OHIO University Mechanical Engineering Conceptual Design Report -Draft 2- THE MILL MULE

Transcription

1 OHIO University Mechanical Engineering Conceptual Design Report -Draft 2- THE MILL MULE Team C: Mill Movers Authors: Adam Barber Andrew Jung Andrew Stockton Cayn Jennice John Bales Mitch Cihon 18 January 2011 Abstract The objective of this project is to develop a cart-like system that will allow the company, White s Mill, to move multiple bags of products (up to 500 pounds) from a two foot tall storage dock and across a gravel driveway, to their customer s vehicles in a less strenuous but more efficient way than simply carrying the products by hand. The proposed concept will incorporate at least six major design components including: 4 wheels, large wheels, locking wheels, a scissor lift, an integrated tailgate, and a steerable front end. The proposed concept will be built around a retrofitted Jet SLT-660F hydraulic lift table cart.

2 1. Concept Generation 1.1. Problem Statement for Concept Generation The customer is in need of a transportation system that is capable of transferring a load of up to 500 lbs of non-fragile farm and garden products from a 2 tall loading dock to the ground and up to a distance of 100 feet across a slightly inclined (~10 ) gravel parking lot. The system needs to be able to function in all-weather conditions and be corrosion resistant, safe in terms of tip-over and structural failure, and easily operated by a single employee Patent Search To conduct a comprehensive and thorough patent search we split up our search among the team members by making a list of terms that described our conceptual system as a whole, as well as separate conceptual subsystems. The terms used for the search relating to the whole system were determined by group consensus. These terms included Folding Cart, Lifting Cart, Ramp Cart, Modular Cart, Stair-Climbing Dolly, Ramp Dolly, and Bridging Dolly. Each term was researched in patent databases by at least two team members each allowing for overlap and thus more thorough searching, and relevant patents were collected and duplicates discarded. We found many patents relating to elevation changes (stair-climbing dollies) but none that could navigate the 2 distance necessary in our mission. The closest results from the patent search regarding the whole system are shown in Table 1. The patents regarding the conceptual design components are discussed in Sections through Table 1: Relevant complete system patent information. Description Number Scissor Lift Cart USPTO # Pneumatic Lift Cart USPTO # Lift System In order to safely load material, a lift system is one of our conceptual design subsystems. In order to allow easy redesign or retrofitting, the current concept utilizes a scissor lift component because the most common and versatile lift design in verticaldisplacement work is the scissor lift. Using scissor lift tables and man-movers as a point of reference, it was found that there exist some special scissor lift systems that are patented, namely US and US

3 The first patent is essentially the most basic scissor lift design and is shown in Figure 1. A hydraulic ram presses the two members vertically while keeping the top plate parallel to the bottom. This is a simple design and a good idea for us to use in our project. Figure 1: Hydraulic scissor lift. The second scissor lift patent is similar to the first in that it is a specialized design, albeit different in operation than the first, and is shown in Figure 2. It functions the same as the first design but substituting a screw for the hydraulic ram. This is much cheaper owing to the lesser degree of engineering involved in the screw but would not work in our system because of the labor it would take to turn the screw. Above all our system is meant to be labor saving so a screw lift such as this would likely not be a reasonable choice.

4 Figure 2: Screw lift Integrated Ramp The only patent, number US 2006/ A1, that we found with an integrated ramp is a copier that includes a paper rack that removes and acts as a ramp during transport. We considered this during our original brainstorming process but the idea was discarded due to the safety issues regarding a heavily loaded wheeled cart being lowered down a ramp. Diagrams displaying the patent are shown in Figure 3. Figure 3: Copier with integrated ramp.

