Tooth Tips: Requirements for Gear Design

December 15, 2016

The following steps help ensure a successful gearbox is designed and specified, and all of the customer’s desired attributes and design inputs are met.


A gearbox design program includes technical and non-technical requirements, including legal and regulatory as well as commercial considerations. The process starts with a description and full understanding of the type of gear or gearbox required and the nature of the application: what type of machine will it be used in, where and how will it be used, and what are the expected performance parameters. The desired cost and estimated volume (total number of units required and per year) are also important. Next, it should be determined who will have ownership rights to the final design: the customer or the service provider/consultant.

For the technical requirements, the design inputs include:

  • Overall gear ratio and number of different reductions and their gear ratios if multi-speed and the ratio tolerance.
  • Size and envelope requirements.
  • Weight restrictions.
  • Input and output interfaces and dimensions.
  • Duty cycle expected for unit, either supplied by the customer or estimated by the customer and service provider/consultant. This consists of a series of torque, speed, and percent-of-time values that are consistent with what the unit is expected to experience in actual field service. Developing this can require instrumenting an actual machine in service and recording the torques and speeds.
  • The type of system loading: Smooth and continuous or with peaks and spikes in torque.
  • Description and specifications of the prime mover: For example, an electric motor developing 50 horsepower at 3,600 rpm with a supplied torque and performance curve.
  • Life requirements, either a Miner’s rule weighted average value expected at this duty cycle or another value at one or more individual conditions.
  • Reliability considerations such as mean time between failures (MTBF).
  • Agreement on gear stress and life calculation methods and the applicable standards and governing bodies, e.g., AGMA 2001, ISO 6336, or DIN.
  • Reliability: Noted as a percent or failures per 100, for example, 99 percent reliability, 1 failure per 100.
  • Product design verification requirements if applicable: Design Failure Mode Effects and Analysis (DFMEA) or design reviews.
  • Product validation testing: Who is responsible for performing these tests and what are the acceptance criteria.
  • Any legal and regulatory requirements.

Next, qualitative and subjective customer preferences should also be discussed and documented, for example, low noise, light weight, smooth operation, high reliability, good serviceability, and low cost.

A Statement of Work can then be created that documents the scope of the project. It should be signed by the customer and service provider/consultant to provide a clear understanding of the work to be performed.

At this stage, the service provider/consultant can begin work on the initial design and engineering on the gear or gearbox system. This includes preliminary engineering calculations and creation of a concept layout drawing showing the initial configuration of the gearbox. These engineering calculations include gear data, stress and life calculations, and other gearbox component selection and calculations such as bearings, shafts, splines, and fasteners. An initial preliminary design specification then can be provided to the customer that includes:

  • Summary of design input information.
  • Concept layout drawing and preliminary design calculations including gear stress and life analysis and results.
  • A preliminary cost and lead-time estimate if required.

The service provider/consultant can select a product design verification plan using one of the following methods: third-party review, alternative calculations, or comparative analysis to past successful designs.

Various design reviews will normally take place during this process, some of which can be jointly attended by the customer and supplier. These include a concept or initial design review; design validation and verification review; manufacturing design review; critical design review; and final design review. A DFMEA may be required by the customer or initiated by the service provider/consultant.

This process may go through several iterations while the customer and service provider/consultant converge on a satisfactory and final design, and then freeze that design. The customer then gives final acceptance and approval to proceed with finishing the details of the final design.

The service provider/consultant completes the design and analysis work. Design outputs are prepared and presented to include:

  • An assembly drawing showing cross-section views of the gearbox with balloons and item numbers corresponding to the bill of material.
  • Installation drawing with interface specifications and overall dimensions.
  • Bill of material.
  • Assembly procedures.
  • If it was determined that the customer will own this design, then the individual part drawings can be provided.
  • Copies of design reviews performed during this product’s development.

After the design is complete and all information is transmitted to the customer, a prototype gearbox can be manufactured, assembled, and tested. Product testing and validation feedback should be provided to the service provider/consultant to ensure the product meets the design requirements and is satisfactory. 

About The Author

Rick Miller

is president/sole member of Innovative Drive Solutions LLC, which provides gear design engineering services for optimizing gears, gearboxes, and other geared devices. Miller has created over 300 original gearbox system designs. He is a member of AGMA, SAE, and ASME, and he holds three patents. He is vice chair of the AGMA Vehicle Gearing Committee. For more information, visit www.innovativedrivesolutions.com.