Theory
The clamp load, also called preload, of a fastener is created when a torque is applied, and is generally a percentage of the fastener's proof strength; a fastener is manufactured to various standards that define, among other things, its strength and clamp load. Torque charts are available to identify the required torque for a fastener based on its property class or grade.
When a fastener is tightened, it is stretched and the parts being fastened are compressed; this can be modeled as a spring-like assembly that has a non-intuitive distribution of strain. External forces are designed to act on the fastened parts rather than on the fastener, and as long as the forces acting on the fastened parts do not exceed the clamp load, the fastener is not subjected to any increased load.
However, this is a simplified model that is only valid when the fastened parts are much stiffer than the fastener. In reality, the fastener is subjected to a small fraction of the external load even if that external load does not exceed the clamp load. When the fastened parts are less stiff than the fastener (soft, compressed gaskets for example), this model breaks down; the fastener is subjected to a load that is the sum of the preload and the external load.
In some applications, joints are designed so that the fastener eventually fails before more expensive components do. In this case, replacing an existing fastener with a higher strength fastener can result in equipment damage. Thus, it is generally good practice to replace old fasteners with new fasteners of the same grade.
Read more about this topic: Bolted Joint
Famous quotes containing the word theory:
“The theory of rights enables us to rise and overthrow obstacles, but not to found a strong and lasting accord between all the elements which compose the nation.”
—Giuseppe Mazzini (1805–1872)
“No theory is good unless it permits, not rest, but the greatest work. No theory is good except on condition that one use it to go on beyond.”
—André Gide (1869–1951)
“We have our little theory on all human and divine things. Poetry, the workings of genius itself, which, in all times, with one or another meaning, has been called Inspiration, and held to be mysterious and inscrutable, is no longer without its scientific exposition. The building of the lofty rhyme is like any other masonry or bricklaying: we have theories of its rise, height, decline and fall—which latter, it would seem, is now near, among all people.”
—Thomas Carlyle (1795–1881)