5 Large Wheels Patent number US is for a tractor with large wheels and is shown in Figure 4. Large wheels are one of our conceptual design components for the same reason they a part of this tractor: they allow the vehicle to navigate over rough terrain. Our system is required to transport a load across gravel up an incline and large tires are necessary to accomplish this. Obviously we could not find a patent for large tires but we feel this one shows our intentions well, albeit on a larger scale. Figure 4: Large-wheeled tractor Steering System The patent search for different steering systems was much more useful in terms of finding ideas. The three patents most helpful were numbers US 2008/ A1, US 2007/ A1, and US 2011/ A1. The first patent, shown in Figure 5, requires the operator to turn the entire front end in order to steer. This design could require a substantial force to move the front wheels and would result in a large turning radius. The upside to this system is it is probably the simplest that we are considering in terms of manufacturing.

6 Figure 5: Cart with front end steering. The second patent is shown in Figure 6 and includes a few sketches showing a precision steering cart. Precision steering of the cart is enabled by a bevel gear train that transfers a rotation of the handle to a rack and pinion gear train. The rack is moved by the pinion to turn the rear wheels left or right. This allows for excellent maneuverability while keeping the stability of a four-wheeled system. However, it is much more complicated the system in Figure 5.

7 Figure 6: Cart with rack and pinion steering. The final patent is shown in Figure 7 and is a three-wheeled steering design where the load would be centralized on the front two wheels. As a result the device has improved maneuverability over the first system but lacks the stability afforded by a fourwheeled system. Figure 7: Cart with three-wheel steering.

8 Locking Wheels We found two patents specifically mentioning locking wheels, numbers US and US 2010/ The first patent is essentially a caster wheel, which has an apparatus inside that contains two gravity-activated pins that lock the wheel when it is on an inclined plane and is shown in Figure 8. This system may be difficult to control and one of our goals is to have a high degree of mobility. Having a complicated system that could limit mobility such as this would likely have a negative impact on our prototype. Figure 8: Gravity-activated locking wheel. The second patent also involves caster wheels but incorporates a sleeve system on the vertical shaft that locks either vertical or horizontal rotation of the wheel and is shown in Figure 9. This concept could apply to our system but would be harder to manufacture than a simple steering system directly attached to the wheels. We plan on taking the challenge of designing and building a steering system, so this concept also will not likely be incorporated in our project.

9 Figure 9: Vertical rotation locked wheel Four Wheels We decided on having four wheels in our design because of the stability and numerous applicable steering systems. Patent research for carts with four wheels results in an incredible number of different designs. Our group has investigated some concepts from patent number US shown in Figure 10 in the conceptual design process. Figure 10: Four-wheeled cart Concept Generation We as a group used a number of concept generation techniques with focus on individual and brainstorming (only 5 members participated because the sixth could not be present for the activity) brainstorming was the most useful of the techniques because it allowed for each group member to freely comment on their own reactions and thoughts in regards to others design ideas. In many cases this led to adaptations to the

10 original drawing to improve the overall design. An example of one team member s diagram including the comments made by the other members is shown in Figure 11. Figure 11: method to generate new ideas for concept design. In the second stage of the conceptual design process, each individual thought of one separate design and presented it to the group. After the presentation of all concepts each member walked around the room making a list of pros and cons for each concept. As a group we discussed each of the concepts presented and narrowed down ideas to six individual design components. The six design components were chosen to be incorporated into our prototype. The six design components include: 4 wheels, lift system, integrated ramp, large wheels, steering, and locking mechanism for the wheels. Once the six design components were chosen, each team member thought of a new design which incorporates all of the six design components. Again, the team presented each other s design ideas and one design was chosen to be the prototype. Examples of the individual diagrams made during this meeting including the comments made by the team members are presented in Figure 12.

11 Figure 12: Second stage examples of conceptual design process. We expect our final design to incorporate the six design components discussed and agreed upon by all six members. The previous exercises did an excellent job of showcasing the best ideas from each member s concept idea. 2. Concept Screening and Evaluation 2.1. Concept Screening We as a team met with the customer to deliver a copy of the completed project proposal report and to go over with them the thought process used to make many of the decisions that are incorporated into the report. After the customer read through the report and gave it some thought, we communicated with them in order to clear up some of the aspects of the proposal that were unclear and to get feedback on what can be changed and/or added in order to make the project fit the needs even better. The customer s biggest concern was that we should focus on loads of less than 500 pounds because for amounts larger than that, they would use the forklift. This will help us narrow down our design decisions to handle loads between 200 and 500 pounds, and not worry about overdesigning the cart to pull larger amounts, however the system should be able to handle slightly higher loads than 500 pounds to account for bed weight or to improve safety Feasibility and Effectiveness Analysis In order to determine the basic feasibility of the proposed conceptual design, we as a team made a trip to White s Mill to perform some basic tests. Using an older pull

