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MECH-
MANUFACTURING / TESTING / ANALYSIS PROJECT
Objective:
A major challenge in delivering a successful design is to ensure that it is manufacturable. This project deals with the design and manufacturing of a device. Through this process, several manufacturing issues will be faced. Once the deliverable is completed, it will be tested and data will be collected on its various sub-systems. The results will be compared to the theoretical knowledge gained in class to see how the manufacturing limitations affect the theoretical design.
Design Procedure:
You are to design a crank / clutch mechanism (see Figure 1) where the crank will work against an extension coil spring (Jones Spring part # or #312), and will be driven by a DC motor (Mabuchi model RF-500TB-
- or other supplied by prof. The power from the motor to the crank will be transferred through a belt / pulley transmission and a clutch that is activated by a compression spring (Jones Spring part #1620 or #1626). Figure 2 shows clutch details. You are strongly encouraged to review clutches by clicking here. Recall that tutorials on DC motors, springs, belt / pulley transmissions and more can be found in the Mechanical Work Session document. The remainder of the construction materials must come from what is available in the Hougen Lab.
BEFORE BEGINNING YOUR DESIGN, PLEASE FAMILIARIZE YOURSELF WITH THE MATERIALS AVAILABLE. YOUR PARTS MUST BE DESIGNED IN A WAY THAT THEY CAN BE BUILT FROM AVAILABLE MATERIAL!
Manufacturing Procedure:
Your drawings will be given to another team to be manufactured for you. Similarly, you will manufacture parts for another team based on their design drawings. Manufactured parts will be returned to their respective design teams when completed. They should be checked to be sure they are in specification and then assembled. Finally, test analysis must be done. A design team will grade their manufacturing team based on the quality of parts received, and the manufacturing teams will grade their design team based on the completeness and quality of the drawings given to them.
PROCEDURE TO DETERMINE DESIGN PARAMETERS (NOTE: Do this in conjunction with the Microsoft Excel design template!)
- Determine torque on the crank mechanism (Tcrank) when the crank angle is 90 degrees:
- Choose an extension spring #312 or #310.
- Choose a spring preload between 1/2-inch and 1-inch.
- Choose a crank length A where 1/2-inch < A < 1-1/2 inch.
- Choose a connecting rod length B where 2A < B < 10A.
Use parameters you’ve chosen above to calculate Tcrank when the crank
angle is 90 degrees. NOTE: the torque calculations tutorial shows calculations for a crank angle of 105 degrees. The computation for a crank angle of 90 degrees is MUCH less complicated!
The Free Body Diagram of the interface between the connecting rod and the spring slider shown below may be helpful to you. Keep in mind that friction force should be kept as small as possible (ideally it should be zero).
- Determine the torque output (Tslip) from the clutch:
- Choose a transmission (pulley diameter) drive ratio (Ratio1) where 2 < Ratio1 < 6.
- Calculate the torque output from the clutch (Tslip) where:
Tslip = Tcrank / Ratio
- Assume an efficiency (η) for this system so that 0.2 < η < 0.
- Adjust values of Ratio2 and η until:
Ratio2 * η = Tslip / Tmotor
When this condition is met, your design parameters are determined. You may proceed to designing the actual parts now based on these parameters. If this condition cannot be met, return to the Microsoft Excel design template and modify various parameters in the yellow boxes (do only 1 or 2 at a time!) until the condition is met.
Testing and Analysis Procedure:
Once your project is assembled and functional, adjust the clutch spring force (Fa) until the clutch slips due to insufficient torque to overcome the maximum crank torque. The crank will not complete a full revolution under this condition.
TEST 1
Determine the motor torque using two different methods and compare the results:
- Apply 3 volts (regulated) to the motor using a one-ohm resistor in series between the motor and the voltage source. Measure the voltage drop across the resistor. Using Ohm’s Law, show in your
Engineering Logbooks how this voltage measurement is equivalent to the current being supplied to the motor.
- Knowing the motor current, use the motor’s specification sheet to determine the motor torque as a function of current.
- Using an oscilloscope, measure the signal across the one-ohm resistor and display it. Adjust the display until you observe a sequence of impulses embedded in this signal. These impulses are “back EMF” generated by the motor. Knowing that there are six pulses per revolution of the motor, and using the oscilloscope to determine the time between pulses, calculate the speed of the motor in RPMs.
- Knowing the motor speed, consult the specification sheet again to determine the motor torque as a function of speed.
- Compare the results of your two torque determinations to the torque value that your team predicted from calculations. Calculate percent differences and comment in your reports.
TEST 2
Determine the clutch slip torque (Tslip) using two different methods and compare results:
- While the motor continues to operate under the conditions of Test 1, adjust the compression spring until the crank angle is 90 degrees when slipping. Measure the spring deformation and determine the value of Tslip based on this.
- Wrap a string around the driven part of the clutch, replacing the belt drive to the crank. Use a spring scale tied to the string to measure the actual force required to make the clutch slip. Calculate the corresponding slip torque (Tslip) based on this measured force and the clutch radius where the string is wrapped around. Make sure your spring scale is calibrated for the angle for which it is being held.
- Compare both experimental Tslip determinations to the calculated value that your team used. Calculate percent differences and comment in your reports.
