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.
Resultado
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Carga no parafuso sob carga externa
A força total de tração no parafuso quando uma carga externa é aplicada à junta é F_b = F_i + C·P, a partir da pré-carga F_i, da constante de rigidez da junta C e da carga externa de tração P. Esta é uma das relações mais importantes — e mais surpreendentes para quem não conhece a teoria — das uniões aparafusadas: quando se aplica uma carga externa P tentando 'separar' as peças, a tração no parafuso não aumenta de F_i para F_i + P (como o senso comum sugeriria), mas apenas para F_i + C·P, onde C é tipicamente 0,2 a 0,4. Ou seja, o parafuso 'sente' apenas uma fração da carga externa! O motivo: a maior parte da carga externa, (1−C)·P, é gasta apenas aliviando a compressão entre as peças (que estavam comprimidas pela pré-carga), e não esticando mais o parafuso. Essa é a genialidade da junta pré-carregada — ela 'esconde' a carga externa do parafuso. Por isso uma junta bem apertada, sob uma carga externa cíclica (que causa fadiga), expõe o parafuso a uma variação de tensão muito pequena (proporcional a C·ΔP, não a ΔP), tornando-a extremamente resistente à fadiga. Esta fórmula vale enquanto a junta não se separa (P menor que a carga de separação); acima disso, o parafuso passa a suportar toda a carga. Informe a pré-carga, a constante de rigidez e a carga externa.
Related Tools
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.
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.
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.
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.