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Client Comments on Report

We received your report. We were very impressed with your analysis, your findings, and the clarity and professionalism of your report. We are surprised to learn that the additional cost and complexity of the compound post hole digger does not provide an advantage to the user. Your FMEA results indicate that the pivot bolts are the most serious failure mode. This may be particularly true after significant use and corrosion and under dynamic loading in difficult applications. The LCA matches intuition - would it be meaningful to try reducing truck transport emissions by shipping the product in pieces to reduce packaging and have our retailers or customers assemble? Given your mechanical analysis, would you conclude that any rational consumer knowing the true properties of both products would choose the simple variant, or are there market segments or applications who would benefit from the compound digger?

Specific comments follow

• You have identified a good list of customer needs, but you seem to be missing some important ones like ergonomics, safety, robustness to variation in ground conditions, and speed/efficiency. Retailer needs may also include the footprint and weight of the product - especially one so bulky. Please provide a more comprehensive list and compile them into a bullet list.
  • Stakeholder list has been revised and updated in report 2, and also readapted into report 1. The format is also provided as a bulleted list. Some key additions involve what was mentioned via safety, ergonomics, footpring/weight conditions for retailers and transportation. Also elimianted design engineers as it was pointed out in class that this was not necessary in the report and not the main focus in terms of stakeholders.

