PAGE 5 • ENTIRE ARTICLE IN PDF FORMAT (985 kb) RELIABILITY As some of the examples which were previously discussed have indicated, the reliability of various equipment can be improved with the proper utilization of Multi-Jackbolt Tensioners. First and foremost is the concept of preload versus working load, which is a widely misunderstood subject. In most bolted joints which have a through bolt, or stud, clamping two members together such as a flange connection, the preload (clamping force) must exceed the working load (or separating force) for the joint to have integrity (See below). General Background of Preload: Preload: although a fully detailed study into bolting principals is above the scope of this paper, some major points need to be discussed. The most important point is the concept of preload vs. working load on a bolted joint. Following is a commonly used example of how preload works:
LEFT: A
fish scale is loaded with a weight of 150 lbs. The spring represents
a bolt tightened to a tensile preload of 150 lbs. MIDDLE: A
block is forced into the indicated position and the weight is
removed. RIGHT: A 100 lb. weight is applied
to the system. The assembly now behaves like a bolt preloaded
to 150 lbs and an external load of 100 lbs. Adding the weight
does not increase the tension in the spring (which represents
the bolt). If the tension were greater than 150 lbs, the scale
would read more than 150 lbs and the block would fall out. Preload: Preload and working load are not additive as often believed. A bolt preloaded to 100,000 lbs induces an equivalent clamping load of 100,000 lbs on the bolted members. Unless the working load or separating force on the joint approaches the preload, the bolt does not feel additional load. In other words, if the separating force of the machine or working load is 50,000 lbs, the bolt will not be loaded to 150,000 lbs but will still only have the initial preload of 100,000 lbs. However, when the working load exceeds the preload, the joint separates and the full working load is carried by the bolt. It is at this situation that the working load cycles against the heads of the bolts or on the faces of the nuts. This is when loosening and bolt breakage occurs.
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