How It Works

Belt Drive Power Transmission

Belt drives transmit power through friction between the belt and pulleys. The key principles:

V-Belt - Wedge shape creates additional friction through wedging action. Most common for industrial use. Efficiency 95-98%.
Flat Belt - Simple and quiet, good for high-speed applications. Requires more tension for same power.
Timing/Synchronous Belt - Positive engagement via teeth, no slip. Used where precise speed ratio is critical.

Speed Ratio: Inversely proportional to pulley diameters. A 2:1 ratio (driven twice driver diameter) halves the speed and doubles the torque.

Belt Tension and Power Transmission

Power is transmitted through the tension difference between tight side (T1) and slack side (T2):

P = (T1 - T2) x v
The ratio T1/T2 depends on friction coefficient and wrap angle. For V-belts, the wedging action increases effective friction by 3-5x.

Service Factors for Applications

Service factors account for load characteristics beyond steady-state operation:

1.0-1.2: Uniform load (fans, centrifugal pumps)
1.2-1.4: Light shock (compressors, conveyors)
1.4-1.6: Moderate shock (positive pumps, mixers)
1.6-2.0: Heavy shock (crushers, mills, presses)

Belt Slip and Efficiency

Creep (1-2%)

Normal elastic stretching as belt moves from slack to tight side. Unavoidable in friction drives.

Slip (>2%)

Indicates insufficient tension or overload. Causes rapid belt wear and heat buildup.

Efficiency

V-belts: 95-98%. Flat belts: 96-98%. Timing belts: 98-99% (no slip).

Quick Select - Common Applications

Belt Drive Calculator

Calculate belt length, speed ratio, belt velocity, wrap angle, and tension for V-belt and flat belt power transmission systems.

Belt Type

V-Belt Section

1/2" top

0.5-10 HP

21/32" top

1-25 HP

7/8" top

5-100 HP

1-1/4" top

20-500 HP

1-1/2" top

50-1000 HP

Driver Sheave Diameter (D1)

Driven Sheave Diameter (D2)

Diameter Unit

Center Distance (C)

Unit

Driver Speed (RPM)

Transmitted Power

Power Unit

Service Factor

Number of Belts

Calculate the required center distance for a given belt length, or recommended center distance range.

Driver Sheave (D1)

Driven Sheave (D2)

Diameter Unit

Known Belt Length (optional)

Belt Length Unit

Recommended Min (D1+D2) --

Recommended Max (2x(D1+D2)) --

Calculated (from belt length) --

Calculate exact belt length and find the nearest standard belt size.

Driver Sheave (D1)

Driven Sheave (D2)

Center Distance

Unit

Exact Belt Length --

Nearest Standard (inches) --

Adjusted Center Distance --

Belt Length Formula

L = 2C + pi(D1+D2)/2 + (D2-D1)^2/(4C)

L = belt pitch length, C = center distance, D1 = driver diameter, D2 = driven diameter

✓

Design Acceptable

Within normal parameters

Results

-- mm

Belt Length

Belt Tension Indicator

Tight Side (T1):

Slack Side (T2):

Effective (T1-T2):

Speed Ratio --

Driven Speed --

Belt Velocity --

Wrap Angle (Small Pulley) --

Output Torque --

Design Power (with SF) --

Power per Belt --

Recommended Section --

Estimated Efficiency --

V-Belt Section Database

Section	Top Width	Depth	HP/Belt @1750 RPM	Min Sheave
A	1/2" (13mm)	5/16" (8mm)	0.5-3 HP	3.0"
B	21/32" (17mm)	13/32" (11mm)	1-8 HP	5.4"
C	7/8" (22mm)	17/32" (14mm)	3-20 HP	9.0"
D	1-1/4" (32mm)	3/4" (19mm)	10-75 HP	13.0"
E	1-1/2" (38mm)	29/32" (23mm)	25-200 HP	21.6"

Power ratings are approximate for 150mm driver sheave at 1750 RPM. Actual ratings vary with sheave size and speed.

Design Guidelines

Parameter	Recommended	Notes
Wrap angle	> 120 deg	< 120 deg requires correction
Belt velocity	15-25 m/s	Max 30 m/s for standard belts
Speed ratio	< 6:1	Use two stages for > 6:1
Center distance	D1+D2 to 2(D1+D2)	Longer = smoother operation
Belt tension	Deflection method	1/64" per inch of span