Variable Loading

Bearing loads are frequently variable and occur in some identifiable patterns:• Piecewise constant loading in a cyclic pattern• Continuously variable loading in a repeatable cyclic pattern

Variable Loading with Piecewise Constant Loading

The loads Fei are equivalent steady radial loads for combined radial-thrust loads.

Variable Loading with Periodic Variation

Shigley’s Mechanical Engineering Design

Shigley’s Mechanical Engineering Design

Variable Loading with Periodic Variation

Shigley’s Mechanical Engineering Design

Selection of Ball and Cylindrical Roller Bearings

We have enough information concerning the loading of rolling-contact ball and rollerbearings to develop the steady equivalent radial load that will do as much damage tothe bearing as the existing loading. Now let’s put it to work.

Example 11-7

shown in Figure 11-12 is a gear-driven squeeze roll that mates with an idler roll. The roll is designed to exert a normal force of 5.25 N/mm of roll length and a pull of 4.2 N/mm on the material being processed. The roll speed is 300 rev/min, and a design life of 30 kh is desired. Use an application factor of 1.2, and select a pair of angular-contact 02-series ball bearings from Table 11-2 to be mounted at 0 and A. Use the same size bearings at both locations and combined reliablitiy of at least 0.92.

Example 11-7

Example 11-7

Shigley’s Mechanical Engineering Design

Example 11-7

Shigley’s Mechanical Engineering Design

Example 11-7

A 02-35 bearing will do.

Bearing Lubrication

The purposes of bearing lubrication

◦ To provide a film of lubricant between the sliding and rolling

surfaces

◦ To help distribute and dissipate heat

◦ To prevent corrosion of the bearing surfaces

◦ To protect the parts from the entrance of foreign matter

Shigley’s Mechanical Engineering Design

Bearing Lubrication

Either oil or grease may be used, with each having advantages in

certain situations.

Shigley’s Mechanical Engineering Design

Shigley’s Mechanical Engineering Design

Some Common Bearing Mounting Configurations

Fig. 11–20

The most frequently encountered mounting problem is that which requires one bearing at each end of a shaft. Such a design might use one ball bearing at each end, one tapered roller bearing at each end, or a ball bearing at one end and a straight roller bearing at the other. One of the bearings usually has the added function of positioning or axially locating the shaft. Figure 11–20 shows a very common solution to this problem. The inner rings are backed up against the shaft shoulders and are held in position by round nuts threaded onto the shaft. The outer ring of the left-hand bearing is backed up against a housing shoulder and is held in position by a device that is not shown. The outer ring of the right-hand bearing floats in the housing.

Some Common Bearing Mounting Configurations

Shigley’s Mechanical Engineering Design

Fig. 11–21

Figure 11–21 shows an alternative method of mounting in which the inner races are backed up against the shaft shoulders as before but no retaining devices are required. With this method the outer races are completely retained. This eliminates the grooves or threads, which cause stress concentration on the overhanging end, but it requires accurate dimensions in an axial direction or the employment of adjusting means. This method has the disadvantage that if the distance between the bearings is great, the temperature rise during operation may expand the shaft enough to destroy the bearings.

Some Common Bearing Mounting Configurations

Shigley’s Mechanical Engineering Design

Fig. 11–22

It is frequently necessary to use two or more bearings at one end of a shaft. For example, two bearings could be used to obtain additional rigidity or increased load capacity or to cantilever a shaft.

Some Common Bearing Mounting Configurations

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Fig. 11–23

Duplexing

When maximum stiffness and resistance to shaft misalignment is

desired, pairs of angular-contact bearings can be used in an

arrangement called duplexing.

Duplex bearings have rings ground with an offset.

When pairs are clamped together, a preload is established.

Shigley’s Mechanical Engineering Design

Duplexing Arrangements

Three common duplexing arrangements:

(a) DF mounting – Face to face, good for radial and thrust loads

from either direction

(b) DB mounting – Back to back, same as DF, but with greater

alignment stiffness

(c) DT mounting – Tandem, good for thrust only in one direction

Shigley’s Mechanical Engineering Design

Fig. 11–24

Preloading

Object of preloading

◦ Remove internal clearance

◦ Increase fatigue life

◦ Decrease shaft slope at bearing

Shigley’s Mechanical Engineering Design Fig. 11–25

Enclosures

Common shaft seals to exclude dirt and retain lubricant

Shigley’s Mechanical Engineering Design

Fig. 11–26

Felt seals may be used with grease lubrication when the speeds are low. The rubbing surfaces should have a high polish.

The commercial seal is an assembly consisting of the rubbing element and, generally, a spring backing, which are retained in a sheet-metal jacket.

The labyrinth seal is especially effective for high-speed installations and may be used with either oil or grease.