• Your test to identify the human applied static load of 30 lbs seems reasonable, but did you consider dynamic loading? I would expect that the most extreme case will be when a user finds a difficult area of clay and applies a dynamic load.
  • Dynamic loading was a concern, but we found it difficult to quantify just how much influence a dynamic load would have on such a tool. We agree with your assumption about when and how such a load could occur, but have struggled with defining just how violent it could potentially be. Based off our mechanical analysis, the tool's lowest factor of safety under the determined 30 lb. maximum static load was roughly 1.7. This is a very common number for a safety factor, as many applications call for a factor of safety between 1.5 and 2. Because of the relative simplicity of the problem and the low risk of fatality resulting from a failure, we feel that this is a reasonable factor of safety for the tool and it helps take into account the possibility of dynamic loading. While it is possible that a user, or combination of users for that matter, could overcome this safety factor and break the digger, the likelihood of such an event is low. If the user must resort to such forceful use, there are likely bigger issues at hand. For example, the soil quality would probably be the main source of need for such abuse. If the soil is very hard, rocky, or even thick and wet, this would potentially cause the user to apply a dynamic load. Most users should be intelligent enough to take this into consideration when they come across such a situation. They'd be much better off waiting until the soil softens, or may even want to consider renting a gas powered auger to perform the task. This will not only overcome the risk of dynamically breaking a mechanical post hole digger, but would also account for the increased physical stress that would be required to dig a post hole in such poor conditions. These concerns were addressed in the CosmosWorks Analysis Conclusions portion of report I.
• Your documentation of use of both products is excellent. One picture shows the dirt that falls out from between the blades when lifting. How much dirt is typically lost?
  • The amount of dirt lost in one scoop depends highly on the soil quality, surrounding conditions, and physical strength of the user. If the dirt is quite moist and thick, it tends to stick to the shovel, making this problem virtually nonexistent. On the other hand, if the soil is very loose and dry, it becomes much more difficult to keep in the digging heads. Also, depending on the strength of the user and his or her ability to hold the tool closed tightly, the amount of dirt lost can vary. Typically, when the soil is soft the user can expect to lose about half of the scoop, regardless of its size. Also, if the user is below average strength, they could potentially lose the entire scoop. These observations are only relevant for the simple post hole digger since the complex version completely seals the dirt into the digging heads upon removal. This problem was addressed in the Product Function/Evaluation section of report I. It can be specifically found in the "Simple Post Hole Digger:" portion of this section, located just after the first five pictures of the user in the blue polo shirt.
• Can you comment on the difficulty of lifting the digger and dirt out of the ground - does it put strain on the back? Is this unavoidable?
  • Because the process involved in digging a post hole is very cyclic, the fatigue a user may face results from the repetitive nature of the tool's use. While the combined weight of the tool and dirt it's removing is only about 10 pounds, this can seem like 100 pounds after using a post hole digger all day. When performing hundreds of repetitions in a full day of digging, it is highly likely that the user will suffer mild to moderate back, shoulder, and possibly joint pain in the hands and elbows. Simple muscle soreness will also be extremely likely in any user after performing several repetitions. The less active the user is in everyday life, the more apparent these symptoms will be for them the following day. For example, when comparing a 21 year old male college athlete with a 70 year old female gardener, it's safe to assume that the affects of using a post hole digger will impact the elder subject much more strongly and rapidly than her younger counterpart. This phenomenon is more or less avoidable unless the process is completely mechanized. This type of solution does exist, as there are tractor mounted post hole diggers that are driven from the tractor's PTO shaft. However, this type of solution is not always applicable for the people and task at hand. These issues were addressed in the first paragraph of the Customer Needs and Stakeholders portion of report I.
• You mention that the compound digger creates additional friction from the additional joints. Approximately how much of the applied force goes to resist this friction?
  • Based on our estimates, approximately 15% of the force goes into overcoming the friction.
• You mentioned that the fiberglass handles are placed into the steel tubes, and the tubes are then compressed around the fiberglass. Is there evidence of this crimping?
  • There is evidence of the crimping; the Complex Post Hole Digger DFA section of the report has been updated with the main observations that evidence the crimping.
• Nice DFMA and FEA analysis
  • Thank you for your compliments. Because some of the major stakeholder needs for the customer include cost efficiency, durability, and reliability, we felt that these two areas of analysis were extremely important to the application of a post hole digger. The DFMA analysis was crucial for understanding the costs involved with making such a tool, and the FEA analysis helped relay the product's reliability and durability. Quite a bit of work was put into these two portions of the report.
• How is it that musical instrument manufacturing shows up as a top category for toxic releases related to post hole diggers? Can you comment?
  • We must apologize for this silly error with a bit of a grin. There was an issue with how the files were saved, specifically with the naming of the images used. The file names were left too generic and were overwritten when an outside source posted another image to the web that utilized the same file name. This should have been foreseen and was simply overlooked on our part. These images have since been replaced and renamed, securing them with more specific names. Again, we apologize for any confusion or inconvenience that this error may have caused.
• You mention that transportation was ignored. Did you use the EIOLCA producer price model, which accounts for transportation?
  • The industry benchmark model used by the EIO-LCA software for our sector accounts for truck transportation. This makes a separate analysis of the impact of delivering the tools to the retailers redundant and wasteful. Also, the purchaser price model in the EIO-LCA software was employed via the truck transportation sector. The results of this research showed that the Hand and Edge Tool Manufacturing sector was outside of the top ten contributors for each particular input considered (Greenhouse Gases, Energy, Toxic Releases, etc.). This is likely due to the relatively small impact that this sector has on the transportation sector when compared to other sectors. For these reasons, this data was omitted from the report, but is mentioned here for clarity. These conclusions have been presented in further detail in the "Transportation" portion of the Design for Environment section of report I.
• Your mechanical analysis is clear except for the free body diagram of the complex digger. There are two pieces, and it is difficult to understand your model without two separate free body diagrams. Are you assuming that the reaction force in the pin at C is only in the vertical direction? What enables you to make this assumption?
  • Because of the symmetric nature of this tool, both handles rotate about point A, which virtually remains still during use. Since the reaction at point A is equal and opposite on both handles of the tool, it should remain still and can be modeled as a pinned joint which only allows rotation and no translation. The same argument applies to point B when it is modeled as a pinned joint in the last free body diagram. These relations are addressed in further detail in the "Complex Post Hole Digger Analysis" portion of the Mechanical Analysis section of report I.
• Your FEA results show a factor of safety of 1.7 at the weakest point. Is this sufficient given the potential for dynamic loading, fatigue, and weathering factors you mention?
  • Fatigue, weathering, and dynamic loading were all concerns we had initially, but we found it difficult to quantify just how much influence these factors would have on a post hole digger. Again, based off our mechanical analysis, the tool's lowest factor of safety under the determined 30 lb. maximum static load was roughly 1.7 -- a common number in the engineering world. Because of the relative simplicity of the problem and the low risk of fatality resulting from a failure, we feel that this is a reasonable factor of safety for the tool and it helps take into account the effects of these various factors. While it is possible that one, or any of these factors could eventually break the digger, the likelihood of such an event is low. The effects of dynamic loading were discussed in detail in the response to your previous question about the 30 lb. max load, and should be referred to here. In regards to fatigue loading, we've concluded that it should not play a major role in a product failure. In most situations, fatigue failures occur as a result of thousands of cycles, a number that seems a bit large for this tool's application. There are two possible reasons a user could be completing this many cycles over the lifetime of the tool, both of which will likely not occur. First, they could be a landscaper or perform some other occupation that requires the digging of many post holes each day. If this is the case, chances are they will not be using a simple, mechanical post hole digger. Rather, they would employ the use of a gas powered auger to expedite the whole process. Secondly, they could own and use the tool for several decades. Again, the number of times the average user would utilize a post hole digger is fairly low, meaning the tool will not undergo thousands of cycles. The concerns regarding weathering and aging have also become less intense. It is safe to assume that most users have some sort of shed or garage that the tool is stored in when not in use, meaning it should be safe from such environmental factors. Again, these concerns were addressed in the CosmosWorks Analysis Conclusions section of report I.

We are looking forward to seeing your research findings and product ideas in the next report.

Client comments on the Mechanical Analysis

A well-done mechanical analysis, which includes both the free body analysis and the finite element analysis on the handle. I have only two comments:

• Please indicate the diameters of the wood rod and pivot bolt in your report.
  • The diameters of the major components of the model have been added to the beginning of the CosmosWorks Analysis of Simple Post Hole Digger portion of report I under "Key Geometry:".
• Please add the boundary condition explanations using textbox in Figure 8.1 because some clients may not be familiar with the graphical representations in the Cosmos software, e.g. the force and zero DOF settings in your model.
  • The boundary condition explanations have been added to the beginning of the CosmosWorks Analysis of Simple Post Hole Digger portion of report I under the "Model" bullet.

The reference for the wood material properties is great. I encourage you post the link onto the discussion board in Blackboard system in order to share the information with other teams.