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Belt Drive Calculator - V-Belt Speed Ratio, Belt Length & Sheave Sizing

Calculate driven RPM, belt length, and sheave diameters for industrial V-belt and synchronous belt drives

Free belt drive calculator for industrial V-belt systems. Enter driver and driven sheave diameters with motor RPM to get speed ratio, driven shaft RPM, belt pitch length, center distance adjustment range, and belt wrap angle. Supports classical (A/B/C/D/E), narrow (3V/5V/8V), and synchronous belt cross-sections. Includes FHP fractional horsepower belt lookup, service factor tables for HVAC fans, conveyors, crushers, and compressors, and multi-belt capacity calculations per MPTA and RMA standards.

Pro Tip: Belt wrap angle below 120 degrees on the small sheave kills drive capacity faster than anything else. Every degree below 180 costs you transmitted horsepower. If your speed ratio exceeds 3:1 on a single reduction, consider a two-stage drive or switch to a synchronous belt that does not rely on friction. A $200 sheave change can prevent $5,000 in unplanned downtime from chronic belt failures.

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Belt Drive & Sheave Calculator
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How It Works

  1. Enter Sheave Diameters

    Input the pitch diameter of the driver (motor) sheave and the driven (load) sheave in inches. Pitch diameter is measured at the belt's pitch line, not the outer diameter of the sheave. Check the sheave casting or catalog for the correct pitch diameter.

  2. Set Motor Speed

    Enter the motor nameplate RPM. Use actual RPM under load (accounting for slip) for precision. A 1750 RPM nameplate motor typically runs at 1740-1760 RPM depending on load. The calculator uses this value with the sheave ratio to determine driven shaft speed.

  3. Select Belt Cross-Section

    Choose your belt type: classical A, B, C, D, or E for legacy drives, or narrow 3V, 5V, 8V for modern high-capacity designs. Narrow belts transmit 30-50% more power per belt than classical belts in the same space.

  4. Review Speed Ratio and RPM

    See the calculated speed ratio, driven RPM, belt pitch length, recommended center distance range, and belt wrap angle on both sheaves. Wrap angle below 120 degrees triggers a capacity warning.

  5. Check Belt Capacity

    Enter required horsepower and service factor to see how many belts are needed. The calculator applies wrap angle correction, belt length correction, and service factor per RMA/MPTA standards to determine the minimum belt count.

Built For

  • Millwrights replacing worn sheaves and needing to verify speed ratio before ordering parts
  • Maintenance techs troubleshooting belt slip and verifying correct sheave combinations
  • Plant engineers designing new belt-driven fan, pump, or conveyor drive systems
  • HVAC technicians calculating blower RPM changes for air balance adjustments
  • Reliability engineers evaluating belt wrap angle and drive geometry for failure analysis
  • Purchasing agents verifying belt part numbers and lengths before placing orders

Features & Capabilities

Speed Ratio & RPM Calculator

Instantly calculate the driven shaft RPM from any combination of sheave diameters and motor speed. Accounts for belt slip factor (typically 1-2% for V-belts) to give you the actual expected output speed rather than theoretical.

Belt Pitch Length Calculator

Computes the required belt pitch length from center distance and sheave diameters using the standard geometric formula. Rounds to the nearest standard belt length and shows the center distance adjustment needed to tension the belt properly.

Wrap Angle Analysis

Calculates belt wrap angle on both sheaves and flags drives where wrap drops below 120 degrees. Shows the horsepower derating factor so you know exactly how much capacity you lose from poor drive geometry.

Service Factor Lookup

Built-in service factor tables for common driven equipment: centrifugal fans (1.2-1.4), positive displacement pumps (1.4-1.6), crushers and mills (1.6-2.0), conveyors (1.2-1.8), and compressors (1.4-1.6). Matches MPTA and manufacturer recommendations.

Multi-Belt Capacity Sizing

Enter your load horsepower and the calculator determines how many belts are required, accounting for wrap angle correction, length correction factor, and the selected service factor. Prevents undersizing that causes premature belt failure.

