Introduction
The sponsor for our senior design project is Daimler Chrysler. We will be working on an upgrade in the paint division of their Newark Assembly Plant, where the Dodge Durango is built.
Production at the Newark plant can be broken down into three overall assembly procedures. The automobile assembly line first begins with the body shop, where the frame of the car is fabricated, molded, and assembled from sheet metal to form the exterior frame of the Durango (i.e. the doors, frame, and hatchback).
Next, this frame is transported to the paint shop, where the Durango frame is sent through half a dozen advanced painting processes.
Finally, the primed, coated, painted, and polished frame is sent back to the assembly shop to install the chassis and the interior features of the car.
FIGURE 1:Durango Plant’s Three Basic Assembly Steps
Introduction to the Paint Shop
The plant, which has been in operation for years, was modified in 1997 to include a state of the art automotive paint shop. This is one of only 215 highly advanced automotive paint shops like it in the world.
The building sits on approximately 800,000 square ft, and is occupied by mechanical conveyor systems, state of the art robots, chemical dip baths and electrically charged painting applications. On average, 896 Durango bodies per day are painted in the shop—each following the same mapped out course through the series of processes.
These refer to the different application processes found throughout the plant. The body is first primed, then painted and finally the protective outer coating is applied. The focus of our project will be in the priming section of the paint shop.
Phosphate and E Coat
Upon entering the paint shop the car body is transferred to a unique carrier designed specifically for the Phosphate and E coat process. On this carrier the body first travels through Phosphate which is an etching solvent used to clean any imperfections left on the raw steel from the previous process. This is done using an ‘Air Bi Dip’ system that utilizes the design of the carrier to suspend the body and dip it in and out of the solvent. A diagram of this process is shown below.
FIGURE 2: Air Bi Dip system
After being cleaned the body then travels to E Coat where the same ‘Air Bi Dip’ process is used to apply the epoxy coating or primer. When this is done, the body is transferred back to its original skid and sent to the oven to cure the primer. From here, the carrier is washed to remove any excess E Coat and then travels to the beginning of the system where it will receive a new body and restart the process.
FIGURE 3: P&E process
What is a skid?
As stated previously, the skids used in the Phosphate and E Coat process are unique to that system only. Their primary mode of transportation are two 16 foot I beam skis that make up the base of the skid. These skis are designed to allow the skid travel down roller tables in a purely linear fashion. Four large cross members protrude from the sides of the skid that allow the carrier to be suspended from hooks that carry it through both Phosphate and E Coat.
When the car body is removed and sent to the ovens the skid goes through a washer that removes a large portion of the uncured E Coat from the skid. During production the skid would complete the loop and restart the process. When production has halted for the day the skids will sit idle on an accumulation line between the washer and initial transfer.
Accumulation Lines
The accumulation area of the P&E process contains 5 lines, each 200 feet long and capable of holding 12 skids. The conveyor works with two motor driven chains that travel beneath the I-beam skis of the skid. The chain has a roller bearing where each link connects to the next. These bearings sit ¼” below the bottom of the link, allowing the chain to roll down the channel of the conveyor. A roller is also present on the top side of the chain. These are less frequent, located every 2 feet or 4 chain links. The purpose of these top rollers is to create a rolling friction between the chain and skid while the skid sits idle on the accumulation line. A picture of the chain link is shown below.
FIGURE 3: Accumulation Chain Link
Problem Definition
The excess E Coat, contained on the skids after exiting the washer, is the source of our design problem. While dormant on the accumulation lines, the skids distribute E Coat to the conveyor chain. The sticky nature of the E Coat gums up the top rollers of the chain preventing them from rotating. This changes the contact between the skid and chain from a rolling to a sliding friction.
The friction between the skid and the much harder rollers causes the skid to wear. This wear has turned out to be detrimental to the skids ability to function and in turn has caused problems in other areas of the P&E process. The picture below shows the wear on the skid caused by the rollers.
FIGURE 4: Chain/Skid Contact Wear
Project definition
Now that there has been enough background information provided on our project, the purpose and aim of our project can be more easily defined. As stated before, our project originates in the paint shop and is focused on the phosphate and E coat skids, which are used to transport Durango car frames through this etching and finishing process.
As stated previously, the 76 skids purchased in 1997 expected to last at least 10 years in the paint shop. In addition, the skids were though to eventually fail from overall wear and from use in production. However, the recent premature failure of these skids after only six years of use is the primary focus of our project.
The skids early failure is attributed to excessive wear rates experienced to these transporters in a localized section of the P& E process. This particular section of the process is called the accumulation lines, which is used to store empty skids during non-production hours or during infrequent delays or backups in production.
Currently, the paint shop is in the process of replacing all 76 worn out skids with a new fleet of 76 slightly modified skids, costing the corporation approximately $790,000.00 in expenses to do so(Appendix G). This substantial cost to fund this replacement project has sparked interest in preventing the new fleet of skids from receiving similar wear rates. DaimlerChrysler Corporation wants to be assured that in six years down the road, these newly purchased skids will not be worn out or failing due to the same excessive wearing rates.
The new fleet of skids will be introduced into the system starting sometime in the month of December 2003. Thus, our project steps or milestones that must be achieved can be defined as:
To identify the concentrated source of wear deformation received by the skids.
