To effectively service fasteners, it is important to ha […]
To effectively service fasteners, it is important to have a working knowledge of screw threads. A thread is a continuous helical ridge formed on the inside (nut) or outside (screw) of a cylinder. This ridge is called the crest. Between each crest is a space, called the root. Threads are set at an angle to the axis of the bolt or nut. This slope is called the helix angle. The angle must be sloped, either upward to the right (for right-hand threaded screws) or upward to the left (for left-hand threaded screws). The thread forms a “V” shape between crests. The angle of this “V” is called the thread angle, and is determined by fastener engineers. Most screw threads used on a bicycle use a 60-degree thread angle.
Bolts and screws are made in different grades of strengths. There is a rating system that is used to mark and identify most industrial bolts. However, the bicycle industry typically uses proprietary bolts without any markings. The SAE (Society of Automotive Engineers) system rates bolts from grade 1 through grade 8. The tensile strength increases with the number. Metric bolts come with a “property class,” a two number system separated by a decimal point.
Even when threads are properly sized, there will be play or slop between external and internal threads when engaged. This play is normal and disappears when the fastener is tightened. The thread can be a bit larger or smaller than ideal, and yet the part will still function adequately. However, if tolerances are exceeded, the part may require excessive force to install, or the fit may be quite sloppy, and the thread may fail during tightening.
A screw Threaded rod, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread. A screw thread is the essential feature of the screw as a simple machine and also as a fastener.