Value Network Mapping for Productivity Improvement: A Case Study Abhijeet K Digalwar* and Rishi Kukreja**
The paper aims at improving the production performance of a transaxle assembly line by lean manufacturing implementation. The production performance has been increased by identifying and eliminating non-value added activities through value network mapping. To achieve maximum productivity while producing 200 pieces per shift instead of 150 pieces per shift, the assembly line will need only 13 operators instead of 17 and the productivity can be enhanced by 62.3% as compared to the current state. On the other hand, while producing 125 pieces per shift instead of 150 pieces per shift, the assembly line will need only 9 operators instead of 17 and productivity can be enhanced by 30.05% as compared to the current state. Both the production plans can be implemented with the minimum setup and changeover times.
Introduction Lean manufacturing is implemented to satisfy customer demand while producing quality products in the most efficient and economical manner by reducing various wastes in the production process. The main goal is to identify the operations that add cost instead of adding value to the product (Tapping et al., 2002). It focuses on elimination of all kinds of waste. According to Ohno (1988), wastes are of seven types—overproduction, waiting, transport, inappropriate processing, unnecessary inventory, waste of motion, and defects. Russell and Taylor (1999) define waste as anything other than the minimum amount of equipment, effort, materials, parts, space and time that are essential to add value to the product. An extended value stream is simply all the actions—both value creating and wasteful—required to produce a product from raw materials and bring it to the customer (Jones and Womack, 2000). Rother and Shook (2003) define value stream mapping as a lean manufacturing technique used to analyze and design the
Plz chk and confirm the author info and also provide the * designations of both the authors ** and email id of the 2nd author
xxxxx, Birla Institute of Technology E-mail:
[email protected]
and
Science,
Pilani,
Rajasthan,
India.
xxxxxx, Birla Institute of Technology E-mail: xxxxxxxxxxxxxxxxxx
and
Science,
Pilani,
Rajasthan,
India.
Value Network for Productivity Improvement: A Case Study © 2013 IUP. AllMapping Rights Reserved.
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flow of materials and information required to bring a product or service to a consumer. At Toyota, where the technique originated, it is known as ‘material and information flow mapping’. According to Jones and Womack (2000), value stream mapping is the simple process of directly observing the flows of information and materials as they now occur summarizing them visually, and then envisioning a future state with much better performance. The present study aims at creating a value stream map for an assembly line assembling transaxles for XYZ Company. A transaxle is a major mechanical component that combines the functionality of the transmission, the differential and associated components of the driven axle into one integrated assembly. The company wants to increase the productivity in order to meet the growth in sales expected in the coming years of the assembly line by reducing manpower. Hence, Value Stream Mapping (VSM) tool is to be applied to identify wasteful operations along the assembly line. The study initiates by closely observing and studying the entire assembly process from raw material to dispatch. All operations performed by different operators are identified. Then complete data, including operation-wise cycle time, operator-wise cycle time and Work in Progress (WIP), is collected. The data is used to make a current state value stream map. Complete flow of materials and information is depicted on the current state value stream map. Then it is analyzed to find the areas of improvement and plot the future state value stream map.
Literature Review Lean Manufacturing Lean manufacturing is a set of principles centered on preserving value with less work. It is a management philosophy derived mostly from the Toyota Production System (TPS) and identified as ‘lean’ only in the 1990s (Womack et al., 1990). There are various tools associated with lean manufacturing such as 5S, just in time, Kanban, production leveling, poka yoke, pull system, TPM, TQM, and VSM. VSM is the most important of them. There are many cases in which lean manufacturing has been implemented in manufacturing industry in different sectors. Muda and Hendry (2002) proposed a world-class manufacturing concept incorporated with lean principles for the make-to-order sector. Singh et al. (2006) presented an integrated fuzzy-based decision support system for enabling the selection of lean tools in a steel industry application. Lander and Liker (2007) presented a case example of a low-volume, highlycustomized artistic clay tile company which has utilized TPS approach for creating highly customized products. Black (2007) used a multiple case study approach to examine the leanness in the UK food and drink companies where the production volume is comparatively low and the variety is high. Yadav et al. (2010) depicted the knowledge gained from implementation practices in the US, the UK and Indian automotive sectors and presented the gaps between the lean principles and actual practices. Hodge et al. (2011) developed a hierarchical lean implementation model 2
The IUP Journal of Operations Management, Vol. XII, No. 4, 2013
and concluded that VSM is generally the first tool used for lean implementation at all levels of hierarchy. Robinson et al. (2012) used lean manufacturing in conjunction with discrete event simulation to improve the delivery of healthcare services. Panizzolo et al. (2012) investigated the improvements due to lean manufacturing adoption in Indian SMEs. Agus and Hajinoor (2012) addressed the key relationships between lean production, product quality performance and business performance within the Malaysian manufacturing industry.
