Coupling Service Factors - Best Practices

When selecting a coupling, identifying and using the correct service factor is one of the three basic inputs (with the other two being horsepower and rpm). Given just these three pieces of information an initial coupling selection can quickly be made.

Unfortunately, not all service factors are created equal. While many manufacturers use AGMA 922-A96: Load Classification & Service Factors for Flexible Couplings as a baseline, this is NOT a standard basis for rating document across all manufacturers. As such, it is critical to check and use the manufacturer's recommended service factor first and foremost. 

Manufacturer's service factors may be specific to both application and the coupling type. Variability may exist because different couplings are more or less well suited for a given application, but they also may exist because of marketing gamesmanship. Without a standard basis for rating, some manufacturers have or may take liberty to present their product in what they believe to be the most favorable light. Efforts may include improving certain ratings (i.e. - increase published max torque) and compensating for it in other ways (i.e. - increasing service factors). As such, again, it is important to pull service factors from the manufacturer, make sure you are considering both the driven and driving equipment, and not assume a "one size fits" all approach.

To determine an application service factor for a Lovejoy branded coupling, please download the product catalog and find the correct application service factor chart for the given coupling type. Should you have any further questions regarding an application, please do not hesitate to contact a Lovejoy application engineer or your favorite Lovejoy authorized distributor. 

A PDF catalog screen capture of what a manufacturer's application service factor chart may look like for a given product range in their catalog has been provided below.

Screen Capture of an Application Service Factor Chart for Lovejoy Disc Couplings

Sizing a Coupling – Pay Attention to the Driver!

One of the steps in properly sizing a coupling is to understand the application.  Continuous operation vs stop-start, high inertia loading, excessive vibration, etc…  The severity of the application is used to modify the calculated application torque to truly represent the usage of the system.  A handy reference guide is the Driven Machine Service Factor charts that coupling manufacturers publish.  This will typically give a number between 1.0 and 3.0. 

An item that is often overlooked but should be considered for specialized applications (infrequently, but particularly pertaining to gear and disc couplings) is the driver.  When a Variable Speed Electric Motor or Internal Combustion Engine is used as the driver within a coupled system, the torque variations experienced by the coupling are more severe than with a standard electric motor system. This variation is included in the formula:

Application Torque x [Service Factor (Driven) + Service Factor (Driver)] = Selection Torque

The Driver Service Factor can vary from 0.0 (standard electric motor) to 1.5 (low cylinder count internal combustion engines).

As you can see, in specific specialized applications, the Driver can potential have a greater impact on sizing the coupling than the Driven unit!

Coupling Sizing Torque - How to Quickly Calculate (inclusive of Service Factor)

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).