Ceiling-mounted storage lift
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This article was contributed as part of a design project for the course 24-441 Engineering Design in the spring semester of 2007 at Carnegie Mellon University.
Team 1 - Report 1: Ceiling-mounted bicycle lift
Team 1 - Report 2: Ceiling-mounted bicycle lift redesign
Contents |
Executive Summary
Project Background
For our senior capstone design project, we were to work in groups to analyze a current product on the market with some engineering involved in the design. Our group of four decided to analyze a ceiling mounted bike lift. For the project, we were to fully understand the functionality of the product and dissect it. Then we were to find opportunities by brainstorming where we could enhance the product or its functionality. Finally, we were to build several prototypes demonstrating our ideas and improvements to the current design.
With our selection of the ceiling mounted bike lift, we found several opportunities where we could improve and change the current design. These improvements include increasing the functionality of the lift, allowing the user more flexibility in the usage of the lift, increasing the maximum lift load and diversifying the product from its competitors on the market. This project has allowed our group to work across several business lines from engineering to marketing to production. The project concluded with our presentation of our final prototype. The details of our opportunity, findings, analysis, design and conclusions will follow.
Opportunity & Market Research
Product Description
Components
The Ceiling-mounted storage lift BOM contains the quantity of each component in this product, the weight of that component, its function, the material it's made of and its most likely manufacturing process. It also contains both a CAD image and a dimensioned drawing of each custom part and a catalog reference for each purchased part. The design is comprised of 121 total parts with 29 unique parts. The total weight is 4.1 kilograms or approximately 9 pounds. The ceiling mounted storage lift has been designed to use a minimal number of parts while still being capable of its desired functionality. The DFMA section of this report includes further detail on some of the considerations that were made in designing many of these parts.
Pre-Packaging Assembly
There are three major sub-assemblies that need to be assembled prior to packaging and then shipping the product. The remainder of the assembly process is to be completed by the consumer. These major assemblies can be seen below: the sliding bracket(1), the lower bracket(2), and the rope locking bracket(3). Please follow the link next to each image for a detailed description on how these components are to be assembled.
1: Storage Lift Sliding Bracket Assembly
2: Storage Lift Lower Bracket Assembly
3: Storage Lift Locking Bracket Assembly
Consumer Assembly
---summary of consumer assembly--- Please refer to the Storage Lift Consumer Assembly page for a detailed description of the steps the consumer will need to follow to properly install the product.
Engineering Analysis
Design for Manufacture and Assembly (DFMA)
In designing the ceiling-mounted storage lift, we employed many of the same DFM techniques that were used to design the ceiling-mounted bicycle lift. Minimizing the number of unique parts lowers production costs by reducing the amount of new machines and tooling to create those parts. Minimizing the total number of parts also reduces costs for raw materials as well as reduces weight and keeps transportation costs low. The ceiling-mounted bicycle lift minimized parts well with only 19 unique parts and 66 total parts. We greatly increased the functionality and complexity in our design while only increasing the number of unique parts to 29. The total number of parts in our design is 121. When possible, we incorporated duplicate parts and subassemblies in our design. For example, the upper and lower bracket assemblies appear twice and are identical. The ceiling mounts are also identical with one mount having two identical locking mechanisms attached. The only non-duplicated part in the design is the single wall rope tie.
In addition to keeping the number of unique parts to a minimum, it is also cost effective to use parts that are already in production for other products and may be purchased. This eliminates the cost of creating new machines. Of our 29 unique parts, 18 are parts that are already in production and may be purchased. Of the 11 parts that must be custom built, there are only 4 parts that are not already custom built for the ceiling-mounted bicycle lift. Ten of the eleven custom parts are stamped and bent from steel sheet metal. The remaining custom built part is the aluminum U-rod that is manufactured using the common manufacturing process of extrusion and bent into shape. The only post machining for the U-rod is to cut a taper on the ends of the rod. By using the manufacturing process of stamping for almost all of our custom parts, we eliminate the need for any post machining. We designed the stamped parts this way to make them very cost effective to be produced in high volume. Stamping will create some waste material, but this material can be gathered and recycled to create more parts. Overall, we feel that the design for the ceiling-mounted storage lift does not require any costly or complex manufacturing processes and is well suited for mass production at low cost.
