Lesson 1 : What is a Mechanical Drive?
Summary of the lesson:
Mechanical drives play a crucial role in various industrial and robotic applications, transforming high-speed, low-torque motion into the opposite: slow, high-torque movement. In this article, we’ll explore what mechanical drives do, clarify key terminologies, and discuss the different types of drives commonly found in robotics. We’ll also touch on the difference between drives used in combustion engines versus those in electric motors.
> Part 1: What Does a Drive Do?
At its core, a mechanical drive—or speed reducer—takes a motor’s high-speed, low-torque output and transforms it into the necessary torque for heavier loads at slower speeds.
For example, imagine a motor spinning at 100 meters per second but generating only 1 Newton meter (Nm) of torque. This may be sufficient for light tasks, but what if a large robot needs 100 Nm of torque to move its joints? That’s where the drive comes in. The drive converts the 1 Nm of torque from the motor into the 100 Nm required for the robot, allowing it to carry out heavy tasks.
However, there’s a trade-off: the increase in torque results in a decrease in speed. If the motor produces 100 rotations per second, the drive will slow down to one rotation per second, but with much greater force.
In essence, a drive adjusts the relationship between speed and torque to meet the demands of different applications.
> Part 2: Terminology - Drive, Speed Reducer, Gearbox
In many contexts, the terms “drive,” “speed reducer,” and “gearbox” are used interchangeably. They all refer to devices that adjust torque and speed.
While you might hear the term “gearbox” often in the mechanical world, you’ll hear it much less when talking about Archimedes Drives. These drives don’t use traditional gears at all, making them distinct from conventional gearboxes. Instead, they employ a traction-based system that eliminates issues like backlash and enhances precision.
> Part 3: Terminology - Actuator vs. Transmission System
The terms “actuator” and “transmission system” are used differently depending on the industry or region, and their definitions can vary. In robotics, particularly in Europe and North America, an actuator typically refers to the components that initiate motion, including the motor and the drive. Sometimes, an encoder is also included to track positioning.
In cases where direct-drive applications are used (i.e., when little torque is required), the drive can be skipped altogether, and the motor alone functions as the actuator.
A transmission system, by contrast, includes everything involved in moving a robot’s components after the motor. The drive is part of this system but also encompasses any belts, pulleys, or joints involved in transferring movement to the robot’s end-effector—the part that performs the actual task. For example, in a robot with a belt after the drive, that belt is considered part of the transmission system.
> Part 4: Different Types of Drives
There are three main types of mechanical drives commonly used in the robotics industry:
Cycloidal Drive: This drive consists of two discs that rotate around each other. We will discuss this type in more detail in the next class, but it’s known for handling high torque with minimal backlash.
Strain Wave Gear (Harmonic Drive): This drive uses a flexible cup called a flex spline that moves within the system. Some people refer to it as a harmonic drive, after the company that invented it. It is widely used in precision robotics.
Planetary Drive: This drive is named for its resemblance to planets orbiting around the sun. The planetary drive offers a high torque-to-weight ratio and is commonly found in various industrial applications.
We will explore these drives in detail in upcoming sections of this series.
> Part 5: Combustion vs. Electrical Motors
Mechanical drives are used in both combustion and electric motors, but their functions differ based on the type of motor.
For combustion engines (like those fueled by gasoline), torque output is limited to a narrow speed range. When driving a car, for example, you shift gears to keep the engine operating within its optimal range. This helps to maintain high torque at different speeds. If the engine reaches a point where torque decreases, you shift to a higher gear to find a new torque peak at a higher speed.
In contrast, electric motors don’t have this limitation. They can deliver constant torque across a wide range of speeds. This makes electric motors more efficient for many applications, as there’s no need to shift gears. For electric motors, a single drive with a fixed gear ratio is typically used, fine-tuned to the motor’s specifications.
