Stretch bolts

What are stretch bolts?

Stretch bolts are specially designed fasteners whose shank diameter is deliberately reduced over a defined length. This so-called reduced shank (or “waist”) is usually thinner than the thread’s minor diameter and allows the bolt to elongate elastically under load. As a result, the bolt behaves like a preloaded spring, reliably maintaining the clamping force in the joint even under changing operating conditions.

Unlike a standard bolt, which absorbs its load largely rigidly, a stretch bolt distributes load peaks through its elastic elongation. This makes it the preferred solution wherever high, dynamic forces must be transmitted safely and durably.

Design and Function

The key feature of a stretch bolt is the reduced shank cross-section between head and thread. This reduced cross-section deliberately lowers the bolt’s spring stiffness, while the clamped components remain comparatively rigid. This ratio is critical to the function.

The Principle of Elastic Preload

When tightened, the bolt elongates elastically, storing energy like a spring. This stored preload holds the joint together. If the joint settles slightly during operation (for example, through the flattening of surface roughness), a rigid bolt loses a disproportionately large amount of preload — a stretch bolt, by contrast, largely compensates for this settling through its greater elastic compliance.

Behavior Under Dynamic Load

Under cyclic loading, the compliant stretch bolt absorbs only a small proportion of the additional operating force as elongation. The result is a significantly lower dynamic stress amplitude in the bolt’s cross-section, which considerably improves the fatigue strength of the joint.

Properties and Advantages

Compared with conventional bolts, stretch bolts offer several design advantages: they maintain preload more stably even in the presence of settling and temperature fluctuations, they increase fatigue strength under dynamic load, and they reduce the risk of self-loosening. This makes them particularly suitable for highly stressed, safety-critical joints.

Typical Applications

Stretch bolts are used primarily where high, often fluctuating forces must be reliably transmitted. Typical examples include connecting rod and cylinder head bolting in engine manufacturing, joints in turbines and compressors, heavy-duty flanges in plant engineering, and highly stressed joints in mechanical and gearbox engineering.

Materials and Standards

Stretch bolts are predominantly manufactured from high-strength quenched and tempered steels, with the strength class matched to the specific application. Depending on the field of use, heat-resistant or corrosion-resistant materials are also employed. Geometry, tolerances and strength are based on the relevant standards as well as on the design specifications from the respective customer drawing.

Assembly and Tightening Methods

For a stretch bolt to deliver its benefits, the correct tightening method is essential. Since a defined elastic elongation is what matters, stretch bolts are frequently tightened using the angle-controlled or yield-point-controlled method rather than purely torque-controlled tightening. This allows the target preload to be set reproducibly and independently of friction variations.

Difference from Standard Bolts

While a standard bolt is designed for maximum stiffness and simple assembly, the stretch bolt is deliberately designed to be more compliant. This targeted elasticity is not a drawback but the actual function: it ensures a stable, fatigue-resistant joint under precisely the conditions where standard bolts reach their limits.

Stretch Bolts from PATTBERG

We manufacture stretch bolts as custom bolts exactly to your drawing — matched to the material, strength class and geometry of your application. For more information, please see our product area Custom Bolts to Drawing.

Frequently Asked Questions

When does a stretch bolt make sense?

Whenever there are high dynamic loads, a risk of settling, or stringent requirements for the operational safety of the joint, and a constant preload must be ensured over the service life.

Can I have stretch bolts manufactured to my own drawing?

Yes. Geometry, waist length, material and strength class can all be tailored to your application. Simply send us your drawing or sample.