Exercise bike
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Contents |
Executive Summary
Stakeholders
There are 5 main stakeholders in the exercise bike: Users, Fitness centers, Maintenance crew, Manufacturers and Equipment stores. The requirements and needs that each has is listed below:
1. Users
• Easy to use
• Storage
• Safety
• Cheap
2. Fitness centers
• Reliability
• Life of product
• Inexpensive
3. Maintenance crew
• Accessible
• Safety
4. Manufacturers
• Compact
• Simple design and components
5. Equipment stores
• Storage
• Aesthetics
• Price
Product Function
Product Use
The product is a stable platform used for cycling practice as well as a smooth, intense workout. The user mounts the bike, sitting on the seat. Feet rest on the pedals and hands on the rubber handles to the front of the bike. While holding the handles, feet on the pedals and sitting on the seat, the rider is provided a stable riding position. To ride, the user pedals forward (backward is not recommended by Schwinn) exerting significant effort to build momentum in the heavy flywheel. Once user reaches a comfortable speed their effort levels to a steady pace. There are a few ways to stop, all not very intuitive to basic bike riders. There is a knob on the frame that can be turned to apply a clamp-brake to the flywheel that will slow the wheel down. The user can also completely remove their legs from the pedal assembly, but this is not recommended since it is dangerous and the momentum in the wheel is too high and will spin for a very long time. The user could also cool down by gradually reducing their pedal speed till it becomes slow enough to dismount. This is recommended anyway in any strenuous workout, stopping abruptly is not good for the body.
Bill of Materials
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| Part/Sub-assembly | Part # | Part Name | QTY | Weight (g) | Function | Manufacturing Process | Material | Image |
|---|---|---|---|---|---|---|---|---|
| Seat | 1 | Seat | 1 | 428 | Allows user to sit on bike | press fit/lay over cloth and stitch | foam, plastic, stainless steel | |
| - | 2 | bolt | 1 | 25 | Assembles seat to seat support | forging, thread rolling | stainless steel | |
| - | 3 | sm. washers | 2 | 15 | Assembles seat to seat support | stamping | stainless steel | |
| - | 4 | lg. washers | 2 | 15 | Assembles seat to seat support | forging, thread rolling | stainless steel | |
| - | 5 | nuts | 2 | 5 | Assembles seat to seat support | casting thread tapping | stainless steel |
- We were not able to dissect the wheel sub-assembly due to the high pressure that the assembly is under at all times. Dissecting the wheel would be extremly dangerous, and once it was dissected, getting the wheel under the same high pressure would have requried special tools that we do not have access to. The conclusion that dissection was not a good idea was supported by the expert machinists in the Carnegie Mellon machine shop in Hamerschlag Hall.
- Removing the bike pedals from the bike frame was not a possibility for our group due to the need of a special bike-specific tool called a crank arm extractor. More information can be found by watching this video: http://www.youtube.com/watch?v=H6aHvq4oD9o. In addition, the sprocket is welded to the pedal crank, making it impossible to remove.
Design for Manufacture and Assembly (DFMA)
Design for Manufacture
This product is heavy and rugged. Looking at all of the components, several design choices were made for manufacturability of Spinners in batch quantities.
The frame consists largely of steel square tubing and is cut at different angles to be welded together. Therefore the frame is very rigid, since almost all joints are welded instead of fastened. The main goal of the frame is to be stable. Therefore the design strategy we see that considered manufacturability is:
- Keep the frame simple and cost effective
- Used inexpensive and simple tubing
- Cut pieces at angles to be welded, no bending
- Use heavy material to add to stability of frame
- Steel tubing
- Steel tubing
The handle and seat assembly have curved steel pieces. Mounting to the frame could have been complicated but instead were designed very simply, not only for assembly but for the sake of manufacturing separate parts at a lower cost. This way only a small piece of material had to be bent in part 7. The handles (part 9) have a large amount of bending required, but this may be due to the requirement of good ergonomics and worth the additional manufacturing cost.
- Complicated parts were made simple by using multiple parts
- These parts assemble easily (pull pins)
- Requires less machinery in-house, more importing of material/components
Along a similar line, many of the other components were bought off the shelf because manufacturing them in-house would be very costly and inefficient. Things like bolts, nuts, washers, and springs would most likely have been purchased from a supplier whose expertise is mass producing such components. This greatly saves costs as long as Spinners are made in batch sizes. If these bikes were in large-scale mass production then some of these common parts would be made in house, but Spinners are special items and not produced in large enough quantities to warrant mass production manufacturing techniques.
Design for Assembly
It became apparent while dissecting the product that the design for assembly of the Schwinn Spinner exercise bike was carefully planned as well as pragmatic. This made the dissection and re-assembly process relatively easy and safe.
In terms of safety the bike is designed so that the heaviest object, the 40 pound wheel, cannot slip out of its supports after any one particular piece is removed. There is at least a second line of defense to support the wheel. If this were not the case, it would be easy for anyone to get hurt or break the bike not knowing which particular piece held the wheel. Once that piece was loosened, the wheel could easily slip and crack another part of the bike or fall on someone’s hand or foot.
