High Torque 12VDC Planetary Gear Motor for Electronic Sporting Equipment

Overview

A well-known designer and manufacturer of hunting equipment was looking to design an auto-cocking crossbow. Their intentions were to eliminate the hassle of cocking the crossbow by hand between each shot by utilization of an electric micro motor. Overall this feature would ease the process and make it a much safer operation by limiting the risk of injury while still allowing users to enjoy the activity.

At the beginning of this project, we worked directly with the design team to determine a suitable electric motor solution that would met both their physical and electrical requirements.

Challenge

The customer needed a compact gear motor with enough speed and torque capable of cocking a crossbow that was also efficient enough to run off a LiPo battery pack providing the gear motor with a rated voltage of 12VDC. The amount of force required to cock the bow was quite high, and the total space available made this a real challenge. Additionally, the battery was limited in size so power had to be utilized efficiently.

The last challenge we overcame was how to effectively deliver the power from our motor to the cocking mechanism while also streamlining the manufacturability of the crossbow.

Solution

High load speed and torque in a compact size was the overall goal for this project. Our solution was a ø38mm planetary gear box paired with a brush dc motor that was robust enough to meet the torque, voltage, current, and speed requirements of the application. This 12 volt gear motor has a peak output power of more than 300W, which is rather substantial for a motor of this size. When mated with our high precision planetary gearbox this planetary gear motor solution was more than capable of accomplishing the task in the required amount of time. At the gear motors peak efficiency, it provides over 2.5Nm of torque output at 252 rpm.

The LiPo battery pack selected had to be compact enough to fit within the confines of the crossbow, so available load current was limited. Our gear motor was optimized so that the crossbow could be used a minimum of ten times per battery charge.

The design team decided on a custom multi-spline output shaft. This allowed for a quick installation process because the output shaft could not be turned by hand to align with the internal gearing mechanism. The multi-spline design provided many contact points for operation once installed in order to limit any slippage. We tooled for the custom output shaft then manufactured it in stainless steel.

In addition to our engineering motor design and manufacturing expertise, our logistics team handles the delivery coordination of completed gear motors with the customers’ contract manufacturer.