Bearing Details

BEARINGS

TERMINOLOGY

 Bearings are used to reduce friction between components having relative motion.

The bearing makes many of the machines we use every day possible. Without bearings, we would be constantly replacing parts that wore out from friction.

The Basics

 The concept behind a bearing is very simple: Things roll better than they slide. The wheels on your car are like big bearings. If you had something like skis instead of wheels, your car would be a lot more difficult to push down the road.

                                                       Bearing Loads

       ·         The bearings that support the shafts of motors and pulleys are subject to a radial load.

       ·         The bearings in this stool are subject to a thrust load

       ·         The bearings in a car wheel are subject to both thrust and radial loads.

                                                  Types  of  Bearings

          
    1)      Ball Bearings – 

·         Single Row Deep Grove Ball Bearing

·         Double Row Deep Grove Ball Bearing

2)      Roller Bearings

3)      Cylindrical roller Bearing

4)      Spherical Roller Bearing

5)      Taper Roller Bearing

6)      Angular Contact Ball Bearing

7)      Ball Thrust Bearings

8)      Roller Thrust Bearings and

9)      Tapered Roller Thrust Bearings.

Ball Bearings :  

These type of bearing. They are found in everything. These bearings can handle both radial and thrust loads, and are usually found in applications where the load is relatively small. In a ball bearing, the load is transmitted from the outer race to the ball, and from the ball to the inner race. Since the ball is a sphere, it only contacts the inner and outer race at a very small point, which helps it spin very smoothly. But it also means that there is not very much contact area holding that load, so if the bearing is overloaded, the balls can deform or squish, ruining the bearing.

Roller Bearings :  

 like the one illustrated here are used in applications like conveyer belt rollers, where they must hold heavy radial loads. In these bearings, the roller is a cylinder, so the contact between the inner and outer race is not a point but a line. This spreads the load out over a larger area, allowing the bearing to handle much greater loads than a ball bearing. However, this type of bearing is not designed to handle much thrust loading. A variation of this type of bearing, called a needle bearing, uses cylinders with a very small diameter. This allows the bearing to fit into tight places.

Ball thrust bearings :

            Like the one shown here are mostly used for low-speed applications and cannot handle much radial load. Barstools and Lazy Susan turntables use this type of bearing.

 Roller Thrust Bearings :

         Like the one illustrated here can support large thrust loads. They are often found in gear sets like car transmissions between gears, and between the housing and the rotating shafts. The helical gears used in most transmissions have angled teeth -- this causes a thrust load that must be supported by a bearing.

Tapered Roller Bearings :

Can support large radial and large thrust loads.

 Tapered roller bearings are used in car hubs, where they are usually mounted in pairs facing       opposite directions so that they can handle thrust in both directions.

Cutaway view of (left) a spherical roller thrust bearing and (right) a radial tapered roller bearing

Cutaway view of (left) a spherical roller thrust bearing and (right) a radial tapered roller bearing

Some very high-speed devices, like advanced flywheel energy storage systems, use magnet bearings. These bearings allow the flywheel to float on a magnetic field created by the bearing. Some of the flywheels run at speeds in excess of 50,000 revolutions per minute (rpm). Normal bearings with rollers or balls would melt down or explode at these speeds. The magnetic bearing has no moving parts, so it can handle these incredible speeds.

Other type of the bearing :

 These are known as BUSH or SLEEVE Bearings. Most of them are made of Brass, Bronze, White metal, Copper. They have grooves and holes for storage of oils/greases in the bearing. Most widely used. Can be used for more than 3000000 DN. Load carrying capacity increases with RPM.

                                                Assembly of Bearings

Inner Race Joint Gaps :

          When the inner race is assembled around a shaft, there shall be a small gap at the joint. The gaps at the joints, typically between 0.015” (0.4mm) and 0.025” (0.5mm) per side ensure contact between the bore of the inner race and the shaft. This is illustrated to the right.

                                                        Nomenclature of Bearings

Bearings are designated by,   XXXX  or  XXXXX

Here the last 02 digits multiplied by 05 gives the bore size of the bearing,viz.,if last two digits are 05 then bore size of bearing is 25 mm,however there is exception for 00 to 03

i.e. if last 02 digits are;

 00 then bore = 10mm ;01 then bore = 12 mm ;

02 then bore = 15 mm ; 03 then bore = 17 mm

 First digit indicates, type of bearing

1 = Angular Contact Ball Bearing

2 = Self Aligning Ball Bearing

3 = Taper Roller Bearing (5 digits); Double Row Ball  

       Bearing (4 digits)

4 = Spherical Roller Bearing

5 = Thrust Ball Bearing

6 = Deep Groove Ball Bearing (4 digits); thin series

(5 digits); Miniature (3 digits)

 Second digit indicates O.D. or Width series i.e. load carrying capacity of the bearing, higher this number higher is the load carrying capacity.

