Thursday May 16, 2002
10:00 am - Noon (and 1:00 - 2:00 pm, if needed)
in the M.E. Shop.

OBJECTIVE

To design and build a single-ratio pulley transmission that effectively couples the rotational power of an electric motor to a rotatable wheel. This project serves as an oportunity to design and fabricate a device that requires preliminary calculations, detailed machining, careful alignment, and extensive testing to yield minimum frictional loses and maximum performance in a design competition format.

IMPORTANT DATES

Thursday May 2, 2002   Design Proposal Due

A summary overview of your design concept is due during class. This page should include a list of your team members, your team name (if you have one), the "name" of your entry (if you have one), a sketch of your design idea, and a discussion of the important physical principles and characteristics of your transmission.

Thursday May 16, 2002   Transmission Contest

Transmissions will be tested at 10:00 am in the M.E. Shop.  Your final score will be the best obtained in a maximum of 2 attempts.

Tuesday May 21, 2002   Design Debriefing

A one (or more) page summary describing the innovative features of your transmission and evaluating the successes and failures of your design.
What would you have done differently were you given more time?

Photograph of transmission evaluation apparatus.

EVALUATION

The performance of each transmission will be measured once during the transmission contest. The same constant-voltage power supply and load wheel will be used for each evaluation. The load wheel will start from rest and its speed will be measured by monitoring the DC-voltage output of an analog tachometer that is connected to the wheel shaft.

Based on the performance of each Transmission a score (S) will be computed according to the following cost function:

where:

N   =   maximum speed (rpm) of the wheel (this can be a peak speed, and need not be sustained)

T   =   time (seconds) required for the wheel to reach 250 rpm (or the time to reach the maximum speed if it is less than 250 rpm).

WORK GROUPS

Students will work in groups of two or three to design and fabricate one single-ratio pulley transmission. Interactions with other students in the class is highly beneficial; therefore, conversations, calculations, analyses, ideas and tests may be shared among the students, but the transmission design and fabrication must be the effort of an individual group. In the end, the effectiveness of a particular design will depend on how well it is fabricated, tested, and tuned by the group.

Note that this collaboration policy does not extend to replicating others ideas. Occasionally, two groups will arrive at a very similar solution independently, sometimes one group will see a great idea in another group's transmission, and finding no superior alternative will want to incorporate it. This duplication is permissible, though not necessarily encouraged. In many respects, you should treat this design project as similar to an ordinary homework set. It is permissible to collaborate with your classmates and seek the advice of the instructor, TA, M.E. shop staff, other class participants, and other students; however, the final product must be your own work. If you are concerned about the acceptable limits to collaboration, discuss the situation with the instructor.

DETAILS

1. SAFETY:

   A transmission that is judged to be a potential safety risk to any participant or spectator will be disqualified.

   It is mandatory that safety glasses be worn at all times while fabricating and testing. It is, of course, also mandatory that safety glasses be worn at all times while in the M.E. Shop.

2. ENERGY SOURCES:

   The power used by the transmission to spin the load wheel is limited to the power supplied by the electric motor. This must be true on an instant-by-instant basis, which means that no (appreciable) energy from the motor can be stored in the transmission.

3. TRANSMISSION PROJECT MATERIALS:

   • Your transmission must be constructed entirely from materials supplied by the M.E. Shop for this class:
     • 1/2-inch and 3/8-inch thick Plexiglas
     • 1.0-inch diameter aluminum round stock
     • braided Dacron fishing line
     • 0.25-inch OD (outside diameter) precision ground steel shafting (limit 24 inches per group).
     • 0.25-inch ID (inside diameter) sintered bronze bushings
     • clear plastic (tygon) tubing stretchable to 0.25-inch ID
     • threaded fasteners (screws) from the M.E. Stock
     • 1/16-inch cross section o-rings with the following inside diameters: 0.5, 1.0, 2.0, 4.0, 5.0 inches.
     • 3/32-inch cross section urethane belt (orange-go) material that can be formed into belts of any length
     • standard office paper clips
   • Additional supplies are available, subject to Rodney's or John's approval.
   • Replacement supplies and materials are available on a limited basis. When in doubt check in the shop.
   • Glues and epoxies may be used only for bonding.
   • These materials may be modified in any way (disassembled, cut, machined, ground, etc.).
   • The project materials may not be altered chemically (except locally by glues for bonding).
   • Soldering and Brazing are permitted.
   • Welding is not permitted.
   • Light machine oil, mineral oil, or vegetable oil can be used SPARINGLY to lubricate. Do not contaminate the evaluation apparatus. For many traction materials, including belts and pulleys, once they are contaminated with oil it is nearly impossible to effectively clean them.
   • It is permissible to build tools, jigs and/or fixtures to help fabricate your transmission, and also to help prepare it for evaluation. For example, you might want to build a template or other device to help ensure that your transmission is set up "perfectly" before each evaluation. This may help provide consistent performance.

