Lesson 5 : history of traction drive
Summary of the lesson:
This session focuses on traction drives—a unique, often overlooked alternative to traditional gear-based transmissions in robotics. The lecture covers their principles, mechanical properties, history, advantages, and limitations, serving as a foundational understanding before exploring the Archimedes Drive.
> Part 1: History of Traction Drives
1870s – Early Industrial Origins
First used in automated factory conveyor systems.
Featured discs and adjustable rollers for controlling conveyor belt speed.
Though clever, the application was narrow and remained niche.
1920s–1930s – Automotive Interest
As automobiles grew, gearless solutions were explored to avoid gear backlash and complexity.
Traction drives were studied but found lacking in reliability and performance, so gears remained dominant.
1970s – The Nasvytis Drive
NASA-backed research led to the invention of the Nasvytis Drive.
Designed as a planetary traction system:
Two layers of planetary rollers were used to spread stress.
Incorporated a torque-sensitive preloading system to adjust normal force.
Despite technical novelty, it was too complex and costly for commercial adoption.
Hollow Roller Concepts
Later, engineers explored hollow planetary rollers:
Offered low weight, efficient stress distribution, and very high efficiency.
But presented practical limitations:
Hollow structure made it impossible to run a stable central axis, essential for gear-grounding.
Although theoretically ideal, the design was impractical to implement in most machines
> Part 2: Terminology - FRICTION VS TRACTION, STRESS, METAL FATIGUE
Friction vs. Traction: What’s the Difference?
Friction involves resistance between sliding surfaces (static or kinetic).
Traction refers to rolling resistance created by contact pressure and motor-driven rotation—like a train on steel rails.
Traction aims to avoid kinetic friction and operate purely through rolling, enhancing efficiency and reducing heat/noise losses
Stress and Material Deformation
Definitions and Concepts
Stress = Force ÷ Cross-sectional area.
Causes strain (deformation), which is either:
Elastic (returns to original shape), or
Plastic (permanent change).
Hertzian Contact Stress
Occurs in rolling systems like traction drives or bearings.
Generates deep, localized stress at contact points.
Leads to subsurface microcracks that gradually cause surface pitting and degradation
Metal Fatigue & Failure
Even under elastic stress, repetitive cycles can cause metal fatigue.
Eventually leads to material failure, even without plastic deformation.
In gears, fatigue can cause sudden failure.
In traction drives, degradation is gradual, allowing predictive maintenance and controlled end-of-life scheduling.
> Part 3: Advantages and Disadvantages of a Strain Wave Gears
Advantages:
| Feature | Benefit |
|---|---|
| High efficiency | Due to rolling rather than sliding contact |
| Low noise | No gear mesh noise |
| No backlash | Continuous contact, ideal for precision control |
| High max speed | Less heat, no lubricant breakdown |
| Lightweight (hollow) | Especially with hollow rollers (if feasible) |
| Controlled EOL | Gradual wear makes maintenance predictable |
Disadvantages:
| Limitation | Drawback |
|---|---|
| Low gear ratio | Can’t reduce speed/torque as much as gears |
| Short lifetime | Especially if poorly designed or stressed |
| Complexity | Preloading mechanisms add manufacturing cost |
| No stable axis (hollow) | Limits use in compound gearing systems |
Conclusion:
The Nasvytis Drive and later hollow designs aimed to increase torque and efficiency, but tradeoffs in durability, cost, and mechanical feasibility limited their adoption. These designs remain important theoretical steps that informed the development of newer solutions like the Archimedes Drive.
