APPENDIX 6 APPLICATION OF QUALITY FUNCTION DEPLOYMENT IN OILFREE COMPRESSOR DEVELOPMENT

Transcription

1 181 APPENDIX 6 APPLICATION OF QUALITY FUNCTION DEPLOYMENT IN OILFREE COMPRESSOR DEVELOPMENT A6. 1 INTRODUCTION Quality function deployment (QFD) is an overall concept that provides a means of translating customer requirements into the appropriate technical requirements for each stage of product development and production (i.e., marketing strategies, planning, product design and engineering, prototype evaluation, production process development, production, sales) described by Sullivan (1986).QFD which is a widely used customer- driven product development method, originated in the late 1960s in Japan by Akao (1990). The basic concept of QFD is to translate the desires of customers into design requirements, which are also known as product technical requirements, product features or engineering characteristics, and subsequently into parts characteristics, process plans and production requirements. In order to establish these relationships, QFD usually requires four matrices. These are product planning, part planning, process planning, and production/operation planning matrices, respectively. Product planning matrix translates customer needs into design requirements; part-planning matrix translates important design requirements into product/part characteristics; process-planning matrix translates important product/part

2 182 characteristics into manufacturing operations; production/operation planning matrix translates important manufacturing operations into day-to-day operations and controls (Shillito 1994). QFD has been successfully introduced in many industries to improve design processes, customer satisfaction, and to create a competitive advantage is explored in Hauser &Clausing (1988). A QFD team collects and treats a set of customer requirements (CRs) for new product development. A number of design requirements (DRs) that affect CRs are also identified and worked out to maximize customer satisfaction. In practice, a QFD team is organized to determine the importance levels of DRs by computing the relationships between CRs and DRs and relationship between the DRs themselves with the importance score of each customer requirement described by Cohen (1995) &Terninko (1997). According to American Supplier Institute, QFD is A system for translating customer requirements into appropriate company requirements at each stage from research and development to engineering and manufacturing to marketing/sales and distribution (ASI 2001). The three main goals in implementing QFD are: 1. Prioritize spoken and unspoken customer wants and needs. 2. Translate these needs into technical specifications and characteristics 3. Build and deliver a quality product by focusing everybody toward customer satisfaction.

3 183 QFD uses some principles of Concurrent Engineering in that crossfunctional teams are involved in all phases of product development. Each of the four phases in a QFD process uses a matrix to translate customer requirements from initial planning stages through production control. Each phase, or matrix, represents a more specific aspect of the product's requirements. The relationships between elements are evaluated for each phase. Only the most important aspects of each phase are deployed into the next matrix. Poor understanding of customer requirement and inaccurate assumptions made during the elicitation and analysis of information required will have a negative impact on product design and manufacturing in terms of cost, quality and delivery time. Quality function deployment focuses on customer expectations or requirements, often referred to as the voice of the customer. It is employed to translate customer expectations, in terms of specific requirements, into directions and actions, in terms of engineering characteristics, that can be deployed through and it is explained in the article given in < - Product planning - Part development - Process planning - Production planning Quality function deployment enables the design phase to concentrate on the customer requirements, thereby spending less time to redesign and modifications.

4 184 A 6.2 HOUSE OF QUALITY There are two popular QFD process models. One is called Matrix of Matrices, developed by Akao (1990). In Akao s model, the QFD structure is normally presented as a system of thirty matrices, charts, tables, or other diagrams. The other one is four-phase model developed by Hauser & Clausing (1988), which is probably the most widely described and used model. Most current QFD papers applied the four-phase model. Usually the first phase of QFD is called house of quality (HOQ). HOQ is also the basic design tool of quality function deployment. The name of house comes from its physical appearance as shown in Figure A 6.1. It is a conceptual map that provides a means of inter-functional planning and communication between customer requirements and technical responses, with its objective to achieve maximized customer satisfaction. Interrelationship between technical attributes Technical attributes Customer requirements (Voice of customers) Relationship between the customer requirements and technical attributes Prioritized customer requirements Prioritized Technical attributes (Absolute weight) Figure A 6.1 House of quality

5 185 The following steps involved in building the HOQs: A) List customer requirements (WHATs) QFD starts with a list of goals/objectives and this list is often referred as the WHATs that a customer needs or expects in a particular product. Customer requirements are also called Voice of Customers (VOC). In industry, the business development team provides the VOC. B) List Technical attributes (HOWs) The objective of the house of quality is to design in a way that meets or exceeds the customer expectations. The customer needs and expectations have been expressed in terms of customer requirements; the QFD team must come up with engineering characteristics or technical descriptors (HOWS) or technical attributes that will affect one or more of the customer requirements. In this work, the major customer requirements like Free Air Delivery (FAD), maximum working pressure, overall size, cost, life of the critical parts, power requirement, noise level and vibration level are converted into technical attributes like Cylinder bore and stroke, configuration, Compression ratio, material selection and subsystem component design etc., as shown in Figure A 6.2.

