A New Take On Overtorque In Speed Reducers
In the field of robotic mechanical engineering, the selection of a drive can profoundly influence the system’s performance, efficiency, and longevity. The transmission system plays a pivotal role in ensuring smooth and efficient operations. Starting, nominal, peak, and maximum torque are fundamental concepts that engineers must understand. Here is a simplified explanation for each term:
- Starting torque is the force necessary to put the output in motion from a standstill.
- Nominal (aka continuous rated/ average/ mean) torque is the mean torque exerted by the drive during its operation at the specified nominal speed.
- Peak (aka acceleration/ braking) torque is the force that can be generated for only a short period of time, usually for acceleration or deceleration or to overcome excessive friction.
- Maximum torque is the force above which the drive will endure critical failure.
Overtorquing (also called overload or shock load) is a critical issue that engineers must grapple with, as it can lead to severe damage to the transmission system and disrupt the overall functioning of the robot. This article delves into the concept of overloading, its implications on different types of drives, and how the innovative Archimedes Drive from IMSystems offers a robust solution to this problem.
Understanding Overtorque
Overtorque refers to the condition where the applied torque exceeds the designed limit of the transmission system. This can occur due to sudden load changes, start-up or shutdown operations, extreme wear of mechanical parts, or mechanical errors. Overtorquing is a significant concern as it can lead to catastrophic failures, such as tooth breakage, which can result in costly repairs and downtime.
A study showed that in robot-automated production lines almost half of robot failures were due to positional error, and a quarter were caused by drive failures. On an industry scale that turns into huge amounts of resources being wasted.
General Consequences of Overtorque
Overtorquing can lead to excessive wear and tear in robots, especially the gearboxes of industrial robots, causing them to fail prematurely. This can result in significant downtime in production as the robot needs to be repaired or replaced. Accidental collisions with other robots, humans, or other objects, or unintended driving conditions can cause overtorque situations, which can affect the ability of the robot to function or affect their repeatability. This can have a considerable negative impact on productivity and maintenance costs. Here are some of the negative effects overtorquing poses on the lifetime of mechanic and mechatronic systems:
- Component Damage: Excessive stress on the mechanical components leads to premature wear and tear, deformation, or even complete failure.
- System Overheating: Overtorque often results in increased heat generation. If the system is not designed to handle this excess heat, it can lead to overheating, which can damage electronic components and reduce the lifespan of the system.
- Decreased Performance: Overtorque can cause the system to operate outside of its optimal range, leading to decreased performance. This can manifest as slower response times, reduced accuracy, or lower output quality.
- Increased Energy Consumption: When a system is overtorqued, it often requires more energy to perform the same tasks, leading to increased energy consumption and higher operating costs.
- Safety Risks: In extreme cases, overtorquing can lead to catastrophic system failure, which can pose safety risks, especially in systems that interact directly with people or operate in hazardous environments.
- System Instability: Overtorquing can cause instability in the system, leading to unpredictable or erratic behavior, which can be difficult to diagnose and correct.
Overtorque in Traditional Gearboxes
Strain Wave Gears
Strain wave gears offer high gear ratios, compact size, and excellent positional accuracy. However, they can suffer from backlash and wear over time, leading to decreased performance. When overloaded, they can suffer from excessive wear and tear, increased heat generation and vibrations, reduced efficiency, and a shortened lifespan.
Precision Planetary Drives
Precision planetary drives are known for their high-power density, dynamics, and precision. They offer low backlash, high torsional rigidity, and high efficiency. However, their performance can be affected by high torque loads, leading to potential damage or failure. Overloading can cause damage to the gear teeth such as cracking or breaking, increased heat generation, and reduced efficiency.
Cycloidal Drives
Cycloidal drives offer high speed reduction ratios and high torque capacity in compact sizes. They are efficient and can feature minimal backlash. However, they can generate vibrations if not balanced properly, leading to increased wear on the exterior teeth of the cycloidal disk and the component bearings. Overtorquing scenarios can lead to damage of the components, increased heat and imprecisions, reduced efficiency, and a shorter lifespan.
How to Prevent Overtorque Damage?
Several methods can be employed to prevent critical damage, such as using torque limiters, installing overload clutches, or implementing electronic control systems. However, each of these methods has its drawbacks. Torque limiters and overload clutches can be expensive and require regular maintenance, while electronic control systems may not respond quickly enough to prevent damage in sudden overtorque situations.