12 cart, we decided to test different weights in order to determine what load could be moved across the gravel parking lot by a single employee. A picture taken at White s Mill during the test is presented in Figure 13. Figure 13: White s Mill prototype test #1. By stacking several bags onto the cart, we determined that any amount of weight up to around 550 pounds is feasible for one employee to pull, and that loads of up to 700 pounds or so are feasible for two employees to pull. This test confirmed that the range that we determined in conjunction with the customer of 200 to 500 pound loads should be easily pulled by a single employee Subconcept Development and Scoring In order to make early subconcept decisions, a list of 16 key design needs were identified, discussed, and rated. Each need was weighted on a scale of 1-5, where 5 is given to the most important needs in terms of success in solving White s Mill s overall need, based on team consensus. The weights were supported by our research and interviews with the partner; we judged each need based on the customer s wishes and our own research. In order to create specification ranges, each customer need was quantified. Based on the general need, a more specific metric was created for each and given units to ensure that it was a quantifiable issue. The metrics with selected units and marginal and ideal quantifications are presented in Table 2.

13 Table 2: Early design metrics with units and marginal and ideal values (outdated). Need # Metric Unit Marginal Value Ideal Value 1 Turning radius in 2X length of cart Length of cart 2 Load capacity lbs > Professionalism of appearance Length of life before failure Years Safety factor psi/psi >2 5 6 Empty cart weight lbs < Ease of use Length of time before corrosion onset Years >3 6 9 Unit manufacturing cost minus labor US $ Special items needed for regular maintenance # of tools < Adaptability as trailer Yes/No No Yes 12 Unit retail price US $ Customer confidence in design Frame width in < Vertical negotiation height in 24 >30 16 Number of operators at maximum capacity # of operators 2 1 Based on the needs and metrics, four design criteria that enveloped them were created. The four design criteria are User Friendliness, Function, Durability, and Cost. In order to quantify the impact of each design criteria on the project, Table 3 was used. Placing each of the needs into the appropriate design criteria and adding the need weights for each design criteria provided a combined need weight. We divided the four compiled need weights by the total weight of all the customer needs to obtain a percentage importance of each separate criterion. Table 3: Comparison of early design criteria and their impact on the project (outdated). Design Criteria Included Need #s Combined Need Weight Priority Function 1, 2, 6, 11, 14, % User friendliness 3, 5, 7, 10, % Durability 4, 8, % Cost 9, % Total: 54 Using these criteria, the first subconcept decisions were made using a customized decision making matrix. Two decisions were made, but only one remained valid after revamping the metrics table the decision for the general type of tailgate. The decision for the type of tailgate we plan to incorporate into the final system was made using the results of the matrix in Table 4.