Design for Usability (DFU)
Design for Safety (DFS)
Safety was a primary consideration in the design of the ceiling-mounted storage lift. We took the time to analyze and verify the design of each component so that the failure mode of the product in extreme conditions is one that will not cause major damage or injury. Similar to the ceiling-mounted bicycle lift, the primary mode of failure in an overloaded condition is slippage of the rope through the locking mechanism. We performed tests on the ceiling-mounted bicycle lift to determine the maximum loading that would cause failure. We gradually increased the weight of the object hung from the lift and applied a dynamic load to see if the lift would fail. The results of our testing showed that the lift failed when a dynamic load was applied to an object with a weight of 240 pounds. A video of the testing can be seen at the following link: http://www.youtube.com/watch?v=2OQ_4o4JSPw .
Since the pulley system in the ceiling-mounted bicycle lift has a mechanical advantage of four to one, the tension in the rope is 60 pounds for a maximum load of 240 pounds. In our design, a tension of 60 pounds in each rope corresponds to a load of 480 pounds. To insure that the primary mode of failure in our design is slippage in the rope, we analyzed and verified that the structural components of concern in our design are capable of withstanding a load of 600 pounds. The components we analyzed were the wheel axles and the pulley brackets. The analyses are shown in detail on our Ceiling-mounted storage lift numerical analysis page. From our axle sizing table, we were able to select the appropriate axle diameter once we determined the separation distance between the wheel and the side of the pulley bracket. To minimize torsion on the L-channel track, we decided to place the wheels as close as possible to the vertical wall of the track. This makes the separation distance 1.25 inches. We sized our axles for steel with a yield strength of 40,000 psi and chose a diameter of 5/16 inches. We also performed a finite element analysis using ANSYS to verify the structural integrity of the pulley brackets. We performed the analysis on simplified shapes and concluded that our pulley brackets would not fail under the same load.
Failure Mode and Effects Analysis (FMEA)
The FMEA for the ceiling-mounted storage lift is effectively identical to the Ceiling-mounted bicycle lift FMEA. In our first report, we identified two distinct failure modes that we felt needed to be addressed. Both failure modes were due to one component, the locking mechanism. Since we re-use this part in our design, we still recommend making the same changes that we suggested in the first report. Our recommendations are restated below. The part numbers are in reference to the Ceiling-mounted bicycle lift parts list.
If the user is standing on the side of the locking mechanism that the bike is on (not the normal standing position) and were to let the rope go, the locking mechanism would not be able to perform it's function of catching the rope and stopping the bike from free falling. The cause of this problem comes from the range of rotational motion of the clutches(9,10) on the locking mechanism. If this range was reduced by roughly 2 to 3 degrees, this entire failure mode could be eliminated. This could be accomplished with a small change to the design of the locking pulley mount(8), in which the bottom tab was made a few millimeters wider. This would not affect the current 8 degree rope angle needed for immediate engagement of the locking mechanism under proper usage. The repeated testing of this component ultimately led us to discover the second most severe failure mode of our analysis.
The components of the locking mechanism that pinch the rope, thus holding it in place, make contact with the rope over a relatively small area. Thus, repeated latch engagement is fairly harsh on the rope and after just several weeks of normal use, obvious signs of rope deterioration become apparent. The rope supplied with this product is sufficiently robust from a tensile load capacity standpoint. Replacing it with a rope made from a less standard material that has better surface wear resistance, although would be simple, we feel as though it would not make fiscal sense. Instead we propose a design change be made to the clutches(9,10) that engage the rope, in which they are given a more blunt surface that makes contact with the rope over a larger area, away from any sharp edges. This will not necessitate any other design changes to accomodate for this modification, which is why we believe this course of action to be the most cost effective change that solves the problem at hand.
Quality Function Deployment
As designed the ceiling-mounted storage lift, we set up a Quality Function Deployment (QFD) to give us quantifiable engineering targets to strive for. Please see the QFD page for a more detailed explanation of this design tool. We met, or came acceptably close to most of our goals such as having less than thirty parts and having a maximum load of 480 pounds. By meeting these goals, we feel that our product has an advantage over the ceiling-mounted bicycle lift and similar products on the market. Our product is attractive to many more buyers and is more useful to each of those individual customers. It is rated to carry a heavier load and is adjustable to carry many different items. Other products on the market do not have the capability of easily adjusting the length between the brackets. Thus, their products limit the types of items one can store once the product is installed. The ceiling-mounted storage lift is especially attractive to buyers who will use the product to store seasonal items. The adjustable length between brackets and interchangeable hooks allow users to store different size and shape items at different times of the year using the same lift system. For example, a user is capable of storing a kayak during the winter months and store a rack of winter closing in the same location during the summer months when the kayak is in use. These features combined with the accomplished goals in our QFD give our product an edge over the competition.
