The slewing turntable allows the upper structure to rotate freely. In a real machine, you would have a large ring slewing gear with roller bearings, rotated by a hydraulic slewing motor and distributor to provide hydraulic oil to drive the travel motors.
In most of the conversion projects, metal ring bearing is used to provide a smooth rotation. Whereas, a large ring gear is bolted on the frame and driven by a motor with a pinion gear. The rotation speed is determined by the rotation speed of the motor and gear ration between the ring and pinion gears. In a real machine, the slewing speed is approx. 10rpm. With a gear ration of 10 times, you would require a motor with speed of 100rpm.
In scale models, some ready-to-play RC models only allowed rotating of 360 or 720 degree. The reason is the wires providing electric power to the travel motors will twist and strangle together after turning a few times. To solve this issue, you can use a mini slip ring which is generally used in the security CCTV to provide continuous power supply. It comes with 6-12-24 wires with 2A rated current. You can easily purchase it in the eBay but it is normally for wholesales as the shipping charge is not worthwhile to purchase only one unit. In addition, you can purchase through a number of suppliers such as Robotshop and Adafruit at US$17.5 + shipping.
It later turned out to be missing during the shipment and another one was ordered causing delay of the project for 2 weeks.
Adafruit: https://www.adafruit.com/products/736
For the slewing gear and bearing, after searching the internet for suitable components and ways of modification, I decided to purchase a VEX turntable bearing kit with motor. VEX (Vexrobotics) is famous for supplying robot components for leisure, education and competition. Their products are pricey but well accepted by the robotic fields.
The turntable and motor are ordered through Robot Market Place together with other components. The turntable is prices at US$19.99 + shipping and the motor at US$12.99 + shipping. The pack comes with two turntables, one 4.25” dia. and one 2.6” dia. The larger one has provision to install the motor so that we do not need to worry about the alignment of the motor and the gear.
The larger turntable proves to be too large and the smaller one is a good fit. The turntable, driving motor and gear are fitted on a single PCB to provide the necessary alignment and rigidity. It was then connected to power and found out to be running smoothly. According to the gear ratio, the maximum slewing is to be 20rpm.
Parts used are:
1 x VEX turntable (60T) US$8.00 (used only the smaller one)
1 x VEX motor type 269 US$12.99
1 x 12T pinion gear US$1.66
1 x bearing flat US$0.50
1 x shaft collar US$0.65
1 x 2” square shaft US$0.60
4 x 10mm stand-off US$4.00
1 x mini slip ring US$17.50
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