When specifying hex head bolts for industrial machinery, structural steel, or heavy equipment, engineers frequently debate between two fundamental German standards: DIN 933 (fully threaded) and DIN 931 (partially threaded). While both conform to ISO 4017 and ISO 4014 respective dimensional standards, using the wrong type in a high-shear joint can lead to premature fatigue failure and costly downtime.
The Core Difference: Shank vs. Threads in Shear Loading
To understand why thread configuration matters, consider how a bolted joint transfers load. When two plates are bolted together and subjected to transverse (sideways) forces, the bolt is placed under shear stress.
A DIN 931 bolt features a smooth unthreaded shank under the head. When installed correctly, this smooth shank acts as a solid cylindrical pin across the shear plane. In contrast, a DIN 933 bolt is threaded along its entire length up to the head. Because threads act as stress concentrators with reduced cross-sectional root areas, placing a thread across a shear plane significantly lowers the joint's fatigue endurance and load-bearing capacity.
Technical Comparison Table
| Standard | Thread Coverage | Primary Mechanical Advantage | Recommended Application |
|---|---|---|---|
| DIN 931 (ISO 4014) | Partially threaded (smooth shank under head) | Higher shear resistance; shank absorbs transverse loads without thread root notch effect. | Structural joints, heavy machinery frames, vibrating screens, bridge connections. |
| DIN 933 (ISO 4017) | Fully threaded to the head | Maximum thread engagement length in blind tapped holes; flexible clamping grip length. | Tapped machine casings, flange connections, cover plates, light-to-medium brackets. |
Engineering Best Practices for Sourcing and Assembly
- Shear Plane Inspection: Ensure the unthreaded shank of a DIN 931 bolt spans the entire thickness of the joined plates. If the threaded portion intersects the shear plane, switch to a shorter bolt or a fastener with a longer shank.
- Preload and Torque: Always calculate tightening torque based on the property class (e.g., Changgu Grade 8.8 or 10.9) and lubrication state, as thread friction differs slightly from shank friction during tightening.
- Thread Engagement: When using DIN 933 in tapped steel holes, ensure a minimum thread engagement length of 1x nominal diameter (1d) for steel-to-steel connections.
Frequently Asked Questions (FAQ)
Is DIN 931 always stronger than DIN 933?
In pure axial tension (pulling), two bolts of the same diameter, material, and property class (e.g., Grade 10.9) offer identical strength. However, in shear or dynamic fatigue environments, DIN 931 is superior because its unthreaded shank avoids thread root stress concentration.
Can I substitute DIN 933 with DIN 931 by adding extra washers?
Adding washers under the bolt head can adjust the grip length, but it does not change the shank length. If the threads still cross the shear plane, washers will not solve the structural weakness.
Does Yuanpai supply both standards in custom lengths?
Yes. Both DIN 933 and DIN 931 are manufactured in our facilities across carbon and alloy steels, complete with full traceability and third-party test reports.
References
[1] ISO 4014:2011. Hexagon head bolts — Product grades A and B. International Organization for Standardization.
[2] ISO 4017:2021. Hexagon head screws — Product grades A and B. International Organization for Standardization.
Related Technical Resources & Sourcing
Explore our manufacturing standards and product specifications to support your next engineering project:
- Core Product: DIN 933 / DIN 931 Hex Head Bolts - Full & Partial Thread
- Recommended Reading: Fatal Overload: Why Torque Calculation Errors Can Destroy a Steel Structure
- Sourcing Guide: 10 Sub-Brands of Yuanpai Fastener – Your Complete Sourcing Guide