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Pulley Calculator

Mechanical advantage, belt length, speed ratio, and effort force

Mech. Advantage

4:1 MA

Effort

55.56 lbs

Rope/ft

4 ft

Mechanical Advantage

4:1

Effort

55.56 lbs

Actual MA

3.6

Rope/ft lift

4 ft

Details

Ideal Effort50 lbs
Friction Loss10%

Example Calculations

14-segment block and tackle, 200 lbs

Inputs

Load200 lbs
Segments4

Result

Effort55.6 lbs (at 90% eff)

Ideal effort = 200/4 = 50 lbs. At 90% efficiency: 50/0.9 = 55.6 lbs.

2Belt drive 6" to 12" at 1750 RPM

Inputs

Driver6 in
Driven12 in

Result

Output RPM875
Belt Length78.28 in

Speed ratio = 6/12 = 0.5. Output = 1750 × 0.5 = 875 RPM.

Frequently Asked Questions

Q

How do I calculate mechanical advantage of a pulley system?

Count the number of rope segments supporting the load. A single fixed pulley has MA=1 (direction change only). A single movable pulley has MA=2. A block and tackle with 4 supporting segments has MA=4.

  • Fixed pulley: MA = 1
  • Single movable: MA = 2
  • Block and tackle: MA = number of supporting rope segments
  • Actual MA is lower due to friction (typically 85-95% efficient)
SystemMAEffort for 200 lbsRope Pull per ft
Fixed (1 pulley)1:1200 lbs1 ft
Movable (1 pulley)2:1100 lbs2 ft
Compound (4 seg)4:150 lbs4 ft
Compound (6 seg)6:133.3 lbs6 ft
Q

How do I calculate belt length for a two-pulley drive?

Belt length L = pi/2 × (d1+d2) + 2D + (d1-d2)²/(4D), where d1 and d2 are pulley diameters and D is center distance.

  • L = π/2(d1+d2) + 2D + (d1-d2)²/(4D)
  • 6" and 12" pulleys at 24" center ≈ 78.3"
  • Equal pulleys simplify to L = πd + 2D
  • Always round up to next standard belt size
Q

How does a belt drive change speed and torque?

Speed ratio = driver diameter / driven diameter. A 6" driver turning a 12" driven pulley halves the speed but doubles the torque. Torque ratio is the inverse of speed ratio.

  • Speed ratio = d_driver / d_driven
  • 6" driving 12": speed halved, torque doubled
  • 12" driving 6": speed doubled, torque halved
  • Power (HP) stays the same (minus friction)
Q

What is the wrap angle and why does it matter?

Wrap angle is how much of the pulley circumference the belt contacts. Minimum 120° is needed for reliable grip. Larger pulleys or closer centers increase wrap angle.

  • Ideal wrap angle: 180° (equal pulleys)
  • Minimum for reliable drive: 120°
  • V-belts grip better than flat belts
  • Tensioner pulleys can increase wrap angle

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This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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