In the March Tooth Tips column, we covered applications — the first 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 second major variable for quality design: specifications.
Before you start to develop the specifications for the gear design, the first step is to thoroughly understand the design requirements. Requirements are what the design needs to accomplish or what objectives it must meet. Oftentimes, they can be mistaken as specifications.
Developing the correct requirements will involve a clear and shared understanding with your customer, as well as good communication and technical skills. Requirements tend to be higher level and describe factors such as price, reliability thresholds, efficiency, noise levels, performance, and weight. The requirements should not only highlight what the design should do, but just as importantly, what it should not do. The requirements can also be unrealistic at times, which will require negotiating with your customer. Recognize ambiguity, assumptions, and unknowns up-front and minimize them. Here are some sample questions to ask:
- What are the operational parameters? What is the input horsepower, efficiency required, noise limit, input and output speeds, and maximum temperature?
- What is the expected life? What is the price, weight, and size?
- Are there any dimensional limitations or contractual requirements?
- Are there maintenance considerations or installation concerns?
- What supporting documentation may be required?
- What is the duty cycle?
- What are the environmental conditions?
- What quality controls are needed?
- Are there any specialty tools required?
Specifications are a detailed description of the work to be done in a project. They identify the specific and quantifiable elements required to produce a gear design that will satisfy the objectives. Specifications describe the “hows” and can vary greatly depending on whether you are designing a new product, reverse engineering an existing design, or developing a manufacturing process for a customer’s design.
The specifications must be considerate of all the requirements. Challenge the customer and your design team to define technically and economically feasible specifications. To illustrate, let’s make some assumptions and look at several example requirements.
Operational Requirements: Speed, Noise, and Size
The input power is provided by an electric motor, so input speed will be 1,200 revolutions per minute (rpm) and the desired output speed is 3,600 rpm. Noise cannot exceed a dBA level of 80, and the support shaft bearings can only withstand minimal axial loading. This will require a speed increase of 1:3. This could be accomplished by one gear mesh with an input gear that is three times the diameter of the output gear. However, you are limited in size, so you need to use two gear meshes. Here is a possible specifications list recognizing these three requirements:
- Gearing to be used: herringbone
- Two gear meshes: ratios 1.5:1 and 1.5:1 with pitch diameters of A:0.67 A and B:0.67 B
- Ground to AGMA 12
- Noise levels to be checked at full power and speed and not exceed 75 dBA
- Package size not to exceed X by Y inches
The customer specifies that the original parts must perform for 20,000 hours with no catastrophic failure. However, they do not set any requirements for downtime or frequency of maintenance. Here are some specifications that may meet the objective:
- The lubrication and filters will be changed every 1,000 hours.
- The unit will be disassembled every 5,000 hours for a complete inspection.
- All seals and bearings will be replaced every 10,000 hours.
- Gearing will be designed using a service factor of 2.
- Gearing will use 8620 material, hardened to 60 HRC and ground.
The customer has set the requirement that gears as received on the dock must pass a 100-percent inspection layout.
- In-process inspection procedures will be set in place at each machining center to validate that feature tolerance falls within 50 percent of the print-allowed tolerances. Final inspection standards will be established for an acceptable quality limit (AQL) level and sampling plan of X to support production run sizes of Y.
In developing specifications, there are numerous standards and tools at your disposal. Military (Mil Spec) standards and industry standards such as ISO, OSHA, ANSI, ABS, API, and AGMA can be used to facilitate the process and provide additional credibility to the design. Tools such as quality function deployment (QFD) can also be used to help define and prioritize customer requirements.