From DDL Wiki

Reducing the Number of Gears in the Geartrain

One very feasible product adjustment that could be made to the rotating tie rack is a reduction of the number of gears in the geartrain. There is sufficient empty space within the product casing to hold gears more than twice the size of those currently used, and little modifications to the base would be required to house a different gear train. However, the whole gear train and motor would need to be moved near the outer edge of the casing (vs. the center) thus, requiring longer wires extending from the circuit.

This change has both pros and cons that would affect the overall design. The advantages are that the product would require fewer total parts, making assembly quicker and easier as well as reducing the number of features needed on the base fixture for holding the geartrain. Larger gears would also reduce the chance of mechanical failure from causes such as loose parts or breaking. For product analysis, the rotating tie rack was assembled and disassembled multiple times. After one re-assembly, a gear was not tight on the shaft which caused it to tilt. This resulted in one of the gear’s pinion teeth shearing off after only three rotations of the rotating belt. This could be solved by utilizing larger gears, allowing the geartrain to withstand larger torque and reaction forces.

Product lists from multiple suppliers were investigated to determine tradeoffs with changing the gear design. It was found that for injection molded spur gears of either nylon or acetal the price increased in even intervals; one large gear equals the cost of two gears half its size. However, the weight of a larger gear is more than twice that of two smaller gears. With most manufacturers, standard number of teeth is not the correct interval needed to reduce to one gear, and specializing would be more expensive even in mass order. Finally, achieving the same torque increase through one idler gear is more difficult. For these reasons we have determined that the tie rack mechanism has been optimized and we would not change the number or size of gears used.

Belt and Pulley versus Meshing Gears

It is important to note that the gear train uses a belt and two-pulley system directly after the driving input rather than two meshing gears. While the same torque and speed ratios can be achieved with either system, the current design is chosen mainly to protect the motor. A feature of belts is that they can slip since their transfer of power relies on friction. In a case that the driven mechanism (belt) or idler gears would jam, this force would not be transferred to the motor shaft because slip would occur before motor damage. The belt used is of wedge type, fitting exactly into the pulley grooves. This optimizes the output of expected ratios under normal functioning conditions, yet, allows the needed failure under extreme loads. This is important because the motor is low-end and could become damaged easily from normal customer use, for example if something gets stuck on a tie hook and jams the gear train.

Another feature from utilizing a belt and pulleys is that it can withstand vibration from the motor without affecting down the line functioning. If a gear were to be mounted directly onto the motor shaft, any vibration or tilting would be transferred to other steps in the drive train and decrease the overall efficiency rating.

This design choice also meets both product and customer needs, and would not be changed in future design.

Mechanical Functioning versus Battery - Operated

A final design consideration is to make functioning mechanical rather than battery operated. This is due to a general assumption that most people do not prefer having to keep batteries on hand or replace them often, and without batteries the rotating function cannot occur. To support this assumption the following analysis was completed.

The current drawn from the batteries by the motor and light (parallel) in the circuitry was measured to be 0.39A. This was found by connecting the feeds of an Agilent® 973A multimeter to the circuitry after the batteries and before the motor.

For our use of the tie rack, we chose Energizer® Industrial™ batteries; the product requires four C-type batteries, two sets of two batteries in series placed in parallel. The specs for these batteries can be found at http://data.energizer.com/PDFs/EN93.pdf which has a chart for milliamp hours-capacity which was used to find the capacity of the battery based on the previously measured current drawn from it. Thus:

Measured:

Discharge: 390 mA

Capacity: 4,200 mA-h

(4,200 mA-h) / 390 mA = 10.77 hours = 646.15 minutes of battery life

Assumptions:

• No loss from the continuous discharge quality of the battery.
• The average person would use this rack for 1 minute twice a day.

Results:

(646.15 minutes) / (2 minutes/day) = 323.08 days of usage with one set of batteries

Conclusion:

Most batteries being used have decreased capacity from shelf life or personal storage, and for our analysis we used one of the top brand and top functioning batteries in the market. Thus, we will conclude that batteries for this device will typically have to be changed twice a year. This agrees with our motives for mechanical functioning for the rotating shoe rack, and future design iterations will be based on this result.