Freewheel Mechanisms, Threads, Welding and Fastening Methods
Freewheel Mechanisms and Ratchet Types
19. What is the freewheel mechanism used for? The freewheel mechanism is used in bicycle wheels and in automobile starter motors. It allows the motor to transmit movement to the engine during starting. Once the engine reaches a higher speed than the starter motor, the freewheel disengages so the starter is not damaged.
20. What types of ratchets exist? A ratchet is a mechanism that allows movement in one direction and prevents it in the other. Ratchets are classified as reversible if the direction can be changed, or locked/non-reversible if they only work in one direction. They can be external, internal or frontal.
Electric Arc Welding and Oxyacetylene Welding
21. Description of electric arc welding. It uses an electrode connected to one of the cables of the transformer. The electrode is brought close to the pieces to be joined, creating an electric arc that melts the electrode and the surface of the pieces. The molten metal mixes and, when cooled, forms the weld bead.
22. Characteristics of oxyacetylene welding. It is obtained by mixing oxygen and acetylene in a torch. It can reach a reducing effect zone of about 3,000 °C.
Threaded Fasteners
23. Types of threaded elements. They are used in all kinds of machines, especially in screws and nuts. Types include: bolts, machine screws, studs, stud bolts, rivets, thread screws and tie rods.
24. Thread description: Metric, Whitworth or Sellers. A thread is the helical ridge on the outside of a screw or inside a nut that allows coupling. Its outer diameter is measured in millimeters. The pitch is the distance a screw advances in one complete turn. The thread angle is formed at 60 degrees.
25. Types of metal welding. Soft, strong, autogenous and electric.
26. Electrode types in arc welding. Acid, basic, cellulosic, oxide, rutile, titanium and miscellaneous (other coatings).
27. What is the function of a voltage reducer in arc welding? It reduces the voltage of the network to a suitable working voltage, providing a constant output voltage and current for the welding process.
Non-Threaded Demountable Connections
28. Demountable non-threaded fasteners. Pins, keys, washers, guides, buttons, Velcro and clips.
Riveting and Alternative Fixing Methods
29. Difference between rivets and riveting. Describe alternative methods of fixing the same two parts together with riveting, and explain the difference in the larger diameter of the rivet hole.
Press Fit Assembly
30. Consistency of a press fit. It consists of introducing a shaft slightly larger than the hole it is going into. If the difference is very large, the hole is heated, expands and then cools, which allows fixing, as in the example of fixing a bolt to the rod and piston. Smaller fits can be done manually or with a press.