Design for 6θ- an introduction to the process and methods

November 2012

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Situation in industry: Product recalls and quality issues. But…

Toyota Gas PedalCause: Variation in design parameters causes pedal to ’lock’

Cost: $2 billion

Offshore WindturbinesCause: Overconstrained design creates parasitic forces that decreases lifetime of critical components from 20 to 2 years.

Cost: DKK 200 mio.

BMW 2012-recallCause: Risk of battery cable cover being mounted incorrectly. Details not released. 1.3 mio cars recalled

Cost: Unknown

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…only the ‘tip of the iceberg’ reaches the market. Most quality issues are solved during product development and production ramp-up. But because the issues are discovered late in the development process, redesign is often costly and challenging. Product complexity makes it difficult to find the root cause for the issue. As a result, product launch is delayed and revenue is lost.

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A quality issue is defined as any deviation of a product’s functional performance from its nominal value - e.g. the force required to pull off the cap of a whiteboard marker.

The markers above no. 10000 and 10001 produced by the same production line with identical specifications. But, they perform differently. The force required to take off the cap is not the same. What causes this variation in performance and how can we reduce it?

• One strategy is to reduce the variation of the components – geometry, materials, surfaces. This is called Six Sigma – a well-proven strategy alraedy adopted by industry.

• Another strategy is to reduce the sensitivity to variation. Valcon has successfully developed 6θ - a coherent set of metrics and methods to reduce the sensitivity of a design.

So where do quality issues come from?

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How did Design for 6θ™ arise?

Design for 6θ™ combines research on quality engineering with years of experience on designing moving mechanics for the automotive and medical industry. Combining the disciplines, and converting them to an operable and coherent design procedure, a new product development paradigm has emerged.

6θ

Kinematics

Robust Design

Precision instrument

design

Statistics

Axiomatic Design

Minimum constraint

design

Robot design

Design of Experiments

(DOE)

Experienced effects:• Transparency in projects• Predictability/precision in time-to-market• Lower sensitivity to variance• Embedded quality – with fewer

specifications• ’Design Freedom’

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Design for 6Theta™

How does 6θ work?6θ has two focus areas: Clarity and Robustness

Desired performance

n (Number of devices)

8N

Method 1: Clarity

Coupling Degree(Kinematics and Design

Clarity)

Method 2: Optimisation

6Theta (Sensitivity and Specifications)

Force required

Source: novonordisk.com

The force required to activate a drug delivery device can vary from batch to batch and device to device. Design Clarity removes this variation by identifying and redesigning ambiguous interfaces and overconstrained designs. Optimisation using robust parameters further enhances the performance of the design.

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A brief introduction to Method 1:Kinematics and Design Clarity

Kinematic design can be applied at an early stage (system level / architecture). Using Kutzbach’s Formula the mobility of a design can be quantified.

A design which is overconstrained is sensitive to variation. For example, a misalignment of the gearbox and shaft in the example to the right, will result in parasitic forces in the bearings, thereby decreasing their lifetime.

Introducing e.g. an Oldham coupling will provide the sufficient degrees of freedom and the system is now insensitive to any misalignments and variation of the components.

The design is overconstrained by 5 degrees of freedom

One possible coupling solution: The Oldham Coupling introduces 5 degrees of freedom.

SHAFTCGEAR-BOX

Z

X

Y

j

iid

j

ii FUnB

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16Principal example of a windturbine-concept

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A brief introduction to Method 1:Kinematics and Design Clarity

The components of a design have intended interfaces, but poor design, e.g. too many constraining surfaces, can lead to abrupt changes of functional surfaces.

The intended and actual number of constraining surfaces can be visualised in a cockpit, thereby providing an overview of the current state of sensitivity in the design.

Designs with ambiguous interfaces should be addressed, because an unintended change of interface results in performance variation.

Sys

tem

Ove

r-co

nstr

aint

s (S

OC

)P

art O

ver-

cons

trai

nts

(PO

C)

No. of interfaces (I)

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A brief introduction to Method 2:Quantifying and improving robustness

With an unambiguous design, it is now possible to derive the transfer functions (the correlation between design parameters and functional performance).

The transfer functions can be derived by

• Analytical derivation

• Simulations

• Experiments (DoE)

The gradient of the transfer function indicates the sensitivity of the design and allows for optimisation.

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Example of Method2:Designing a press-fit

Which design is best?

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Example of Method2:Designing a press-fit

The design parameters that contribute to the holding force of the press fit in the two designs are identified and the Theta-value is calculated. Is is seen, that the force is particularly sensitive to the hole and shaft diameter. However, in the second design, the Theta value is relatively higher, indicating that this is a better design.

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Example of Method2:Designing a press-fit

98.13% 99.92%

A Monte Carlo-simulation confirms, that the second design has a higher yield rate.

