SELECTING THE RIGHT GEAR DRIVE FOR THE APPLICATION
Horsepower, torque and duty cycle (operating conditions) are three of the most important variables to consider when helping a user select the correct gear drive(s). In addition, there are two other important variables – center distance and ratio – that you will need to know in order to meet speed (rpm) requirements and space limitations.
When you know the five variables listed above – horsepower, torque, duty cycle, center distance and ratio – you can select the right spur gears for any application using a three-step process. Let’s walk through that process using the following variables:
• Center distance = 3"
• Ratio required = 3:1
• Horsepower required = 5.5
• Velocity of pinion = 1,800 rpm
• Velocity of gear = 600 rpm
• Service factor = 1.25
Step 1
Find the pitch diameter (PD) of the pinion and gear (assuming the center distance and ratio are fixed) using the following formulas:
PD of pinion = 2 x center distance ÷ ratio + 1
PD of gear = PD of pinion x ratio
Now let’s insert the figures from our sample set of variables and do the math:
PD of pinion = (2 x 3") ÷ (3 + 1) = 6 ÷ 4 or 1.5
PD of pinion = 1.5"
Now that we know the PD of the pinion (1.5) and the required ratio (3:1), we can figure the PD of the gear.
PD of gear = 1.5" x 3 or 4.5"
Step 2
Multiply the required horsepower by the service factor to determine the horsepower rating for the pinion and gear (making sure to check the horsepower rating sheets in the appropriate Gear catalog). Select the pinion and gear according to these known specifications.
Required horsepower = 5.5
Service factor = 1.25
5.5 x 1.25 = 6.88, therefore:
Horsepower rating for pinion = 6.88 at 1800 rpm
Horsepower rating for gear = 6.88 at 600 rpm
Step 3
Check the horsepower ratings of both the pinion and gear selected against the ratings in the appropriate Gear catalogs. Using the horsepower calculations for the pinion and gear (as determined in Step 2), select Gear stock pinion and gear that should be used for this application of the Gears catalog.
