Preload (engineering)

Preload is a mechanical engineering term with several meanings. In the general sense, it refers to the internal application of stress to certain mechanical systems.

Fastener preload

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 when the joint is in service, 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.

Bearing preload

When it is provided an internal stress to a bearing in order to apply a negative clearance it is know as bearing preloading or just preloading. Advantages for preloading are to keep position in axial and radial direction for accurate displacements of angular movements; preloading increase rigidity in bearings; it prevent slinding by giroscope-like movements, especially with high acceleration or speeds; keeping relative position of bearing elements. Preloading methods include position preload and constant pressure preload.^[1]

Testing preload

It is also used in testing a specimen, for a process where the crosshead moves to load the specimen to a specified value before a test starts. Data is not captured during the preload segment.

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.

Applications

Preload becomes very important for large mechanical and high performance system such as large Telescopes.^[2] In the general sense, it refers to the internally application of stress to certain mechanical system. By tensioning preloading increase the natural frequency of a structure avoiding resonance due to external disturbances; prevent buckling by different stress in different position on certain systems; as in the particular case for bearing and fasteners, preload reduces or cancel backslash or dead zones; in addition, preload aids to limite applied loads to a system.

See also

• Bolted joint

External links

• Tutorial on the Basics of Bolted Joints
• Fundamentals of Basic Bolting

Notes