Meaning
Mechanical compaction of surface microscopic irregularities under the action of tightening forces represents a primary source of initial preload loss in bolted connections. During the tightening phase, bearing pressure embedment occurs as the peaks of the surface roughness flatten under the extreme local contact stresses. This flattening reduces the effective thickness of the joint, causing an immediate decrease in the tension of the bolt.
Contact Roughness
High local pressure at the washer face and thread flanks exceeds the yield strength of the materials even under moderate clamping forces. The bearing pressure embedment in these areas is most pronounced immediately after assembly and can continue under external dynamic loads. Polishing or grinding the mating surfaces helps minimize the height of the asperities and limits this relaxation.
Fastener Relaxation
Threaded joints suffer from an initial drop in clamping force that occurs without any rotation of the nut or bolt. In many electrical terminal blocks, bearing pressure embedment accounts for the majority of early preload loss during transport and thermal testing. This behavior requires designers to specify higher initial torque values or utilize spring washers to absorb the displacement.
Mitigation Strategy
Hardened washers distribute the clamping force over a larger area to reduce the localized stress below the compressive yield limit of the clamped parts in the assembly. When softer housing materials like aluminum or plastics are used, bearing pressure embedment is controlled by sizing the fastener head or inserting steel bushings to keep the contact pressure within safe margins. This precaution prevents the structural collapse of the joint during long-term service under vibrating conditions, which otherwise leads to loose connections and complete failure of the electronic enclosure.
