Final Report Summary - HARMLES (Dry lubricated Harmonic Drives for space applications)
Executive Summary:
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.
Harmonic Drive® gears are based on the same, unusual principle of a flexible element used to transmit torque. That offers the following advantages: they provide low or zero backlash in combination with excellent precision. The large gear ratio in combination with the ability of zero stick-slip movement at slow speeds makes them an often used choice for space applications: a small actuator motor (meaning low mass and low power consumption) may be used to move large components like antennas or solar panels.
A specification for such a gear was setup on basis of a questionnaire to European end-users. The transmission accuracy shall remain below 60arcsecs as specified for terrestrial gears. The repeatability is specified with a maximum of 3arcsecs. The minimum gear stiffness is foreseen to be at a level of approx. 1.1*104 Nm/rad which is close to the stiffness of a HFUC gear size 20. Beside quantitative properties, a main qualitative feature of the Harmonic - Drive® gear is seen in the zero backlash, which is a decisive advantage compared to other gear types. The lifetime was required with 17,000 output revolutions at an output torque of 4Nm.
Within harmLES, the geometry of the Harmonic - Drive® gear could be optimised 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 !
Project Context and Objectives:
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.
Harmonic Drive® gears are based on the same, unusual principle of a flexible element used to transmit torque. That offers the following advantages: they provide low or zero backlash in combination with excellent precision. The large gear ratio in combination with the ability of zero stick-slip movement at slow speeds makes them an often used choice for space applications: a small actuator motor (meaning low mass and low power consumption) may be used to move large components like antennas or solar panels.
The main drawback for the use of Harmonic Drive® gears is that they presently need liquid lubricants (including greases) in order to satisfy the operational requirements. Due to vacuum and temperature in space, liquid lubricants and greases bear the risk of outgassing and evaporation. This leads on one hand to loss of lubrication efficiency (some components are lost in the lubricant), but on the other hand to depositions of these outgassed products on other sensible surfaces. Thereby, optical components might be degraded. This may even lead to loss of a full mission, e.g in earth observation.
Since 2002, there has grown a demand for dry lubricated gears due to various needs. These cover extended temperature ranges and applications with critical items close to the gear like optics or telescopes. Harmonic Drive AG has increased its effort in supporting space applications by installing a dedicated team of engineers for these demanding applications. This was based on the need defined by the customers to receive a broader and more profound support and the necessity to adapt the products more to the specific applications. Over the years, this did allow to pool a lot of application experience but being as well faced with more demanding requirements. In the last years, the need for providing more and more customisation and support did increase and the number of inquiries based on the request of using dry lubrication became more apparent. However, no sufficient solutions could be given on durability and suitability of such a dry lubrication scheme and the customers were left alone with the qualification risk.
The project is application triggered. The members of the consortium – manufacturer of Harmonic Drive® gears, developer of coatings and a material developer having a contract as “space materials testhouse” – are on one hand closely related to the space market and its applications. They know also the related requirements for use in space, but act also in non-space markets. This enables “spin-in”, i.e. bringing in of concepts from other fields.
The main research need follows the limited success to use “commercial off the shelf (COTS)” coatings, as a result from previous projects and efforts. The project shall overcome the problems in the low endurance of dry lubricated Harmonic Drive® gears by primarily improvement of dry lubricant coatings and necessary design modifications on the gear. Herein, composite coatings with MoS2 as main solid lubricant but having additional constituents are seen as a most promising option for soft coatings, while Hydrogenated diamond-like carbon (DLC) films are a good candidate for hard coatings.
As outlined in the proposal, the objectives of harmLES were the following:
The major technical objectives of the harmLES project can be summarized as follows:
• Understand the wear mechanisms, that lead to early failures in present dry lubricating coatings
• To develop coatings which enable long life times in Harmonic Drive® gears.
• Re-design of the Harmonic Drive® gear towards the needs for dry lubrication
• Re-assess the materials presently used in the Harmonic Drive® gears, for better mechanical support of coatings
• Establishment of a coating process ready to be qualified for space applications
• Validate dry lubricated Harmonic Drive® gears for long life time by field tests under thermal vacuum
• Install a user-group to setup requirements for future missions
The most promising concepts which will be targeted prepared during harmLES are:
• Solid lubricant coatings based on MoSx-WC (<3µm) prepared by PVD.
• Solid lubricant coatings applied on thermal diffusion treated substrates to improve load-bearing capability.
• Application of an interlayer prior to the solid lubricant coating to increase adhesion.
