PaMiLoCELL **#08** – Robot Transmission Mechanisms

While assembling the AR4-MK5 and studying industrial robots, I am realizing that robot motion is not only about motors, software and kinematics.

Between the motor and the joint there is a fundamental element:

the mechanical transmission.

Its architecture directly influences torque, stiffness, inertia, backlash, vibration, accuracy, payload and dynamic behaviour.

Main transmission families used in robotics include:

• Gear trains – robust and stiff torque transmission.

• Planetary gearboxes – compact, efficient and with high torque density.

• Cycloidal / RV reducers – high stiffness, low backlash and high load capacity, often suited to the main robot axes.

• Harmonic / Strain-Wave drives – compact, lightweight and with very low backlash, often used where low inertia is important.

• Timing belt & pulley systems – transfer motion between separated shafts and allow more flexibility in motor positioning.

• Direct drive – eliminates intermediate transmission stages, but requires high-torque motors.

A useful distinction is also between robot axes.

J1–J3 mainly position the arm and normally manage higher loads and moments, so torque, stiffness and load capacity are critical.

J4–J6 mainly control wrist orientation, where compactness, low mass and low inertia become increasingly important.

Where does the AR4-MK5 fit?

The AR4-MK5 uses a hybrid mechanical transmission architecture rather than a single transmission technology.

During assembly, several axes clearly show a chain based on:

Stepper Motor → Pulley → Timing Belt → Pulley → Shaft / Joint

combined with couplers, shafts, keys/keyways, bearings and mechanical reduction stages.

So the AR4 can be associated mainly with a:

Timing Belt & Pulley + Mechanical Reduction architecture.

This allows the motor to be positioned away from the joint and the transmission ratio to be adapted to the required motion.

The basic principle is:

Motor: high speed / lower torque → Reduction → Joint: lower speed / higher torque

or, in simplified form:

Tjoint ≈ Tmotor × i × η

where i is the reduction ratio and η the transmission efficiency.

This is also why small assembly details matter.

Incorrect belt tension, coupler tightening, key fit, shaft alignment, bearing preload or backlash can later appear as:

lost motion, vibration, reduced repeatability or positioning errors.

Key takeaway

Same 6-axis architecture does not mean the same mechanical behaviour.

Robot performance depends not only on software and kinematics, but also on what happens mechanically between each motor and joint.

For PaMiLoCELL, assembling the AR4-MK5 is therefore also a practical way to understand how belts, pulleys, shafts, bearings and reduction systems work together to create robot motion.

#PaMiLoCELL #PaMiLoLAB #AR4MK5 #Robotics #IndustrialRobotics #RobotMechanics #MechanicalTransmission #TimingBelt #Gearbox #MotionControl #RobotKinematics #MechanicalEngineering #Automation