Final Prototype
Our second prototype showed what we wanted to do very accurately but was unusable because many of the part weren't very strong or precise. We decided to start our final prototype from scratch and add parts from previous prototypes that we felt were up to the quality that we wanted. The rest of the parts were either bought or machined from aluminum. The parts that we reused from the second prototype were the locking mechanisms, string, hooks, and pulleys, which originally came from the ceiling mounted bike lift, as well as the L-brackets, and carabineers. The parts that we bought were the springs that were used in the lock for the moving pulleys and all of the loading devices(the bin, clothes rack, etc.). Everything else we machined out of aluminum and put together ourselves. Building these parts, which consisted of the upper brackets, lower brackets, the u-bolt, and the mounting brackets, took a lot of time but ended up working much better than we originally expected. The final prototype was fully functional, it just had a much lower loading capacity than our product will have. It demonstrates the pulley system, the movable brackets, the locking mechanisms, and the interchangeable lifting mechanisms very well. After the competition of the final prototype we learned that our strings can be very confusing and that we need to have different colored strings for the different parts that they move. Also, we learned that we needed longer strings for moving the brackets so that the end of them didn't get caught in the pulleys. The lock for the moving brackets was made out of three different parts in our prototypes and therefore would occasionally get caught on the L-brackets. In the design this part is made out of 1 U-bolt and will not have this problem.
User-testing
Many of the users of our final prototype expressed very positive opinions of it. They were generally surprised with the ease of use and the wide range of items they could use the device on. One was actually so please with our prototype she offered to buy it from us. Our test subjects were all highly educated college students and were all able to pick up on how to use the lift very easily, even without directions. Also, all or our test subject were able to change the lifting mechanism quickly and without trouble. If we had the resources to test the lift on a wider range of subjects we assume that the result would vary much more and directions or even an example of how to use the lift would be necessary. A couple of the minor problems that we found from our user testing included with the most common on top:
1.) Confusion of which rope to pull in order to move the lift in different ways.
2.) Confusion of how to lock the lift at the top.
3.) Inability to lock the brackets in place after moving them.
4.) Tangling the bracket moving ropes in the pulleys.
Conclusion
We believe that our design should be pursued for production. Generally, our design opens the market of the lift from just bike users to all home owners. It does this at a very minimal cost(less than 10 dollars, for a total around 20-30 dollars), that most people said they would be willing to pay. The product may lose some of its appeal towards bike owners, but with the addition of a much larger market the gain should far out way the losses. The design is also safer than the bike lift in that it can hold more weight because of the dual locking mechanisms being used. This provides a failsafe in that if one of the mechanisms starts to slip the other mechanism may still be able to hold up whatever is being lifted until the problem is fixed. Finally, the design encourages many side products that can be offered. Designs of new lifting mechanisms and ceiling bracket attachments can be produced and sold separately for extra flexibility in the product.
Production Plan
The design is almost completely ready for production. Some cost/revenue research must be done first in order to minimize the cost to produce the product and maximize the revenue received. In order to minimize cost some parts may need to be exchanged for more economical parts that will do the same job. Some user testing on a prototype that completely represents what the product will look like and do will also be necessary. This user testing will have to have a much wider representation of the general population than previous user testing in order to find problems that may have been missed in the past. Market research on packaging also needs to be done. We need to find out whether to package multiple lifting mechanisms with the lift, increasing price of the lift, or to package them separately, lowering the price of the lift and allowing people to buy just the specific lifting mechanism that they want would be more advantageous. Obviously people would want to be able to buy just the lift and the lifting mechanisms that they want, but it may be more profitable to sell everything in the same package and forcing people to buy the lifting mechanisms that they do not need. After this final research is done and nothing comes up that would hinder production, the design will be ready to be produced.
Group Members: Jeff Polack, Kevin Pruzinsky, Davey Quinn, Russel Verbofsky
