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Client Comments on Report

We received your report. The two DFMA suggestions of improving insertion of the motor and reducing unnecessary parts sound interesting. Your failure modes of axle failure, suspension deformation, and gear slippage make sense. You mention that the gear ratio is designed for torque rather than speed, but isn't it the combination of motor and geartrain that matters? It seems that the geartrain alone doesn't say much.

Detailed comments:

• Nice list of customer needs, but it seems you are missing some important ones like performance, range, safety, noise, etc.
• Please explain user interaction with the product in greater detail. Also, please expand on the mechanism functions - how is speed regulated? How does the differential function? How does the suspension function?
  • User Interaction (Control System): The Wheely King is controlled with a radio frequency transmitter. This transmitter has a trigger to control acceleration; pulling the trigger back causes the truck to move forward while pushing the trigger away from you is brake/reverse. When the car is in motion, pushing the trigger will bring the Wheely King to a stop. Releasing the trigger and pushing it forward again will move the truck backwards. To control the steering system, a wheel is mounted on the side of the transmitter. To calibrate these functions, there are 2 potentiometers that adjust the “zero” of the control. Steering can be zeroed by moving the truck in a long straight path and adjusting the steering pot until the truck drives perfectly straight. Adjusting the throttle pot is even easier: just adjust the pot till the truck is stationary. Onboard the RC truck there is a RF receiver that receives the signal from the transmitter. This signal is then relayed to the servo and to the ESC (electronic speed controller) which conditions the signal for transmission to the motor.
  • Steering System: The steering system is actuated by a servo mounted in the upper front chassis area. It’s input signal comes from the transmitter via the electronic speed controller. The servo is attached to the front differential’s steering mechanism through a series of links. Since the servo is roughly above the differential, the linkage has a pivot point near the center of the chassis on the center gearbox mount. Between the servo and the front differential are three links. The front differential also has a three-link assembly to rotate the wheels for steering. Firstly, the wheel hub carriers are linked so that they turn in unison. A 90-degree bent member translates the front/back motion of the servo linkage to the left/right motion of the steering arm. Lastly, a link connects the bent member to the right wheel hub carrier to actuate the wheels.
  • Suspension System: The front and rear differentials are attached the chassis by two suspension links (lower) and one sway bar (upper). This allows the differential some limited movement. On the rear (non-steering) differential’s suspension links, a cross brace is added to further restrict motion. To complete the suspension system, a shock (composed of a viscous fluid damper and linear spring) is added that connects the differential to the shock tower. This pushes the differential down toward the ground, but allows the wheels vertical motion when an obstacle is encountered.
  • Powertrain: Torque is delivered to the wheels from the motor through the powertrain. The motor takes signals from the electronic speed controller and rotates accordingly. This rotation is sent through the center gearbox, reducing the rotations per minute while creating a mechanical advantage, increasing torque. At the bottom of the gearbox is a single rotating shaft that powers both differentials. The shaft has two universal drive shafts mounted to it that allow movement in the suspension while still providing power to the differentials. Inside the differentials is a set of gears that translates the rotation of the drive shafts to the rotation of both wheels in the same direction. This is accomplished with the use of bevel gears.
• Your summary that some components are consolidated while others are not is interesting - can you be more specific?
  • In general, most design functions of the Wheely King were accomplished using as few parts as possible. This makes assembly easier, makes the product less complicated, and reduced manufacturing cost. In some cases however, it is actually more favorable to increase the part count when it has a desirable afect on those factors. For example, take the suspension links. The front and rear suspensions differ in that the rear is locked with a cross brace. These two suspension links and cross brace could have been manufactured as a single part, but they weren't. With the design they used, four identical suspension links are used in both the front and back. This is good from a manufacturing standpoint, as well as from the owner's standpoint because his parts are now interchangable. If any one fails, it can be replaced easily by a single component.
• Great summary of DFMA - it looks like many of the guidelines were followed, and the assembly issue is interesting.
• Great FMEA findings. What scale are you using?
  

The evaluation criteria and ranking system in the textbook 'Design for Manufacturing and Assembly' by Jeremy Michalek was used.

• How did you determine that the battery charging is 30W? Your use assumptions seem like very heavy use - please comment. You appear to have assumed a carbon tax of $200/ton. This seems unlikely - why did you assume this? In any case, it looks like the GHGs could be a significant cost even at a lower tax rate. Thank you for the clarity on how representative the EIO sector is.
• You mention that the gear ratios give you above average torque and below average speed. What is average, and where are you getting those numbers? Is the motor itself standard across all cars? If not, shouldn't the motor and gear ratio be examined together before being able to make such conclusions? Your final results of a 9:1 torque ratio seem reasonable, but what does it tell us?

We look forward to seeing your market analysis and new ideas in the next report.

Client comments on the Mechanical Analysis

This mechanical analysis mainly focuses on the gearbox design of the RC truck. My comments are in follows:

• Please state your assumptions for the mechanical analysis.
• As a client, I would like to an analysis with more depth or more applicative. For example, you can predict the steepest slope that this RC truck can climb (maximum gradability). The value can be simply calculated by using the output torque to wheel, wheel radius, vehicle weight and an assumed friction coefficient between the wheels and the ground.