Fractional horsepower v-belts 4L and 5L (FHP) can be interchanged to the equivalent "Classical" A or B v-belt. You can also use AX or BX notched style v-belts.  Notched v-belts are about 3% more efficient than wrapped style v-belts.

While A and B belts can be an excellent upgrade and substitute for 4L and 5L v-belts, the opposite is not true. 4L and 5L belts have lower horsepower ratings and design life compared to Classical A and B series v-belts. 

Using a 4L or 5L belt in an A or B application will result in reduced performance and belt life.  Upgrading from 4L and 5L v-belts to A/AX or B/BX can improve performance of your belt drive system with increased horsepower capability.

Horsepower Improvement

To demonstrate the improvement of horsepower, let’s compare 5L FHP belts to B Section wrapped belts. FHP belts are typically for low horsepower applications whereas B Section classical belts are for medium horsepower applications. Both belts will fit in the same sheave groove. Using a 5.4" pitch diameter sheave, the 5L belt will transmit 2.39 horsepower.

Using a B section belt on the same 5.4" pitch diameter sheave can transmit 7.21 horsepower, which is three times the horsepower of the 5L belt. The 5L and the B Section belt will both fit in the same sheave, but they provide dramatically different horsepower capacities. 

In addition, FHP v-belts only have about 1/3rd the design life of a Classical v-belt.  Using a BX belt in this example would increase rating to 9.19 horsepower (a gain of 285%) and provide an efficiency gain of about 3%.

* The v-Belt HP ratings are supplied as illustrations. The actual ratings will vary  somewhat depending on the drive ratio and the center distances.

** When upgrading to classical or notched belts, check to make sure the sheaves you are using are appropriate for the higher HP belts. 

Some examples follow: Use only FHP belts in die cast sheaves. Pressed steel sheaves cannot transmit the HP available in notched belts. There are often problems using notched belts in moveable side sheaves.