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Application and Load Analysis for Large Diameter, Heavy Duty Turntable Bearings Many factors must be considered in selecting and applying an antifriction bearing. Chief among these are type and magnitude of loading, speed of rotation, and accuracy. For most applications in construction and material handling equipment, load is the primary concern. Speed and accuracy are relatively unimportant but deserve consideration along with other items such as friction torque, gearing, and mounting. Other applications, such as precision medical equipment, require a high degree of accuracy and close control of torque, but have relatively low loading. Load Because a turntable bearing accepts all types of loading, the main concern with load is its magnitude. See Pages 9-11 for load determination and bearing selection. Turntable bearings are designed primarily for dominant axial (thrust) and/or moment loading. In applications where radial load is significant or the dominant load, it may be advisable to use a bearing with a reduced contact angle. Radial load of a magnitude equal to 10% or less of the axial load may be neglected. For a tentative selection, radial load in excess of 10% may be converted to equivalent thrust load by using a multiplication factor of 5. Speed The application of a standard large-diameter bearing is normally limited to intermittent rotation at a maximum speed of 500 feet per minute at the pitch line (about 50 RPM for a bearing pitch diameter of 3 feet). Where continuous rotation under load occurs or the speed of rotation is greater than that recommended, the standard bearing design can be modified. This modification may include revisions in contact angle and manner of ball separation. In applications where the speed of rotation is greater than 1100 feet per minute, a different type of bearing must be used. Accuracy Positioning of the rotating member relative to the stationary structure may be of concern. With the bearing races securely fastened in a round condition on flat mounting surfaces, the main source of positioning error is internal bearing clearance--bearing runouts being small by comparison. See Page 22. Four-point contact bearings are furnished with sufficient internal clearance to allow for some imperfections of mounting surfaces and for small amounts of deflection under load. Bearings can be furnished with reduced internal clearance to minimize "rock". Extra care should then be taken to assure the installed bearings will be round and flat. Friction torque In most applications of large-diameter bearings the force required to overcome bearing friction is small compared to that required to overcome the inertia of the mass being supported--provided the bearing is properly mounted and contains the standard internal clearance. Bearing clearance is designed to minimize the possibility of tight spots resulting from ordinary imperfections in the mounting. A bearing distorted by out-of-flat or out-of-round mounting surfaces may require a tremendous amount of turning torque. The same is true for a bearing mounted on a structure which deflects locally under load. Unfortunately, this phenomenon is not always recognized until actually experienced. Other factors affecting bearing friction are bearing contact angle, separator and lubricant. A low torque requirement should be referred to Kaydon for special attention. Gears Gears furnished integral with turntable bearing races commonly have an AGMA Standard 20° full depth or stub tooth form the some provision for backlash. Where required, however, modifications of the basic tooth forms and alternate pressure angles can be furnished. For additional strength or where surface hardening is required, a full-round fillet can be provided. Safe tangential tooth loads are given for those bearings listed; however, it is recommended that the machine designer verify the adequacy of the gear for his application based upon his own methods of calculation and past experience. Bearing and pinion mountings lacking in rigidity can result in tooth end loading under the heavier loads. Many designers find it desirable to crown the pinion to compensate for this undesirable effect. *Mounting holes The preferred method of attaching turntable bearings is to bolt through both races with full circles of equally spaced fasteners. It is recognized, however, that the design of the mating structures may dictate the use of special bolt patterns and that assembly procedures may require tapped holes. There is no objection to such mountings, providing it is determined by actual testing, as well as analysis, that the fasteners will have adequate strength to sustain the maximum moment loads possible. See Pages 26-27 for more on bolts. Weld rings and weld bands Welding offers an optional method of attaching one of the races of turntable bearings. The bearing is furnished with a low carbon steel weld ring or band welded to the race. The ring can be welded to the machine without injury to the bearing, provided proper procedures and precautions are exercised. While welding has certain advantages, it is inconvenient to effect major maintenance or replacement of the bearing if damage should occur. Seals Seals are normally included in Kaydon large-diameter bearings and are recommended for bearing protection even where external seals or shields are provided. Gear protection is also important and should be considered when designing the bearing mount. Loading hole The rolling elements in Kaydon bearings may be inserted through a hole drilled radially through the ungeared race and then plugged. The area of the raceway interrupted by this hole is relieved to prevent it from receiving load. Whenever possible, however, the loading hole should be positioned out of the maximum load zones. Lubrication One or more grease fittings or lubrication holes are provided in all turntable bearings. Additional lube holes can be furnished on request. For further discussion of lubrication, see Page 26.Mounting and installation This topic is discussed in detail on Pages 25-31. Normal application Special attention must be given to bearing application whenever conditions are different from those considered normal. For a "normal application" of turntable bearings the following conditions should apply:
