Mechanical Joining

Question 1

What are the reasons we may opt for mechanical joining?

Answer 1

• Easiest way to join different materials
• The final component may have huge internal voids, so having a “box” construction may be the most efficient method
• Component too large to make in one piece
• We may wish to dismantle component later, e.g. for servicing

Question 2

What factors should be considered when we are designing joints?

Answer 2

• Is the joint/s necessary? It introduces a weakness and incurs a cost
• Is the joint location suitable? think of appearance and/or stress max/min
• Are the components designed for assembly? e.g. access to the joint location, How would you assemble it? Are there simultaneous locations of several parts?(This is bad)

Question 3

What are the types of mechanical joining?

Answer 3

• Screws
• Pins
• Rivets
• Friction
• Clips
• Interlocks

Question 4

What are the advantages of using screws to join?

Disadvantages?

When should we use fine threads and coarse threads?

Answer 4

Advantages

• Predictable strength
• Easy to dismantle to repair
• Very Robust

Disadvantages

• Relatively expensive compared to other joining processes
• Difficult to locate and assemble in mass production

Coarse threads used when at least 3 full turns of engagement possible

Fine threads used when fewer than 3 turns possible, or in places of excess vibration

Question 5

In locations with high vibration, what may we do to ensure that the screw doesn’t come loose?

Answer 5

We use a locking device. These include:

• Split pin and castle nut
• Spring washer
• Lock nut
• Fibrous/plastic insert(like a high friction kinda thing)

Question 6

What are the main types of pin fasteners? The strongest ones?

What are the advantages of pin fasteners?

Answer 6

• Dowel
• Tapered pin
• Spring/roll pin
• Split pin
• Cotter pin

The strongest ones are dowel and tapered pins, however they are also the most expensive ones as the holes need to be more accurate, usually needing a precision fit

Advantages : Can bear quite a lot of shear load, but weak in other directions

Question 7

What is a rivet?

What are the advantages?

Where do we usually use them?

What do we use when there is only access to one side of surface?

Answer 7

• Essentially, they are like pins but both ends flattened after insertion to prevent coming loose

Advantages: High strength:weight ratio. Very predictable performance

We use them where shear loads dominate, e.g. aircraft. We usually use them to join sheets together. , and we have managed to automate the process for mass production

When only have access to one side, we use pop rivets. These use the process of necking to their advantages to lock the rivet in place.

Question 8

What are the types of rivets?

Where would we use each one?

Answer 8

• Pop rivet: Where only access to one side of sheet
• Shrink Fittings: These are “force fittings” where we usually heat one component up to permit engagement. These fittings are really strong, but they do exert high stresses on the components that are engaging, and holes must be very accurately dimensioned.
• Circlips: these are a subsection of spring fasteners. They basically snap onto a cylindrical component, either directly onto, or onto a groove/ into a hole. They allow for easy assembly and can support light axial loads

Question 9

What are snap locks and interlocks?

Answer 9

Are you stupid- They use the elasticity of materials to their advantage, and fold sheets onto into one another mechanically.