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Effects: Selected cases

The table contains selected cases of 6θ applied in industry.

Project Issue Source Change & Effect

Industrial grinder Vibrations and noise Mechanism design was overconstrained, resulting in large and varying internal forces.

Overconstraints removed. Noise and vibrations disappeared.

Large-scale scanner Noise on scanner images

Overconstrained and ambiguous design led to varying torque on motor

Overconstraints removed. Noise removed from images.

DVD Tray Motor unable to drive tray in & out. Bigger motor used, but new motor is noisy.

Overconstrained design leading to sensitivity of parallellity of bearing shafts

Overconstraints removed. Old motor re-installed. Noise removed.

Medical device Milestone missed. Lack of overview in tolerance analysis. Variation in functional performance.

Ambiguous design. Redesign with focus on ambiguity. Improved tolerance overview.

Medical sampler Leakage between liquid reservoir and flow channel

Ambiguous design. Unintended (and hence uncontrolled) component elements influenced positioning of parts

Redesign of parts (minor changes) enhancing design clarity. Leakage stopped.

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KPI TARGET NEXT ACTIONS STATUS LEGEND WEEK 32

COUPLING DEGREE

Through interface analysis of all body and part interfaces a coupling degree for the product can be calculated as follows: (System improvements*interfaces + part improvement + number of interfaces)/number of interfaces. This provides a number describing how many improvements per interfaces the product currently has

Coverage 100%Normalized coupling degree: 3 or below.

Primary target. Must be completed (100%) in week 34

Subsystem XXX must be included into the kinematic cockpit - DEADLINE XXX

Main interface groups have been solved. Detail modelling and cleanup still to do

Normalized couplling degree: number of improvement points per interface

MATERIALS

A measure of how many components have had an approved material assigned, living up to all requirements

Coverage 100%100% materials assigned

Materials is secondary. Begin process after week 34

Find / create a standard material list - DEADLINE XXX

Critical components have materials. Springs and visual components still to do. Visual components need input from designer

Percent of components with assigned material

STRUCT

URE

A measure of the distribution of safety factors against relevant failure criteria for all identified calculations

Coverage 100%All safety factors higher or equal to 1,4

Process parallel with results from coupling degree.Begin process week 22

Areas which are dependent on strength and flexibility should be found - DEADLINE XXX

There are interfaces which have not yet been solved . A short review estimates that most of these interfaces can be solved with a small effort at level 1. Calculations below safety factor 1 needs redesign

Green: safety factor > 1.4Yellow: 1.4>safety factor > 1Red: Safety factor < 1

TOLERA

NCE

A measure of the distribution of all tolerances specified. Tolerances are divided into categoryies by their IT grade equivalent compared to the process necessary to produce the component in question

Coverage 100%No critical tolerance specificationsMinimum 85 % acceptable tolerances

All weeks. Must be determined in week 44 at the latest

• X, Y, Z. Alignment tolerance chain must be solved, this will tell us about the concept - DEADLINE XXX

• Sensor tolerance chains (unsolved)• 2,5 N packing spring (unsolved)• 1 N constant force spring (unsolved)

The focus has been on the tolerances concerning the precision of the alignment of the gasket over the solution pack. Remaining calculations are mostly fittings . Snap calculations should have priority

Green: acceptable tolerance specification (IT grade higher or on par with recommendation)Yellow: Challenging tolerance specification (IT grade between 0 and 1 lower than recommended)Red: Difficult tolerance specification (IT grade more than 1 lower than recommended)

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Valcon

Coupling degree

Advanced methods:6θ™ contains more methods and metrics to help identify and obtain quality in design:

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How to make your organisation design 6θ-solutionsValcon has gained experience from many succesful product development projects working with the 6θ-methods and metrics, and offers a variety of services to clients that are interested in improving their quality and predictability of their product development process.

• Implementation of 6θ™ in the organisation.

• Training of engineers, project managers and directors.

• Integration into corporate NPD-process

• Executing 6θ™-product development in a specific project.

• Ongoing documentation and improvement of 6θ™-level.

• 6θ™-training of relevant personnel – from engineers to top management

• Certification (green, yellow & black belts)

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6θ™ in an academic context

6θ™ is also being applied in academia

• Valcon and DTU (Technical University of Denmark) are sponsoring a PhD Research Project on robust design.

• DTU is developing a course on robust design scheduled to launch in the fall semester 2013. Valcon will contribute with knowledge and cases.

• Master Thesis Projects on robust design are currently underway.

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Contact

For further information, contact Valcon:

Janus Juul RasmussenDirector of Valcon DesignMail: jjr@valcon.dkTelephone: (+45) 24 43 97 69

Martin Ebro6θ™-specialistMail: mec@valcon.dkTelephone: (+45) 24 43 97 86