6 Figure A 6.2 List of technical attributes

7 187 C) Develop a relationship matrix between WHATs and HOWs The next step in building a HOQ is to compare the customer requirements and technical attributes and determine their respective relationships. The QFD team now fills the inside of the house of quality, called the relationship matrix. The relationship matrix is used to represent graphically the degree of influence between each technical attributes and each customer requirement. It is common to use symbols/letters to represent the degree of relationship between the customer requirements and technical attributes. A h represents a strong relationship. (Weight assigned: 9) A m represents a medium relationship. (Weight assigned: 3) A l represents a weak relationship. (Weight assigned: 1) If the box is left blank, then there is no relationship exists. D) Develop an interrelationship matrix between HOWs The roof of the house of quality, called the correlation matrix, is used to identify any interrelationships between each of the technical attributes. The matrix lies in the roof of HOQ. The technical attributes are not orthogonal. Usually the change of one technical attribute will affect the values of other technical attributes. The interrelationship matrix depicts the impact of this effect and helps designers construct models and make trade-offs between technical attributes. E) Importance weight of customer requirements The QFD team ranks each customer requirement by assigning it a rating. Numbers 1 through 10 are listed in the importance to customer column

8 188 to indicate a rating of 1 for least important and 10 for very high important. In other words, the higher the rating is given to more important customer requirements. F) Develop prioritized technical attributes The absolute weight for each technical attributes is determined by taking the dot product of the column in the relationship matrix and the column of importance to the customer. In this work, QFD is applied to know the part characteristic and its importance. This work will guide the new designers to identify Critical to Quality Characteristics (CTQs) of the parts and process of oil free reciprocating trunk piston compressor. CTQ is a product feature or process step that must be controlled to guarantee that you deliver what the customer wants. The part characteristics will be addressed either through design or process. This will help in doing the Failure Mode and Effect Analysis (FMEA) with respect to design. A 6.3 QFD - OIL FREE COMPRESSOR AND DISCUSSION The business development team provide the voice of the customer and the same is converted into product requirements in the first phase of QFD. In order to meet the entire customer requirement, the product requirements are listed in the house of quality and product requirements are divided into seven subsystems as shown in Table A6.1. The importance is assigned only to the relevant product requirements corresponding to subsystem level requirements. Subsystem level technical attributes are prioritized based on the absolute weight of each attributes and the same is plotted in Pareto chart. From the Pareto (shown in Figure A6.3) 80% of the technical attributes at the top of the

9 189 chart classified as critical to quality (CTQ s) to meet the product requirements. More importance was given to CTQ s during the manufacturing process. After the subsystem level HOQ, the part level house of quality is created from each subsystem. In this work, more focus was given to the transmission system to address all relevant parameters which contribute the life of critical parts like crankshaft, connecting rod and bearings.

10 190 Table A 6.1 Subsystem classification and its components Main system Subsystem Components Crankcase Free end cover Housing system Fly end cover Breathing arrangements Seals Crankshaft Oil free reciprocating compressor Connecting rod Power transmission system Bearings Motor Flywheel Cylinder Cylinder head Compression system Piston assembly Piston rings Valves Filter element Air intake system Filter housing Inlet manifolds Cooling Fan assembly Cooling system Inter cooler After cooler

11 191 PARETO - COMPRESSOR MAIN SYSTEM Absolute weight Component design Material selection Cooling system design Parts from proven / international suppliers Self lubricated rings Coated pistons Coated cylinders Cylinder bore and stroke configuration Sealed bearings selection Valve design Leak proof joints Housing system design Volumetric efficiency Design of filters Compression ratio Cost of consumable parts Design of drive system Surface finish of parts Configuration/shape of parts Less consumable parts Mechanical balancing No of stages Motor selection Receiver capacity Should meet statuatory & regulatory requirements Volume balance Rust prevent coating of parts Use of std tools Ease of accessability to parts Mistake proof assembly of parts Control system design Vibration Emergency stop Mechanical efficiency Figure A 6.3 Pareto Compressor

12 192 Figure A 6.4 HOQ Bearings Absolute weight Figure A6.5 Pareto - Bearings

13 193 The part level QFD (Shown in Figure A6.4) is done to know technical attributes which contributes the performance and other requirements of the subsystem. Since this work is focused on bearing failure, bearing QFD is considered for the discussion. From the Pareto of the bearing QFD (Shown in Figure A6.5), it is understood that absolute weights for the technical attributes like bearing life, internal operating clearance, geometric tolerance, lubricant properties and operating temperature range to be given for more importance to meet the customer requirements. A 6.4 CONCLUSION QFD analysis was carried out to find the importance of the parts characteristics or technical attributes from the system level to the part level. From this QFD, it was found that the bearing related parameters are important to enhance the life of the transmission system. Therefore, in order to meet the customer requirements, the technical attributes of bearing like bearing life, internal clearance, lubricant properties and operating temperature range should be given importance during bearing selection. This document will guide the new designers to prioritize the technical attributes of oil free reciprocating compressor design.