PART I STRESS CONCENTRATION 1 Stress concentration l

PART ”I” STRESS CONCENTRATION

1. Stress concentration l Stress concentration appears in mechanical elements due to sudden change in cross section. l It has negative effects: - Decrease the strength of element. - Tends to crack initiation. l Dimensions of stress raiser.

l It is affected by: - Value of load. - Type of load. - Dimensions & shape of stress raiser.

Shape of Stress Raisers Single Vnotch Single Sharpedge Notch

Curved shoulder Single Semi-circular Notch

Effect of Double notch Shape

Effect of Combined stress (Load Type)

1. Shoulder

2. Single Sharp-edge Notch

3. Single V-shape Notch

4. Single Semi-circular Notch

5. Double V-shape Notch

6. Double Sharp-edge Notches

7. Double Semi-circular Notch

7. Examples of Failed Elements Fatigue failure of a shaft

Pitting corrosion resulting from elevated temperatures on an aluminum housing.

A brittle fracture that originated at a surface defect.

Three conveyor rollers: q. The center one is “as supplied, ” q. The outer two have failed from contact stresses. q. The left roller was case hardened and cracked. q The right roller was unhardened and ductilly deformed.

Fracture due to Torsional effect.

Keyseat distortion Torsional force Keyway deformation due to shaft overloaded.

Ductile deformation due to shaft overloaded.

S t r es s C o n c en t r a t i o n F a c to r

2. Stress Concentration factor: Assume a simple plate (b x t x L). l The plate has a hole of diameter ”d”. l It is under axial force “F”. l Two different stress appears in member. t L l b

- Average stress “ av” - Average stress “ max” L max t av b

3. Stress Raisers: 3. 1 Holes 3. 1. 1 Holes in shafts 3. 1. 2 Holes in plates

3. 2 Notches 3. 2. 1 Notches in shafts 3. 2. 2 Notches in plates

3. 3 Sudden Change of Dimensions 3. 3. 1 900 Angle 3. 3. 2 Stepped Shafts 3. 3. 3 Stepped Plate

4. Relieving stress Reducing of stress concentration in the mechanical members. Factors affecting the efficiency of stress relief • Element (member) dimensions. • Raiser dimensions. • Load (Type, Value, Direction)

Design Considerations R el a t ed T o M a c h i n g

1. Drilling 1. 1 Drilling on inclined surfaces - Avoiding of drilling on inclined surfaces to prevent effect of tangential drilling (Cutting) force.

1. 2. Drilling of holes in relative small heights. X > D/2 H > D/2 X H

2. Tool tolerance for Threading - External Thread - Internal Thread Tolerance undercut External Thread Tolerance undercut Internal Thread

3. Tool tolerance for Milling

X

4. Tool tolerance By Grinding wheel

5. Base for assembly Guide for disc or washer assembly Guide for pin assembly

6. Perpendicular Surfaces Tool - Tip curvature of single point cutting tools deforms the corner of the perpendicular surfaces

3 mm 450 d

2 1 1 X 2 1 2 2 1

7. Pin Hole relationship Sharp Corner 450 x 1. 5

L 2 L L 1

A B L A B

8. Avoiding of shoulder by tapered shaft ends - Hub fixation occurs when the contact surface between hub and both of washer and shoulder.

9. Assembly of Elements on Parallel Surfaces Cover Cylinder - Clearance between cover and cylinder is used.

10. Assembly of Elements on Parallel Surfaces - Partial contact between flat contact surfaces is better to avoid effect of out of flatness or distortion.

11. Bases of flat casting Elements - Bases of casting elements in the same level is recommended.

12. General Recommendations by Casting

- Casting of perpendicular surfaces.

- Casting of wheels with webs.

12. Strengthen of Vertical column

12. Strengthen of Vertical column

12. Strengthen of Vertical column

13. Gear Wheels F F F - Improvement of external gear strength by means of strengthening single (A) or double (B) web.

- Improvement of bevel gear strength.

13. Bolts Connections - M 1 : M 8 Each 1 - M 8 : M 24 Each 2 L 1 0. 8 d Bolts series - M 48 Each 4 H (bolt Head) = 0. 7 d H (Nut) = 0. 8 d L 1 = Variable Design- Make Nuts and bolts 0. 7 d - M 24 : M 48 Each 3 2 d 5 d d 1.

Bolts Connections 1) Hole drilling Drill Work Piece L 1 : Hole Depth

Bolts Connections 1) Hole drilling 2) Thread Cutting L 2 : Thread Depth L 3 : Bolt Thread Depth Thread Cutting tool

13. Bolts Connections 1) Hole drilling 2) Thread Cutting 3) Bolt (Stud) assembly.

Steps of Stud Assembly Washer Nut

A. Through Bolt DHole = 1. 2 Dbolt • Bolt is under tension. • Elements are under compression.

B. Tap Bolt • Upper part has a hole (1. 2 d) • Lower part is threaded. • Connection occurs due to the mutual forces between: • 1 - Contact between Bolt head and surface of upper part. • 2 -Teeth of the bolt end

C. Stud Consist of two threaded parts and a cylindrical neck. One end is assembled in the base element and the other element is pressed by a net assembled in the other bolt end. Hole diameter of the upper part is (1. 2 d).

Relative Dimensions of Hole and Bolt D : Bolt Diameter ( D=30 For Bolt M 30) H 1 = 0. 8 D ( Nut Height) t (Washer height) = 0. 15 D D 2 = 2 D ( Washer Biggest width) A = 3 D ( Min. Surface of contact width) L 1 = 2 D +1. 25 (Thread of bolt length) L 2 = L 1 + 0. 25 D (Hole thread length) L 3 = L 1 + 0. 4 D (Hole depth)

Washer placed under nut.

Pressure

A=2 D

Applications

Belts & Pulleys

Belts & Pulleys

Belts & Pulleys Design- 3 D CONVEYOR ANIMATION - FINAL PROJECT -

END