• Optimisation of the coating thickness and lay-up to get maximum endurance life.
The consortium decided not to include “one or the other” end user, but is targeting to serve the whole European space industry. Therefore, a so-called end-user-group was installed. Several European End-users from space industry took part in specifying requirements for Harmonic Drive® gears in order to meet demands for future missions. Letter of Interests (LoI) were attached to the proposal showing the interest of space industry.
The industrial objectives guide the choice of coating processes (PVD) and the compositions towards long life time of Harmonic Drive® gears in space applications. The major (industrial) objective of the harmLES project is to improve performances in the following applications:
• Establishment of new generation of dry lubricated Harmonic Drive® gears with proper life time
• Improvement of Life Time from currently a few output revolutions to several hundred with the target of several thousands
• Establishment of new coating for Harmonic Drive® gears (to be transferred to industrial coaters)
Improved lifetime opens automatically the number of applications which can be served by this:
- a few output revolutions are sufficient for launchers and deployment mechanisms (solar panels, … )
- several hundred output revolutions will open the application to planetary exploration and scientific applications
- several thousand output revolutions are then sufficient for antenna pointing mechanisms, thruster vectorisation mechanisms, antenna pointing mechanisms first on Low Earth Orbit and the upper end of output revolutions even for geo-stationary satellites.
The need for advanced dry lubricated Harmonic Drive® gears is driven by the space industry. The need is:
• No dry lubricated Harmonic Drive® gears are available on commercial market,
• Strategic1: “Non-dependence from US suppliers”, i.e. being not dependent on export restrictions (ITAR)
• Strategic2: Solution of this problem would lead to European and even world-wide leadership
• New: Harmonic Drive® gears could be used in non-ambient temperatures
• New: No risk of contamination of sensible parts of spacecrafts (outgassing)
• Spin-offs into similar, industrial applications such as vacuum applications (semiconductor industry, large tests stand such as DESY) and cryogenic application (which is a growing industrial sector) are clearly visible.
Project Results:
As planned, at beginning of the project a workshop was organised together with an end user group (European space industry). Following that, the requirements for Harmonic Drive® gears were established.
The project was setup in two lines running partly in parallel:
• development of coatings based on sample level
• development on component level
The development of coatings based on sample level was started with a survey of coatings and the selection of “Starters” (COTS and a space proven coating by TECNALIA) was done. After manufacturing of Lab-samples by HDAG, TECNALIA prepared first coatings and did structural characterisation like XRD and scratch testing. HDAG purchased reference coating from commercial suppliers. Testing of these coatings was performed by TECNALIA and AAC on tribometer level including post-inspection by SEM. AC2T investigated the surface chemistry on these coatings by XPS, as well as on components from previous projects as benchmark. The developed coatings were benchmarked with dry lubricant coatings based on MoS2 and DLC (commercial and prototype) and WC (commercial).
Main findings on sample level are that benchmark testing showed that the most promising solid lubricant coating is based on MoS2 being reinforced with WC, being referred to as “MoS2-WC”. Alternatives e.g. based on DLC, WS2 or WC did not offer reasonable lifetimes (<10.000 cycles). For the actual application based on high strength steels like SS17-4PH, even standard (pure) MoS2 coatings offer lower lifetimes (range <50.000 revs) than MoS2-WC coatings: repeatability by TECNALIA shows a “repeated lifetime” over 200.000 revs, but tests at AAC under vacuum achieved more than 700.000 revs. Among the variations in process parameters and layup, an optimum coating type (with reference “25988”) was identified as the most promising with regard to endurance. Also changes in the interface (thickness/gradient) did not show significant improvement of endurance. Besides that, an increased subsurface strength seems to further increase lifetime, which was shown by fretting and PoD-tests under vacuum.
Besides coating properties, also the substrate plays an important role. First tests on MoS2-WC coatings failed because of the high roughness of the substrates (Ra<0,6µm). This initiated a change the machining of the gear components using new machining parameters and revealing in lower “medium” roughness in range of Ra = 0,1-0,4µm. The achieved roughness on the gear components was the one applied to discs. It was finally proven to be the optimum range with regard to lifetime of the MoS2 –WC coatings. Tribological tests on coatings applied on samples with medium roughness showed significantly increased lifetimes in contact to pins relevant to Harmonic Drive® gears. When testing those coatings on polished gear steel life time decreased. Even further increase in lifetime of the MoS2-WC coatings on samples was obtained, after performing pre-treatment of the steel surface. This leads to improved mechanical support of the solid lubricant coating. The findings were transferred stepwise to the second line, i.e. onto the gears.