Comparison

Belt Type Cross-Section HP Range Speed Range Best Application
Classical A 1/2" top width 0.25-10 HP Up to 6,500 FPM Light-duty fans, small pumps, shop equipment
Classical B 21/32" top width 1-25 HP Up to 6,500 FPM Medium fans, blowers, centrifugal pumps
Classical C 7/8" top width 10-100 HP Up to 6,500 FPM Large fans, compressors, heavy conveyors
Narrow 3V 3/8" top width 0.5-25 HP Up to 10,000 FPM Modern compact drives, high-speed equipment
Narrow 5V 5/8" top width 5-200 HP Up to 10,000 FPM High-capacity industrial drives, large pumps
Synchronous Toothed profile 0.1-500+ HP Up to 16,000 FPM Positive drive, no slip, precise speed control

Frequently Asked Questions

Speed ratio equals the driven sheave pitch diameter divided by the driver sheave pitch diameter. For example, a 4-inch motor sheave driving a 12-inch fan sheave gives a 3:1 ratio. The driven RPM is the motor RPM divided by the ratio, so a 1750 RPM motor produces 583 RPM at the fan. Always use pitch diameters, not outside diameters, for accurate calculations.
The practical maximum speed ratio for a single-stage V-belt drive is about 6:1 to 8:1, though 3:1 is preferred for reliability. Higher ratios cause the belt wrap angle on the small sheave to drop below 120 degrees, dramatically reducing power transmission capacity and belt life. For ratios above 5:1, consider a two-stage drive, a cogged belt, or a synchronous belt system that does not depend on friction.
Pitch diameter is not the outside diameter of the sheave. It is the diameter at which the belt rides in the groove, measured at the belt's pitch line. For V-belts, pitch diameter is typically 0.2-0.5 inches larger than the groove OD depending on belt cross-section. The easiest method is to check the casting number stamped on the sheave and look up the pitch diameter in the manufacturer's catalog.
V-belt slip results from insufficient tension, worn sheave grooves, oil contamination, or overloading. Normal slip is 1-2%; above 3% indicates a problem. Check tension with a belt tension gauge (15-20 lbs deflection force per strand for most industrial belts). Inspect sheave grooves for wear. Worn grooves let the belt ride too deep, losing wedging action. Replace sheaves if the belt sits flush with or below the sheave OD.
Narrow belts (3V, 5V, 8V) transmit 30-50% more power per belt than equivalent classical belts because of their deeper cross-section and better wedging action. Use narrow belts on new installations, high-speed drives (above 3,500 FPM), and anywhere space is limited. Classical belts (A, B, C, D) are still common in legacy equipment. Switching from classical to narrow often lets you reduce belt count while improving drive performance.
Belt pitch length = 2C + 1.57(D+d) + (D-d)^2/(4C), where C is center distance, D is the large sheave pitch diameter, and d is the small sheave pitch diameter. All measurements in inches. The result is the theoretical pitch length. Round to the nearest standard belt length (belts come in 1-inch increments for most cross-sections) and adjust center distance to compensate.
Wrap angle is the arc of contact between the belt and sheave, measured in degrees. On the small sheave, wrap angle decreases as speed ratio increases or center distance decreases. Maximum power transmission occurs at 180 degrees. Below 120 degrees, capacity drops sharply because the belt does not have enough contact area to grip the sheave. For every degree below 180, the effective horsepower capacity is reduced by the wrap correction factor published in belt manufacturer catalogs.
In a well-maintained drive, V-belts typically last 3-5 years or 20,000-30,000 operating hours. Replace belts when you see cracking, glazing, fraying, or when tension cannot be maintained. Always replace all belts in a multi-belt drive as a matched set. Mixing old and new belts causes uneven loading and premature failure of the new belts. Inspect sheaves for groove wear at every belt change.
Disclaimer: This calculator provides engineering estimates based on standard MPTA, RMA, and manufacturer formulas. Actual belt drive performance depends on installation quality, alignment, tensioning, environmental conditions, and equipment loading. Always verify sheave compatibility and belt ratings with the belt manufacturer's engineering data. ToolGrit is not responsible for equipment damage or failure resulting from drive design decisions.

Learn More

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Belt Drive Sizing: Speed Ratios, Belt Length & Sheave Selection

How V-belt drives work, calculating speed ratios, selecting belt cross-sections, determining belt length, and troubleshooting common belt drive problems.

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