Value Stream Mapping There are many case studies highlighting the implementation of VSM in various industries in various sectors. Rauniyar (2007) developed a value stream map for XYZ Company, which is a lead acid refining and recycling company located in Minnesota and servicing in North America. Singh and Sharma (2009) applied VSM in an Indian manufacturing firm to reduce the lead time, processing time, WIP inventory, and manpower requirement. Chen and Meng (2010) proposed a VSM-based production system for Chinese enterprises to help them deploy lean production systematically for better productivity by eliminating the roots of wastes. Sawhney et al. (2009) used VSM to evaluate breakdown maintenance operations for enhancing the performance of equipments. Anand and Kodali (2009) presented an application of VSM with simulation during the design of Lean Manufacturing Systems (LMS) using a case study of an organization following a job shop production system to manufacture doors and windows. Simulation was used to show that the case organization can achieve significant improvement in performance and can meet the increasing demand without any additional resources. Vinodh et al. (2010) applied VSM for enabling leanness in an Indian camshaft manufacturing organization. Hedberg and Lindström (2011) applied VSM in new product introduction. They generated a customized VSM tool adapted to manage two-fold customer value within new product introduction environments, Author: plz processes that construct products and information gathered from the production and provide complete incorporated into next generation products. Recently, Singh et al. (2011) presented a ref. as it is not review of literature on VSM in which they classified the literature into four categories— there in ref. list conceptual work, empirical/modeling work, case studies and survey articles. It becomes difficult to study a value stream when it has various streams merging together during the flow. Such is the situation in the case of an assembly line, where many raw materials are functioned upon and many subassemblies and assemblies are done at different stages. Rother and Shook (1999) stated that “Many value streams have multiple flows that merge. Draw such flows over one another. But do not try to draw every branch if there are too many. Choose the key components first, and get the others later if you need to.” Hence, to analyze the current state of the assembly line, a network map is plotted starting with all raw materials as the beginning of a unique value stream and all the operations and assemblies are represented by separate boxes also mentioning their specific cycle times. Value Network Mapping for Productivity Improvement: A Case Study
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Methodology A review of literature on lean manufacturing, VSM and value network mapping is followed by the selection of an appropriate manufacturing organization for carrying out the implementation study. After identifying the organization, operations in assembly line are studied. Then the current state map is developed after collecting the relevant data. Subsequently, the current state map is analyzed and various improvement proposals for improving the leanness of the organization are formulated and discussed with the company executives to develop the future state map. After the development of the future state map, various inferences that would improve the value addition of the processes are derived based on the experiences derived from the case study.
Discussion The case company is one of the largest manufacturers of components of power transmission, with one of its plants located in Uttar Pradesh, India. The company’s name is not disclosed to maintain confidentiality. The company has a line dedicated for assembly of transaxles. The annual turnover of the company is around US$10 mn.
Operations in Assembly Line To draw the current state map it is most essential to understand all the operations and processes of the assembly. It should be done by going to the floor and observing and interacting with the staff. No pre-fabricated data and process charts should be trusted. In the transaxle assembly line, there are three stages of assembly:
Gear Preparation In this stage, the gears and other parts are washed and prepared for assembly. This stage includes washing, lapping/deburring and rolling. • Washing is done to remove any impurities on the components. Castrol Techniclean SF is the chemical used for washing the components. The parts that are washed are: – Gears – Differential Housing – Aluminum Box Housing – Input Shafts – Extension Tubes • Lapping refers to the process where a grinding wheel is meshed with a gear to remove any kind of burrs on the gear teeth. Hysolex oil is used as a coolant. For input shaft, deburring is also performed using a Gratomat to remove burrs. 4
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• Rolling is performed to remove any burrs or dents that cause noise at a later stage. The tooth-to-tooth error and total composite error are checked. The parts which undergo rolling are: differential gear, double gear, and input shaft.
