Bearings and Seals

Post Author : Blue Collar Post Date : 12/4/20 Post Time : December 04, 2020


The bearings in gas and steam turbines provide support and positioning for the

rotating components. Radial support is generally provided by journal, or roller

bearings, and axial positioning is provided by thrust bearings. Some engines,

mainly aircraft jet engines, use ball or roller bearings for radial support, but

nearly all-industrial gas turbines use journal bearings.

Along service life, a high degree of reliability, and economic efficiency are the

chief aims when designing bearing arrangements. To reach these criteria, design

engineers examine all the influencing factors:

1. Load and speed

2. Lubrication

3. Temperatures

4. Shaft arrangements

5. Life

6. Mounting and dismounting

7. Noise

8. Environmental conditions

Rolling Bearings

The aero-derivative gas turbine design, with its low-supported weight

rotors—for example, the LM 5000 HP rotor weighs 1230 lbs (558 kg)—

incorporates roller bearings throughout. These do not require the large lube oil

reservoirs, coolers and pumps, or the pre-and post-lube cycle associated with

journal bearing designs. Roller bearings have proven to be extremely rugged

and have demonstrated excellent life in industrial service. Most bearings provide

reliable service for over 100,000 hours. In practice, it is advisable to replace

bearings when exposed during major repairs, estimated at 50,000 hours for gas

generators and 100,000 hours for power turbines.

There are many roller bearing types. They are differentiated according to the

direction of the main radial loads (radial bearings) or axial loads (thrust bearings),

and the type of rolling elements used, balls or rollers. Figure 13-1 shows

the different types of bearings. The essential difference between ball bearings

and roller bearings are that ball bearings have a lower carrying capacity and

higher speeds, while the roller bearings have higher load carrying capacity and

lower speeds.

The rolling elements transmit loads from one bearing ring to the other in the

direction of the contact lines. The contact angle α is the angle formed by the

contact lines and the radial plane of the bearing. α refers to the nominal contact

angle, i.e., the contact angle of the load-free bearing as seen in Figure 13-2. Under

axial loads, the contact angle of deep groove ball bearings, angular contact ball

bearings, etc., increases. Under a combined load it changes from one rolling

element to the next. These changing contact angles are taken into account when

calculating the pressure distribution within the bearing. Ball bearings and roller

bearings with symmetrical rolling elements have identical contact angles at their

inner rings and outer rings. In roller bearings with asymmetrical rollers, the

contact angles at the inner rings and outer rings are not identical. The equilibrium

of forces in these bearings is maintained by a force component, which is directed

towards the lip. The pressure cone apex is that point on the bearing axis where the

contact lines of an angular contact bearing (i.e., an angular contact ball bearing, a tapered roller bearing, or a spherical roller thrust bearing) intersect. The contact

lines are the generatrices of the pressure cone apex. In angular contact bearings,

the external forces act, not at the bearing center, but at the pressure cone apex.

Rolling bearings generally consist of bearing rings, inner ring and outer ring,

rolling elements that roll on the raceways of the rings, and a cage that surrounds

the rolling elements as seen in Figure 13-3. The rolling elements are classified

according to their shapes. Into balls, cylindrical rollers, needle rollers, tapered

rollers, and barrel rollers as shown in Figure 13-4.

The rolling elements’ function is to transmit the force acting on the bearing

from one ring to the other. For a high load carrying capacity it is important

that as many rolling elements as possible, which are as large as possible, are

accommodated between the bearing rings. Their number and size depend on

the cross section of the bearing. It is just as important for load ability that the

rolling elements within the bearing are of identical size. Therefore, they are sorted

according to grades. The tolerance of one grade is very slight. The generatrices of

cylindrical rollers and tapered rollers have a logarithmic profile. The center part

of the generatrix of a needle roller is straight, and the ends are slightly crowned,

this profile prevents edge stressing when under load.

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