Fasteners

Question 1

Typical applications of Fasteners:

Answer 1

• Structures for buildings and bridges
• Automotive, aerospace, electrical and marine assemblies
• Domestic and office appliances
• Machine tools
• Pipework and ducting
• Furniture

Question 2

Forms of Fastening:

Answer 2

Non-permanent

Permanent

Question 3

Non-permanent fastenings:

Answer 3

• Screws
• Bolts
• Keys
• Pins
• Nuts

Question 4

Permanent Fastenings:

Answer 4

• Welding, brazing and soldering
• Adhesives
• Rivets

Question 5

Design Considerations

Answer 5

• Strength
• Weight
• Cost
• Corrosion
• Magnetic properties
• Life
• Maintenance
• Assembly

Question 6

Threaded fasteners:

Answer 6

Bolts vs Screws

• Many differing definitions but the concern here is with non-tapered, externally threaded fasteners, intended to engage with a matching internally threaded nut or other component.
• A ‘bolt’ may or may not have a section of plain shank.
• A ‘machine screw’ will generally be threaded for its full length to the head.
• With the exception of the last usage above, a ‘screw’ will generally have a taper, and a thread designed to cut into the material in which it engages.

Question 7

Bolt Types:

Answer 7

Hex

Domed

Bent

Square

Pentagon

Flat

Anchor

Blind

Double end

Carriage

Eye

Penta-head

Shoulder

T Head

U-bolt

J-bolt

Elevator

Flange

Hanger Lag

Question 8

Hex bolt:

Answer 8

Can be grabbed by tools from all angles and if necessary, even installed by hand. Many different bolt heads incorporate a hex design.

Characterised by its size-sided head, there are many different types of hex bolts. Typically used in machinery and construction applications and available fully or partially threaded.

Question 9

Domed bolt:

Answer 9

These decorative bolt heads are difficult to wrench externally, which adds a level of security.

Question 10

Bent bolts:

Answer 10

The example shown in the slides is an eye bolt for lifting, but bolts are available in other shapes for speciality applications.

Question 11

Square bolts:

Answer 11

Enables an easy grip for wrenches for tightening nuts.

Question 12

Pentagon bolts:

Answer 12

Designed to resist tampering by not accommodating torx and hex tools.

Question 13

Flat bolts:

Answer 13

Flat head bolts are countersunk, which enables them to save space on applications:

Question 14

Anchor bolts:

Answer 14

Used in the construction industry to secure a structural component to a concrete slab or poured foundation.

Question 15

Double end bolts:

Answer 15

A bolt with a thread on both ends, also called stud bolts. Used to fasten two flanges or pipes more effectively.

Question 16

Blind bolts:

Answer 16

For structural applications where access is restricted to one side, enabling the lock to still be completed.

Question 17

Carriage bolts:

Answer 17

A self-locking bolt with a domed head and square section underneath. Used mostly in hinges and locks to provide a level of security, permitting removal from the bolt’s nut side.

Question 18

Eye bolts:

Answer 18

used for lifting applications. Some are designed for heavy loads while others are for non-load bearing uses.

Question 19

Penta-head bolts:

Answer 19

Highly specialised bolt with five-sided head. Typically used for security on manhole covers and ground lids.

Question 20

Shoulder bolts:

Answer 20

Performs as a shaft or axle that can hold a rotating part, such as a bearing. Also called a shoulder screw and stripping bolt.

Question 21

T Head bolts:

Answer 21

Also known as a T-slot. Inserts into a recess and with the applied nut, the bolt is secured against turning. Common in construction and automotive.

Question 22

U-bolts:

Answer 22

Functions in applications ranging from supporting pipes to use on automotive drive shafts and exhaust systems.

Question 23

J-bolts:

Answer 23

Also referred to as a hook bolt and typically used in structural applications, such as roofing and securing walls to concrete foundations.

Question 24

Elevator bolts:

Answer 24

Named after its original use in grain elevator systems. Today it’s used on assembly lines, furniture and various consumer products. Large circular head and low profile provides generous clearance.

Question 25

Flange bolts:

Answer 25

The flange acts as a washer to distribute the load. Commonly used to connect plumbing pipes, a vehicle's transmission and engine and other mechanical applications.

Question 26

Hanger bolts:

Answer 26

Connects two surfaces while hiding the bolt. Often used to suspend electrical conduits, fixtures and sheet metal.

Question 27

Lag Bolts:

Answer 27

Also known as lag screws, these bolts are typically used to connect heavy lumber and lag machinery to wood floors.

Question 28

Terminology:

Answer 28

* Pitch * Crest * Root * Flank * Major diameter * Minor diameter * Pitch (or effective) diameter * Thread angle

Question 29

Thread Profile:

Answer 29

ISO metric thread: designated by the letter M, followed by the nominal diameter and the pitch required (in mm): e.g., M6 × 1.5

Question 30

Bolt Strength: Tensile:

Answer 30

Movement of the joint members is prevented or limited by generating a tensile preload. General design case is for predominantly tensile loading. Most of the bolt load taken on the first few threads of the bolt, greatly increasing the stress intensity. Minimising the stress amplitude can be achieved by a number of techniques, e.g., by maximising the flexibility of the bolt compared to the joint.

