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Unformatted text preview: MECH 325 Assignment for Module #2: Flex Drives and Bearings
Due October Due at start of class on October 14, 2010 Summary Due to Vista at 1:00pm on October 13, 2010 Scenario
The purpose of this assignment is to design a flexible drive train and bearing set for some machine. The machine, shown schematically in Figure 1, has a 20 kg carriage that moves left and right using a 0.75 in lead power screw. A motor is to drive the system using a flexible drive (to be determined). The flexible drive should provide drive ratio of 2:1 (that is, the power screw should turn at 50% of the speed of the motor). The weight of the carriage is supported by a 2 in diameter shaft through a bushing. The allowable wear on the bushing is 0.02 in. Bearings or bushings A and B need to be selected for the power screw ends. For the purpose of this assignment, ignore issues of carriage control (e.g. assume switches and mechanical stops and exist for the extremes of the carriage movement). 32" approx. Carriage 20 kg, 3" wide Power screw 0.75" lead 8" approx. A B 18" to 22" Bushing 2" Flex drive To be sized Assume the system is to be designed for steady state operation with motor output torque of 0.25 N∙m at 250 rad/sec (to overcome friction in the system). Furthermore, assume the system is to be designed for changes in carriage direction based on a maximum 1.0 N∙m of motor output torque. The carriage deceleration when approaching the right side (i.e. motor side) of the device can be assumed to be 20 m/s2, and the carriage deceleration when approaching the left side can be assumed to be 5 m/s2. Assume these accelerations can be considered to be “heavy” and “light” loading, respectively. The motor shaft has a diameter of 0.5 in and the motor can be repositioned vertically to accommodate centre‐to‐centre distances between the motor shaft and power screw of 18 to 22 in. The diameter of the unthreaded portions of the power screw shaft is 1 in (and can be turned down as needed to fit bearings). The motor orientation can also be reversed (i.e. shaft pointing to the right in the figure above) and the power screw shaft can be extended to the right by 12 in for a cost of $100. The desired life of the retrofitted machine is at least 6 years (based on 7 hrs of operation per day and 250 days per year). A safety factor of 2.5 is required for the belts and bearings. The metric used to evaluate the design is the total cost based on component cost (flexible drive and bushings / bearings) and maintenance cost (as described in the “Further Notes and Simplifications” section). Thus, the performance metric is: Performance = Total Cost [US$] Further Notes and Simplifications You do not need to design or specify mounting hardware for bushings, bearings or flexible drives; you may assume that any components you specify will be appropriately mounted. Likewise, you do not need to account for the cost of the mounting hardware. You must supply a total cost for your design over its 6‐year life. The total cost includes the purchase price for all flexible drive components (belt, chain, pulley, sprocket, etc.) and bushing/bearing components. If components need to be replaced during the 6 year period due to insufficient life, the replacement parts must be included in the cost along with $50 for each component removed to account for labour. To simplify the process, specify parts from the following catalogues and use prices in US$: o Gates (belts and roller chain): pdf file on Vista o Manhattan Supply Company: http://www1.mscdirect.com o McMaster‐Carr: www.mcmaster.com o Reid Supply: www.reidsupply.com o W.W. Granger: http://www.grainger.com Reporting Requirements
There are two graded elements for this assignment: a report for formal marking (85% of mark) and the performance of your design relative to others in the class (15%). To allow in‐class comparison of designs, and to be eligible for the performance marks, you will need to submit a one‐page summary of your design on Vista (see below). Report The team report is due at the beginning of class on Oct 14. It will be convenient for you to have a copy of your report in class, (either in hardcopy or on a laptop). Your report shall consist of: A title page with the assignment number, your group number, and names and student numbers for all team members. A summary of your approach to the problem, your assumptions and methods, your final design, and the design and performance information requested below. Point‐form writing, tables, and figures are all encouraged. The summary must not exceed 2 pages and text should be computer‐generated. An appendix outlining your detailed calculations. The appendix can be hand‐written or computer‐generated and must not exceed 10 pages. The report must contain the following design and performance information in the summary (supporting calculations must also be provided, either directly in the summary or in the appendix): Catalogue name, part numbers, and prices for all specified components Drivetrain geometry (diameters, pitch, centre‐to‐centre distance, etc.) Bearing and/or bushing geometry (types, sizes of bearings, spacing, inner and outer bore, etc.) Drivetrain and bearing life Drivetrain and bearing component cost (main components only) Performance Relative to Class When you hand in your report, there will be an opportunity to view the performance of all teams and engage in a discussion / debate. All teams should be prepared to support and justify the assumptions and design decisions they made. The top performing team (i.e. the team with the design of lowest total cost) will receive the top performance mark on the assignment unless another team can identify an error, flaw, or oversight in their design. The top performing team will have to respond to questions from the rest of the class. If an error, omission, or unrealistic assumption is identified, the team with the next lowest cost design will be considered the top performing team and the process will repeat. Performance marks for the rest of the class will be determined by the instructor using either a linear or non‐linear scaling. A working design will at minimum receive a performance mark of 50%. One-Page Design Summary To facilitate the class discussion critiquing different designs, each team will submit a 1page summary of their design in .pdf format. The summary is due by 1:00pm on Wednesday, October 13. The summary should show a sketch or schematic highlighting the component choices and layout. The component specifications, as well as a summary of costs and component life should also be shown. The summary can be prepared in Powerpoint, Word, or even a scanned image of a neat hand‐drawn slide, as long as the submitted file is in .pdf format. Further instructions for uploading your summary page can be found on the assignments button from the “Course Tools” menu in the MECH 325 Vista site. Addendum After reviewing other teams’ designs, if your team feels that your report could be improved substantially due to oversights, you may submit a 1‐page, 12‐pt font addendum with clarifications and/or amplifications. This can be handed in as a hardcopy to the Mech Office (CEME 2054) or emailed as a PDF file but must be received by Dr. Ostafichuk within 24 hours of the end of the class. This is not intended as an opportunity for you to completely change your design. Any extra points gain in the addendum will be worth ½ of their value had they been provided in the original report. ...
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This note was uploaded on 02/01/2011 for the course MECH 325 taught by Professor Peteostafichuk during the Fall '10 term at The University of British Columbia.
- Fall '10