Basis for bearing load ratings The turntable bearings in this catalog are designed primarily for use in applications where applied loads may be high but speed of rotation is slow and operation is usually intermittent. In such applications, bearing fatigue life is of little concern and selection of the bearing may be based on its static rating. Static rating is defined as the maximum load which may be applied to the bearing while it is stationary without impairing the smoothness of subsequent operation. Load rating curves are supplied for most bearings listed herein. These curves represent the maximum combined axial and moment loads which may be applied to the bearing. When selected from the curves for a crane or application with similar operating characteristics most bearings can be expected to last for the life of the machine if the loading used in the selection is based on the maximum machine rating. Use of the curves is explained under Selection Procedures below. Typical applied loads To select a bearing for a given load condition, the actual bearing loads must be determined from the forces applied to the equipment in which the bearings will be installed. These forces will commonly be applied perpendicular to the axis of the bearing (radial force) or parallel to the axis (axial or thrust force). If not applied in either manner, the force can be resolved into components acting along similar lines. Location of the applied forces relative to the bearing will determine the moment load on the bearing. Radial forces must be located relative to the plane of the rolling elements with axial forces located relative to the bearing axis. Figures 1, 2, and 3 illustrate typical applications of external forces and the resulting bearing loads. Bearing load analysis
In this analysis the loaded race is considered to constitute a freebody in space acted upon by the applied loads and stabilized through the ball contacts by the other race. A plane is passed through the axis and the lines of action of the applied loads. For purposes of calculating the reactions R1, R2, R3, and R4, they are assumed to act only on the two balls whose centers are in the selected plane. Once the reactions are determined, the maximum reaction is assumed to be distributed over a limited number of balls based on the eccentric nature of the applied loads. The latter is determined from a comparison of the magnitudes of the reactions. While four possible reactions are indicated, only three of these will occur due to bearing deflections under the applied forces. To solve for the reactions, one must be assumed equal to zero. The three remaining reactions are then determined by the summation of moments about points selected from A, B, C, and D. If one of the calculated reactions is found to be negative, the original assumption of the inactive reaction is incorrect and a new assumption must be made. In general, bearings for construction, material handling, and similar types of equipment may be selected from the load rating curves. However, with radial load exceeding 10% of the thrust load is present, Kaydon should analyze all load data and recommend the bearing. Significant or dominant radial load may dictate the selection of a larger bearing or a modification of the contact angle. A detailed Kaydon load analysis is also recommended for those applications in which there is an appreciable variation in the load and operating conditions, and maximum loading is infrequent. This analysis can result in selection of a smaller, more economical bearing than that selected on the basis of maximum loading only. Calculated data includes maximum ball load, race size change, ball contact deflection, change in contact angle, size of the contact area, stress in the contact area, subsurface shear stress, static factor of safety, dynamic factor of safety, and bolt factor of safety. Selection procedure 1. Review preceding material, especially NORMAL APPLICATIONS before proceeding with selection.
2. Determine the preferred mounting arrangement-pinion and gear location, etc. 3. Determine the maximum bearing loads (see Figure 1-3). Consider all applied forces including work loads, wind loading on large superstructures, and gear loads if significant.
4. Refer to the list of bearings and their load rating curves. Pages 12-24. (Curves based on service factor = 1.00.) 5. Select a bearing on the basis of preferred mounting arrangement and maximum load condition. 6. Check capacity of the gear see Pages 27-29. 7. Check capacity of the mounting bolts see Pages 26-27. 8. Submit to Kaydon for engineering review. Typical mountings Kaydon bearings can be designed to suit a number of mounting arrangements. View the six basic arrangements here. These can be varied to suit requirements peculiar to a specific application. Such variations include types of holes, location and number of lube holes, omission of integral gear, incorporation of special seals, etc. The mounting structures shown are intended to be illustrative only. Important details in design such as mounting plate thickness, location and number of stiffening members, and bolt lengths must be determined by the equipment designer. We invite you to consult with Kaydon prior to selecting or mounting your large bearings. Bearings Application and Load Analysis
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To determine the effects of combined
loading, Kaydon uses a unique freebody
analysis. This analysis was developed as
part of a study of large diameter anti-friction
bearings conducted for the Massachusetts
Institute of Technology under
a United States Air Force contract. As
illustrated in