Question 31

Bolt Strength: Tensile stress and stress area:

Answer 31

Simple method would be to take minor diameter, however this has been shown to be pessimistic. Testing of threaded rods in tension shows that their strength is better defined by the average of minor and pitch diameters. * Area for tensile stress: 𝐴𝑡 =(𝜋/16)(𝑑𝑝 +𝑑𝑟)^2 * And tensile stress: 𝜎𝑡 =𝐹/𝐴𝑡 Where: dp – pitch diameter; dr – minor diameter; F – tensile load

Question 32

ISO Metric Threads:

Answer 32

For ISO threads: 𝑑𝑝 =𝑑−0.6495𝑝 and 𝑑𝑟 = 𝑑−1.2269p Where: d - nominal bolt diameter p = pitch

Question 33

Pre-loads (Bolt Strength: Tensile):

Answer 33

* Pre-load for reusable joints: 𝐹𝑖 = 0.75𝐴𝑡𝜎𝑝; where: 𝜎𝑝 − 𝑝𝑟𝑜𝑜𝑓 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑡ℎ𝑒 𝑏𝑜𝑙𝑡, 𝐴𝑡 − 𝑎𝑟𝑒𝑎 𝑓𝑜𝑟 𝑡𝑒𝑛𝑠𝑖𝑙𝑒 𝑠𝑡𝑟𝑒𝑠𝑠 * Pre-load for permanent joints: 𝐹𝑖 = 0.90𝐴𝑡𝜎𝑝; where: 𝜎𝑝 − 𝑝𝑟𝑜𝑜𝑓 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑡ℎ𝑒 𝑏𝑜𝑙𝑡, 𝐴𝑡 − 𝑎𝑟𝑒𝑎 𝑓𝑜𝑟 𝑡𝑒𝑛𝑠𝑖𝑙𝑒 𝑠𝑡𝑟𝑒𝑠𝑠 * If detailed information concerning the proof strength is not available, it can be approximated as: 𝜎𝑝 = 0.85𝜎𝑦; 𝑤ℎ𝑒𝑟𝑒: 𝜎𝑦 −𝑦𝑖𝑒𝑙𝑑 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ

Question 34

Bolt Strength: Torque:

Answer 34

Once the pre-load is determined, then the torque can be calculated from: 𝑇 =𝐾𝐹𝑖𝑑 𝑤ℎ𝑒𝑟𝑒: 𝑑−𝑡ℎ𝑒 𝑛𝑜𝑚𝑖𝑛𝑎𝑙 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟; 𝐾−𝑐𝑜𝑛𝑠𝑡𝑎𝑛t 𝐹𝑖- pre-load

Question 35

Elasticity in Bolted Joints:

Answer 35

The relative stiffnesses of the bolt and joint affects what happens when an external load is applied to a joint. If the bolts are relatively stiff, most of the additional force, above the clamping load, is taken by the bolt and the bolt should be designed to take the clamping force and any additional force. Such a joint can be classified as a soft joint. But if the bolts are relatively flexible compared to the joint, then, most of the additional force, above the clamping load will initially go towards decreasing the clamping force until the components separate. The bolt will then carry all of the external load. This kind of joint is classified as a hard joint.

Question 36

Elasticity in Bolted Joints Equations:

Answer 36

Final force in the bolt: Fb= Fi + (kb/(kb+kc)) x Fe Final force in the component: Fc= Fi - (kc(kb+kc)) x Fe where: Fb- final force in bolt Fi- initial force in bolt Fe- external force (per bolt) kb- bolt stiffness kc- component stiffness Fc- final clamping force

Question 37

Bolt Strength: Shear:

Answer 37

In other typical applications, shear loading largely transmitted through friction of bolted faces. Where bolt is explicitly designed for shear load, use ‘fitted’ bolt shank.

Question 38

Fitted Bolts:

Answer 38

Specifically for shear load applications. Fitted bolts are installed in pre-drilled or pre tapped holes, typically in a joint that requires a high degree of accuracy and alignment.

Question 39

Power Screws:

Answer 39

Square thread Acme thread Buttress thread

Question 40

Rivets:

Answer 40

Non-threaded, permanent fasteners. On installation, the rivet is placed in a drilled hole of the material to be joined, and the other end of the rivet, called tail is deformed, and expands to about 1.5 times the original shaft diameter, holding the rivet in place. Made from ductile material such as carbon steel, aluminium and brass. A variety of coatings are available to improve corrosion resistance.

Question 41

Design Factors for Riveted Joints:

Answer 41

* Rivet -Size -Type -Material * Joint -Type -Spacing

Question 42

Riveted Joint and Joint types:

Answer 42

Riveted joint: Lap joint, Butt joint Types: Single riveted lap joint, double riveted lap-joint, double riveted butt-joint

Question 43

Adhesives:

Answer 43

Adhesive joints are achieved by bonding with natural or synthetic substances which form a rigid or semi rigid interface without the use of mechanical fasteners.

Question 44

Advantages of adhesives:

Answer 44

* More uniform distribution of stresses over the bonded area * Stiffer structures * Smooth surface finishes * Fatigue resistance, low temperature fabrication, etc.

Question 45

Disadvantages of adhesives:

Answer 45

* Lower strength * Decreasing strength with temperature * Oxidation * Not easy to dismantle

Question 46

Types of Adhesives:

Answer 46

* Anerobics * Cyanoacrylates * Acrylics * Ceramics * Epoxies * Hotmelts * Phenolics * Plastisols * Polyurethanes * Polyvinyl Acetates * Rubber adhesives * UV Curable * Silicones

Question 47

Adhesive Joints:

Answer 47

Butt joint (very poor) Lap joint (good) Scarf joint (very good) Double strap joint (excellent)

Question 48

Welding:

Answer 48

Joining achieved by heating metals to the point of being molten or plastic. Equivalent process applicable to thermoplastic polymers. Brazing and soldering use dissimilar fillers such that the parent material remains well below the melting temperature.

Question 49

Summary:

Answer 49

* Permanent, non-permanent fasteners. * The choice of fasteners is not generally independent of the design of the components being joined. * Fasteners are generally bought in, off the shelf items, and in an industrial context the suppler will be able to provide critical data for the design process.