ARCO Pulleys

The outer diameter (OD) of a pulley directly affects the rotational speed (RPM) of the alternator or accessory it drives. Here's how it works:

Basic Relationship:

• Larger OD pulley → slower alternator RPM
• Smaller OD pulley → faster alternator RPM

This is because the pulley acts as a gear: the engine crankshaft turns the belt, and the belt turns the alternator pulley. The ratio between the crankshaft pulley and the alternator pulley determines how fast the alternator spins.

Example:

If your engine crank pulley is 150 mm and your alternator pulley is:

• 50 mm → alternator spins 3× faster than the crank
• 75 mm → alternator spins 2× faster

⚠️ Why It Matters:

• Too small a pulley: Alternator may overspeed at high engine RPMs, risking damage.
• Too large a pulley: Alternator may undercharge at idle or low RPMs.

Here’s how different alternator pulley outer diameters (ODs) affect alternator RPM, assuming a crankshaft pulley OD of 150 mm and engine speed of 3000 RPM: