How does one go about sizing a gearbox? This simple question can generate a plethora of answers and myriad of opinions. Here are a few ideas and tips on the various methods used to size gearboxes.

Know the Performance Requirements

Whether they are design inputs from a potential customer or specs for a new standard product line, the performance requirements of your application are essential in determining gearbox size. Examples of performance requirements are as follows:

• Speed – how fast does the application require the output shaft to turn?
• Torque – What is the torque load requirement?
• Overhung Load – External radial loads applied to the output shaft may require more gear load capacity and therefore larger gears.
• Duty Cycle – How long will the gearbox operate before turning off? Depending on life requirements, intermittent duty applications can utilize smaller gearboxes.
• Life – How many cycles or continuous hours are required to meet the desired life? Designing for 400 hours makes a big difference in the sizing compared to designing for 4000 hours.

Environment & Size

Application location and size requirements may influence gearbox size.

• Operating Temperature – will the temperature stay relatively the same or have large swings in variation?
• Space– does the gearbox need to fit within a specified area?
• Weight – must the gearbox weigh no more than a certain amount?

Efficiency

When coupled with a motor, does the overall efficiency and/or current draw need to be under a certain value? In general, gearboxes have a torque range in which they operate at optimal efficiency. If overdesigned (i.e. the gearbox has larger gears than necessary), the gear set will not achieve optimal efficiency at the operating torque. However, care must be taken not to under-design the gearbox when trying to achieve optimal efficiency and thereby sacrifice life.

Cost

Often the most critical factor in any product, the cost ceiling impacts many design choices. Material and processing selection comprise a high percentage of the overall cost.

• Machining Steel – while many variations exist with different mechanical properties, machinability and cost, machining steel is a very common material used to produce gears, pinions and shafts. Machining steels come in various forms such as castings, forgings, and cold or hot rolled bar stock. High accuracy gears can be produced from machining steels for high capacity and low noise using relatively inexpensive cutting tools. Machining steels can be heat treated to increase load capacity and potentially reduce the size of the gear set. After heat treatment, gears can be machined again (hard hobbing, skiving, grinding, etc.) to account for distortion. However, each additional operation (heat treat, grinding, etc.) adds both tooling and piece price cost.
• Powdered Metal – a process by which metal is ground, mixed into a powder, compacted in a die and then heated in ovens. Powdered metal gears potentially offer a lower cost solution as opposed to machining steel. On the other hand, powdered metal gears have lower load carrying capacity and cannot achieve as tight of tolerances compared to machined steel gears. Lower load capacity will likely affect the gearbox size but is not as critical with smaller gear sizes.
• Plastic – while having approximately 10 percent of the load capacity compared to steel, injection molded plastic gears excel in cost-sensitive, low-torque applications. Intricate mold designs can provide weight savings versus steel.
• Bronze – the use of bronze gears is highly dependent on the application; bronze gears are most commonly used in worm gear sets. The sizing of worm sets is primarily dependent on the performance requirements.

As discussed, many factors must be considered to correctly size a gearbox. Working closely with the supplier is essential in providing a gearbox solution that meets your specific needs and expectations.

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