What You Need to Know About Gearbox Service Factor

Friday - 16/10/2020 11:59

What You Need to Know About Gearbox Service Factor

Service factor plays a crucial role in a long-lasting, reliable gearbox. Service factor is a way to measure how well a gearbox or a gearbox motor will handle specific demands and operating conditions, depending on the application. Let's take a look at why the gearbox service factor is essential, what it means, and how to optimize it for your requirements.
Why Is Gearbox Service Factor Important?

When choosing an industrial gearbox, it's essential to account for typical usage and operating demands. Doing this will enable you to select a gearbox that won't experience rapid wear and tear or premature failure, costing your business lost productivity and parts replacement.

Two electric motor drives of the same size doing the same primary function may be subject to entirely different stress levels when operated in other conditions. Especially in harsh industrial environments, it can be challenging to predict the maximum stress levels that a gearbox will experience. Gearboxes that start and frequently stop, cyclic loads, high peak loads, vibration, high duty cycles (i.e. running 24 hours/day), and high ambient temperatures will experience a shorter lifespan. High-stress environments greatly accelerate theaverage rate of wear and tear.

This massive usage results in a service life that is much shorter than expected. Typically, choosing the correct gearbox designed for higher horsepower applications extends its life span exponentially. But higher horsepower gearboxes are more expensive, so you may be wondering how much oversizing you need for your application. To answer this question, let's first examine what gearbox service factor is.

What Is Gearbox Service Factor?

The gearbox service factor is the ratio between the horsepower that a gearbox is rated to handle and the horsepower required for the application. In practical terms, it defines a performance safety margin that may be required by incredibly demanding tasks to help ensure long-lasting and trouble-free operation.

For example, a service factor of 1.0 means that a gearbox only meets the application's design horsepower requirements, without any safety margin. A service factor of 2.0 indicates a gearbox that can handle double the required application horsepower and is highly oversized for the task.

The gearbox's service factor directly impacts the durability and resistance to pitting and bending fatigue of the gear teeth. As a general rule, the gears' longevity is proportional to the increase in service factor raised to the 8.78th power. For example, a 30% additional service factor will result in a 10x increase in the gears' lifetime. But how exactly is the service factor calculated for different types of gearbox applications?

How Do I Calculate My Gearbox Service Factor?

To choose the right gearbox, calculate the service factor you require and then match it to the rating on the gearbox to determine if it will be suitable. Calculating the service factor required of a gearbox is not an exact science. Numerous operating conditions have different impacts on how well the gearbox will perform. The gearbox properties, including gearing, the construction materials, bearing quality and design characteristics, affect its ability to withstand its demands. Calculating the service factor is based on practical experience guidelines rather than empirical formulas.

The AGMA provides widely used standards for determining a gearbox's service factor. These are guidelines that primarily consider the type of application and the duty cycle to provide a good value based on gearbox manufacturers' extensive experience supplying different industries. Manufacturers use these guidelines to determine the service factor rating for a gearbox used in a specific application.

For example, for typical industrial tasks, a value of 1.4 is adequate. However, this rating can increase depending on how long the gearbox is in use and load characteristics. Suppose your gearbox will be operating in exceptionally stressful circumstances, or you're unsure of the operating conditions. In that case, select a higher service factor, or consult with a trusted gearbox supplier to determine whether it can handle the demands you'll put on it.

What Is Gearbox Service Class?

Many manufacturers use the concept of service class rather than a service factor value to simplify matching a gearbox to an application. A service class essentially corresponds to a range of service factor values, and each service class is around a commonly used service factor. Because calculating service factor isn't precise, using a service class instead can be more useful than a numerical value and provide a small margin of safety when selecting the right gearbox for the task.

There are three main service classes in common usage. Service class I corresponds to a minimum service factor of 1.25. Service Class II corresponds to a minimum service factor of 1.40, and Service Class III corresponds to a minimum service factor of 2.0. Class I gearboxes would be used in typical industrial tasks, while Class III gearboxes uses would be in a very heavy-duty application that requires exceptional resilience.

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