1. Fastening (more complex shapes = better function)
Non-permanent
Bolted
Permanent
Welded
Bonded

2. Outline General Thread Nomenclature & Types Power Screws
Stresses in Threads
Preloading Fasteners/Joints
Fasteners in Shear

3. Threads p pitch in./thread d diameter (major) in.
dp pitch diameter in.
dr minor diameter in.
L Lead in.

5. Screw Classifications
Unified National Standard
ISO (Metric)
Thread Pitch
UNC –coarse
UNF –fine
UNEF –extra fine
coarse
fine
Tolerance
Class 1
Class 2
Class 3
several levels
d=12mm
fine
Class 2
d=0.25”
metric
¼-20 UNF –2A
M12 x 1.75
external threads
20 threads/in.
p=1.75 mm/thread
**see Tables 14-1 and 14-2 for standard sizes**

6. Tensile Stress F F
At also in Tables 14-1 and 14-2

7. Outline General Thread Nomenclature & Types Power Screws
Threads
Loads
Self-locking
Efficiency
Stresses in Threads
Preloading Fasteners/Joints
Fasteners in Shear

8. Power Screw Applications
Where have you seen power screws?
jacks for cars
C-clamps
vises
Instron material testing machines
machine tools (for positioning of table)

9. Power Screw Types Square Acme Buttress (contrafuerte) strongest
no radial load
hard to manufacture
Acme
29° included angle
easier to manufacture
common choice for loading in both directions
Buttress (contrafuerte)
great strength
only unidirectional loading

10. Load Analysis What “simple machine” does a power screw utilize? P y x
dp L  N f P F
LIFTING y x

11. More Completely…
LIFTING
LOWERING y P x f F N L 
dp

12. For Acme Threads 
LIFTING
LOWERING

13. Friction Coefficients
oil lubricated= collar w/ bushing=0.15 ± 0.05
collar w/ bearing=0.015 ± 0.005

14. Self-Locking / Back Driving
self-locking – screw cannot turn from load P
back-driving – screw can be turned from load P
for self-locking:
would square or Acme of same dimensions lock first?

15. Efficiency -for lifting- higher efficiency for lowering
(also derive with frictionless torque/torque)

16. Ball Screw

17. Outline General Thread Nomenclature & Types Power Screws
Stresses in Threads
Body Stresses
Axial
Torsion
Thread Stresses
Bearing
Bending
Buckling
Preloading Fasteners/Joints
Fasteners in Shear

18. Tensile Stress F F
At also in Tables 14-1 and 14-2

19. Torsional Stress depends on friction at screw-nut interface
For screw and nut,
if totally locked (rusted together), the screw experiences all of torque
if frictionless, the screw experiences none of the torque
For power screw,
if low collar friction, the screw experiences nearly all of torque
if high collar friction, the nut experiences most of the torque

20. Thread Stresses – BearingF
p/2
p/2
Abearing=(p/2)(dpnt)

21. Thread Stresses – BendingF
p/2
p/2
transverse shear is also present, but max stress will be at top of tooth
For both bearing and bending, F and nt are dependent on how well load is shared among teeth, therefore
use Factual=0.38F and nt=1 (derived from experiments)

22. Mohr’s Circle F
p/2
p/2 z y x

23. Buckling
use dr

24. Outline General Thread Nomenclature & Types Power Screws
Stresses in Threads
Preloading Fasteners/Joints
Proof Strength
Spring Behavior
Loading & Deflection
Separation of Joints
Fasteners in Shear

25. Preloading & Proof Strength
Sp  stress at which bolt begins to take a permanent set
Preloading
static loading: preload at roughly 90% of Sp
dynamic loading: preload at roughly 75% of Sp

26. Spring Behavior BOTH material being clamped and bolt behave as springs
(up to yield/permanent set stresses)
for the bolt, threaded vs unthreaded have different spring constants:
applied load P

27. Affected Area of Material
For material, basic model is as follows (shown for 2 materials being clamped)
Area is hard to define… from experiments, the following is accurate:
When no edges nearby and same materials, even simpler form can be used:
A and b are from Table 14-9, pg 916

28. Papplied relieves compression in material &
Loading & Deflection Pm Pb Fm Fb F
b1
m1 P Fi m b 
MATERIAL
BOLT
P=Pb+Pm
Papplied relieves compression in material &
adds tension to bolt
Fm=Fi-Pm
Fb=Fi+Pb

29. Distribution of Applied Load
b= m

30. Applied Load to Equal Sp
How many times more would the loading on the bolt need to be to incur permanent set?
(assuming no material separation)

31. Yielding Safety Factor
Fm = Fi + P(C-1)
Fb = Fi + CP
Ny=Sy/b

32. Separation
Separation occurs when Fm=0
Fm = Fi + P(C-1)

33. Strategy Reviewed Given: joint dimensions Find: bolt
See Example 14-2, p. 906
Given: joint dimensions
Find: bolt
set preload equal to 90% Sp
find lt so that you can find kb
find km
calculate C, then Pb, Pm, then Fb, Fm
find stress in bolt and separation load
Such that: factors of safety>1

34. Dynamic Loading of Fasteners
Bolt only absorbs small % of P
Stresses
Bolt is in tension
Material is in compression
Fatigue is a tensile failure phenomenon
 Preloading helps tremendously in fatigue

35. Outline General Thread Nomenclature & Types Power Screws
Stresses in Threads
Preloading Fasteners/Joints
Fasteners in Shear
What is Shear?
Straight Direct Shear

36. Direct Shear

37. Doweled Joints
“It is not considered good practice to use bolts or screws in shear to locate and support precision machine parts under shear loads”
Norton
Shear can be handled by friction caused by bolts… but, better practice is to use dowels
Bolts need clearances… at best 2 out of a 4 bolt pattern will bear all of load
dowels support shear, but not tensile loads
bolts support tensile loads, but not shear

38. Direct Shear Ashear=2x(cross sxn of dowels)
dowels support shear, but not tensile loads
bolts support tensile loads, but not shear

39. Outline Revisited General Thread Nomenclature & Types Power Screws
Stresses in Threads
Preloading Fasteners/Joints
Fasteners in Shear

40. Chapter 9
Welding, Brazing, Bending, and the Design of Permanent Joints
From Shigley & Mischke, Mechanical Engineering Design
Part 3
Design of Mechanical Elements

41. Welding Symbols

42. Butt Welds

43. Fillet Welds

44. Welding Issues Requires Can Cause Careful Design Skilled Welder
Weakened adherends
Thermal distortion
Removal of heat treatment

45. Welding References AWS (American Welding Society) Lincoln Electric
ASME Codes & Standards
Pressure Vessels & Piping
Nuclear Installations
Safety Codes
Performance Test Codes

46. Bonded Joints (thin members)

47. Bonded Joint Types

48. More Types

49. Peel Stresses

50. Good Practices

51. Bonding Issues Can achieve Beware Lighter joint Less costly joint
Better sound absorption
Beware
Peel stresses
Environmental effects
Thermal mismatch

52. Bonding References
SAMPE (Society for the Advancement of Material & Process Engineering)
ASTM Committee D-14 on Adhesives