14 Table 4: Tailgate decision making matrix (format outdated, results still valid). This process was fine for a minor subconcept decisions, but proved to be inadequate in making major decisions. After reanalyzing each metric and the way they were presented, we modified the metrics list, making several improvements, discussed them with our advisor and customer, and decided to reevaluate the steering system using the new metrics while abandoning the original design criteria for the rest of the project, and adopting a much more comprehensive decision making process. Notable changes include: adjustment of values to be more applicable to the project and need, replacement of the term marginal with acceptable, using a 1-10 scoring as opposed to 1-5 for the decision making matrix and retaining the 1-5 weights in the metrics table, disregarding of the original design criteria, removing some metrics that were unwieldy or unimportant, and using all updated metrics in the decision making process separately as opposed to using lumped groups. Up to this point, we have only needed to use this new decision making system to aid in making one major subconcept decision (the steering system) but plan to implement it in its current form from now until project completion. The new metrics list is presented in Appendix A, along with the decision making matrix used to evaluate the steering system. No concrete decision has yet been made on the steering system, but two candidates have been decided upon based on the findings: the wagon steering system and a bevel gear steering system. 3. Proposed Design Concept As stated in Section 1.3, important components in our conceptual design include: 4 wheels, lift system, integrated ramp, large wheels, steering, and locking mechanism for the wheels. The whole team agreed that these six components were critical to the success of our project and that whatever the final design happened to look like, these six components needed to be included. Several sketches and drawings that included the six concept design components were created and bits and pieces of each were taken in order to create a drawing of what the final design concept should look like. An early rendition of the proposed design concept selected for further refinement is shown in Figure 14.

15 Figure 14: Early basic prototype concept. As shown in the figure, the cart has four wheels, which are large enough to travel over rocky, gravel surfaces without getting stuck or slipping. It also includes an integrated ramp that can be used to load materials into the cart from the loading dock using the existing dolly. Additionally, it includes a scissor lift system to raise and lower the bed of the cart to the height of the dock or to the height of trunks for unloading purposes. Aspects that are not modeled in this rendering are the steering system and the wheel locking system, as well as a decided-upon modification made to the tailgate system where the tailgate is actually placed on one of the lateral (side) bed surfaces to allow easier and more stable loading as opposed to being placed on the rear surface. The steering system and chassis, along with the wheels and wheel locks, have not yet been decided upon in terms of concept. We plan to utilize mock-ups and sub-prototyping during winter quarter, using careful experimentation in ME 488, to make these decisions since they all rely heavily on chassis design. In order to create a safe prototype, a risk analysis pitting the building of a scissor lift versus the purchasing of a lift for retrofitting was conducted and is presented Appendix B. The results of the analysis and the accompanying team discussion with our advisor led us to decide on purchasing a lift system and modifying it, subsequently centering our final concept on the lift. Several candidates for the lift system were proposed by several members of the team, with about one week of work going into finding government/education/research or discount prices for the lifts from wholesalers and other suppliers. The decision making matrix used to guide the purchase of one of these lifts is presented in Appendix C. The decided-upon lift system was the Jet brand model SLT-660F with a 660 lb operating capacity due to its superior value, company confidence (5-year warranty, even though it may be voided in the retrofitting), dimensions, and empty weight. On January 12 the lift system was ordered and is scheduled to arrive so that the prototyping process can begin officially on January 19. A copy of the order receipt is presented in Appendix D.

16 Table 1: Risk analysis between buying or building lift system. Risk Factors Options Technical Scheduling Cost Safety Build High (Complicated design) High (Many Sub-Systems) Medium ($$) High (Testing components) Buy Low (Retro fit current design) Low (Reasonable shipping time) High ($$$) Low (Tested, Guaranteed, Warranty) Table 2: Risk analysis totals for buying or building lift system. : Level of Risk Options High Medium Low Build Buy 1 0 3

17 1.0 Concept Generation 1.1 Problem Statement for Concept Generation Our customer is in need of a transportation system that is capable of transferring a load of up to 500 lbs of non-fragile farm and garden products off a 2 tall loading dock and up to 100 across a slightly inclined (~10 ) gravel parking lot. This system needs to be able to function in all-weather conditions and be corrosion resistant, safe in regards to tip-over and structural failure, and easily operated by a single employee. 1.2 Patent Search To get a comprehensive and thorough patent search we split up our search among our team members by making a list of terms. These terms were determined by group consensus on what similar systems are called. These terms included Folding Cart, Lifting Cart, Ramp Cart, Modular Cart, Stair-Climbing Dolly, Ramp Dolly, and Bridging Dolly. Each term was searched as relating to patents by at least two team members and relevant patents were collected and duplicates were discarded. We found many patents relating to elevation changes (stair-climbing dollies) but none that could navigate the 2 distance we are faced with without resorting to expensive solutions (hydraulic lifts) that would not mesh with our team goal of completing this project with a budget under $1000. Table 1 shows the closest results from this patent search. Table 1: Relevant Patent Information. Description Number Scissor Lift Cart USPTO # Pneumatic Lift Cart USPTO # Concept Generation We as a group used a number of concept generation techniques with focus on individual and brainstorming (only 5 members participated because the sixth could not be present for the activity) brainstorming was the most useful of the techniques because it allowed for each group member to freely comment on their own reactions and thoughts in regards to others design ideas. In many cases this led to adaptations to the original drawing to improve the overall design. Figure 1 shows an example of one team members method produced in class by five out of six team members.

