Eight Incorrect Fastener Application Practices

Many personnel assume that minor fastener substitutions will not cause obvious adverse impacts during equipment assembly and maintenance. This mindset is incorrect. As critical mechanical components, fasteners must be selected and installed in strict accordance with technical specifications. Arbitrary replacement with similar-looking fasteners may trigger equipment malfunctions and serious safety accidents. This article summarizes eight common misoperations that are mistakenly regarded as standard fastener usage.

1. Substituting fine-thread bolts with coarse-thread bolts

Key mechanical connections such as transmission shafts mostly adopt fine-thread bolts. During maintenance, some technicians replace missing fine-thread bolts with coarse-thread ones, which must be prohibited. Fine-thread bolts feature larger core diameter, smaller pitch and thread flank angle, delivering high tensile strength and excellent self-locking performance. They also withstand impact, vibration and alternating loads far better. Replacing them with coarse-thread bolts will easily lead to loosening, thread stripping or bolt fracture, further inducing mechanical failures.

2. Mismatched hole clearance

Bolts bearing transverse loads and shear forces (including transmission shaft bolts and flywheel bolts) require transition fit with mounting holes to realize firm assembly and stable lateral load resistance. If assembly proceeds without inspection when excessive clearance exists between the bolt shank and hole wall, the bolt will easily loosen or be sheared off under operating loads.

3. Increasing nut thickness to improve connection reliability

A common misconception is that thicker nuts engage more thread turns and enhance joint stability. In fact, thicker nuts lead to extremely uneven load distribution across engaged threads, making the threaded connection more prone to loosening under vibration.

4. Multiple spring washers under a single nut

When the bolt shank remains excessively long after fastening, some installers stack extra spring washers under the nut. Stacked washers bear uneven compressive force during tightening, which may cause washer fracture, reduce bolt preload and generate eccentric load, severely weakening connection reliability.

5. Over-tightening or improper tightening tools

Many operators follow the wrong principle of "tighter is better" and apply excessive torque, resulting in thread stripping. For critical bolts requiring controlled torque tightening, using adjustable spanners for convenience leads to insufficient preload, subsequent bolt loosening and equipment breakdowns.

6. Using oversized inner-diameter washers

If matching washers are unavailable, some staff use washers with excessively large inner bores. This reduces the contact area between the bolt head and washer, lowering bearing capacity and anti-loosening performance. Under vibration and impact operating conditions, the bolt will loosen rapidly.

7. Improper anti-loosening locking measures

Critical bolts must be equipped with standard anti-loosening locking devices; four typical non-compliant operations are listed below:

1,Cotter pin locking: Do not use undersized cotter pins or half-cut cotter pins;

2,Spring washer locking: Do not adopt washers with excessively small split opening gap;

Lock tab locking: Do not bend the lock tab only against the nut chamfer edges;

Double nut locking: Do not install the thinner nut on the outer side.

8. False fastening caused by unclean mating surfaces

All rust, scale, iron filings on bolts, nuts and threads must be thoroughly removed before assembly. Burrs, sediment and foreign debris on component joint surfaces also require complete cleaning. If foreign contaminants remain during tightening, the joint only appears compressed visually while actual clamping pressure is insufficient. Under vibration, impact loads or temperature fluctuation, the bolt will quickly loosen due to this false fastening state.