Most of the conversion projects are using electric geared motors or servo motors to drive the excavator. The simple one is to bolt the motors on the travel frame and connected to the driving shaft. As the motors are installed outside of the frame, it is easier to be damaged and looking not so pretty. Another option is to install inside motors inside the frame similar to what the CTI-Modellabu’s kit with pulley-belt system.
Before ordering the parts and components, you would require to determine the size of the motors required. Normally, a real excavator travels at max. 5.5 km/h or 90 m/min. With a scale of 16:1, you would require the model to travel at least 5.6 m/min. As the diameter of the driving wheel is approx. 4 cm, each revolution of the wheel is equivalent to 12.5 cm of travel. It will require the motor to have at least 45 rpm to achieve 5.6/min. speed.
Then, we need to decide the power output of the motors. By pulling the model, the friction coefficient is found out to be approx. 25%. With weight of the model and other components added up to be around 2 kg, you would require to have a pulling force of at least 0.5kg in order to enable the model starting to move. To ensure the excavator model traveling at top speed, I added a factor of 100% which turns to be 1.0kg in total or 0.5kg per motor. Therefore, you would require motors with torque of 9.3N.cm.
I decided to adopt the pulley-belt system and taking the above as consideration, the following parts were ordered from Robot Market Place, where you can use your calculator to determine the belt length, gear ratio and pulley sizes:
2 x 30T pulleys
2 x 15T pulleys
2 x 67T belts
2 x 150:1 micro geared motors
2 x motor mounting brackets
Total cost: US$72 + shipping
Robot Market Place website:
Step 1
You would need to cut out spaces at the undercarriage structure to house the motors, pulleys and belts. After cutting out the space, the bracket that allowing the drive shaft becomes weaker and an extra support bracket is required.
Step 2
The original shaft is only an aluminum pin, it is then replaced by a stronger 4mm bolt. The larger pulleys are machining at the edge to reduce the O.D. in order not to interfere with the travel frame and the bore is enlarged to 4mm dia. The pulley is fixed by a set-screw to fix at the shaft and bolt is screwing into the sprocket wheel.
The driving motor is fixed by a T-shaped mounting block of which is bolt on the undercarriage structure. You might need to use shims to adjust the height and position of the mounting bracket so as to achieve the necessary tension of the belt.
Step 3
By putting together the travel frame and tracks, the motors are connected with a power supply of 6V to test the driving speed and other functionality.
During the assembly, it found out that the larger pulley could not fit in the travel motor hub. It is better to a smaller pulley (26T) and shorter belt (64T) in the future. Furthermore, if you can find a slower motor, it is better to order two small pulleys so that you do not need to cut out so much space of the undercarriage frame to accommodate the pulley-belt system.
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