"INTRODUCTION This Data Item presents a procedure for assessing the suitability of a user-specified commercial needle-bearing roller follower driven by a disc cam. The choice of an appropriate roller follower is an important step in the design of cam systems used in many industrial machines and the use of the procedure presented in this Item@ in conjunction with a suitable manufacturer's catalogue@ will ease this choice. A decrease in design time can be achieved together with improvements in life@ load/speed capacity and hence machine reliability. Cam roller followers are subject to conditions of continuously varying angular velocity and load during operation. To assess the performance of a given roller it is necessary to determine@ at a series of cam angular positions@ the values of load and velocity and then@ using the method recommended in manufacturers' catalogues@ to convert them to equivalent load and equivalent velocity. This analysis is carried out by a computer program (ESDUpac A9126) supplied with this Data Item. The program determines the estimated life@ the maximum contact force at the cam oller interface@ the maximum load on the roller stud or spindle and the maximum angular velocity of the roller. These values are then used to assess the suitability of the chosen roller. Assumptions The procedure presented in this Item is applicable to both translating and swinging roller followers driven by disc cams with spring force closure. It is assumed that: (a) forces acting on the roller are due only to follower system inertia@ action of the return spring and externally applied load acting on the follower system (friction in the follower system and gravitational force may be modelled as part of the external load if required)@ (b) friction force at the roller pivot is negligible@ (c) slip does not occur between the cam and roller?@ (d) the cam and roller are designed and manufactured so that the roller can accurately follow the cam profile at all times (undercutting does not occur)@ (e) the cam and roller remain in contact throughout the operating cycle (see Section 3.4)@ and (f) the follower system is sufficiently stiff to be considered rigid. This list of assumptions is not comprehensive; further specific assumptions are described in Section 3. Limitations and Precautions The use of cam laws that exhibit infinite jerk (see Reference 5) is excluded because such motion causes shock loads on the roller and possibly roller slip. The procedure in this Item assumes that the dominant failure mode for cam roller followers is fatigue of the rolling elements. This is true for perfectly aligned@ well lubricated bearings at normal temperatures operating in a clean environment. In practice the life of a roller may be limited by wear or corrosion. Careful choice of cam material@ heat treatment and surface finish using Data Item 94017 (Reference 14) will minimise the possibility of premature failure by these modes. To evaluate the roller fully it is necessary to check that contact stress at the cam surface is acceptable (see Reference 13) and that curvature and pressure angle constraints (see References 8 and 12) are not infringed. Use of This Data Item The theoretical background is described in Section 3 of this Data Item. It is recommended that the necessary calculation is carried out using the accompanying computer program (ESDUpac A9126) described in Section 7. A guide to the choice of values for the inputs to this program is presented in Section 4. The examples in Section 6 provide valuable hints on the effective use of the program. Analysis of roller performance may be useful: (a) during a feasibility study (see Data Item 94017 ?C Reference 14)@ (b) as part of a detail design (see Data Item 94017 ?C Reference 14)@ (c) to investigate the cause of roller failure@ or (d) to upgrade mechanism performance by retro-fitting. It is likely that the nature of the information available in the above cases will vary. For instance during a feasibility study the mass of the follower and the externally applied forces may not be known exactly. The analytical procedure is applicable when uncertainty surrounds values of inputs but the accuracy of the results is obviously dependent on the quality of the input information. It is recommended that the program be used repeatedly during the design process as increasingly accurate information becomes available. It may be necessary to run the program with different rollers to identify the most suitable roller (see Section 4.2). A bearing manufacturer's current catalogue is essential to obtain values for inputs to the program and to assess the program output. ? Assumption (c) is obviously questionable. Slip sometimes occurs between the roller and the cam especially at high speed but this is very difficult to predict. Use of the ""no slip"" assumption results in a good first-order approximation of roller speed and roller life under most operating conditions."