Fasteners & Lead Screws Flashcards
What are the main purposes of threads in machinery?
Threads are primarily used for fastening (as in bolts, nuts, and screws) and for linear movement and power transmission (as in lead screws).
What are the three main types of threaded fasteners?
Bolts/nuts and screws, friction (taper) joints, and splines.
Describe the role of a bolt’s external thread.
The external thread on a bolt is designed to mate with an internal thread in a nut, securing components together.
What are the key characteristics of bolts?
Bolts generally do not have threading up to the head (having a plain shank) and are fastened with a nut.
What is an internal thread cutting tap?
It’s a tool used to create internal threads within a nut or a hole to accommodate a threaded bolt.
Explain the two types of ISO Metric threads.
Coarse (most common) and fine (specialized for specific applications).
What’s the best practice for bolted joints?
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
Describe the general characteristics of screws.
Screws are typically threaded up to the head and are not fastened with a nut but screwed into a tapped hole.
Explain the function of a stud in fastening.
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.
What does the British Standard BS 3692 specify?
It determines the strength value of ISO Metric fasteners, specifying tensile and yield strength values.
Explain the concept of a power screw jack.
It uses a threaded screw to convert rotational movement into linear motion, used in devices like car jacks or lathe beds.
What’s the formula to calculate total torque required to raise a load with a square thread?
The formula considers the torque to overcome collar friction and the torque to raise the load and overcome friction in the nut.
What is the significance of the lead in a screw thread?
Lead, which is equal to the pitch in single-start threads, is crucial for determining the linear movement generated per screw rotation.
What are the different types of threads for power transmission?
Square thread (0° included angle), Acme thread (29° included angle), and Buttress thread (variable angle, optimized for force in one direction)
How does pre-tensioning affect bolted joints?
Pre-tensioning stretches the bolt, reduces fluctuations under repeated loading, and lowers bolt fatigue failure risk.
What factors limit the amount of pre-tensioning possible in a bolt?
The amount of pre-tensioning is limited by the proof stress of the bolt material.
Define ‘nominal diameter’ in the context of bolt dimensions.
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.
What is the difference between the minor and pitch diameters in a threaded fastener?
The minor diameter is the smallest diameter of the thread, and the pitch diameter is at the midpoint of the thread profile.
How are the different grades of bolts distinguished according to British standards?
Different grades (like 4.6, 5.8, 8.8, etc.) indicate the bolt’s tensile strength and yield strength, defining their load-bearing capacities.
What’s the importance of calculating the torque for bolt pre-tensioning?
Correct torque ensures adequate pre-tensioning without exceeding the bolt’s strength, preventing failure under load.
What are the primary functions of a screw thread?
Screw threads are mainly used for holding parts together, adjusting or moving parts relative to each other, and transmitting power.
Describe the ‘pitch’ of a screw thread.
The pitch is the distance from a point on one thread to a corresponding point on the adjacent thread, measured parallel to the axis.
What does the ‘major diameter’ of a screw thread refer to?
The major diameter is the largest diameter of a screw thread, typically measured from the crests of external threads or roots of internal threads.
Explain the concept of ‘thread angle’ in screw threads.
The thread angle is the angle between the flanks of the thread, which determines the thread profile and affects its strength and wear properties.
What is the significance of the ‘root’ in screw threads?
The root is the bottom of the thread groove, with its shape and depth affecting the strength and fatigue resistance of the thread.
