1. Friction Wedges & Screws
ENGR B36 - Statics
Pat Aderhold
11/24/2014

2. News & Notes

3. News & Notes

4. Today’s Agenda More friction... Practice Problems on white board
Wedges
Screws
Practice Problems on white board
Review / project time

5. Working with Wedges Nothing new Text emphasizes graphical solutions
Same use of F = µN
Same equilibrium equations
Text emphasizes graphical solutions
Easier to work with “friction angle” φ
µ = tanφ
Resultant “inclined from normal” by this amount

6. Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 357
Working with Wedges
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley p. 357

7. (That we aren’t going to concern ourselves with)
Other Details
(That we aren’t going to concern ourselves with)
Sliding and locking
Slides only if slip simultaneously top & bottom
Otherwise “self-locking”
No slip if
Pull out from locked position?
Friction opposite direction
Just another set of FBDs and equil. eqns.

8. Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 363
Practice Problem 6/53
The 10° wedge is drive under the spring-loaded wheel whose supporting strut C is fixed. Determine the minimum coefficient of static friction µs for which the wedge will remain in place. Neglect all friction associated with the wheel.
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley p. 363

9. Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 363
Practice Problem 6/56
The 10° doorstop is inserted with a rightward horizontal force of 30 lb. If the coefficient of static friction for all surfaces is µs = 0.20, determine the values NU and NL of the normal forces on the upper and lower faces of the doorstop. With the given information, can you determine the force P required to extract the doorstop?
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley p. 363

10. Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 359
Screws
Many applications
Derive basic equation on white board
Square threads
Jack with load W and moment M
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley p. 359

11. “Unwrapping” the Screw
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley p. 360

12. Scenarios With At “on the verge of unwinding” Raising the load
Lowering the load
Screw unwinds itself
Can prevent by adding moment
At “on the verge of unwinding”

13. Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 365
Practice Problem 6/62
Determine the torque M which must be applied to the handle of the screw to begin moving the 100 lb block up the 15° incline. The coefficient of static friction between the block and the incline is 0.50 and the single-thread screw has square threads with a mean diameter of 1 in. and advances 0.4 in. for each complete turn. The coefficient of static friction for the threads is also Neglect friction at the small ball joint A.
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley p. 365

14. For Exam #3 Can fill out a note card Three problems Up to 4” x 6”
One side only
Bring to me for approval on Wednesday
Three problems
Distributed loading / shear & moment diagrams
Buoyancy
Friction

15. Group Design Project due next Wednesday!
For Next Class
Read through Articles A1 & A2
Practice Problems 6/63 & 6/66
Think of questions and review problems
Make/bring “cheat sheet” for Exam #3
Exam #3 next Monday!
Group Design Project due next Wednesday!

16. Open Time
Ask individual questions on HW
Work on group design project