Second line was the development on component level, i.e. gear parts were coated with the new coating types and tested. This second line was split again in two paths: influence of coating types (thickness, compositions, pre-treatment) but also influence of gear design. Also these lines were followed in parallel, taking findings of the coating development into account. As first step (prior to testing of the gears itself), the test devices at HDAG and AAC were upgraded: it was needed to establish a kind of stiffness testing which can be done in-situ in the vacuum chamber, as the efficiency alone was not sufficient to assess the condition of the Harmonic Drive® gears. While upgrading the test devices, the re-design of the Harmonic Drive® gear was started with a detailed FEM-simulation. These findings were introduced step-by-step to the gear components. After testing in air and vacuum, wear was investigated by SEM (AAC) and XPS (AC2T).
Besides the gear development also the test devices needed upgrades, which were already used for testing the gear components. Especially, the ability to measure in-situ the stiffness was used to assess the integrity of the gears. In order to ease testing, a so-called “standard test box” was utilized. This was used to just change the main gear components (Flexspline, Circular Spline and Wave Generator bearing) without the need to build a complete gear with housing. Measurement of axial forces enabled to verify proper assembly of the test gear, as well as the assessment of the friction coefficient between FS and WGB outer ring.
The main outputs are prototypes of optimized solid lubricated Harmonic Drive® gears, which showed increasing lifetime with more development steps implemented.
First tests using the new coating failed very early, after few hundreds of revolutions. Increasing the thickness did not help. Then a first step of the re-design of the gears was finalised and inserted to the components. Then life times in the order of magnitude of few thousands of output revolutions were achieved with dry lubricated gears. The main conclusion was that changes in coating thickness do not improve life time but design changes (leading changes in geometry, contact pressure, internal preload) do lead to significant increases in life time. As a further step, the lubrication of the Wave Generator bearing was improved: the races were coated with MoS2, which lead to a significant improvement of the gears efficiency. In parallel, a next step in design was done. As final design changes a new tooth profile in combination with a decreased gear preload showed high influence on the life time. (Such a decrease of life time when increasing the contact pressure was also seen in pin-on-disc tests, and is well known for MoS2 based coatings.) With that, lifetimes in the range of the requirements were firstly achieved: gears were stopped at 17.500 and 22.000 revolutions without having failed. Changes in coating, e.g. thicker coating did not led to significant improvements. Also pre-treatment of the Circular Spline (although without the last step on design change) improved the performance, but not as significant as the final design change did. For WP 4, the joining of both developments (latest design plus pre-treatment) was introduced.
The final work package had the objective to merge the findings from WP 3 which means a combination of the latest design with the pre-treatment of the surfaces. However, initial tests with pre - treated, uncoated parts at HDAG showed some anomalies. Even a second trial did not lead to successful initial testing on gear level, leading finally to the decision to omit the pre-treatment. On the other hand, the pure MoS2 in the WG-bearing was replaced by a reinforced MoS2-WC. At last, also the design was slightly changed (i.e. the engagement of the toothing). Two demonstrators were finally assembled for testing under vacuum. The first failed before achieving 17.500 OPRs. As main reason, the WG-bearing was identified. Therefore, on short hand in the second demonstrator the WG-bearing was replaced by a pure-MoS2 coated one. Testing was extended also to low temperature (-150°C) which would never be possible by grease lubrication.
Main output was that 2 demonstrators were finally tested under thermal vacuum. Some more gears were manufactured, but they had to be ruled out, after the initial testing under air showed no proper operation. It had to be concluded that pre-treatment of the new toothing could not be successfully introduced. Hence, for both gears no pre-treatment was selected. Further improvement of this process would be necessary before applying it. Secondly, in the WG-bearing the use of the (pure) MoS2 was found to be the better choice, as the MoS2-WC failed before 10.300 output revolutions. The second gear, showed promising high efficiency (~80%). This did not decrease too much when running at -150°C to ~50% (grease lubricated gears cannot go below a range of -50 °C, as the greases turn into solid-type condition that makes relative motion between parts impossible as it acts as a sort of glue. A gear would get stuck). Finally, it failed around 5700 OPR with decrease in efficiency and stiffness. This leads to the conclusion that the last design change was self-defeating with regards to the endurance and that in order to further increase the lifetime alternative solutions need to be considered.