To developing a working concept, as well as a working model that decreases wear experienced by the new skid rails.
Finally, to provide experimental findings and a report to validate a decrease in wear rate experienced by the skid rails or skids bottoms by use of our design concept.
Currently, there have been a few projects that have been constructed to identify the cause of wear on the bottom of the skids and the findings have concluded that the accumulation lines can be held accountable for the sustainable wear rates. However, this must be validated to make sure our solution doesn’t overlook evidence that these reports findings have missed as well.
In summation, our project entails increasing the functionality, efficiency, and life of the body transport systems, called “skids” that are used in the assembly plant’s paint shop. First, identifying the exact cause behind the accelerated wear rates of the original skids is a major milestone of our project.
In addition, once the cause and source of wear are identified, our team objective is then to develop a concept and design that will protect the new skids from receiving the same wear rates as the original skids. Lastly, provide a technical report of all of our project findings on the system, our design calculations, our prototype experiments and findings, and finally, a complete design package including design drawings, design set up, materials and equipment used, manual instructions, costs, etc.
Customers
The customers for this project are all employees of Daimler-Chrysler. Scott Tamblyn, our first customer, is the Area Manager of the paint shop at the Newark assembly plant. He was our top contact for higher order information. He acted as our liaison between all workers in the paint shop and introduces us to various specialists in the plant that could assist us with our project. Scott also provided us with specific information on contact companies, reports and analysis documents, pictures, project costs, and price projections, to name a few.
The next two customers are Mike Cullen and Matt Savage, who are the Paint Maintenance Advisors for the plant’s second shift. Initially, they introduced us to the project and the actually issues that surround our project. Both of these advisors have much experience with the conveyor systems in the P&E process and the mechanical systems in this section of the plant.
In the beginning of the project, they helped to clarify any questions we had about the mechanical systems we were required to work with and understand. They also helped us to identify the constraints and system limitations associated with the project. They provide us with clearance to work in the specific area of the plant, assign specific plant workers to supervise our visit in the plant as well as helped us to gather relevant information and/or materials that we needed for the project.
Finally, our last sponsor, a millwright employee of the plant, was Carl Witt. Carl’s job as a millwright is defined as one who maintains and cares for the mechanical equipment in a factory or plant. Therefore, Carl was extremely familiar with the all the mechanical systems in the paint shop of the plant.
Carl also was very familiar with the realms of our assigned project and offered much clarification on questions we had about the nature of the accumulation lines, the conveyor systems, and the skids in the phosphate and E coat processes. In addition, he advised us on the feasibility of various developed concepts, as well as design issues, such as machining and manufacturing feasibility, clearance tolerances, system compatibility factors, and safety concerns, to name a few.
Wants
Our final concept selection must accommodate and satisfy all of the customer’s wants as a result of our final design. After questioning each individual mentioned above, we formed a categorized list that includes each customer’s list of specifications that needed to be met with our final design.
This list includes the top wants specified by each customer and is categorized under the following categories: (1) Performance, (2) Cost, and (3) System Compatibility (Figure 10). Cost is weighted the most since it was most important for the final prototype. Weighted equally are performance and system compatibility, which implies a design with the least effect on other parts of the P&E system.
Metrics and Target Values
In order to relate the wants of Chrysler to each concept, a set of measurable metrics was needed. The metrics created by the team are as follows: cost and wear rate. A target value for each metric was then created, to aid in relating the ability of each concept to reach each metric.
The target value for cost, namely price, will simply follow a least-cost comparison model. The most affordable solution to solving this problem will be viewed as the target value. The cost of each concept will be discussed later, but will include initial, future, maintenance, and system incompatibility costs into the estimation.
Wear rate is the second metric used to compare concepts and is used to determine the concept that provides minimal destruction due to friction and wear. Two wear rates must be modeled. The two situations are the sliding, abrasive contact occurring between skids and the high rollers and the rolling wear occurring between the skids and the cast iron roll tables.
Because the skids are rapidly wearing from the chain, the groove along the skid rails is in turn causing accelerated wear on the rollers. This must also be considered when comparing possible solutions. Both wear models used the following parameter to determine the wear rate. The measurable parameters used to evaluate each design and its ability to meet our customer wants are shown in the figure below.
Figure 10: Customer wants and Metrics
They are described more in the Appendices A-C. The wear rate will also allow us to predict life of each concept.
Constraints
Like all projects, there are constraints that limit the development of possible solutions and concepts. The following constraints that significantly impact and limited our possible solutions during our concept development process were the following: safety issues, maintaining a required delivery rate in production of the plant, and other system constraints dealing with weight capacities, size limitations, and system compatibility issues.
Safety is a very important constraint and is, for the most part, self explanatory. Our concept must be safe for workers who monitor the system as well as those who maintain it. All components of our design should be compatible with all existing chemicals common to the accumulation lines and all mechanical machinery existing on the accumulation lines.
Another constraint that is very important is the issue of maintaining the delivery rate set by our sponsors. Our design should allow for at least 56 jobs per hour, meaning 56 Durango’s sent through the P&E process on an hourly basis. Any concept that may slow this average production rate must not be considered acceptable.