This post is divided into two parts. Part one is the fundamentals and equations of calculating coupling sizing torque... followed immediately by part two, short cuts to calculating coupling sizing torque.
Part 1: Fundamentals of Calculating Coupling Sizing Torque
One key component of sizing and selecting nearly every coupling is calculating the Nominal Torque as well as the Design Torque. Using inch (imperial) units of measurement...
Nominal Torque = Horse Power x 63025 / RPM = Application Torque (in-lbs)
While system nominal/application torque is important to know and understand, it is imperative that this NOT be the torque used to select the coupling. Rather, the application torque must be multiplied by a service factor (application specific) to ensure the coupling is robust enough to handle the unique challenges of that specific system.
Design Torque = Application Torque x Service Factor = Coupling Sizing Torque (in-lbs)
Application service factors are generally available and listed in each manufacturer's PDF product catalog... and it is this service factor that should be used when selecting your coupling. Different manufacturers do rate their couplings differently (i.e. - some companies like publishing/marketing high catalog performance values... and will compensate for their higher catalog values by insisting on higher service factors), so it is important that you use the manufacturer's published rating and not a rating found elsewhere. That said, if you want to learn more, and become an expert on coupling service factors... there is an AGMA information sheet readily available to download and review (AGMA 922-A96: Load Classification and Service Factors for Flexible Couplings).
Part 2: How to Quickly Calculate Coupling Sizing Torque
Given how frequently coupling users (and application engineers) are required to calculate coupling sizing torque, Lovejoy built a free coupling sizing torque calculator that is readily available both on the internet, as well as in iPhone/Android smart phone form (pictured left). In both cases, the user enters the horsepower, RPM, and service factor... and the required coupling sizing torque is automatically calculated. Values can also be calculated or converted between metric and inch (imperial) units by a toggle button at the top of the screen.
(Because the tool is supplied by Lovejoy, the tool also auto selects a coupling for review within 9 major coupling types... just tap the coupling type in question... and, if that coupling type cannot accommodate the required coupling sizing torque, it will automatically gray itself out.)
To download the iPhone or Android app, search for "Lovejoy" on your app store and/or learn more about the app by clicking here. To use the online web tool, click here.
Assuming you are pulling the service factor from the appropriate manufacturer's catalog, the torque calculator aspect of the Lovejoy tool can be used for any coupling application (independent of if you are using a Lovejoy solution or a solution from another manufacturer).