Assembly The electric transaxle assembly involves the following steps: • Torque Calibration: Torque is applied on the bolt till a ‘click’ sound occurs. The applied torque value is then noted from the digital meter. The torque value observed should be mean of a specific torque value range of each station where torque wrench is required. • Differential Assembly: The side bevel and pinions are arranged and the differential pin holds the assembly in place. • Bearing Assembly: Bearings are press fitted on both the ends of the differential assembly. • Complete Differential Assembly: The differential assembly and the differential gear are assembled. The bolts are ‘tacked’, ‘tightened’ and ‘torque’. During bolt assembly, the ‘3T’ process is followed: – Tacking: Tack bolts manually upto 2-3 threads by hand. – Tightening: Tighten bolts using pneumatic air gun of the specific torque value. – Torque: The required torque value is confirmed using torque wrench (‘click’ sound). • Extension Tube Assembly: An oil seal is press fitted into both the extension tubes to avoid any kind of oil leakage. • Shaft Assembly: The circlip, bush and bearing are press fitted on the shafts. • Pressing of oil seal and dowel pin: Dowel pins are used to fix the two halves of the gear box. Oil seal is press fitted and FUCHS RENOLIT HLT 2 grease is applied. The oil seal prevents oil leakage as well as dust contamination. • Sealant Application: Since the sealant is liquid gasket, it can take the shape of the profile easily. Using this type of sealant makes the process faster. • Bearing Assembly in Double Gear: Bearings are press fitted on both ends of the double gear. • Bearing Assembly in Input Shaft: Bearings are press fitted onto the input shaft. • Complete Box Subassembly: The input shaft, double gear and differential gear are assembled such that the following gear pairs are formed: – Input shaft (Z=17) and double gear (Z=53). Value Network Mapping for Productivity Improvement: A Case Study
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– Double gear (Z=20) and differential gear (Z=79). • Final Complete Assembly of Transaxle: First, the valve body is attached to the box. Then the extension tube and shaft are assembled.
Testing and Dispatch This stage involves the following steps: • Air Leakage Test: The leakage adapter is fixed onto the breather hole. Machine measures the differential pressure and the leakage pressure. Inlet pressure is 300 mbar 50 mbar. Acceptable maximum leakage pressure is 0.3 mbar.. Rejected axles are immersed in water tank to identify the region of leakage. • Noise Test: First, it is tested for ‘unbalance’ condition. The axle is clamped and shaft is coupled with motor. The shaft is rotated clockwise and anticlockwise at 220-240 rpm. – Manually checked for any abnormal noise like dent in gear. – Noise meter reading: less than or equal to 74.5 dB. • Oil Drainage: The oil filled during testing is drained out. A magnet plug is fixed in place of the valve body. The magnet attracts any scraps of metal that form when the axle is running. Breather plug is fixed onto the axle breather hole. • Pre-Dispatch Inspection (PDI): This is a step to ensure an error-free assembly. The operator re-checks every bolt by using the respective torque wrenches. He also checks for markings on all components. • Packing: After PDI, the axle is placed into a carton. In one carton, 18 axles are placed. A bar code label is printed, which contains complete information about the axle. This barcode is pasted onto the axle.
Designing the Current State Map After understanding the complete process of the assembly, the takt time is calculated and cycle times of all operations are recorded. Takt time determines the pace of customer demand. It is the rate at which the company must produce to satisfy the customer demand. Producing at takt time means synchronizing the pace of production with the pace of sales. Cycle time is the time that elapses from the beginning of an operation until its completion; in other words, it is the processing time. Total cycle time is the total of the cycle times for each individual operation in a value stream. Cycle time differs from takt time as it is the actual time it takes to do the process. Our goal is to synchronize takt time and cycle time, to greatest extent possible. To calculate the takt time for a particular product family or value stream, we divide the available production time by the total daily quantity required.
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Takt Time
Available Production Time Total Quantity Required
First, we calculate the production time available per shift, i.e., 460 min (Shift working time = +510 min; Lunch break = –30 min; Tea Breaks = –20 min) Total quantity required per shift = 150 pieces
Takt Time
Available Production Time Total Quantity Required
Takt Time
460 3.066 min 184 s 150
Hence, current takt time is 184 s The cycle times of all operations are mentioned in Table 1. After recording the cycle times of all operations and the WIP, the current state map is plotted as shown in Figure 1.
Designing the Future State Map
After recording the cycle time and takt time data of the operations and operators (as shown in Figure 2), it is observed that the work is very unevenly divided amongst the operators. So, along with value network mapping for removal of non-value added activities, line balancing is implemented for improvement of work allocation amongst operators. There is seasonality in the demand for the product, hence it cannot be made in same quantity all throughout the year. So, two different levels of production are chosen according to which the production can fluctuate, and two separate plans are made according to each level of production. The plans have to be constrained because of the fact that production has to take place according to the same (or almost similar) assembly floor layout in both the conditions. So that the setup time or changeover time is not huge.