4. TRANSMISSION SPECS

   Size:

   When each evaluation begins, your transmission must fit into a 6 x 6 x 6 inch (inside dimensions) Plexiglas box. This rule requires your entire transmission to actually fit inside the box at t=0, when electrical power is applied, at the start of the evaluation. A plexiglas testing box is available in the M.E. Shop to check the size of your transmission.

   
   Photograph of 6 x 6 x 6 inch size box.

   Mass:

   There is no limit on the transmission's mass; however, you must be able to install/deinstall it within the allocated time.

   Physical Interaction:

   No manipulation of, or interactions with, a transmission is allowed during evaluation.

   Installation:

   Four threaded mounting holes are provided to simplify the installation of your transmission. It is recommended that you use at least one of these to fasten your transmission to the mounting platform shown in the figure below. Both the motor and wheel have 1/4-inch OD shafts and flexbile couplings to connect the motor and wheel to the 1/4-inch OD input and output shafts of your transmission. Note that the transmission may be placed anywhere within the 6 x 12 inch outline on the mounting platform at the start.

   
   Photograph of installed transmission.

   

5. SPATIAL RULES:

   Each transmission must be designed to functionally interact with nothing other than: the mounting platform, the motor shaft, and the wheel shaft.

6. EVALUATION APPARATUS:

   • Flexible couplings with 1/4-inch ID split clamps are provided on both the motor and wheel shafts. These flexible couplings can accommodate 0.020-inch of radial misalignment, and 5^o of angular misalignment. They require 3/8-inch of axial length for clamping to the input and output shafts of your transmission.

   • The load wheel has the following experimentally measured performance properties:
     (note that these are approximate, limited by our measuring capabilities).
     (Updated: May 3, 2002)

     Speed (rpm)	76.2	156.6	232.8	301.2	337.5
     Drag Torque (N-m)	0.00905	0.02450	0.04880	0.07790	0.09840

   • A curve fit to the measured drag torques yields the following fit equation (w is the wheel speed in radians/second):
     (Updated: May 3, 2002)

     Drag Torque [N-m] = 7.29390 x 10^-7 w³ + 2.91430 x 10^-5 w² + 8.37900 x 10^-4 w

   • The rotational inertia of the load wheel is approximately:   I = 0.27 [kg-m²]

   • The rotational inertia of the motor is approximately:   I = 45 [g-cm²]

   • The motor performance at its normal operating voltage of 24V is characterized by the following (approximate) specifications:

     Stall Torque (T_s) = 0.287 N-m
     No-Load Speed (w_o) = 4550 rpm

     For this contest, we will drive the motor at 20 Volts (Voltage set with no load on motor shaft). The stall torque and the no load speed values are approximately proportional to the operating voltage; therefore,

     Stall Torque (T_s) = 0.239 N-m
     No-Load Speed (w_o) = 3792 rpm

   • Intentional damage to the evaluation apparatus will result in disqualification.

   • The evaluation apparatus will be available, in 04 Spaulding, for transmission testing for the duration of the project.

7. SIMULATIONS:

   Here is some Mathematica code for simulating the motor/transmission combination. This code has been verified by Paul Novak (April 2000) to be correct. Unfortunately, Paul has long since graduated from Caltech, so he is unavailable to answer questions about his code.

   (New May 10, 2002)
   Here is some C-code and data that may be useful in running simulations of your transmission. This code was written by Prof. E. Antonsson, and was used to generate the data shown in the PDF plots below.

   (New May 10, 2002)
   Several sets of data and plots of that data. from from the C-code above are available here.

8. TIME:

   The timing of the evaluation procedure is:

   • 45 seconds to set-up your transmission, starting from when a group is called to start.
   • 240 seconds maximum duration of each evaluation starting with the application of power.
     No action of the transmission is permitted prior to the application of electrical power to the motor.
   • 30 seconds to remove all of your transmission after the evaluation.

9. GRADING:

   Remember winning isn't everything. Having the transmission with the highest "score" is not the goal of this class. The primary goal is to learn something about engineering design, and applying the engineering material that you have learned in your other classes to a design problem. Your term grade will be only very slightly influenced by your transmission's "score". Instead, your grade will depend much more heavily on what your have learned about solving engineering design problems. Hence, the grading will be based as follows:

   • 40%  Design Concept:   How innovative and sound are the basic principles of your design?
   • 30%  Implementation:   How well did you implement your design in hardware?
   • 30%  Performance.
   The Transmission Project Grade Sheet contains details of the grading criteria.

10. MISCELLANEOUS:

    Be sure to test your transmission under the most realistic evaluation conditions possible. Note that there will be some unavoidable variations from evaluation to evaluation (temperature, air currents, etc.)

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    Last Updated 9:47:20 AM PDT, Tuesday, May 21, 2002