In brief, the project claims to have been successful. It could be shown that the selected coating concept (reinforced MoS2) enabled the best reasonable lifetimes on the steels used in Harmonic Drive® gears. Based on simulation several changes in the design were evaluated step-by-step to optimise the Harmonic Drive® gears towards solid lubrication. Finally, for a set of prototypes, the lifetime could be increased from a few hundred output revolutions to the required 17.500 and even more. Two gears were stopped without failure, indicating that even higher endurance might be feasible.
Potential Impact:
For space activities one of the main European strategies is “non-dependence”. In the past several missions were in danger, because products which were not available from European suppliers had to be bought from US-suppliers, but export was blocked by ITAR-regulations. ITAR is a permanent danger to European space activities. Hence, it is one of the top-priorities in ESA-Programs. Moreover, if the project is successful, Europe would reach world-wide leadership in this specific product.
Economic impact is seen in two ways: in sense of entities like industry and researchers, but also in terms of “public money”. European space programs are financed by public bodies (e.g. member states of ESA, EU). 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. This means, that space programs can be made more cost efficient and the tax money can be re-directed to social issues.
Second economic impact, is of course the improvement of European industry. Space market is a world-wide market. Leadership or USP have to be gained in world-wide competition. This project would definitely enable such a world-wide-USP. HDAG would become the only supplier world-wide of these high-end dry lubricated Harmonic Drive® gears gears. Other low-technology solutions with much, much lower performance (and therefore not usable for most of the applications described in the proposal) are existent. The project will ensure that HDAG is able to protect these high-end solution against attempts from outside Europe, which do enter into the low-demanding applications. So it will protect European competitiveness in a high-value market.
This will also be a sustainable strengthening and be held for several years close to a decade, since space industry is strongly relying on so-called “space heritage”: the first successful product will be bought for several years, therefore future competitors will not easily enter the market. Besides industry (HDAG as supplier of the product), also researcher will benefit from the project: TECNALIA as supplier of a new coating (in terms of reputation as developer but also in terms of selling this coating in terms of spin-offs, licensing, ..), also AAC benefits from this project as being proven partner for validation of mechanisms with Harmonic Drive® gears and will gain its position as space materials testhouse in contract to ESA/ESTEC.
The results of this project will in second level also strengthen the competitiveness of European space industry. Companies like Astrium, Sener, Kongsberg … will be able to offer new mechanisms to satellite manufacturers. Moreover, spin-offs into similar, industrial applications such as vacuum applications (semiconductor industry, large tests stand such as DESY) and cryogenic application (which is a growing industrial sector) will follow. Further fields could even be the machining sector, where attempts are made to reduce of cutting lubricants. By improving the competitiveness of industry, employment is secured in Europe and dependence on USA or Japan is reduced.
List of Websites:
www.harmles.eu
Contact:
Project Coordinator:
Ernst-Dieter JANOTKA
email: janotka@ac2t.at
Scientific Coordinator:
Dr. Andreas MERSTALLINGER
andreas.merstallinger@aac-research.at
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.
Harmonic Drive® gears are based on the same, unusual principle of a flexible element used to transmit torque. That offers the following advantages: they provide low or zero backlash in combination with excellent precision. The large gear ratio in combination with the ability of zero stick-slip movement at slow speeds makes them an often used choice for space applications: a small actuator motor (meaning low mass and low power consumption) may be used to move large components like antennas or solar panels.
A specification for such a gear was setup on basis of a questionnaire to European end-users. The transmission accuracy shall remain below 60arcsecs as specified for terrestrial gears. The repeatability is specified with a maximum of 3arcsecs. The minimum gear stiffness is foreseen to be at a level of approx. 1.1*104 Nm/rad which is close to the stiffness of a HFUC gear size 20. Beside quantitative properties, a main qualitative feature of the Harmonic - Drive® gear is seen in the zero backlash, which is a decisive advantage compared to other gear types. The lifetime was required with 17,000 output revolutions at an output torque of 4Nm.
Within harmLES, the geometry of the Harmonic - Drive® gear could be optimised 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 !
Project Context and Objectives:
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.
Harmonic Drive® gears are based on the same, unusual principle of a flexible element used to transmit torque. That offers the following advantages: they provide low or zero backlash in combination with excellent precision. The large gear ratio in combination with the ability of zero stick-slip movement at slow speeds makes them an often used choice for space applications: a small actuator motor (meaning low mass and low power consumption) may be used to move large components like antennas or solar panels.
