Fasteners & Lead Screws

Question 1

What are the main purposes of threads in machinery?

Answer 1

Threads are primarily used for fastening (as in bolts, nuts, and screws) and for linear movement and power transmission (as in lead screws).

Question 2

What are the three main types of threaded fasteners?

Answer 2

Bolts/nuts and screws, friction (taper) joints, and splines.

Question 3

Describe the role of a bolt’s external thread.

Answer 3

The external thread on a bolt is designed to mate with an internal thread in a nut, securing components together.

Question 4

What are the key characteristics of bolts?

Answer 4

Bolts generally do not have threading up to the head (having a plain shank) and are fastened with a nut.

Question 5

What is an internal thread cutting tap?

Answer 5

It’s a tool used to create internal threads within a nut or a hole to accommodate a threaded bolt.

Question 6

Explain the two types of ISO Metric threads.

Answer 6

Coarse (most common) and fine (specialized for specific applications).

Question 7

What’s the best practice for bolted joints?

Answer 7

Ensure the bolt passes through clearance holes, has a plain shank across the joint face, uses a washer beneath the nut, and has at least two threads protruding from the nut

Question 8

Describe the general characteristics of screws.

Answer 8

Screws are typically threaded up to the head and are not fastened with a nut but screwed into a tapped hole.

Question 9

Explain the function of a stud in fastening.

Answer 9

A stud has a plain shank on one end (permanent part) and a longer threaded part on the other (removable part), providing a stable, semi-permanent fastening solution.

Question 10

What does the British Standard BS 3692 specify?

Answer 10

It determines the strength value of ISO Metric fasteners, specifying tensile and yield strength values.

Question 11

Explain the concept of a power screw jack.

Answer 11

It uses a threaded screw to convert rotational movement into linear motion, used in devices like car jacks or lathe beds.

Question 12

What’s the formula to calculate total torque required to raise a load with a square thread?

Answer 12

The formula considers the torque to overcome collar friction and the torque to raise the load and overcome friction in the nut.

Question 13

What is the significance of the lead in a screw thread?

Answer 13

Lead, which is equal to the pitch in single-start threads, is crucial for determining the linear movement generated per screw rotation.

Question 14

What are the different types of threads for power transmission?

Answer 14

Square thread (0° included angle), Acme thread (29° included angle), and Buttress thread (variable angle, optimized for force in one direction)

Question 15

How does pre-tensioning affect bolted joints?

Answer 15

Pre-tensioning stretches the bolt, reduces fluctuations under repeated loading, and lowers bolt fatigue failure risk.

Question 16

What factors limit the amount of pre-tensioning possible in a bolt?

Answer 16

The amount of pre-tensioning is limited by the proof stress of the bolt material.

Question 17

Define ‘nominal diameter’ in the context of bolt dimensions.

Answer 17

It’s the basic diameter of the thread, denoted as ‘d’ for bolts and ‘D’ for nuts, used as a reference point for other measurements.

Question 17

What is the difference between the minor and pitch diameters in a threaded fastener?

Answer 17

The minor diameter is the smallest diameter of the thread, and the pitch diameter is at the midpoint of the thread profile.

Question 18

How are the different grades of bolts distinguished according to British standards?

Answer 18

Different grades (like 4.6, 5.8, 8.8, etc.) indicate the bolt’s tensile strength and yield strength, defining their load-bearing capacities.

Question 19

What’s the importance of calculating the torque for bolt pre-tensioning?

Answer 19

Correct torque ensures adequate pre-tensioning without exceeding the bolt’s strength, preventing failure under load.

Question 20

What are the primary functions of a screw thread?

Answer 20

Screw threads are mainly used for holding parts together, adjusting or moving parts relative to each other, and transmitting power.

Question 21

Describe the ‘pitch’ of a screw thread.

Answer 21

The pitch is the distance from a point on one thread to a corresponding point on the adjacent thread, measured parallel to the axis.

Question 22

What does the ‘major diameter’ of a screw thread refer to?

Answer 22

The major diameter is the largest diameter of a screw thread, typically measured from the crests of external threads or roots of internal threads.

Question 22

Explain the concept of ‘thread angle’ in screw threads.

Answer 22

The thread angle is the angle between the flanks of the thread, which determines the thread profile and affects its strength and wear properties.

Question 22

What is the significance of the ‘root’ in screw threads?

Answer 22

The root is the bottom of the thread groove, with its shape and depth affecting the strength and fatigue resistance of the thread.