How to Calculate the Suitable Gear Ratio for Planetary Gearboxes?

The gear ratio, also known as the transmission ratio of a gear mechanism, refers to the ratio between the input speed and the output speed in the gear mechanism. It is commonly denoted by the symbol "i." The formula for the gear ratio is as follows:

Gear Ratio (i) = Motor Output Speed / Gearbox Output Shaft Speed

 

Example 1:

If the motor output speed is 2000 rpm and the output speed of the planetary gear reducer (final operating speed) is 100 rpm, then the gear ratio is i = 20:1.

 

Recommendation:

It is recommended to calculate the gear ratio based on the actual highest operating speed of both the motor output and the output end of the planetary gear mechanism. This is because if a lower final speed is required, it can be achieved by adjusting the motor speed. The explanation is as follows:

 

Example 2:

If the motor output speed is 2000 rpm and the planetary gear reducer output speed (final operating speed) is 120 rpm, then the calculated gear ratio for the planetary gear reducer is i = 16.6667:1.

GearKo produces planetary gearboxes with approximate gear ratios of 15:1 and 20:1. You can choose the smaller gear ratio of 15:1. The reason for choosing a smaller gear ratio is explained below.

 

Explanation 1: Choosing 15:1

Gearbox Output Shaft Speed = Motor Output Speed / Gear Ratio (i)

Gearbox Output Shaft Speed = 2000 / 15 = 133.333 rpm

At this point, simply lower the motor speed to 1800 rpm to achieve the desired final speed of 120 rpm.

Gearbox Output Shaft Speed = 1800 / 15 = 120 rpm

 

Explanation 2: Choosing 20:1

Gearbox Output Shaft Speed = 2000 / 20 = 100 rpm

At this point, the motor speed would need to be increased to 2400 rpm to achieve the desired final speed of 120 rpm.

Gearbox Output Shaft Speed = 2400 / 20 = 120 rpm

 

In summary, if the rated speed of the motor is 2000 rpm, choosing a gear ratio of 20:1 would require the motor to be frequently operated at an overspeed, which can shorten its lifespan. This explains why, after calculation, the desired gear ratio specification from GearKo gearboxes may not be achievable, leading to a selection of a gear ratio that is close but slightly smaller.

 

Understanding the working principle and calculation method of planetary gear ratio can help you choose the appropriate gear ratio for your machinery and maximize its efficiency. If you have any requirements or inquiries regarding the calculation of gear ratios for planetary gearboxes, please feel free to contact us.