As the most demanding yet common mechanical system in a gear train, gearing provides a mechanical interaction to transmit power while changing torque and speed. To achieve a quality design, there are many variables to consider. Here, an overview of four major elements (application, specifications, design, and process) is presented, while subsequent Tooth Tips articles will cover each in more detail.
A complete understanding of where the gearing will be used is an essential starting point. The designer must comprehend the surrounding environment, if there are any external influences, and what are the primary requirements of performance. For example:
- Inputs: What will be driving the input gear/shaft — an engine, electric motor, turbine, or other means? What are the expected input speeds and at what horsepower? Is rotation clockwise or counterclockwise? Will there be any external controls for speed, shifting, or torque? Will the input produce any impulse or shock loading?
- Outputs: What is the sense of rotation? Is it opposite of the input? What is the desired output rpm? Will a gear train be needed to provide different ratios, and if so, how will clutching or speed changes be accomplished? Is the output axis parallel to, skewed, or at right angles to the input axis?
- External: Are environmental conditions a factor? Will the gearing be exposed to excessive heat, corrosive chemicals, condensation, high pressure cleaning, or periodic or poor preventive maintenance practices? Will the gearing need to be preserved for any extended storage periods, and if so, how will it be brought back online? How will the customer use the product?
The required performance characteristics are the driving force in the design. This greatly depends on whether you are designing a new product, reverse engineering an existing design, or developing a manufacturing process for a customer’s design. Determine the following questions:
What are the expected duty cycles? Are there limitations on size, weight, and cost? What is the expected life of the gearing? Will the product be a throwaway or rebuildable? Will the design require in-field serviceability, and will a standby be needed? What are the recommended service factors for the application, and are they adequate for the end use?
Once you have a good understanding of the desired outcome, you can start the design. There are many gear design software packages available on the market, and some basic programs are available free online. The more common software from Gleason, KISSsoft, or UTS (Universal Technical Systems, Inc.) is powerful, and some interface with your manufacturing equipment. Most are iterative and can provide the geometry, dynamics, life, and load detail based upon the input data.
- Determine the gear type and system. The relationship of the input and output shafts will help direct what type of gearing may be best — spur, worm, bevel, or hypoid. If larger changes in speed or torque are needed, you may need a gear train or planetary approach. For each mesh, consider that there is roughly a 2-percent parasitic loss.
- Determine the best materials to be used. The type and combination of materials can vary. In some applications, the driver and driven gears may need to be different materials for wear. For steel applications, 4340 or 8620 will meet most needs; at higher stress levels, a 9310 may be needed. Engineered plastics are also a solution typically used in lower load applications.
- Is lubrication required, and if so, what will the lubrication system look like — spray, splash, or grease? Lubrication is essential to prevent scuffing, wear, and pitting damage to gear tooth surfaces, and it helps dissipate heat. The right amount of lubrication is important. Too much in a splash system can create excessive foaming and heat.
- What are the speeds of the individual components? If pitch line velocity exceeds certain gear type minimums, it is designated as high speed. This can affect the decision on lubrication type and may introduce windage concerns.
- What quality level is required to deliver the life, noise, and accuracy of the intended design? Refer to the AGMA 2015 standard.
In designing the gear or gear train, consider the method of manufacturing. The processes and machines used can greatly affect the quality, cost, and life of the parts. Consider the following:
- How will the gear blanks be produced? Will you use forged, cast, or turned blanks? Each has its own characteristics when it comes to residual stresses, heat treat distortion, and cost.
- Scrutinize all operations after blanking to be sure nothing is introduced that could degrade design performance. Heat treating is probably the most critical and sometimes the most difficult to predict and control. Grinding after heat treat can help reduce distortion variability, but be sure to check for burning.
- Quality control is essential. In-process and final inspection protocol must be in place and validated. Don’t overlook handling or packaging.
When designing a gear, the key is to be aware of all the factors and make your design decisions accordingly.