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An external static load producing a strain in a test specimen. For example, a preload is imposed prior to forced vibration testing. Preload is usually expressed in units of force instead of units of deflection.
For any fastener to work properly, it must be stretched sufficiently to produce a static pre-load (clamping force) that is greater than the expected external loads.
As an example, imagine two blocks that are held together by two screws. These blocks are subjected to a force F trying to separate them. If both screws are hand tightened, that is, tightened just enough that the blocks touch, without any gap between them, the screws will not exert any force SF on the blocks; the combined total screw force SF equals zero (SF = 0). As soon as the force F is applied, and because F is greater than FS (F > FS), the blocks will separate and the screws will be stretched until the resistance (or force) in thescrews SF equals F (SF = F). But by then, the blocks have separated and left a gap between them. In molds, any undesired gap means flashing or leaking, and is not acceptable. To prevent such gaps, the screws must be tightened to such an extent that they will be stretched to a desired preload. FS must be greater than the expected force F (FS > F). When the force F is now applied, the blocks will not separate unless F becomes greater than FS. In practice, there are two types of preload:
1. The preload exerted by screws. Screws must always be tightened to the manufacturers suggested values, that is, to about 60–70% of the yield strength of the screw. The resulting force (or holding power) of the screw can be found in allscrew tables.
2. The preload can be provided by stretching the steel of mold parts, suchas tapers, wedges, stripper rings, and so on, ormold plates, as in the following example, and by press fits, which are a kind of preload, or by shrinking of rings or bars over usually cavities, for building up cavities from sections. When specifying preload on tapers or wedges, it is common practice to indicate the distance (which is also called preload) that the tapers are allowed to move (and thereby stretching the steel) before coming to a stop. This preload is especially important where cavities split in two or more sections.