It is important to note that the assembly makes it easy to remove pieces which would do not have easy access. For example, the back casing which is sandwiched between the pedal cranks and the bike frame can be slipped out from behind due to its design, which incorporates slots into the axle holes. This allows the user to remove the casing without completely removing everything else from the axles.
Many of the bike parts are sub assemblies themselves which is a great DFA because it creates a layering affect for dissection. In order to replace a small part on the bike there is never a time when the entire bike would need to be taken apart. Instead, the user would remove the particular sub-assembly the part belongs to and then dissect that assembly, keeping the remainder of the bike intact. The design also follows a format of high-quality DFA by minimizing the part count and using larger, welded parts rather than piecing these larger parts together with nuts and bolts. In most cases, the large welded parts like the handlebars and seat assembly have easy attachment and adjustment to the base frame with a pin and slot design.
FMEA
The Failure Mode and Effects Analysis is used to establish problems with the existing product design, suggest improvements to decrease severity and/or occurrence rate of the specified problem, or increase the chance of identification of the problem during the manufacturing process. The effect of the change on the product is predicted by the analyst.
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| Item and Function | Failure Mode | Effects of Failure | S | Causes of Failure | O | Design Controls | D | RPN | Recommended Action | Responsibility and Deadline | Actions Taken | S' | O' | D' | RPN' | ||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Pedals-User applies force to drive product | Torsional stress failure | Product no longer functions | 8 | Inadequate welding | 2 | Post-welding stress testing at different rotational speeds | 3 | 48 | -Higher-quality welding | -Welder in manufacturing process | N/A | 8 | 1 | 4 | 32 | ||
| Gear #1-Converts user-driven tranlational energy to rotational energy | Wear from chain contact | Increased slippage | 3 | Lack of proper maintenance | 4 | Fretting-wear testing at different speeds | 5 | 60 | -Lubricant coating to prevent wear | -Gear #1 Designer -Manufacturer | N/A | 3 | 3 | 5 | 45 | ||
| Chain-Converts Gear #1 rotational energy to translational energy | Slippage | Loss of energy, possible consumer injury | 4 | Radical change in speed | 6 | Record instances of slippage at different speeds, accelerations | 3 | 72 | -Increase surface contact between gear and chain | -Chain Designer -Manufacturer | N/A | 3 | 4 | 3 | 36 | ||
| Wear from gear contact | Increased slippage | 3 | Lack of proper maintenance | 4 | Fretting-wear testing by measuring friction at different speeds | 6 | 72 | -Lubricant coating to prevent wear | -Chain Designer -Manufacturer | N/A | 3 | 3 | 5 | 45 | |||
| Pin shearing | Chain breaks, product is unable to function | 6 | Poor choice of pin material | 2 | Apply different forces upon chain, measure results | 4 | 48 | -Change pin material -Increase pin diameter | -Chain Designer -Manufacturer | N/A | 6 | 1 | 4 | 24 | |||
| Gear #2-Converts Chain translational energy to rotational energy | Wear from chain contact | Increased slippage | 3 | Lack of proper maintenance | 4 | Fretting-wear testing by measuring friction at different speeds | 6 | 72 | -Increase surface contact between gear and chain | -Gear #2 Designer -Consumer | N/A | 3 | 4 | 3 | 36 | ||
| Gear #2-to-Wheel Shaft-Translates Gear #2 rotational energy to Wheel | Torsional stress failure | Product no longer functions | 8 | Poor choice of shaft material | 1 | Torsional stress testing by measuring deformation at different angular velocities | 2 | 16 | -Add safety cage around wheel and shaft assembly to prevent injury | -Gear #2-to-Wheel Shaft | Designer | N/A | 4 | 1 | 2 | 8 | |
| Shear stress failure | Product no longer functions | 8 | Poor choice of shaft material | 1 | Shear stress testing by measuring deformation with different wheel weights | 2 | 16 | -Change shaft material | -Gear #2-to-Wheel Shaft Designer | N/A | 6 | 1 | 2 | 12 | |||
| Handlebar-Assembly-User comfort | Loose casing | Slightly uncomfortable for consumer | 2 | Vibrations from use of product | 9 | Measure vibrational results of different speeds, assembly dimensions | 1 | 18 | -Revision of assembly dimensions | -Handlebar-Assembly Designer -Frame Designer | N/A | 1 | 9 | 1 | 9 | ||
| Seat-Assembly-User comfort | Loose casing | Slightly uncomfortable for consumer | 2 | Vibrations from use of product | 9 | Measure vibrational results of different speeds, assembly dimensions | 1 | 18 | -Revision of assembly dimensions | -Seat-Assembly Designer -Frame Designer | N/A | 1 | 9 | 1 | 9 | ||
| Spring in Wheel-Helps prevent slippage caused by rapid deceleration | Broken spring | Overloads spring, no longer prevents slippage | 6 | Excessive rapid deceleration | 2 | Measure stress in spring under a variety of pressure to identify actual stress limit | 5 | 60 | -Increase length of spring -Increase thickness of spring -Change spring material | -Spring Designer | N/A | 6 | 1 | 5 | 30 |
Design for Environment (DFE)
Abe and Casey
Mechanical Analysis
Shobhit
