Power transmission in a direct drive
A direct drive connects the motor directly to the axis to be driven – without a gearbox, belt, or any other mechanical transmission stage. Torque, speed, and motion are thus transmitted directly. The motor speed and load speed are always identical.
The ILM and ILM-E series from TQ-RoboDrive can be used as a direct drive if the available torque, speed, and thermal characteristics match the application’s load profile.
Requirements for a direct drive
The use of a motor without a gearbox requires that it can provide the required output torque directly. For the design of such a direct drive, not only individual characteristic values, but also the complete load and motion profile with the following parameters are relevant:
If the motor torque is not sufficient or if the required motor size results in unacceptable weight, combining it with a gearbox may be more sensible. The choice of drive concept therefore always depends on the interaction of the requirements.
Technical effects of a direct drive
The most important advantage of a direct drive is that gear-related backlash and additional losses due to a gearbox stage can be avoided. Other components such as belts, gear meshing, or additional couplings can also be eliminated in a direct drive, provided they are not required in the respective drive concept.
However, motor and controller losses, bearing friction, and compliance in the mechanical structure remain. The achievable positioning accuracy and dynamics therefore depend on the interaction of motor, bearings, encoder, mechanics, power electronics, control system, and load.
In addition to these mechanical and control-related factors, thermal design also plays a central role.
Exploded view of the HPR40 with Harmonic Pin Ring gearbox
Thermal design
Continuous and holding torques are thermally demanding. Even at zero speed, electrical losses are generated in the winding, although no mechanical power is being delivered. This heat must be dissipated through the stator and the structure.
The thermal behavior therefore depends heavily on the installation situation. The stator’s thermal connection must be taken into account in the design just as much as the duty cycle, ambient temperature, and cooling concept.
Whether a direct drive makes sense cannot therefore be assessed solely on the basis of the required torque. Depending on the application, a gearbox or a quasi-direct drive may also be the more suitable solution.
Direct drive, QDD or gearbox?
A quasi-direct drive (QDD for short) combines a motor with high torque density and a low-ratio gearbox. In this way, the output torque can be increased while dynamics and backdrivability are largely preserved compared with gearbox concepts with higher reduction ratios.
The following overview compares the three concepts:
| Concept | Translation | Typical Advantage | Characteristic Properties | To Consider |
|---|---|---|---|---|
| Direct drive | 1:1 | Direct mechanical coupling without a gear stage | No gear-related backlash and lower mechanical complexity | The motor must provide the full output torque on its own |
| Quasi-direct drive (QDD) | Low | Combines high torque density with low reduction | Can support high dynamics, good backdrivability, and force-controlled motions – suitable for humanoid robotics | Gear losses, hysteresis, and elasticity remain |
| Geared drive | Medium to high | Very high output torques with a smaller, lighter, and faster-running motor | Lower motor-side torque and current requirement; depending on the concept, high gear ratios and holding torques | Friction, backlash, stiffness, and reflected inertia of the gearbox affect the system |
Which concept is suitable depends in particular on the application’s torque, speed, dynamics, installation space, weight, and control behavior.
Typical applications
Direct drives are used, among other things, in the following areas:
Which drive architecture is suitable for a specific application must be evaluated based on the technical and, where applicable, regulatory requirements.
In contrast, for heavily loaded joints in humanoid robots or quadrupeds, a QDD concept is often more suitable, since a pure direct drive would require a motor that is too large or too heavy.
A direct drive connects the motor directly to the driven shaft without a gearbox or other mechanical transmission stages. The motor and load speed are fundamentally identical. This eliminates gear-related backlash and the additional losses of a gear stage. However, the motor must provide the required output torque entirely on its own.
A motor for a direct drive must be able to provide the required continuous, peak, and holding torque without mechanical transmission. In addition, speed, installation space, supply voltage, thermal connection, and load profile must suit the application. Heat dissipation is particularly important, as even at high holding torques and zero speed, electrical power losses occur in the winding.
A quasi-direct drive combines a high-torque motor with a low-ratio gearbox. Compared with a pure direct drive, this can achieve higher output torque. At the same time, dynamics and backdrivability can remain relatively high compared with gear drives with higher reduction ratios. However, the actual characteristics depend on the motor, gearbox, and mechanical design of the overall system.
A direct drive can be useful if the motor can provide the required output torque directly and requirements such as precise controllability, immediate power transmission, or avoiding gearbox-induced backlash are important. If the motor torque is not sufficient or if the motor would be too large and heavy, a QDD or a more heavily geared transmission drive may be more suitable.
A direct drive does without an additional gearbox stage and can avoid gearbox-related backlash, gearbox losses, and additional mechanical transmission elements. This can offer advantages in control behavior, maintenance effort, and mechanical design. Motor losses, bearing friction, and mechanical compliance, however, remain. Therefore, the complete drive system must always be considered.
With a direct drive, the motor must generate the required torque directly. This can result in significant thermal loads, especially at high continuous or holding torques. Even at zero speed, electrical power losses occur in the windings. Heat dissipation through the stator, housing, and cooling concept is therefore an essential part of the design of a direct drive.
