Understanding Functions – Part 2

Beginner’s Solution

In an FMEA, which of the following is true about a “function”? (Select all that apply)
1. A “function” is what the item is intended to do, and can be listed with or without respect to any standard of performance. (False. A function description needs to include the standard of performance. It is the function statement including the standard of performance that allows the FMEA team to determine the failure modes.)
2. A “function” is what the item is intended to do, usually to a given standard of performance. (True)
3. There is always one function for each item in an FMEA. (False. There can be many functions for an item.)
4. The function description in an FMEA must include the consequence or impact on the end user. (False. An effect must include the consequence or impact on the end user, not a function.)

Intermediate/Advanced Solution

Scenario: You are preparing to lead a System FMEA on the all-terrain bicycle. The “All-Terrain Bicycle Functional/Technical Specifications” includes the following verbiage:

Front suspension: Should be able to absorb forces up to 1000N without reaching the end of the travel distance. Should use a 75mm-travel coil-sprung fork with hydraulic damping for greater adjustability and control. Should withstand g-force acceleration to 3g, above which it is considered abusive usage. The fork shock absorber should have adjustable spring rates and damping. The damper should be oil-filled (not air-compressed).
Color offerings: Grey-black or silver-red with scratch-resistant coating.
Frame strength: Should be able to withstand 2x the 95th percentile rider (in terms of weight) on the top part, plus 2000 N of force on points of contact with rear and front wheel, 1500 N on point of contact with handlebars.
Frame weight: Should be less than 25 pounds (maximum).
Frame material: B1457 premium aluminum. No visible corrosion on frame for 8 years.
Frame diameter: Maximum diameter of frame is 2 inches at down tube, top tube 1 inch, seat tube 0.75 inch, rest of tubes up to 0.5 inches.
Frame welding: Tungsten Inert Gas (TIG) welding.
Gears: ABC 9-speed Ease of pedaling: Should be able to move bicycle with 5Nm torque on first gear.
Rims: Double-walled 26-inch alloy rims should be able to withstand 2x the rider’s weight, plus 3g-force deceleration impact on solid surfaces such as rocks. Each rim should weigh no more than 10% of bicycle’s total weight.
Tubes: Should be designed for nominal pressure of 40 psi (2.8 bars), but should be able to withstand temporary increase of 50% in pressure.

Answer the following three questions:

Problem 1: The Front suspension has a requirement: “Should withstand g-force acceleration to 3g, above which it is considered abusive.” Should this requirement be part of the all-terrain System FMEA, and if so, how would it be used?

Answer 1: During the ground rules and assumptions portion of FMEA preparation, the team will need to consider whether abuse of the all-terrain bicycle will be within the scope of the analysis. The specification clearly says that g-forces above 3g are considered abusive. The team may wish to ignore any g-forces above 3g. In this case the 3g limit would carry over to the function column of the System FMEA as one of the primary functions, such as “the bicycle must be able to safely and reliably with-stand g-forces up to 3g while riding on all surfaces, environments, and maneuvers, according to the customer usage document ABC.” However, if the FMEA team decides to address g-forces above the 3g limit (abusive) for safety or any other reason, they will need to document this decision (with approvals as needed) and generate the appropriate function verbiage.

Problem 2: The Gears have a requirement: “Ease of pedaling—should be able to move bicycle with 5 Nm torque on first gear.” Should this requirement be part of the all- terrain System FMEA, and if so, how would it be used?

Answer 2: The ease of pedaling requirement should be used in the function column of the System FMEA as one of the primary functions. For example, one of the all- terrain bicycle functions might be “The rider should be able to easily pedal the bicycle forward in first gear applying 5 Nm of torque.”

Problem 3: What type of requirement seems to be missing from the All- Terrain Bicycle Functional/Technical Specifications”?

Answer 3: The requirements in the document cover subsystem usage, subsystem-to-user interface, and subsystem-to-environment interface. Missing are requirements related to subsystem-to-subsystem interfaces. Example might be steering while braking maneuvers to avoid loss of control of bicycle by rider.