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Containment Ring Problem
Inventive Problem Solving Case Study

NOTE: Click here for a detailed case study of this problem using the Innovation WorkBench (IWB) software.


Step 1. Document the Problem Situation
Step 2. Formulate the Problem
Step 3.1. Prioritize Directions
Step 3.2. Generate Ideas
Step 4. Develop Concepts
Step 5.1. Evaluate Results
Step 5.2. Plan for Implementation
 

STEP 1. DOCUMENT THE PROBLEM SITUATION

The containment ring is part of a fan used in an aircraft air conditioning system. This ring is designed to hold fragments of the fan in case the fan should burst. The ring must be strong and, as a result, is thick and heavy.

The problem is to reduce the weight of the containment ring. However, reducing the ring's weight reduces its strength to an unacceptable degree.

The high rotational speed of the fan creates centrifugal forces that can cause the fan to break. Should this occur, the fan fragments (some of which may be relatively large) are propelled away at high speed. These fragments can destroy surrounding equipment and injure anyone nearby.

Primary Useful Function: The Primary Useful Function of the containment ring is to prevent the fragments produced by an impeller burst from flying away.

Drawback: A thin containment ring is light, but cannot stop the flying fragments. A thick containment ring prevents fragments of the fan from flying away, but is also heavy.

STEP 2. FORMULATE THE PROBLEM

The Problem Formulator model describing the containment ring problem is shown below. Note that this problem contains two contradictions, making it difficult to solve:

  • The high rotational speed of the fan provides air movement, but also causes the centrifugal forces that pull apart the impeller.
  • The thickness of the ring provides the ring’s strength, but also results in the ring being heavy.

The Problem Formulator™ automatically creates Directions for Innovation from the model, among which are included the following:

  1. Find a way to eliminate, reduce or prevent the (Ring is very heavy), under the condition of the (Ring is very thick).
  2. Find an alternative way to obtain the (Ring is very thick), that provides or enhances the (Ring is very strong), and does not cause the (Ring is very heavy).
  3. Find an alternative way to obtain the (Ring is very strong), that provides or enhances the (Ring holds fragments), and does not require the (Ring is very thick).
  4. Find a way to eliminate, reduce or prevent the (Impeller bursts), under the condition of the (Centrifugal forces pull impeller parts) and (Material not strong enough).
  5. Find an alternative way to obtain the (Fan moves air), that provides or enhances the (Air conditioning inside aircraft), and does not require the (Fan rotates quickly).

STEP 3.1. PRIORITIZE DIRECTIONS

The above Directions are divided into three categories, as shown below.

Category 1 – Directions important for solving the problem. These Directions are directly related to resolving the main problem: reducing the weight of the containment ring:

  1. Find a way to eliminate, reduce or prevent the (Ring is very heavy), under the condition of the (Ring is very thick).
  2. Find an alternative way to obtain the (Ring is very thick), that provides or enhances the (Ring is very strong), and does not cause the (Ring is very heavy).
  3. Find an alternative way to obtain the (Ring is very strong), that provides or enhances the (Ring holds fragments), and does not require the (Ring is very thick).

Category 2 – Directions important for improving the system. This Direction is not directly related to reducing the weight of the containment ring. However, it would result in improving the system to the extent that the containment ring becomes unnecessary—and a containment ring that is not present weighs nothing. Thus, improving the system can indirectly resolve the problem.

  1. Find a way to eliminate, reduce or prevent the (Impeller bursts), under the condition of the (Centrifugal forces pull impeller parts) and (Material not strong enough).

Category 3 – Directions too general to be considered. This Direction pertains to completely substituting the entire system (i.e., the rotating fan) with a system based on some other principle of operation. Such a Direction is not of interest to a company that produces fans, and thus it is considered "out of scope" in terms of solving the problem of reducing the weight of the containment ring.

  1. Find an alternative way to obtain the (Fan moves air), that provides or enhances the (Air conditioning inside aircraft), and does not require the (Fan rotates quickly).

