SMITHs - ENGINEERING DATA

The following information is intended to assist the machine designer or bearing user after the basic decision to use a cam follower type bearing has been made. This engineering data should be considered in selecting the size and style of cam follower that will provide adequate service life. Other factors such as alignment, lubrication, shock and track capacity will affect performance and their effect must also be considered in order to achieve the most favorable design.

The ratings and calculations are theoretical values based on ideal conditions. Most applications will involve less than fully optimum conditions, and in a situation where a particular size bearing may appear marginal in capacity, it is suggested that the next larger bearing be selected.

Please consult our engineering department when specific application conditions are not covered in this section.

BASIC DYNAMIC CAPACITY

The Basic Dynamic Capacity rating (B.D.C.) is that radial load in pounds that 90 percent of a group of bearings will support for one million revolutions at a speed of 33 1/3 rpm without failure. this is equivalent to a B-10 life of 500 hours. All catalog load ratings will be stated in terms of Basic Dynamic Capacity.

BEARING LIFE

Assuming that cam followers have been suitable mounted and lubricated, they will operate with nominal internal wear until such time as fatigue failure of the rolling elements takes place. This is evidenced by spalling or flaking on the surfaces of these components. Bearing life is expressed as the number of revolutions, or the number of hours at a given speed, that a bearing will complete before fatigue failure occurs. Life will vary from bearing to bearing; however, it will stabilize into a definite pattern when the same size and type of a large number of bearings are considered.

The B-10 or minimum life is the most common expression of bearing life. It is defined as the number of revolutions (or hours as a constant speed) that 90 percent of the tested bearings will complete or exceed before failure. The life attained or exceeded by 50 percent of the same group of bearings is called B-50 or average life.

EQUIVALENT LOAD

Since the Basic Dynamic Capacity seldom relates to actual speed and life conditions present in a given application, a formula for the conversion of this data has been developed. The Equivalent Load formula allows the designer to establish the load capacity required to operate within any set of life or speed conditions. The life and speed factors may be taken from the following tables.

Basic Dynamic Capacity = Equivalent Load * Speed Factor * Load Factor

Example:
Find the Basic Dynamic Capacity needed to carry a 2000 pound load at 250 rpm for 600 hours B-10 life.

2000 * 1.83 * 1.064 = 3895 pounds (load)(SF)(LF)
Select a cam follower with B.D.C. of at least 3895 pounds.

MAXIMUM ALLOWABLE LOAD

The amount of radial load a stud type cam follower can support is limited by the bending and shear strength of the stud which supports the bearing. This value is distinct from other capacity ratings which are based on rolling elements only. Proper installation will keep the bending moment to a minimum. The Maximum Allowable Load should never be exceeded and is listed for each bearing in the dimensional tables.

TRACK CAPACITY

The track or cam capacity for both cam followers and cam yoke rollers is the load which a steel track at a certain tensile will withstand continuously with negligible deformation. This will affect life and performance and may be the limiting condition in an application. While the track capacity can be increased with a harder track material, the working loads of the bearing should not be exceeded.

Table 1, below, lists steel track capacities for all cam follower bearing sizes with a track hardness of Rockwell "C" 40 or a tensile strength of 180,000 PSI. To obtain the capacity rating for other track hardnesses, multiply the track capacity by the factor shown in Table 2.