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Revision as of 15:04, 12 October 2008

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.
• 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. Most likely, the soil quality would 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 such 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 breaking a mechanical post hole digger, but will also account for the physical stress that would be required to dig a post hole in such poor conditions.
• 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 "Simple Post Hole Digger:" portion of report I, located just after the user pictures in the Product Function/Evaluation section.
• 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 issue was 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?
• 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?
• Nice DFMA and FEA analysis
• How is it that musical instrument manufacturing shows up as a top category for toxic releases related to post hole diggers? Can you comment?
• You mention that transportation was ignored. Did you use the EIOLCA producer price model, which accounts for transportation?
• 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?
• 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?

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.
• 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 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.