Residual Member Clamping Force
Calculate the residual clamping force on the members (clamped parts) of a bolted joint under external load, F_m = F_i − (1 − C)·P, from the preload F_i (N), the joint stiffness constant C and the external tensile load P (N). When an external load P tries to separate the parts, it does not go entirely to the bolt — most, (1−C)·P, acts to RELIEVE the compression between the members. The residual force F_m is how much clamping STILL holds the parts together after the external load is applied. This value is crucial for several reasons: while F_m stays POSITIVE (compression), the joint is closed and tight, and the bolt is protected (feels only C·P); if F_m reaches ZERO, the joint SEPARATES (and the bolt takes the whole load). In SEALED joints (gaskets, engine joints, pressurized pipe flanges), the residual member force is what keeps the seal compressed and prevents leaks — so it must stay above a minimum value, even under maximum service load (internal pressure, for example). Computing F_m is essential to ensure the joint stays tight and sealed in operation, and it is the criterion that sets the minimum required preload. Enter the preload, the stiffness constant and the external load.
Resultado
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Força residual de aperto nos membros
A força de aperto residual nos membros (peças unidas) de uma junta aparafusada sob carga externa é F_m = F_i − (1 − C)·P, a partir da pré-carga F_i, da constante de rigidez da junta C e da carga externa de tração P. Quando uma carga externa P tenta separar as peças, ela não vai inteira para o parafuso — a maior parte, (1−C)·P, atua aliviando a compressão entre os membros. A força residual F_m é o quanto de aperto ainda resta segurando as peças unidas depois de aplicada a carga externa. Esse valor é crucial por várias razões: enquanto F_m permanecer positivo (compressão), a junta está fechada e estanque, e o parafuso fica protegido (sente só C·P); se F_m chegar a zero, a junta separa (e o parafuso passa a sofrer a carga inteira). Em juntas com vedação (gaxetas, juntas de motor, flanges de tubulação pressurizada), a força residual nos membros é o que mantém a vedação comprimida e evita vazamentos — por isso ela deve permanecer acima de um valor mínimo, mesmo sob a carga máxima de serviço (a pressão interna, por exemplo, que tende a separar os flanges). Calcular F_m é essencial para garantir que a junta permaneça apertada e estanque em operação, e é o critério que define a pré-carga mínima necessária. Informe a pré-carga, a constante de rigidez e a carga externa.
Related Tools
Joint Separation Load
Calculate the external load that causes a bolted joint to separate (open), P_0 = F_i ÷ (1 − C), from the preload F_i (N) and the joint stiffness constant C. The separation load is the external tensile load at which the compression between the clamped parts fully vanishes — the point where the joint starts to OPEN. Below it, the parts stay compressed and the joint behaves 'smartly' (the bolt feels only C·P of the external load, with small stress variation); ABOVE it, the parts separate, and from then on ALL additional external load goes straight to the bolt (which then takes the whole load, with severe fatigue and failure risk). Joint separation is thus a condition the design must AVOID with margin: a safety factor against separation is applied (the separation load must be well above the maximum expected external load). The formula shows the separation load grows with preload (well-tightened joints separate later) — another reason to use high preloads. Separation also causes leaks (in sealed joints), loss of stiffness and loosening. Ensuring the joint never separates under service load is a fundamental bolted-joint design criterion. Enter the preload and the stiffness constant.
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.
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.
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.