Belt Length Acronyms (La, Le, Ld, Lp, Lw and Li)

We’re putting it here as a quick reference.

• L_a: Outer (circumference) length¹ 
• L_e: Effective length² 
• L_d: Datum length³ 
• L_p: Pitch length^{3, 4} 
• L_w: Working length³ 
• L_i: Inner (circumference) length¹ 

Certain conditions: 

¹ No belt stretch / tension applied 

² Length relative to the effective outside diameter of a sheave with belt stretch / tension applied 

³ Length at the pitch diameter of the sheaves used with belt stretch / tension applied 

⁴ Technically defined as “any circumferential line which keeps the same length when the belt is bent perpendicularly to its base” (ISO 1081:2013) 

 Here’s a more detailed explanation of belt lengths, according to the GatesFacts Technical Information Library.

Different length measurements are used due to the complex geometry of belts and their interaction with pulleys:

• Outer/Inner length: The physical circumference of the belt's outer and inner surfaces*
• Effective length: The length around the neutral axis of the belt (where it experiences neither tension nor compression during bending)
• Datum length: A standardised measurement often used for V-belts, close to the effective length*
• Pitch length: For toothed belts, the length of an imaginary line along the pitch circle of the belt (where the teeth are evenly spaced)^

* Commonly used for measuring V-belts

^ Commonly used for measuring timing belts