Thrust and guide bearings
General Information
The turbine-generator unit presents remarkable masses under rotation and considerable inertia forces, that increase if masses are not balanced, come into play. The runner may be subject to lateral forces due to the asymmetric flow that strikes it and the generator may be subject to magnetic pull. These forces associated to the weight of the rotating parts must be controlled by means of the BEARINGS. In the case of HORIZONTAL machines, the main loads on the bearings are due to the weights that act in perpendicular with the machine shaft and the axial loads, also present, are smaller than those in the corresponding vertical unit because in this case there is no weight along the shaft. Making reference to a vertical unit, the axial loads are discharged by the THRUST BEARING and the radial ones by the GUIDE BEARINGS. The number of guide bearings depends on the type of turbine-generator unit and essentially by the length of the shaft line. A verification if the number adopted is correct is carried out by means of a calculation of the flexural critical speeds. The bearings may be oil, grease or water lubricated and, depending on the application, we can find sliding smooth (the most used) or rolling (rollers, balls) surfaces.
Guide bearing
In HORIZONTAL UNITS, the definition of guide maybe improper because they counteract also the weight of rotating parts. Anyway, once made this clarification, we can say that, depending on their behavior, in this type of machines we can identify two main types of bearings: STIFF and TILTING. The stiff ones are used if there is no appreciable deformation of the shaft line, while the tilting ones may self-align making use of spherical seats (fig. 1). Very often they are combined with the thrust bearing. The (fig. 2) illustrates one combined bearing with two opposite counterthrusts that prevent the axial sliding in both directions. Depending on the specific pressure on active parts and on the rotation speed of the shaft, the lubrication may be forced, by the aid of pumps, or inside the tank itself by using rings placed on the shaft that are dragged in rotation by the shaft itself. Their use is limited in first approximation to the following values of the peripheral speed U of the shaft: 2,5 m/s <U< 13 m/s out of this interval it is necessary to carry out the forced lubrication. Indicating by D the shaft diameter, the length of the active part of the bearing is on average L= (0,8 ÷ 1)
In vertical unit, the main function is to keep the shaft aligned vertically and support any radial force that can take place during the operation. The tilting guide pads (fig. 3.1 and 3.2) are usually adopted, even if we can find bearing with bushings in case of mean-small units. The lubrication may be natural with a thermal exchange coil placed inside the tank or forced by the aid of pumps and external exchangers. In general the use of this type of bearings requires the presence of a manifold, integral or placed on the shaft and, according to the dimension of the machine the number of pads may vary from 6, 8, 10, 12, .... The lining materials that can be used of the bearings are very different: rubber, bronze, resin, white metal, the last one is the most used one. An example may be SnSb8Cu7 UNI 4515 that presents in room temperature a hardness BRINNEL of 23÷27 MPa.
Fig 1 Fig 2 Fig 3.1 Fig 3.2
Thrust bearings
In general, in vertical machines, the thrust bearings are positioned above the generator and often they are combined with one guide bearing (fig. 4). Very important is the resting point of each pad with respect to the load barycenter. In monodirectional machines, the position of the resting point is not barycentric and this favors the formation of an oil film between pad and thrust runner. In reversible turbines (pump-turbines) the resting point must be located halfway of the pad length and therefore this will cause a more critical situation with respect to the monodirectional machines. As a general dimensioning criterion, we keep a specific pressure on the pad surface of 2,5÷4 MPa, (areaching even 6 MPa) depending on the material used. For what concerns the lubrication and the cooling system, it is valid what already said for vertical guide bearings. Generally, the maximum temperature measured on the pads does not exceed 80°C and that of the oil bath in which they are submerged 50÷60 °C. We can summarize 3 main aspects for a good operation of a thrust bearing:
1) limited specific pressure on the pad
2) homogeneous and generous cooling
3)surfaces in relative motion (pad and thrust runner) machined with narrow tolerances
General Information
The turbine-generator unit presents remarkable masses under rotation and considerable inertia forces, that increase if masses are not balanced, come into play. The runner may be subject to lateral forces due to the asymmetric flow that strikes it and the generator may be subject to magnetic pull. These forces associated to the weight of the rotating parts must be controlled by means of the BEARINGS. In the case of HORIZONTAL machines, the main loads on the bearings are due to the weights that act in perpendicular with the machine shaft and the axial loads, also present, are smaller than those in the corresponding vertical unit because in this case there is no weight along the shaft. Making reference to a vertical unit, the axial loads are discharged by the THRUST BEARING and the radial ones by the GUIDE BEARINGS. The number of guide bearings depends on the type of turbine-generator unit and essentially by the length of the shaft line. A verification if the number adopted is correct is carried out by means of a calculation of the flexural critical speeds. The bearings may be oil, grease or water lubricated and, depending on the application, we can find sliding smooth (the most used) or rolling (rollers, balls) surfaces.
Guide bearing
In HORIZONTAL UNITS, the definition of guide maybe improper because they counteract also the weight of rotating parts. Anyway, once made this clarification, we can say that, depending on their behavior, in this type of machines we can identify two main types of bearings: STIFF and TILTING. The stiff ones are used if there is no appreciable deformation of the shaft line, while the tilting ones may self-align making use of spherical seats (fig. 1). Very often they are combined with the thrust bearing. The (fig. 2) illustrates one combined bearing with two opposite counterthrusts that prevent the axial sliding in both directions. Depending on the specific pressure on active parts and on the rotation speed of the shaft, the lubrication may be forced, by the aid of pumps, or inside the tank itself by using rings placed on the shaft that are dragged in rotation by the shaft itself. Their use is limited in first approximation to the following values of the peripheral speed U of the shaft: 2,5 m/s <U< 13 m/s out of this interval it is necessary to carry out the forced lubrication. Indicating by D the shaft diameter, the length of the active part of the bearing is on average L= (0,8 ÷ 1)
In vertical unit, the main function is to keep the shaft aligned vertically and support any radial force that can take place during the operation. The tilting guide pads (fig. 3.1 and 3.2) are usually adopted, even if we can find bearing with bushings in case of mean-small units. The lubrication may be natural with a thermal exchange coil placed inside the tank or forced by the aid of pumps and external exchangers. In general the use of this type of bearings requires the presence of a manifold, integral or placed on the shaft and, according to the dimension of the machine the number of pads may vary from 6, 8, 10, 12, .... The lining materials that can be used of the bearings are very different: rubber, bronze, resin, white metal, the last one is the most used one. An example may be SnSb8Cu7 UNI 4515 that presents in room temperature a hardness BRINNEL of 23÷27 MPa.
Fig 1 Fig 2 Fig 3.1 Fig 3.2
Thrust bearings
In general, in vertical machines, the thrust bearings are positioned above the generator and often they are combined with one guide bearing (fig. 4). Very important is the resting point of each pad with respect to the load barycenter. In monodirectional machines, the position of the resting point is not barycentric and this favors the formation of an oil film between pad and thrust runner. In reversible turbines (pump-turbines) the resting point must be located halfway of the pad length and therefore this will cause a more critical situation with respect to the monodirectional machines. As a general dimensioning criterion, we keep a specific pressure on the pad surface of 2,5÷4 MPa, (areaching even 6 MPa) depending on the material used. For what concerns the lubrication and the cooling system, it is valid what already said for vertical guide bearings. Generally, the maximum temperature measured on the pads does not exceed 80°C and that of the oil bath in which they are submerged 50÷60 °C. We can summarize 3 main aspects for a good operation of a thrust bearing:
1) limited specific pressure on the pad
2) homogeneous and generous cooling
3)surfaces in relative motion (pad and thrust runner) machined with narrow tolerances