Engineering Analysis Presentation

1. Engineering Analysis Presentation ME 4182 Team: 5 Guys Engineering + 1 Nathan Bessette, Rahul Bhatia, Andrew Cass, Zeeshan Saiyed, Glen Stewart YJ Chok

2. Automatic Whiteboard Wiper • Last Time • Layout Drawings • Layout or assembly drawings • How individual parts or subsystems fit together as a whole • Encouraged to use computer modeling software • Drawings for actual design, not prototype • This Time • Present a critical analysis of the design • Determine the areas that are most likely to fail • Potential engineering and/or manufacturing problems

3. Calculations Using the situation with the heavy writing 2” 5” For 9.6 erasers to span the height of the board For 4.8 erasers to span half the height of the board

4. Material Analysis • Deflection Analysis For Vertical Slider Bar • -Assume Circular cross section • -5 ft. long • -Half the normal force from the board acts at the center of the rod (5.5 lbs)

5. Eraser sub-assembly Weight Calculation

6. Eraser Subassembly Weight Calculation • Density: ρ= 0.5097 lbs. / Ft. of bar • Quantity of bar 2 x 60” bars = 120” 4 x 5” supports = 20” 1 x (2” x 24”) eraser backing = 48” TOTAL = 15⅔ ft • Aluminum Weight = 15⅔ ft ·0.5097 lbs. / Ft. ≈ 8 lbs. • Motor Assembly Motor ≈ 2.3 lb Rack & Pinion ≈ 1 lb Extras ≈ 0.5 lb • Motor Assembly Weight ≈ 3.8 lbs. • TOTAL WEIGHT, W ≈ 11.8 lbs.

7. Statics Analysis (Eraser at bottom) • Forces on A and B are reactions on Sliding Assembly from sliding rails. • Total of two sliding rails attached together • Weight acts at the center of gravity (3.85 inches from the wall, 2.75 feet from the bottom of the assembly) • Normal force from board, Nx is 11 lbs and acts at the center of the eraser (1.5 feet from the bottom of the assembly) • Reactions calculated by summing forces and summing moments about a fixed point

8. Statics Analysis (Eraser at Top) • Forces on A and B are reactions on Sliding Assembly from sliding rails. • Total of two sliding rails attached together • Weight acts at the center of gravity (3.85 inches from the wall, 1.90 feet from the bottom of the assembly) • Normal force from board, Nx is 11 lbs and acts at the center of the eraser (3.5 feet from the bottom of the assembly) • Reactions calculated by summing forces and summing moments about a fixed point

9. Possible points of failure

10. Shear Analysis on the wheels Side View Front View Track Wheels Wheel Support Support Attachment Area of the wheels under shear Top View

11. Shear stress on the wheels due to weight Area of one wheel under shear, Aw = 0.1266 in2 Total Area under shear, A T,w = 0.5063 in2 Shear stress due to Normal force, τN = Fs,W / A T,w = 5.825 psi Shear strength of Nylatron, Sy = 10,500psi Factor of safety for the wheels, n = 1803 Fs,N = 2.950 lb Area of the wheels under shear

12. Shear stress on the wheels due to Normal Force Area of one wheel under shear, As = 0.0765 in2 Total Area under shear, A T,s = 0.3061 in2 Shear stress due to Normal force, τN = Fs,N/ A T,s = 13.816 psi Shear strength of Nylatron, Sy = 10,500psi Factor of safety for the wheels, n = 760 Fs,N = 4.229 lb Area of the wheels under shear

13. Horizontal Torque Requirements • Required Torque calculation: T = Fmax tension due to friction·rpulley = (7 lbs.)(2.25 in.) = 15.75 lb-in = 1.3 lb-ft Treq = 1.3 lb-ft minimum 4.5” ID Pulley Treq = 2.6 lb-ft Motor Fmax, tension = 7 lbs.

14. Vertical Torque Requirements Ffriction = 7 lbs. Motor 3.75” ID Gear Fweight = 5 lbs. Fweight = Weight of Motor Assembly (3 lb est.) plus Eraser Backing (2 lb est.) Required Torque calculation: T = Fmax(friction+weight)·rgear = (12 lbs.)(1.875 in.) = 22.5 lb-in = 1.875 lb-ft Treq = 1.875 lb-ft minimum

15. Motor Analysis Minimum Torque Requirements: Horizontal Sliding = 1.3 lb-ft Vertical Sliding = 1.875 lb-ft

16. Motor Analysis MINIMUM TORQUE NOT MET MINIMUM TORQUE NOT MET MINIMUM TORQUE NOT MET BULKY COMPARED TO GLOBE MOTOR FACTOR OF SAFETY TOO SMALL Minimum Torque Requirements: Horizontal Sliding = 1.3 lb-ft Vertical Sliding = 1.875 lb-ft

17. Motor Analysis Free Speed Calculations: Across the board Fisher-Price: Up/down board Globe:

18. Can the cable subassembly overcome frictional forces without breaking? Proposed materials: Bicycle brake cable (steel) Rubberized Nylon cable Here, we will analyze the 5 mm cable with the lowest tensile strength to ensure a sufficiently high factor of safety for the stationary board. Weight considerations are largely ignored for this analysis as they not pertinent to the direction of motion. A distributed load of 7 lbf is applied against the direction of motion of the cable due to the board friction present. Thus the motor force must overcome the friction force. Since there are 2 pulleys (top and bottom) aiding the path of motion of the eraser, the stress on the cables is halved indicating that the cable tension in summation must overcome eraser assembly friction, pulley/bearing friction, and applied motor stress. With the chosen motor (maximum torque of 34 N-m = 25.077 ft-lb.) at a distance of 2 ft, the cable has a F of S of at least 20 which is ample to ensure that the cable, even with the smallest tensile strength, will not stretch or deform and will definitely not snap. This means that cost can largely dictate the cable material that is chosen.

19. Automatic Whiteboard Wiper • Next Time • Part Drawings • Prepare a complete set of part drawings • Must contain enough information so the part can be fabricated • Drawings are for the actual design, not for the prototype