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Approximate horsepower ratings for selected sizes (number of teeth) of helical gears operating at various speeds (RPM) are provided for hardened steel gears on the horsepower and torque charts. (A sample chart is shown in Figure below)

The horsepower ratings are based on the beam strength of the gear teeth. These ratings are for parallel shaft applications under normal operating conditions (defined as smooth load, “shockless” operations for 8-10 hours per day where gears are properly mounted and lubricated). Ratings for gear sizes and speeds not listed in your catalog may be estimated from the values indicated.
Note: Ratings for bronze gears are approximately 33% of the values indicated for hardened steel.

Helical Gears Operating on Parallel Shafts
Below step how to select the right helical gears when the gears are operated on parallel shafts. Let’s walk through the selection process using the following variables:
• Shafts = Parallel
• Ratio = 3:1
• Speed = 1,800 RPM, pinion
• Horsepower required = 5
• Center distance = 4”
• Hand, pinion = Right hand
• Hand, gear = Left hand

Step 1
Find the pitch diameter (PD) of the pinion using the following formula:
• PD Pinion = 2 x CD (center distance) ÷ Ratio + 1
• PD Pinion = 2 x 4 ÷ 3 +1
• PD Pinion = 2 inches
Find the pitch diameter (PD) of the gear using the following formula:
• PD Gear = PD Pinion x Ratio
• PD Gear = 2 x 3
• PD Gear = 6 inches

Step 2
Referring to the horsepower ratings (RPM) in your Gear catalog, look down the column labeled “1800” until you find a 2-inch pitch diameter gear with a rating of 5 – or more – horsepower.
If you have followed along correctly, it appears as though a 10-pitch, 20-tooth gear (H1020) will be capable of carrying this horsepower. Upon further checking, however, you will find that there is no stock helical gear with 60 teeth available to complete the drive.

Accordingly, the next gear with a 2-inch pitch diameter capable of carrying your load is the 8-pitch, 16-tooth gear (HS816R). Given that there is a 48-tooth gear available from stock (HS848L), these gears are the ones to use to meet the specifications set forth in our example.