Table of Contents - Rainbow Precision Products, Inc.

Useful Formulas for V-Belt DrivesTorque to horsepowerconversionLoad requirements for a drive may be given in terms oftorque (turning effort) rather than horsepower. For drivedesign purposes, use the formulas below to converttorque (expressed as pound-inches or pound-feet) tohorsepower. Be sure to use the rpm of the shaft for whichthe torque is known, and do not mix the torque of oneshaft with the rpm of another shaft.Horsepower =Horsepower =(Torque, pound-inches) (rpm)63,025(Torque, pound-feet) (rpm)5,252Power to or from machineryWithout accurate horsepower requirement data for a drive,the formulas below may be used. Efficiency must beknown or estimated to use these formulas, and is shownas a decimal (i.e., if a pump is 75% efficient, use .75 in theformula). When a drive is providing power to a pump orgenerator, estimate a low efficiency for the driveNmachine. For power input to a drive from a motor or turbine,estimate a high efficiency for the driveR machine.A.C. MachineryKilowatts =(volts) (amps) (pf)YWhere: pf = power factorSingle phase: Y = 1000Three phase: Y = 577Power required for generator/alternatorHorsepower =(volts) (amps) (pf)(Z) (eff)Where: eff = overall mechanical and electrical efficiencypf = power factorSingle phase: Z = 746Three phase: Z = 431Power from motorHorsepower =(volts) (amps) (pf) (eff)ZWhere: eff = overall mechanical and electrical efficiencypf = power factorSingle phase: Z = 746Three phase: Z = 431D.C. MachineryKilowatts =(volts) (amps)1000Power required for generatorHorsepower =(volts) (amps)(746) (eff)Where: eff = overall mechanical and electrical efficiencyPower from motorHydraulic MachineryPower required by pumpsHorsepower = (Q) (P)1714 (eff)Where: Q = flow rate, gal./min.P = discharge pressure for pumps, inlet pressure forturbines, lb./sq. in.eff. = overall mechanical and hydraulic efficiencyPower from turbineHorsepower = (Q) (P) (eff)1714Where: Q = flow rate, gal./min.P = discharge pressure for pumps, inlet pressure forturbines, lb./sq. in.eff. = overall mechanical and hydraulic efficiencyV-Belt Tension FormulasEffective PullT 1 -T 2 = 33,000 ( HP )VWhere: T 1 = tight side tension, poundsT 2 = slack side tension, poundsHP = design horsepowerV = belt speed, feet per minuteT 1 +T 2 = 33,000 (2.5-G) ( HP )GVWhere: T 1 = tight side tension, poundsT 2 = slack side tension, poundsHP = design horsepowerV = belt speed, feet per minuteG = arc of contact correction factorTension RatioT 1 /T 2 =11-0.8G(Also, T 1 /T 2 = eK)Where: T 1 = tight side tension, poundsT 2 = slack side tension, poundsG = arc of contact correction factore = base of natural logarithmsK = .51230, a constant for V-belt drive designq = arc of contact in radiansTight Side TensionT 1 = 41,250 ( HP )GVWhere: T 1 = tight side tension, poundsHP = design horsepowerV = belt speed, feet per minuteG = arc of contact correction factorSlack Side TensionT 2 = 33,000 (1.25-G)( HP )GVWhere: T 2 = slack side tension, poundsHP = design horsepowerV = belt speed, feet per minuteG = arc of contact correction factorBelt SpeedV = PD (rpm) = (PD) (rpm) (.262)3.82Where: V = belt speed, feet per minutePD = pitch diameter of sheave or pulleyrpm = revolutions/minute of the same sheave or pulleyHorsepower = (volts) (amps)(746)Where: eff = overall mechanical and electrical efficiency120