And so we continue our discussion on threaded fasteners that we started last month. Without further adieu, let's get right down to the nuts and bolts of it all.
How Fine Is Coarse?
Essentially, a thread is an inclined plane cut along the surface of a fastener. Varying the angle of the plane determines the cut or thread-increasing the angle produces a coarse thread, while decreasing it results in a fine thread.
Fasteners are graded according to how many threads there are to the inch for SAE fasteners, or centimeters for metric ones. Most threaded fasteners are available with either coarse threads conforming to Unified National Coarse (UNC) standards or Unified National Fine (UNF) threads. Fine threads have more threads per inch than coarse.
The long and short of it is this: Coarse threads are easier and faster to use. They provide an easier "start" of the fastener, with less likelihood of cross-threading. Nicks and burrs from handling are less likely to affect assembly, and they are less likely to seize in temperature applications and in joints where corrosion is likely. Additionally, coarse threads are less likely to "strip" and are more easily tapped into brittle materials.
On the other hand, fine threads provide superior fastening (typically 10-percent stronger holding power than coarse) in hard materials, and they can be adjusted more precisely due to their shallower helix angle. They are also better in situations where length of engagement (depth) is limited and where wall thickness is limited, again because of their smaller thread cross-section (coarse threads are cut deeper into the shaft of the fastener than fine.)
While all of this is very interesting, it isn't worth a hill of beans if the fastener isn't properly installed, and that leads us to a different twist on the subject, if you will.
How Tight Is Tight?
All fasteners have to be correctly tightened in order to perform the job for which they are intended, and that degree of tightness is referred to as the torque specification. Let's say, for instance, that two steel plates need to be tightened together with a force of 100 pounds. Let's also say that a single 3/4-inch-diameter bolt is used to create the binding force. Once tightened, the two plates are held together just as if a 100-pound weight were sitting on top of them.
However, if the bolt was loose, external loading, vibration, and temperature change would eventually cause the plates to come apart because they would fatigue (the simplest form of fatigue is that of metal being bent back and forth, but in this case the bolt would be elongated enough for the joint to loosen). The strength of any joint is dependent upon the two factors: the strength of the fastener itself, and the degree to which it is tightened. Tightness can be accurately controlled by the measurement of the torque (twisting force, measured in pound-feet) to which it is tightened. Torque applied to a fastener creates inner tension (stretching) that, in turn, creates the holding power desired.
Torque wrenches perform this task. The strength of the fastener is determined by the raised grade markings on the head of the bolt or screw. These head markings were developed by SAE International (formerly the Society of Automotive Engineers) for automotive applications, and ASTM International (formerly the American Society of Testing and Materials) for structural applications.
A one-pound weight or force applied to a lever arm one-foot long is equal to one 1 lb-ft, or 12-inch pounds, of torque.