STEP 3.2. GENERATE IDEAS

The following Direction is considered:

  1. Find a way to eliminate, reduce or prevent the (Ring is very heavy), under the condition of the (Ring is very thick).

The "map" below illustrates the procedure for selecting the Operators by which the above Direction can be carried out. Options available at each step are arranged in columns. The selection made is shown in blue. Each selection leads to the column to the immediate right. The right-most column contains the Operators – the two selected Operators are described in detail following the map.

Operator: "Abandon Symmetry"

Recommendation: If an object is symmetrical, consider reducing its weight by abandoning the symmetry. Consider, for example, excluding a part of the object that does not bear the main load.

Illustration: Designing asymmetrical mounts

For aesthetic reasons, motor and generator mounts are often designed with symmetrical shapes. But because the machines rotate, the load on the mounts is actually asymmetrical.

To reduce the weight and conserve material, mounts for non-reversible units should be designed to support only the loads they must actually bear.

Ideas Generated:

  • Vary the thickness of the ring, reducing the thickness where permissible.

Operator: "Strengthen Individual Parts"

Recommendation: Consider strengthening those parts that bear the main load, and also reducing the weight of the parts that do not bear the main load.

Illustration: Pump housing made of standard pipe

The housing of a high pressure pump, made from a steel casting, was complicated in shape. Since cavities and uneven surfaces were frequently found in the casting (necessitating repairs made by welding), a very thick, heavy casting was used.

The defects were eliminated, and the weight reduced by nearly half, when the housing was fabricated by welding several pieces together. High-strength rolled pipe was used for the section that was required to withstand the greatest pressure.

Ideas Generated:

  • Make a multi-layer ring
  • Add "ribs" to a thin ring

STEP 4. DEVELOP CONCEPTS

From the above ideas, the following concepts are developed:

Concept #1: Make the ring thin, but composed of several circular ribs. These ribs can significantly increase the strength of the ring with a less significant increase in mass.
Concept #2: Make a multi-layer ring as described by the following:

Internal layer made of a thin steel ring

Intermediate layer to absorb the energy of the fragments, made of:

  • radial brush
  • balls
  • honeycomb

External layer to hold the fragments, made of:

  • steel pipe
  • coil
  • textile or wire needles

STEP 5.1. EVALUATE RESULTS

Concept #1 requires the following for implementation:

  • calculations of strength and weight
  • redesign of one part (the ring itself)
  • additional manufacturing operations (making the ribs)

All resources for producing such a ring exist.

Concept #2 requires the following for implementation:

  • research to determine what type of intermediate layer is best
  • calculations of strength and mass
  • redesign of the containment ring as an assembly unit
  • changes in the materials used
  • changes in the manufacturing process
  • testing of the new design

As it happens, all materials suggested for this design are already used in the manufacturing processes of producing the aircraft; thus, all the required resources exist.

A comparison of these two concepts shows that:

  • Concept # 1 is easier to implement quickly; however, it achieves a limited reduction in mass.
  • Concept # 2 promises a more significant reduction in mass, but requires more time for implementation.

STEP 5.2. PLAN FOR IMPLEMENTATION

The implementations of Concepts # 1 and # 2 can be planned in the following manner:

Concept # 1

 

Concept # 2

calculate strength and mass of thin ring with ribs   test strength of several types of intermediate layers
create design of ring with optimal strength and mass   calculate several variants of intermediate layers
produce rings with new design for testing   create designs of ring with different intermediate layers
test rings with ribs   produce rings with different intermediate layers
correct design according to results of test  

test rings

correct manufacturing process for producing rings with new design   correct design according to results of test

start producing rings with ribs

  correct design according to requirements of mass production
    create manufacturing process for mass production of multi-layer containment ring
    produce rings for testing
    test rings
    correct design according to results of tests
    correct manufacturing process
    start producing multi-layer rings

Problem which can appear during the implementation of Concept #1:

Additional manufacturing operations (for example, machining ribs from thick pipe or stamping and welding ribs onto thin pipe) increases labor and cost.

This is a typical secondary problem; Operators from the group "Reducing cost" can help to solve this problem.

(END OF CASE STUDY)

 


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