SKF 6208/HC5C3 Hybrid Ceramic Deep Groove Ball Bearing

A hybrid bearing with ceramic balls for electric motors and high speeds

The SKF 6208/HC5C3 is a hybrid ceramic deep groove ball bearing. The rings are bearing steel and the balls are bearing grade silicon nitride, which makes the bearing electrically insulating: current cannot pass from the shaft through the balls to the housing. The bearing has a 40 mm bore, 80 mm outside diameter and 18 mm width and is an open design without seals.

SKF recommends hybrid bearings for smaller electric motors and for any machine where shaft currents or high speeds are an issue. The product page also lists protection against electric current damage and suitability for difficult and contaminated environments as the main features.

Technical specifications

Why silicon nitride balls change the performance

The SKF brochure compares the materials directly. Silicon nitride has a density of 3.2 g/cm3 against 7.9 for bearing steel, a hardness of about 1 600 HV against 700, a modulus of elasticity of 310 GPa against 210 and a thermal expansion of 3 against 12 times 10 to the power minus 6 per kelvin. Electrical resistivity is 10 to the power 12 ohm metres, and dielectric strength is 15 kV/mm.

In practice the lower density reduces inertia during rapid starts and stops, the low friction cuts temperature at high speed, the hardness resists wear in contaminated lubricant and the stiffness is higher. SKF also reports a reduced risk of false brinelling, the small indentations that appear on stationary bearings exposed to vibration. The reference speed of this size is 22 000 r/min and the limiting speed 14 000 r/min.

Designation, lubrication and ordering notes

In the brochure suffix list, HC5 is rolling elements of silicon nitride and C3 is a radial internal clearance greater than normal; the 6208 base number is a 40 mm bore deep groove bearing. Standard sealed hybrid bearings are filled with SKF WT grease, but this part is an open version with no seal suffix, so it must be lubricated and housed accordingly. SKF recommends relubricating open hybrid bearings with LGHP 2 grease and says the interval is generally 3 to 5 times longer than for an all-steel bearing.

Hybrid bearings are handled and mounted like standard steel bearings, with the right tools. The shaft abutment must be between 47 and 52.5 mm, the housing abutment at most 73 mm and the fillet radius at most 1 mm. Confirm cage type and the voltage you need to block; SKF gives no voltage rating for the ceramic balls in this table. See Electric Motors for other motor bearings and the SKF TLMR automatic lubricant dispenser for lubrication.

Documents and quotation

The attached SKF brochure on INSOCOAT and hybrid bearings sets out the material properties, the cages and seals, the grease choices and the installation recommendations. Dimensions and speeds in the table come from the SKF product page for the 6208/HC5C3. Seven Star LLC, an independent supplier in Muscat, will quote on the designation and quantity you send.