April 06, 2016

Stronger metals will make lighter and better harmonic gears for improved robotic arms and drives for demanding applications light space vehicles

Several new series of Low Weight Harmonic Drive gear units have been introduced, using lightweight housing materials and an optimized design. These gear units, based upon existing product lines, maintain the same torque capacity with up to 30% lower weight. Additionally, a new small Harmonic Drive hollow shaft actuator has been developed, expanding the size range of the SHA Series of brushless actuators.

New stronger metallic glass steel (three times higher elastic limit than tungsten carbide) should enable even stronger and lighter and higher performing harmonic drive gear units.

Unlike other gear technologies, harmonic gears have the same size, weight, and form factor regardless of gear ratio. This feature allows design flexibility or revisions without the need to redesign the entire mechanism. Reduction ratios of 30:1 through 160:1 are commonly available.

The torque density of Harmonic Drive products is exceptionally high. This is a significant benefit when the gears are used on a robotic arm. The low weight of the gear reduces both the mass and inertia of the arm, allowing faster positioning and/or higher payloads. This benefit has a cascading effect when moving from the sixth axis back toward the first axis, reducing the torque required merely to support or accelerate the mass of the gearing.

Finally, because of the operating principle that virtually eliminates tooth wear, Harmonic Drive gears provide zero backlash for life and have a long, maintenance-free life, reducing costly downtime.

Stronger and lighter robotic arms

Articulated robots are frequently used for welding, manufacturing, and packaging. Harmonic Drive gears reduce inertia of the robot arm to improve dynamic performance through faster acceleration and faster settling time. They also increase payload capacity and reduce power requirements.

Mobile robots have the capability to move around in their environment and are not fixed to one physical location. Because they are powered by a battery, compact size and light weight are often the primary design considerations for any component used in them. When used in the arm of a mobile robot, such as for an explosive ordinance disposal (EOD) robot, harmonic gearing allows the payload capacity of the arm to be increased.

The low weight of the Harmonic Drive LLC gears enables the overall weight of the prosthetic limb to be similar to that of a human limb. This is critical for the comfort, usability, and battery life of the prosthetic.

Lightgears for space rovers and solid lubricants for vastly reduced consumable lubrication

There has been work to apply solid lubricants to Harmonic Drive® gears. Based on these trials it was found that the gears can be operated even at -269°C. Although being used in various cryogenic applications, the reachable lifetime is comparably short. So as to improve the achievable endurance an essential development was necessary. Hence the EU – funded project HarmLES was executed in order to significantly increase the accessible lifetime. Following an integrated approach covering gear design, materials and coating, the prototype of a new Harmonic Drive® gear type was developed.

Harmonic Drive® gears are used for more than four decades for space applications. In fact, this gear principle was originally developed for space applications. This does cover both planetary exploration and satellites. Based on the application, a large variety of configurations has been developed over time, following as well the product improvements achieved. Today, gears from size 5 (12.7 mm pitch circle diameter, PCD) to size 100 (254 mm PCD) are available in gear ratios ranging from 30:1 to 320:1 and in different types such as cup type, flat type, large hollow shaft type and lightweight versions.

the HarmLES project developed new the geometry for the Harmonic - Drive® gear. It is optimized towards solid lubrication for space applications. This resulted in a new gear type called ZirconLine of size 20 with ratio 100. Throughout the project, measures were step – by step introduced to the gear, decreasing as well the contact stresses and the sliding path especially within the toothing. Finally the prototype of a gear size 20 ratio 100 with a new tooth profile was developed.

Besides the re-design also a new coating could be developed by TECNALIA. This is a composite coating based on a WC-interlayer plus a solid lubricant top-layer being a reinforced MoS2. During development on basis of simple discs, an optimum variant could be achieved, which showed superior lifetime compared to standard MoS2 coatings on the steels being relevant to the Harmonic - Drive® gear. The coating process was adopted to gear components (toothing and WG-bearing). Finally, several gear sets were coated and tested. For the final gears, the characteristics of the gear were in line with the above mentioned requirements. This result could be approved by the visual appearance and microscopic investigation by SEM. No wear was visible, therefore the test could be evaluated to be passed.

HarmLES enabled within its duration to increase the achievable endurance of the gears, whereas the characteristics of the prototypes were in line with the above mentioned requirements:
• at 4Nm lifetimes of even up to 20.000 output revolutions were achieved in vacuum testing (partly without failure at end of test, compared to a few hundreds at begin of the project).
• in contrast to grease lubricated HDs, the efficiency course of the gear during the vacuum endurance test was almost stable throughout the whole test

The HarmLES project has been successful.

One major part in satellite costs are the costs for launch. Typical price range is 10.000 to 15.000 €/kg. That’s why mass decrease is one of the main design drivers in space. Using Harmonic Drive® gears instead of planetary gears enable mass decreases by factors of 2 to 3. Assuming a medium harmonic gear box with a mass of 1 kg, the cost reduction is already in a range of 10.000€ to 20.000€ per unit. Consider new planetary exploration missions with a rover for low temperatures (jovian moons) or high temperatures (mercury). A rover similar to the mars rover would use 9 Harmonic Drive® gears, this would save in total 9kg, i.e. more than 90.000€ launch costs.

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