Belt Contact Arc
Calculate the contact-arc length of a belt on the smaller pulley, L_arc = (d ÷ 2)·θ, from the smaller pulley diameter d (mm) and the wrap angle θ (radians). The contact arc is the length of the belt portion actually in contact with the pulley (touching it), along the wrap angle — simply the pulley radius times the angle (in radians), the arc-length formula. This length matters for several reasons: it sets the CONTACT AREA between belt and pulley (with the width), governing contact pressure and friction distribution; it influences heating (friction × area) and wear of both belt and pulley; and it is relevant to elastic slip (creep), where the belt, changing tension from T₁ to T₂ along the arc, elastically stretches and contracts, sliding microscopically over the pulley — a small INEVITABLE slip (1-2%) occurring even without gross slipping, making the output speed always slightly below theoretical. A larger contact arc (bigger pulley or more wrap) distributes friction better and reduces the slip tendency. This calculation complements the geometric and friction analysis of a belt drive. Enter the smaller pulley diameter and the wrap angle.
Resultado
—
Arco de contato da correia
O comprimento do arco de contato de uma correia na polia menor é L_arco = (d ÷ 2)·θ, a partir do diâmetro da polia menor d e do ângulo de abraçamento θ (radianos). É o comprimento da porção da correia que está efetivamente em contato com a polia, ao longo do ângulo de abraçamento — simplesmente o raio da polia vezes o ângulo (em radianos), a fórmula do comprimento de arco. Esse comprimento é importante por algumas razões: ele determina a área de contato entre a correia e a polia (junto com a largura), que governa a pressão de contato e a distribuição do atrito; influencia o aquecimento (atrito × área) e o desgaste tanto da correia quanto da polia; e é relevante no escorregamento elástico (creep), em que a correia, ao mudar de tração de T₁ para T₂ ao longo do arco, estica e contrai elasticamente, deslizando microscopicamente sobre a polia — um pequeno escorregamento inevitável (de 1-2%) que ocorre mesmo sem escorregamento grosseiro, e que faz a velocidade de saída ser sempre ligeiramente menor que a teórica. Um arco de contato maior (polia maior ou maior abraçamento) distribui melhor o atrito e reduz a tendência ao escorregamento. Este cálculo complementa a análise geométrica e de atrito da transmissão por correia. Informe o diâmetro da polia menor e o ângulo de abraçamento.
Related Tools
Belt Wrap Angle
Calculate a belt's wrap (contact) angle on the smaller pulley, θ = π − 2·arcsin((D − d) ÷ (2·C)), from the larger D and smaller d pulley diameters (m) and the center distance C (m). The wrap angle is the angle of the arc over which the belt actually WRAPS the pulley, in contact with it — and it is a critical parameter, since it is along that arc that the friction (transmitting the force) acts. The LARGER the wrap angle, the greater the contact area and the greater the force the belt can transmit without slipping. In a drive between two DIFFERENT-DIAMETER pulleys, the belt wraps LESS around the smaller pulley (angle below 180°) and MORE around the larger — and slipping always starts on the pulley with LESS wrap (the smaller), which therefore limits capacity. The wrap angle decreases when the diameter difference grows or the center distance shrinks (close, very different pulleys 'wrap' little). So drives with large reduction (very different pulleys) or close centers have reduced capacity, and sometimes use an IDLER (tensioner) pulley to increase wrap. The wrap angle enters directly into the tension ratio (e^(μθ)) and the belt-count correction factors. Enter the pulley diameters and the center distance.
Belt Transmission Ratio with Slip
Calculate a belt's real transmission ratio accounting for slip, i = (D ÷ d)·(1 − s/100), from the driving D and driven d pulley diameters (mm) and the slip percentage s (%). A belt's THEORETICAL transmission ratio is simply the pulley diameter ratio (D/d) — a large driving pulley turning a small driven one multiplies the rotation. But in practice, a belt drive is NOT exact like a gear drive (which has interlocking teeth): the belt transmits by FRICTION, and there is always a small SLIP between belt and pulleys. This slip has two components: ELASTIC slip (creep, inevitable, ~1-2%, from the belt stretching and contracting as tension changes between the two sides) and GROSS slip (occurring under overload, when the belt loses grip — undesirable and harmful). Slip makes the driven pulley's real rotation SLIGHTLY LOWER than theoretical, and the real transmission ratio a bit different from nominal. In applications needing exact synchronism (engine timing shafts, positioning), V-belt slip is unacceptable, and TIMING (toothed) belts or chains, which do not slip, are used. This calculation quantifies the slip effect on the transmission ratio. Enter the driving and driven pulley diameters and the slip percentage.
Belt Maximum Tension
Calculate a belt's maximum (tight-side) tension, T₁ = T_e·r ÷ (r − 1), from the effective tension T_e = T₁ − T₂ (the power-transmitting force, N) and the tension ratio r = T₁/T₂ (at the slip limit). Knowing the force the belt must transmit (the effective tension, from power and velocity) and the maximum tension ratio the belt sustains before slipping (from friction, wrap and, in V-belts, the wedge effect), the individual side tensions can be computed. The maximum tension T₁ (tight side) is the larger, and it SIZES the belt's strength (which must not break) and the load on the BEARINGS and pulley shafts (which feel the sum of both side tensions, bending the shaft). Knowing T₁ is essential to: check the belt resists (versus its tensile strength), size the bearings for the radial load imposed by the belt (which can be significant and shortens bearing life), and set the correct installation tension. The LOWER the tension ratio r (worse friction, less wrap), the HIGHER the T₁ needed for the same power — hence the advantage of V-belts (high r) in reducing loads. Enter the effective tension and the tension ratio.
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.