Plan 1 Production = 200 pieces per shift Takt time = 138 s The changes proposed according to Plan 1 are: 1. Only two operators instead of three on Operation No. 150 (Final complete assembly of transaxle). 2. Only one operator instead of two on Operation No. 180 and 190 (Completion and oil drainage + PDI). Value Network Mapping for Productivity Improvement: A Case Study
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3. Operation No. 90 (press fitting bearings on input shaft) to be done by the same operator who is doing operation No. z-17 (rolling input gear). 4. Operation No. 100 (press fitting bearings on double gear) to be done by the same operator who is doing operation no. z-20 (rolling of double gear). 5. Operation No. 60 (fitting dowel pin and lip seal in semi-housing) to be done by the same operator who is doing the Operation Nos. 10 and 20 (subassembly of differential housing and press fitting bearings on differential housing). Table 1: Cycle Time Data Operation No.
Operation Name
z-79
Rolling Differential Gear
108.11
260
z-17
Rolling Input Gear
101.00
163
z-53
Rolling Double Gear 1
98.67
166
z-20
Rolling Double Gear 2
63.33
81
10
Subassembly of Differential Housing
66.56
77
20
Press fitting bearings on differential housing
26.89
35
30
Fitting differential gear on differential housing
102.00
120
35
Press fitting of oil seals in extension tubes
45.67
54
40
Fitting components on semi-axle shaft
90.00
90
60
Fitting dowel and lip seal in semi-housing
37.56
44
90
Press fitting bearings on input shaft
22.78
26
100
Press fitting bearings on double gear
24.22
27
80 m + 110 + 120
Complete box subassembly
125.44
140
150
Final complete assembly of transaxle
257.50
270
155
Air leakage test
132.67
143
160
Noise test
82.00
93
180 + 190
Completion and oil drainage + PDI
132.50
144
Total Cycle time 8
Avg. Cycle Max. Cycle Time (s) Time (s)
1,516.89
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Figure 1: Current State Map for Transaxle Assembly Line Differential Gear
Input Shaft
Double Gear
Extension Tube
Semi-Axle Shaft
z-79
z-17
z-53
35
40
108
101
98.67
45.67
90
20
20
20
10
90
z-20
66.56
22.78
63.33 20
20
100
26.89
24.22
10
20
20
20 60
20
37.56 10 30
150
80 – 110 – 120
102
10
125.44
6
257.5 6 155 132.67 6
Operation No.
160
Cycle Time (s)
82
Inventory
20 180 – 190 108
Value Network Mapping for Productivity Improvement: A Case Study
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Figure 2: Operation-Wise and Operator-Wise Cycle Times 184
132.66
125.44
108.11
102
101.00 98.67
93.45
90
84.56
85.83 85.83 85.83
8.2 66.25 66.25
Cycle Time
63.33 45.67
0.00
z-79
(1)
z-17 z-53 z-20
10 +20
30
35
40
(2)
(5)
(6)
(7)
(8)
(3)
(4)
150 80 60 +90 +110 +100 +120
(9)
(10)
(11)
150
150
155
160
180 + 190
180 + 190
(12)
(13)
(14)
(15)
(16)
(17)
Operation No. (Operator No.)
6. Operation No. 40 (fitting components on semi-axle shaft) and Operation no. 35 (press fitting of oil seals in extension tubes) to be done by the same operator. 7. Shift-wise production target to be increased from 150 to 200 pieces which will reduce the takt time from 184 s to 138 s. According to this plan a future state map is plotted (as shown in Figure 3). Figure 4 shows the graph of operation-wise and operator-wise cycle time according to this plan. It can be closely observed that the white area under the takt time line has considerably reduced, hence decreasing wastage of time and increasing productivity. Productivity enhanced, according to Plan 1, can be calculated as:
Productivity Before
150 pieces 17 8 man hours
= 1.10 pieces/man hour
Productivi ty Proposed
200 pieces 13 8 man hours
= 1.92 pieces/man hour 10
The IUP Journal of Operations Management, Vol. XII, No. 4, 2013
Figure 3: Future State Map for Plan 1 Differential Gear
Input Shaft
z-79
z-17+90 123.78
108 20
Extension Tube
Double Gear
z-53 135.67
Semi-Axle Shaft
35 + 40 135.67
20 10 + 20 + 60
z-20+100 87.55
131.01 10
20
20
20
20
80 + 110 + 120
30 10
102
150 6
125.44
257.5 6 155 132.67 6
Operation No.
