T2-2 NT 63,000 3 2 1 1/2 1/3 1/4 0 4 12 25 28 40 50 TORQUE lb. in. HP = Figure 1    Relationship Between                   Horsepower, Torque and                   Rotational Speed 1.0    INTRODUCTION A coupling is a design component intended to connect shafts of two mechanical units, such as an electric motor and a hydraulic pump or compressor driven by this motor, etc. As stated in the Resolution of the First International Conference on Flexible Couplings [1, 3], "...a flexible coupling, although it is relatively small and cheap compared to the machines it con- nects, is a critical aspect of any shaft system and a good deal of attention must be paid to its choice at the design stage." The following is a brief engineering data on couplings. More details are available in [1, 3]. The application considerations for couplings are numerous. The most important are the following: • Torque and Horsepower • Allowable Shaft Misalignment • Lateral and Axial Flexibility of Coupling • Torsional Flexibility • Backlash • Rotational Velocity Error • Service Conditions 2.0    APPLICATION CONSIDERATIONS Flexible couplings are designed to accommodate various types of load conditions. No one type of coupling can provide the universal solution to all coupling problems; hence many designs are available, each possessing construction features to accommodate one or more types of application requirements. Successful coupling selection requires a clear understanding of application conditions. The major factors governing coupling selection are discussed below. 2.1     Torque and Horsepower The strength of a coupling is defined as its ability to transmit a required torque load, frequently in combination with other factors. Hence, a coupling may be selected whose rated torque capacity is many times greater than needed. For example, in a coupling subject to wear and increasing backlash, a useful torque rating would depend chiefly on back- lash limitations rather than strength. For manually operated drives, the torque imposed through improper handling may be in excess of the drive torque required. Couplings are frequently specified in horsepower capacity at vari- ous speeds. Horsepower is a function of torque and speed, and it can be readily de- termined from the formula: HP = where N = rotational speed in rpm and T= torque in lb. in. This relation- ship is graphically represented in Figure 1. 2.2     Shaft Misalignment Shaft misalignment can be due to unavoidable tolerance build-ups in a mechanism or intentionally produced to fulfill a specific function. Various types of misalignment, as they are defined in AGMA Standard 510.02, are shown in Figure 2. NT ______ 63,000