FrictionWedges & Screws

ENGR B36 - StaticsPat Aderhold11/24/2014

News & Notes

News & Notes

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

Working with Wedges

Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 357

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

“Unwrapping” the Screw

Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 360

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!

Open Time

• Ask individual questions on HW

• Work on group design project