Bolt Preload
Calculate the recommended preload (initial clamping force) of a bolt, F_i = 0.75·A_t·S_p, from the bolt tensile stress area A_t (mm²) and the material proof strength S_p (MPa); the result is the tensile force installed in the bolt on tightening. Preload is perhaps the MOST important and most misunderstood concept in bolted joints: a well-designed bolt is tightened to be strongly TENSIONED (stretched), clamping the joined parts together. This clamping force keeps the joint tight and, counterintuitively, PROTECTS the bolt from fatigue. The 0.75·A_t·S_p value (75% of proof load) is the classic recommendation for NON-permanent (reusable) joints; permanent joints use 0.90·A_t·S_p. A HIGH preload is desirable because it: keeps the joint together under varying external load; prevents loosening from vibration; and, mainly, makes an external tensile load be absorbed mostly by DECOMPRESSION of the (stiff) parts rather than additional bolt stretch — so the bolt stress variation (which causes fatigue) is very small. That is why well-tightened bolts rarely fail by fatigue, and loose bolts fail. Enter the tensile area and the proof strength.
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Pré-carga de parafuso
A pré-carga (preload, força de aperto inicial) recomendada de um parafuso é F_i = 0,75·A_t·S_p, a partir da área de tensão A_t e da tensão de prova do material S_p — o resultado é a força de tração instalada no parafuso ao apertá-lo. A pré-carga é talvez o conceito mais importante e mais mal compreendido das uniões aparafusadas: um parafuso bem projetado é apertado para ficar fortemente tracionado (esticado), comprimindo as peças unidas entre si. Essa força de aperto mantém a junta firme e, contraintuitivamente, é o que protege o parafuso da fadiga. O valor 0,75·A_t·S_p (75% da carga de prova) é a recomendação clássica para uniões não permanentes (que serão desmontadas); para uniões permanentes usa-se 0,90·A_t·S_p. Uma pré-carga alta é desejável porque: mantém a junta unida sob carga externa variável; impede o afrouxamento por vibração; e, principalmente, faz com que uma carga externa de tração seja absorvida majoritariamente pela descompressão das peças (rígidas) e não pelo alongamento adicional do parafuso — de modo que a variação de tensão no parafuso (que causa fadiga) é muito pequena. É por isso que parafusos bem apertados quase não falham por fadiga, enquanto parafusos frouxos falham. Informe a área de tensão e a tensão de prova.
Related Tools
Bolt Load under External Load
Calculate the total tensile force in the bolt when an external load is applied to the joint, F_b = F_i + C·P, from the preload F_i (N), the joint stiffness constant C and the external tensile load P (N). This is one of the most important — and most surprising to the uninitiated — relations of bolted joints: when you apply an external load P trying to 'separate' the parts, the bolt tension does NOT rise from F_i to F_i + P (as intuition suggests), but only to F_i + C·P, where C is typically 0.2-0.4. That is, the bolt only 'feels' a FRACTION of the external load! The reason: most of the external load (1−C)·P merely RELIEVES the compression between the parts (which were compressed by the preload), rather than stretching the bolt more. This is the genius of the preloaded joint — it 'hides' the external load from the bolt. So a well-tightened joint, under a CYCLIC external load (causing fatigue), exposes the bolt to a very small stress variation (proportional to C·ΔP, not ΔP), making it extremely fatigue-resistant. This formula holds while the joint does NOT separate (P below the separation load); above that, the bolt carries the whole load. Enter the preload, stiffness constant and external load.
Bolt Stiffness
Calculate a bolt's stiffness (spring constant), k_b = (A_t·E) ÷ L, from the tensile area A_t (mm²), the material elastic modulus E (MPa) and the grip length L (mm, the effective length under tension between head and nut). When tensioned by the preload, the bolt behaves as a very stiff SPRING: it stretches an amount proportional to the force (Hooke's law), and its stiffness is force per unit elongation. This stiffness is one of two essential ingredients of bolted-joint analysis — the other is the stiffness of the clamped PARTS (members). The ratio between these two stiffnesses (the joint stiffness constant C) determines how an external load splits between the bolt and the parts. Typically the parts (massive, with large effective compression area) are MUCH stiffer than the bolt (thin and long), which is the DESIRED situation: stiff parts absorb most of the external load, protecting the bolt from stress variation and fatigue. Long, thin bolts have low stiffness (good for sharing load), while short, thick bolts are stiff. Knowing k_b is the starting point of fatigue and joint-separation analysis. Enter the tensile area, elastic modulus and grip length.
Joint Stiffness Constant
Calculate a bolted joint's stiffness constant, C = k_b ÷ (k_b + k_m), from the bolt stiffness k_b (N/mm) and the members' (clamped parts) stiffness k_m (N/mm). The constant C (also called bolt load fraction) is the heart of bolted-joint analysis: it tells what FRACTION of an external tensile load is carried by the BOLT, the rest (1−C) being carried by decompression of the MEMBERS. The value of C reveals the elegant, protective behavior of a preloaded joint: since the (massive) members are usually much stiffer than the (thin) bolt, k_m >> k_b, so C is SMALL (typically 0.2-0.4). This means that when an external load P is applied, only a small portion C·P adds to the bolt tension — most of the load (1−C)·P is absorbed by RELIEF of the compression between the parts. That is why the bolt stress variation is small (good fatigue resistance) and why preload is so beneficial. The smaller C (stiff parts, flexible bolt), the better the bolt protection. The constant C appears in all subsequent formulas: bolt load, residual member force, separation load and fatigue safety factor. Enter the bolt stiffness and the members' stiffness.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.