breakdown voltage up to 1,000 V.

To ensure smooth, trouble-free operation, it is necessary to prevent the bearing from electric current passage. The contact areas between housing, outer ring, rolling elements, inner ring and shaft can act as electric contacts. The resulting arcs – characterised by very high temperatures similar to that produced by melting and welding processes – will damage rolling elements as well as raceways of bearing rings. Typical damage is characterised by craters (fig. 2) and false brinelling.

The risk for damage to bearings can occur when transient voltages exceed 0.5V or the electric current flow is more than 0.1A/mm2 (related to the contact area of rolling elements). Also, electric current does not normally flow evenly through several bearings connected together, for example. Electric current flow tends to be concentrated in only one bearing.

To overcome this problem SKF has developed INSOCOAT bearings. Leading manufacturers of electrical machinery confirm that the use of electrically-insulated bearings offers the most economical solution when compared to other protective measures.


Typical applications of the new bearing are in traction motor protection. Potential cases are:

  • case 1: where electric current could flow from the housing to the shaft. In this case all bearings have to be electrically insulated (fig. 3)
  • case 2: for electric current flow inside the rotor, normally caused by asymmetrical electric motors. Here protection of only one side of the bearing arrangement is necessary. Mainly the smaller bearing, situated on the non-drive side, is selected for electrical insulation (fig. 4)
  • case 3: in wheel bearings for low floor tramcars (fig. 5)

Bearing design

The insulating coating on the outer ring of the INSOCOAT bearing is made from aluminium oxide and applied using plasma spraying technology. SKF has carried out extensive research and development in this area and is pursuing further improvement of the process. The latest phase of the development is the ability to handle a 1,000V dielectric breakdown voltage.

Design characteristics of the bearing include:

  • ISO standard dimensions and tolerances
  • Interchangeability with non-insulated bearing types
  • Suitability for all common housing types
  • Dielectric breakdown voltage optional up to 500V or 1,000V.
  • Coating thickness of about 0.05 mm
  • Durability of coating
  • High thermal stability

Common suffixes used for INSOCOAT bearings are:
VL024 and VA309 breakdown voltage to 500V and VL0241 and VA3091 breakdown voltage up to 1,000V. VA309 and VA3091 are applied to bearings for traction motor applications.

To ease mounting in case of tight housing fits, it is recommended that the housing bore is provided with an additional chamfer of 15° on the side at which mounting should commence. The ceramic coating has sufficient hardness and adhesive power to resist damage during mounting if all the bearings are handled with reasonable care.

In principle, electrically-insulated bearings can be of any type. Most common bearing types include cylindrical roller bearings and deep groove ball bearings with outside diameters larger than 75 mm for traction motor designs, stationary electric motors (medium and large types), generators and special applications including submersible pumps. Also electrically-insulated taper roller bearings could be used as wheel bearings for tram and suspension tube bearings for electric railway vehicles.

Series of electrically insulated bearing

The standard scope of supply of insulated bearings includes the most common sizes and variants of bearings:

  • Single-row deep groove ball bearings
  • Single-row cylindrical roller bearings

In addition to the standard classification, SKF can also provide special INSOCOAT bearings and bearing units with complex ring geometries such as:

— Flanged tapered roller bearings

— Traction motor bearing unit

CoatMaster as the China Bearing Manufacturer, it focus on Research & develop the Electrically Insulated Bearing since 2009, Through 10 years learning from SKF, FAG, NSK Such top quality Bearing manufacturer, CoatMaster Mastered the core technology of insulation technology and the quality of Electrically insulated bearing reached the world class quality level and save the cost for the insulated bearings end-user. its quality are proved by our regular customers on the market.

It not only can produce the top quality Insulated deep groove ball bearings, insulated cylindrical roller bearing, insulated tapered roller bearings, but also the inuslated spherical roller bearing, insulated spherical roller thrust bearings, insulated angular contact ball bearings, insulated linear motion bearing, insulated self-aliging ball bearings, Hybrid ceramic bearing etc. CoatMaster would like to supply the complete bearing industry solution for Electrical Motor of Railways,Industrial electric motors & generators,Wind energy,Construction,Industrial pumps.

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