In the February Tooth Tips column, an overview was presented of the four major elements — application, specifications, design, and process — that dictate the ultimate accuracy of gearing and how well it satisfies the customer. Within this series, we take a closer look at each, starting with application.
Application
During the application phase, there is no need to sharpen the pencil or fire up the computer just yet. It is time to rely on common sense and engage your team’s accumulated years of “street smarts.” Apply a basic methodology, such as the flow diagram shown in Figure 1, and ask many seemingly simple questions. The more questions asked, the greater your understanding will be of what has to happen and what may influence the outcome.
The designer’s comprehension of these factors will be essential in establishing a solid footing to guide the gear design and ultimately satisfy the customer’s requirements.
Input
The first question to ask is, what will be driving the gearing? Typically, the horsepower and speed are provided, but these alone do not tell the entire story. It is essential to look deeper to fully understand the application and expose additional details.
What will the input power source be? Will the input power be from more than one unit? Will it be diesel, gas, turbine, inline, V 8/10/12, or an electric motor?
The sample output curves shown in Figure 2 and Figure 3 illustrate some of the differences to be aware of.
Additional questions you should ask include: How will multiple inputs be governed? How much will the power fluctuate? What are the speeds at which the unit will be operating within, and if at a set rpm, what are the torque pulses? Will the rpm vary? Will the power be directly coupled, belt-driven, or clutched? Will the engagement be quick or gradual and at what speeds, and will it be manually or electronically controlled? What happens in the event of an abrupt shutdown, loss of fuel, or a mechanical failure?
Engagement types can directly affect the presence and magnitude of shock loading during start-up or transitional events. Secure the actual horsepower curves for the power sources and performance specifications for any coupler or clutching devices. One other consideration (not related to performance) is whether there are any footprint or envelope restrictions.
Output
Once you have a firm understanding of the input side, it is then necessary to scrutinize the output. In most cases, the customer will specify the basic requirements such as output speed, the load to be driven, output orientation to input axis, and the typical duty cycle. The relationship of the output to input speeds and whether multiple speeds or change in direction are necessary has to be defined. If multiple speeds are required, consideration of shifting mechanisms, multiple gear meshes, and gearing configurations may be required. The coupler questions previously raised are repeated here for the same reason, but with a focus on the driven member. The shock loading to the driven member and the rate at which the load is applied to the gearing should be appreciated. Noise of the gearing along with whether the driven load will vary over time from first predicted is also a concern. Be mindful of the output placement and if it satisfies packaging constraints.
External Influencers
This area is often overlooked yet it is frequently the cause of many failures. Be persistent in asking questions, and look for the unusual. Consider the environment bilaterally. What might the incoming influencers be, and how might the design impact the environment?
Incoming: Check out the practices and habits of the customer. How will the gearing be handled, cleaned, assembled, lubricated, and maintained? If assembly is required, is the design asymmetric to ensure proper assembly? Anticipate misuse and compensate for it in the design.
How will the gearing be supported or mounted? Will the gearing or gearbox be subjected to long-term storage, and if so, how will it be preserved and brought back online? Will the gearing be exposed to any corrosive chemicals, excessive heat, cold, gases, or foreign liquids? Can the gear enclosure breathe to the outside and entrap moisture or create condensation? If so, how will the design accommodate it? A simple practice, such as pressure-washing a product, while meant to keep it clean, can almost certainly drive cleaning fluids and water past seals and into the assembly.
What will the immediate surroundings be, and can you isolate the gearing from adverse conditions? Considerations for vibrations, isolation mounting, protective surroundings, filtration, and a robust maintenance schedule can be invaluable.
Outgoing: A holistic design also takes into consideration how it may impact the environment. (See Figure 4.) Is the gearing designed to minimize noise, will any oil vapors be released, is the design safe for operation, and if a failure should occur, has risk mitigation been considered? Is there an opportunity to use less materials, increase recyclable content, and use materials with lower environmental impact? Are manufacturing processes considered for energy efficiency, and can the gearing be reconditioned and reused? Don’t forget to consider end-of-use options as well. These considerations can be even more relevant when working with plastics.
Conclusion
A comprehensive and balanced understanding of the application and how it interfaces with the environment is essential in developing a solid design. The key, as stated before, is awareness to help guide your design decisions.
