friction wedges & screws engr b36 - statics pat aderhold 11/24/2014
TRANSCRIPT
Today’s Agenda• More friction...
Wedges Screws
• Practice Problems on white board
• Review / project time
Working with Wedges• Nothing new
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
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.
Practice Problem 6/53
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 363
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.
Practice Problem 6/56
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 363
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?
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 2012. p. 359
Scenarios• With
Raising the load Lowering the load
• With Screw unwinds itself Can prevent by adding moment
• At “on the verge of unwinding”
Practice Problem 6/62
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 365
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 0.50. Neglect friction at the small ball joint A.
For Exam #3• Can fill out a note card
Up to 4” x 6” One side only Bring to me for approval on Wednesday
• Three problems Distributed loading / shear & moment diagrams Buoyancy Friction
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!