Boltspenning

Boltspenning, commonly referred to as bolt preload or bolt tension, is the axial force developed in a bolt when it is tightened. This tension creates a clamping force between the joined parts and helps resist external loads that would otherwise separate them. The amount of preload depends on the tightening method, bolt size, material properties, and friction in the threads and under the bolt head.

The preload, clamp load, and joint stiffness determine how a joint responds to service loads. Under external loading, part of the load is carried by the bolt as tension while the parts experience compressive stresses. If the joint or bolts are too compliant, preload can decrease and slip or separation can occur. Over-tightening can yield the bolt or damage threads, and excessive preload increases bearing stresses in the joint.

Control and measurement: Torque is commonly used as a proxy for preload, but friction variability makes it an imprecise indicator. Direct methods include strain gauges attached to bolts or mechanical/ultrasonic systems that estimate preload from bolt elongation. Designing for target preload or clamp load also considers temperature, lubrication, and vibration. Locking methods or special fasteners may be used to reduce loosening risk.

Applications and considerations: Bolt tension is critical in automotive, aerospace, machinery, and structural bolted joints. Maintenance aims to preserve adequate preload; hence fastener selection, proper lubrication, tightening sequences, and inspection practices are essential to ensure reliable clamping under expected service conditions.