Preload is a civil engineering term with several meanings. In the general sense, it refers to the internal application of stress to certain mechanical systems.
The most common usage is to describe the load applied to a fastener as a result of it being installed, before any external loads are applied (e.g. tightening the nut on a bolt). Preload in such cases is important for several reasons.
First, a tightened bolt experiences only a small fraction of any external load which is applied later, so that a fully tightened bolt can (depending on the exact application) sustain a much greater load than loosely tightened bolt.
Second, a nut which is correctly tightened will resist coming loose under the influence of vibration, temperature cycling, etc.
Internal stress to a bearing through application of negative clearance is known as bearing preloading. Advantages of preloading include the following: maintain axial and radial position for accurate displacements of angular movements; increase bearing rigidity; prevent sliding or gyroscope-like movements, especially with high acceleration or rotation rates; maintain relative position of bearing elements. Preloading methods include position preload and constant pressure preload.
When tensile specimens are initially placed into testing grips, they can be subjected to small compressive forces. These forces can cause specimens to bend imperceptibly, causing inaccurate and inconsistent results. Establishing a small preload as a part of the test method eliminates those compressive forces on specimens and improves the repeatability of results.
Preload becomes very important for large mechanical and high performance system such as large Telescopes. In the general sense, it refers to the internal application of stress to certain mechanical systems. By tensioning, preloading increases the natural frequency of a structure, avoiding resonance due to external disturbances. It also prevents buckling if stresses change depending on position in certain systems. In the particular case for bearings and fasteners, preload reduces or cancels backlash or dead zones. In addition, preload aids to limit applied loads to a system.