Friction
Wedges & Screws
ENGR B36 - Statics
Pat Aderhold
11/24/2014
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”
a < 2f
• Pull out from locked position?
• No slip if
 Friction opposite direction
 Just another set of FBDs and equil. eqns.
Practice Problem 6/53
The 10° wedge is drive under the springloaded 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 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 2012. p. 363
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
a <f
 Raising the load
 Lowering the load
• With a
>f
 Screw unwinds itself
 Can prevent by adding moment
• At
a = f “on the verge of unwinding”
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 0.50. Neglect friction
at the small ball joint A.
Meriam, JL and Kraige, LG. Statics 7th Ed. Wiley 2012. p. 365
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