The Archimedes Drive: Taking a Load Off
The Archimedes Drive offers a unique combination of high performance, precision, low noise, and efficiency, making it ideal for factory automation equipment. Its intrinsic mechanical transparency results in smooth controllability, opening new design opportunities for a multitude of applications. Moreover, its unique overtorque protection ensures that the drive won’t suffer from critical damage when overloaded, increasing the drive’s (and thus the whole system’s) lifespan and reducing maintenance costs.
Unlike conventional drives, the Archimedes Drive uses a unique traction mechanism that allows it to handle momentary high torque loads without suffering critical damage. The drive’s design ensures that the load is evenly distributed, eliminating the risk of tooth breakage or deformation.
Microslip in the Archimedes Drive
The Archimedes Drive has an inherent fail-safe mechanism: the presence of microslip. Microslip refers to the slight slippage that occurs between the contact surfaces of the drive components (input annulus, compound traction rollers, and output annulus) during operation. Traditionally, microslip is not desired as it causes imprecisions, but in this case it allows the Archimedes Drive to act as a brake and not suffer critical failure when overloaded. Because of this slip characteristic the drive needs an output encoder to ensure the traction rollers do not drift, but the combination of the constant tractive contact and the use of encoders provides extreme precision by fully eliminating backlash as well as the risk of critical failure. Through allowing a small amount of controlled slippage, the drive can absorb and dissipate the excess force to the drive components. This unique feature of the Archimedes Drive makes it highly resilient and reliable in overload or shock load situations.
Unlike conventional teethed gearboxes, with their risk of tooth-fractures, the toothless Archimedes Drive restrains freewheeling of the output during failures. This holds for both the expected fatigue failure for normal operation and its resilience to overloads.
The failure mode for normal operation is subsurface-stress-initiated fatigue, very similar to for instance the failure mode of bearings. Typically, this can be detected by a degraded performance in a variety of easily measurable data sources. In case of overtorque the drive will fail to a “jammed” condition, rather than a “freewheel” condition. This is an important consideration for safety cases. In the event of a failure leading to a “jammed” condition, the drive will immediately begin acting as a friction brake. The drive will apply a constant peak (or braking) torque equal to Tslip – the torque at which full slip occurs.
In case the drive is heavily overloaded beyond its rated nominal torque, it will enter the full-sliding regime of the traction curve. Yet, it still delivers its maximum slip torque. The drive operates in a braking fashion as it inhibits the motion of the output and dissipates power. As long as the overload peak is relatively short, no permanent damage is to be expected. Provided that the lubrication layer is not so disrupted as to allow for local heating to the point where friction welding occurs, the drive should be able to resume operation with no technical deficit.
Below you can see a demonstration of our DELTA-15 Archimedes Drive being overtorqued. Once this happens, the drive acts as a friction brake, and when the extra load is removed it recovers its normal operating regime.
Benefits of the Archimedes Drive
The Archimedes Drive offers several benefits, particularly in overtorque situations. Its innovative design allows it to handle high torque loads efficiently, reducing the risk of damage and extending the drive’s lifespan. The concept of microslip not only prevents critical failure during overload situations but also contributes to the drive’s overall efficiency and performance. Furthermore, the Archimedes Drive offers higher precision and efficiency compared to conventional drives, making it an ideal choice for industrial robotics.
Another important thing to note is that most drives or gearboxes are designed to withstand a peak torque many times greater than the nominal torque, to account for possible cases of overloading. This makes the drives often unnecessarily bulky. This is not the case for the Archimedes Drive, since there is no need for high amounts of safety maximum torque. As an example, our DELTA-250 model was designed with a peak torque of 250 Nm and a maximum torque of around 270 Nm, making for a better drive size and weight. This has consequences for the whole system, allowing for a lighter design. You are then able to also incorporate lighter motors and less complex safety features.
Conclusion
In conclusion, the Archimedes Drive offers a unique solution to overtorque situations, outperforming other drives in terms of performance, efficiency, and longevity. For robotic mechanical engineers seeking a reliable, efficient, and durable drive, the Archimedes Drive is an excellent choice. Its intrinsic overtorque protection not only enhances the safety of manufacturing environments, but also reduces potential downtime in production, ultimately extending the lifespan and reducing the energy consumption of industrial robots.