160
Cycle Time (s)
82 20
Inventory
180 + 190 108
Figure 4: Operation-Wise and Operator-Wise Cycle Time According to Plan 1
138.00
Cycle Time
108.11
135.67
131.01
123.78 98.67
0.00
125.44
128.75 128.75 132.66
132.50
102 87.55
82
z-79
z-17 + 90
z-53
z-20 + 100
10 + 20 + 60
30
40 + 35
80 +110 +120
150
150
155
160
180 + 190
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
Operation No. (Operator No.) Value Network Mapping for Productivity Improvement: A Case Study
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Productivity Enhanced = 62.3% Hence the productivity has been increased by 62.3% of the current value.
Plan 2 Production = 125 pieces per shift Takt time = 220 s The changes proposed according to Plan 2 are: 1. Only two operators instead of three to perform Operation No. 150 (final complete assembly of transaxle) and also Operation No. 155 (air leakage test). 2. Only one operator instead of two on Operation No. 180 and 190 (completion and oil drainage + PDI), and who also does Operation No. 160 (noise test). 3. Operation No. z-79 (rolling differential gear) and Operation No. z-17 (rolling input gear) to be done by the same operator. 4. Operation No. z-53 and z-20 (i.e., complete rolling of double gear) to be done by one operator. And he also does Operation No. 90 (press fitting bearings on input shaft) and Operation No. 100 (press fitting bearings on double gear). 5. Operation No. 60 (fitting dowel pin and lip seal in semi-housing) to be done by the same operator who is doing Operation No. 10 and 20 (assembly of differential housing). 6. Operation No. 40 (fitting components on semi-axle shaft) and Operation No. 35 (press fitting of oil seals in extension tubes) to be done by the same operator. 7. Shift-wise production target to be decreased from 150 to 125 pieces which will increase the takt time from 184 s to 220 s. According to this plan a future state map is plotted (as shown in Figure 5). Figure 6 shows the graph of operation-wise and operator-wise cycle time according to this plan. It can be closely observed that the white area under the takt time line has considerably reduced, hence decreasing wastage of time and increasing productivity. Productivity enhanced according to plan 2 can be calculated as:
Productivity Before
150 pieces 17 8 man hours
= 1.10 pieces/man hour
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The IUP Journal of Operations Management, Vol. XII, No. 4, 2013
Figure 5: Future State Map for Plan 2 Differential Gear
Extension Tube
Double Gear
Input Shaft
Semi-Axle Shaft
z-53 + z-20 + 90 + 100
z-79 + z-17 210
35 + 40
209
10
135.67
20 10 + 20 + 60 131.01
20
20
20
10 80 + 110 + 120
30 10
102
150 + 155 6
125.44
196 20 160 + 180 + 90
Operation No. Cycle Time (s)
108
Inventory
Figure 6: Operation-Wise and Operator-Wise Cycle Time According to Plan 2
220.00
210.00
214
209.00
196
138.00 136
131.04
Cycle Time
98.67
102.00
196
128.75 125.44
102 82
0.00
z-79 + z-17
z-53 + 90 + 100
10 + 20 + 60
30
(1)
(2)
(3)
(4)
35 +40
(5)
80 +110 +120
150 + 155
150 + 155
160 + 180 + 190
(6)
(7)
(8)
(9)
Operation No. (Operator No.)
Value Network Mapping for Productivity Improvement: A Case Study
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Productivity Proposed
125 pieces 9 8 man hours
= 1.73 pieces/man hour Productivity Enhanced = 30.05% Hence, the productivity has been increased by 30.05 % of the current value.
Conclusion This paper introduces a value network mapping approach that, unlike VSM, is able to map value streams that have multiple flows that merge through a case study that is focused on productivity improvement of electric transaxle assembly line. It has been observed that current total cycle time of electric transaxle assembly line is 1,516 s and current takt time while producing 150 pieces per shift is 184 s. Only 49% of the total time is being utilized for value addition and hence there is definitely scope for productivity enhancement. According to the company requirement, the assembly line should function at two levels of production. It should either produce 200 pieces per shift or 125 pieces per shift to achieve maximum productivity. While producing 200 pieces per shift instead of 150 pieces per shift (current state), the assembly line can operate with only 13 operators instead of 17, and productivity can be enhanced by 62.3% as compared to the current state. On the other hand, while producing 125 pieces per shift instead of 150 pieces per shift (current state), the assembly line can operate with only 9 operators instead of 17, and productivity can be enhanced by 30.05% as compared to the current state. Both the production plans can be implemented with minimum setup and changeover times.
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