18 Figure 1: method produced in class to generate new ideas for project solution. In the second stage of the conceptual design process, each individual thought of one separate design and presented it to the group. After the presentation of all concepts each member walked around the room making a list of pros and cons for each concept. As a group we discussed each of the concepts presented and narrowed down ideas to six individual design components. The six design components were chosen to be incorporated into our prototype. The six design components include: 4 wheels, lift system, integrated ramp, large wheels, steering, and locking mechanism for the wheels. Once the six design components were chosen, each team member thought of a new design which incorporates all of the six design components. Again, the team presented each other s design ideas and one design was chosen to be the prototype. Figure 2 is an example taken from concept meeting.

19 Figure 2: Second stage example of conceptual design process. We expect our final design to incorporate the six design components discussed and agreed upon by all six members. The previous exercises did an excellent job of showcasing the best ideas from each member s concept idea. 2. Concept Screening and Evaluation 2.1 Concept Screening We as a team met with the customer to deliver a copy of the completed Project Proposal Report and to go over with them the thought process used to make many of the decisions that are incorporated into the report. After the customer read through the report and gave it some thought, we communicated with them in order to clear up some of the aspects of the proposal that were unclear and to get feedback on what can be changed and/or added in order to make the project fit the needs even better. The customer s biggest concern was that we should focus on loads of less than 500 pounds because for amounts larger than that, they would use the forklift. This will help us narrow down our design decisions to loads between 200 and 500 pounds, and not worry about overdesigning the cart to pull larger amounts. 2.2 Data and Calculations for Feasibility and Effectiveness Analysis In order to determine the basic feasibility of the proposed conceptual design, we as a team made a trip to White s Mill to perform some basic tests. Using an older pull cart, we decided to test different weights in order to determine what load could be moved across the gravel parking lot by a single employee. Figure 3 was taken at White s Mill during testing.

20 Figure 3: White s Mill prototype testing. By stacking several bags onto the cart, we determined that any amount of weight up to around 550 pounds is feasible for one employee to pull, and that loads of up to 700 pounds or so are feasible for two employees to pull. This test confirmed that the range that we determined in conjunction with the customer of 200 to 500 pound loads should be easily pulled by a single employee. 3. Final Design Concept As a result of several concept generation team meetings, a list of six Concept Design Components was created. The whole team agreed that these six components were critical to the success of our project and that whatever the final design happened to look like, these six components needed to be included. Table 2 shows the required design components. Table 2: Required design components decided on by team. Concept Design Components 1 Four Wheels 2 Lift System 3 Integrated Ramp 4 Large Wheels 5 Steering System 6 Locking Wheels

21 Several sketches and drawings that included the six concept design components were created and bits and pieces of each were taken in order to create a drawing of what the final design concept should look like. A rendition of the final design concept selected for further refinement is shown below in Figure 4. Figure 4: Final Design Concept Drawing As shown in the figure, the cart has four wheels, which are large enough to travel over rocky, gravel surfaces without getting stuck or slipping. It also includes an integrated ramp that can be used to load materials into the cart from the loading dock using the existing dolly. Additionally, it includes a scissor lift system to raise and lower the bed of the cart to the height of the dock or to the height of trunks for unloading purposes. Aspects that are not modeled in this rendering are the steering system and the wheel locking system. These aspects will be further refined and added to this model in the coming weeks.