V-Belt Size Table

Because power capacity scales with cross-sectional area and wedge pressure, not width alone. SPZ (10 x 8 mm, 34 degrees) carries roughly half again the power of Z (10 x 6 mm, 40 degrees) on the same pitch diameter: the extra 2 mm of height adds tensile cord capacity, and the tighter wedge jams deeper into the groove for more normal force per unit of tension. Machine designers exploited this to shrink drives - three narrow belts in the space of four classic ones - and that packaging advantage, not any change in belt chemistry, is what made narrow profiles the post-1980 default. For the reader the practical consequence is in the table: identical width columns with different height and angle columns are the fingerprint of two genuinely different drive families, and the pulleys are not interchangeable even when the widths match on a ruler.

Nominally yes, historically no. The A section was standardized at exactly 1/2 inch = 12.7 mm; the 13 mm figure is the rounded metric name adopted when the profiles went international. The same story runs down the classic table: B is named 17 mm but was cut at 21/32 in (16.5 mm), C named 22 mm but really 7/8 in (22.2 mm), D named 32 but cut 1-1/4 in (31.75), E named 38 but really 1-1/2 in (38.1). For a buyer this is trivia with one sharp edge: an old imperial-market pulley groove was cut to the inch dimension, and a belt that is genuinely 13 mm (rather than 12.7) can sit slightly proud or tight in it. Measuring with calipers and buying to the measured millimeters - not to the letter alone - is the safe habit when mixing old imperial hardware with modern metric-named belts.

The belt's 40-degree wedge is its free-standing shape; bending it around a pulley compresses the inside of the section and squeezes the wedge tighter toward the pulley's axis of rotation. Groove angles are therefore cut smaller than the belt angle - commonly 38, 36 or 34 degrees depending on pulley diameter - so that the bent belt bottoms its flanks fully in the groove instead of touching only at the outer edges. The remark column in the classic table records exactly this for the A section: 38-degree pulley angle in imperial practice. This is also why the narrow family dropped to a 34-degree belt wedge: narrower, taller sections bend more sharply around the same minimum sheave diameters, so the whole geometry shifts down a few degrees to keep flank contact. Match belt family to groove family and the angles take care of themselves.

Not on the same pulleys - same width, different world. The groove for an SPZ is cut for an 8 mm tall, 34-degree section; a Z (6 mm, 40 degrees) in that groove sinks too deep, wedges weakly at the wrong angle and slips under load, while the reverse swap wedges an oversized section at the groove's rim and burns the belt crown. Length designations also shift between families. What you can do one-for-one is upgrade inside the narrow family: XPZ for SPZ, XPA for SPA - the XP (high-performance) grades keep the identical cross-section in the table and add a cogged bottom and stronger cords, which run cooler on small diameters and transmit more power. So the migration path from classic to narrow is a sheave-set replacement done once, and after that, performance upgrades stay within the letters.