In the April Tooth Tips column, we covered specifications — the second major element that dictates the ultimate accuracy of gearing and how well it satisfies the customer. In this column, we continue the series by taking a look at the third major component for quality design: the design process.
Design, by its nature, is an iterative process taking into consideration many variables. It also involves a series of steps that engineers and designers follow to find a solution to a need.
Once you have a comprehensive understanding of the application, specifications, and desired objectives, it’s time to start the design process. There is no one right approach. The process you choose and its rigor is a culmination of your experiences, technical academia, past field performance, historical warranty analysis, industry standards, and company policies. There are, however, basic fundamentals that should be part of your approach:
- A basic understanding of gearing theory.
- An understanding of the terminology used in gearing (see reference in Figure 1). A complete and detailed list of the terminology with definitions can be found in many gear design resources. One such reference is ANSI/AGMA 1012-G05 (Gear Nomenclature, Definitions of Terms with Symbols).
- An understanding of materials and their mechanical properties.
- Basic-to-moderate mathematical skills.
- Fluency with the related equations and software options.
- Familiarity with the various gear types available and the benefits of each.
- A working knowledge of various manufacturing approaches and their effect on gear performance.
- Consideration for reliability, safety, and efficiency.
- The ability to be open-minded and recognize that the first design solution may be too costly or impractical and that refinements may be necessary.
Repeatability in the design process and thereby higher confidence in the expected outcome is essential to your continued design success. Repeatability can best be achieved by developing and institutionalizing a well-documented and detailed procedure that your design team will use for each gear design project. Referral to industry standards are excellent references when developing your own process. Here are a few key items that a design procedure should consider.
- Commit to a well-defined and repeatable design approach. Identifying and applying a comprehensive gear or gearbox design approach will be imperative. Most companies have developed a process over years of experience, successes, and failures, while some companies may just be starting out. Regardless of the origin, challenge the process and make it intuitive and easy to replicate. Industry standards such as AGMA ISO 22849-A12, ANSI/AGMA 6001-E08, ANSI/AGMA 2005-D03, and ANSI/AGMA 6123-B06 will provide a comprehensive background. Also, gear design and simulation software packages can help offer procedural guidance, consistency, and accuracy. Most packages are inclusive and provide the geometry, dynamics, life, and load detail based upon the user’s input.
- Review and consider gearing options. There will more than likely be numerous solutions to meet the design objectives. Consider the appropriate gearing options, which can range from simple to complex, and review the pros and cons of each. At a higher level, the relationship of input-to-output shaft orientation, packaging constraints, and speeds will help define what type of gearing may be best. Simple parallel axis, right angle, or planetary solutions are just a few possibilities.
- Interpret the application and specifications. Referring to the March and April Tooth Tips columns covering application and specifications, use that collected information to quantify the parameters that will be required to perform the required calculations. These parameters — such as horsepower, speeds, center distances, and envelope size — are typically prompted by most software fields. In many cases, the inputs can be interdependent.
- Determine the best materials to be used. In order for the gear design to provide the expected objectives, it must be able to withstand both static and dynamic loading and resist contact surface failure. The mechanical properties of the materials to be considered for each member have a significant influence on each gear’s performance. Consequently, each gear and gear set should be scrutinized for optimum and compatible materials. The materials can also be altered through heat treating, which will affect the case and core hardness relationship and resultant performance. When selecting your material, also keep in mind manufacturability and availability.
- Perform the design calculations. Performing the calculations and analyzing the results will be an iterative process to optimize your design. The simultaneous consideration of factors such as stress, size, life, weight, cost, and manufacturability are just a few of the items that should be balanced. Others include:
- Power density
- Boundary conditions
- Starting torques
- Dynamic loading during running conditions
- Proper alignment and compactness of drive
- Pitch line velocities
- Contact stresses
- Service/safety factor
- Low noise
Keep in mind the objectives that you identified in the application and specifications stages, and challenge your design team to ensure that the objectives have been met. If compromises have to be made, quantify the impact and communicate that to your customer.
- Prototype and test when possible and identify improvements. Where project budgets and timing allow, test the design under conditions as close to real world as possible. Identify areas for refinement, and integrate them back into the design process. Even if you cannot apply the results to the current design, capture that knowledge for future opportunities.
Gear and gearbox design can be a challenging undertaking, and its success is dependent upon the thoroughness, consistency, and discipline of your design team’s approach. Having a design process in place is indispensable in delivering consistent results.