In engineering, preload is the deliberate application of a permanent internal force or stress to a mechanical component or system prior to the application of external operational loads. The primary objectives of preloading include increasing the stiffness of an assembly, eliminating mechanical play (backlash), improving the distribution of loads, and enhancing the fatigue life of components. Preload is a fundamental concept in several fields, including mechanical, structural, and precision engineering.
Fasteners and Bolted Joints
In the context of threaded fasteners, preload is the tension created in a bolt or screw when it is tightened. This tension generates a corresponding compressive clamping force on the joined components. A properly calculated preload ensures that the joint remains tight and that the connected parts do not separate when subjected to external tensile forces. Furthermore, it protects the fastener from fatigue failure by reducing the magnitude of the fluctuating stress cycle experienced by the bolt during operation. If the external load does not exceed the preload, the bolt experiences minimal additional stress.
Bearings
Preloading in rolling-element bearings (such as ball or roller bearings) involves applying a sustained axial load to the bearing components. This process removes internal clearances or "play" between the rolling elements and the races. The benefits of preloading bearings include:
- Increased Stiffness: Reducing the elastic deformation under external loads.
- Improved Accuracy: Enhancing the precision of shaft rotation by maintaining consistent contact.
- Noise and Vibration Reduction: Preventing the rolling elements from sliding or vibrating within the races.
- Load Sharing: Ensuring that multiple bearings in a set share the applied load more uniformly.
Structural Engineering
In structural engineering, preloading is often manifested as "prestressing." The most common application is prestressed concrete, where high-strength steel tendons are tensioned within the concrete. This places the concrete under a permanent state of compression. Since concrete is strong in compression but weak in tension, the preload allows the structure to counteract tensile stresses that occur when the member is loaded (e.g., a bridge beam under the weight of traffic), thereby preventing cracking and allowing for longer spans.
Mechanisms and Motion Control
In mechanical systems such as lead screws, gears, and linkages, preload is used to eliminate backlash—the clearance between mating parts that causes a lag in motion when direction is reversed. By applying a preload (often through springs or adjustable nuts), constant contact is maintained between surfaces, ensuring high positional accuracy and repeatability in CNC machinery and robotics.
Measurement and Control
Achieving the correct preload is critical, as excessive preload can lead to premature wear, increased friction, and overheating, while insufficient preload can result in joint separation or mechanical instability. Common methods for controlling preload include torque measurement, the "turn-of-nut" method, direct tension indicators, or the use of specialized load-sensing washers.