In Taper Roller bearing both second and third digit are used to indicate load carrying capacity.

In Needle Roller Bearings I.D. and Width are mentioned e.g. 35x10; 40x15

For cylindrical roller bearing and special bearings prefixes and suffixes are used which specifies the material of the cage

e.g. Steel;Brass;Nylon etc.

Bearings may also be designated by O.E.M. drawing number.

Ball bearings are available with grease filled in it and shield on either one side or both sides, the shield is either steel or rubber, if steel shield are used then suffix Z is used and if shield is rubber then suffix is RS,if shield is on one side then it is denoted by Z or RS,if it is on both sides then designation is 2Z or 2RS respectively.

                                                        Lubrication of Bearings

         Friction and wear are reduced by separating rollers and races with a lubricant film to minimize metal to metal contact. The major factors in selecting a lubricant are speed, lubricant base oil viscosity and temperature

Building a Lubricant Film:

 As speed and viscosity increase, thickness of lubricant film increases. As temperature increases, lubricant film thickness decreases. The lubricant film should be sufficient to cover the average peaks on the bearing surface by a ratio of at least 1.25. Ranges from 3 to 7.5 are described as elasto - hydrodynamic lubrication (abbreviated EHD or EHL). Adequate lubrication is defined as a ratio of 1.25 to 3.0. As the ratio falls below 1.25, some metal to metal contact will occur.

Grease Lubrication :

      Grease lubrication is easier than oil to retain in the bearing offering lower lubricant losses and improved sealing. Grease also offers better protection against corrosion to the roller surfaces. A grease typically consists of three components; a thickener (sometimes called a soap), a base oil and additives. The oil in the grease has an ISO-VG rating. In most cases, this is the key to selecting the grease. At speeds in excess of 200,000mm dn, greases with synthetic base oils are recommended.

Oil Lubrication :

    Oil lubrication can be broken down into three major categories; recirculating oil systems, constant level and oil mist.

Recirculating oil systems use a pump to provide a continuous flow of oil to the bearing which is then recaptured, cooled, filtered and recirculated.

A constant level oiler is the simplest method for delivery of oil lubrication to a bearing. The oiler maintains a constant level in the bottom of the bearing. Ideal conditions for oiler use would be bearing temperature less than 60°C, load through center, with low to moderate speeds.

                                                         Sealing of Bearings

 Aluminum Triple Labyrinth (ATL) :

 Machined aluminum bodied triple labyrinth seal for high speed and general applications.

 Temperature limits:; -20°C to +100°C

Maximum speed: Bearing maximum

Shaft surface finish: 3.3 µm Raa.

 High Temperature Packing (HTP) :

 A PTFE filament yarn impregnated with graphite and lubricated with silicon. A direct replacement for felt in high temperature applications.

 Temperature limits:  -70°C to +260°C

Maximum speed: 6000dN; 150000mm dN

Shaft surface finish: 0.8 µm Ra.

Triple labyrinth with Viton rubber cord insert (TL HT) :

Suitable for high speed and high temperature applications.

Temperature limits: -20°C to +175°C

Maximum speed: Bearing maximum

Shaft surface finish: 3.2 µm Ra

Single lip with spring loaded retaining plate (SRS RP) :

Suitable for severe splash or completely submerged applications.

Temperature limits: -20°C to +100°C

Maximum speed: 6000dN; 150000mm dN

Shaft surface finish: 0.4 µm R

Labyrinth grease groove (LAB) :

Standard seal for bearings over 300mm. Particularly successful on marine applications. Suitable for low or high speed operation.

Temperature limits: As bearing

Maximum speed: As bearing

Shaft surface finish: 3.2 µm Ra.

Synthetic nitrile rubber single lip (SRS) :

For wet but not submerged applications. Can be used to retain bearing lubricant by mounting lip innermost.

Temperature limits: -20°C to +100°C

Maximum speed: 6000dN; 150000mm dN

Shaft surface finish: 0.8µm Ra.

Felt (F) :

Made from wool and selected fibers.

Temperature limits: -70°C to +100°C

Maximum speed: 6000dN; 150000mm dN

Shaft surface finish: 1.6 µm Ra.

Neoprene rubber triple labyrinth (NTL)

 For applications with a maximum speed of 3300rpm for shaft diameters up to 65mm, 2000rpm for shafts from 70mm to 90mm and 1800rpm for shafts up to 105mm. Can be used where an explosive or corrosive atmosphere prevents the use of aluminum.

Temperature limits: -20°C to +100°C

Maximum speed: 6000dN; 150000mm dN

Shaft surface finish: 3.2 µm Ra.