The main drawback for the use of Harmonic Drive® gears is that they presently need liquid lubricants (including greases) in order to satisfy the operational requirements. Due to vacuum and temperature in space, liquid lubricants and greases bear the risk of outgassing and evaporation. This leads on one hand to loss of lubrication efficiency (some components are lost in the lubricant), but on the other hand to depositions of these outgassed products on other sensible surfaces. Thereby, optical components might be degraded. This may even lead to loss of a full mission, e.g in earth observation.
Since 2002, there has grown a demand for dry lubricated gears due to various needs. These cover extended temperature ranges and applications with critical items close to the gear like optics or telescopes. Harmonic Drive AG has increased its effort in supporting space applications by installing a dedicated team of engineers for these demanding applications. This was based on the need defined by the customers to receive a broader and more profound support and the necessity to adapt the products more to the specific applications. Over the years, this did allow to pool a lot of application experience but being as well faced with more demanding requirements. In the last years, the need for providing more and more customisation and support did increase and the number of inquiries based on the request of using dry lubrication became more apparent. However, no sufficient solutions could be given on durability and suitability of such a dry lubrication scheme and the customers were left alone with the qualification risk.
The project is application triggered. The members of the consortium – manufacturer of Harmonic Drive® gears, developer of coatings and a material developer having a contract as “space materials testhouse” – are on one hand closely related to the space market and its applications. They know also the related requirements for use in space, but act also in non-space markets. This enables “spin-in”, i.e. bringing in of concepts from other fields.
The main research need follows the limited success to use “commercial off the shelf (COTS)” coatings, as a result from previous projects and efforts. The project shall overcome the problems in the low endurance of dry lubricated Harmonic Drive® gears by primarily improvement of dry lubricant coatings and necessary design modifications on the gear. Herein, composite coatings with MoS2 as main solid lubricant but having additional constituents are seen as a most promising option for soft coatings, while Hydrogenated diamond-like carbon (DLC) films are a good candidate for hard coatings.
As outlined in the proposal, the objectives of harmLES were the following:
The major technical objectives of the harmLES project can be summarized as follows:
• Understand the wear mechanisms, that lead to early failures in present dry lubricating coatings
• To develop coatings which enable long life times in Harmonic Drive® gears.
• Re-design of the Harmonic Drive® gear towards the needs for dry lubrication
• Re-assess the materials presently used in the Harmonic Drive® gears, for better mechanical support of coatings
• Establishment of a coating process ready to be qualified for space applications
• Validate dry lubricated Harmonic Drive® gears for long life time by field tests under thermal vacuum
• Install a user-group to setup requirements for future missions
The most promising concepts which will be targeted prepared during harmLES are:
• Solid lubricant coatings based on MoSx-WC (<3µm) prepared by PVD.
• Solid lubricant coatings applied on thermal diffusion treated substrates to improve load-bearing capability.
• Application of an interlayer prior to the solid lubricant coating to increase adhesion.
• Optimisation of the coating thickness and lay-up to get maximum endurance life.
The consortium decided not to include “one or the other” end user, but is targeting to serve the whole European space industry. Therefore, a so-called end-user-group was installed. Several European End-users from space industry took part in specifying requirements for Harmonic Drive® gears in order to meet demands for future missions. Letter of Interests (LoI) were attached to the proposal showing the interest of space industry.
The industrial objectives guide the choice of coating processes (PVD) and the compositions towards long life time of Harmonic Drive® gears in space applications. The major (industrial) objective of the harmLES project is to improve performances in the following applications:
• Establishment of new generation of dry lubricated Harmonic Drive® gears with proper life time
• Improvement of Life Time from currently a few output revolutions to several hundred with the target of several thousands
• Establishment of new coating for Harmonic Drive® gears (to be transferred to industrial coaters)
Improved lifetime opens automatically the number of applications which can be served by this:
- a few output revolutions are sufficient for launchers and deployment mechanisms (solar panels, … )
- several hundred output revolutions will open the application to planetary exploration and scientific applications
- several thousand output revolutions are then sufficient for antenna pointing mechanisms, thruster vectorisation mechanisms, antenna pointing mechanisms first on Low Earth Orbit and the upper end of output revolutions even for geo-stationary satellites.
The need for advanced dry lubricated Harmonic Drive® gears is driven by the space industry. The need is:
• No dry lubricated Harmonic Drive® gears are available on commercial market,
• Strategic1: “Non-dependence from US suppliers”, i.e. being not dependent on export restrictions (ITAR)
• Strategic2: Solution of this problem would lead to European and even world-wide leadership
• New: Harmonic Drive® gears could be used in non-ambient temperatures
• New: No risk of contamination of sensible parts of spacecrafts (outgassing)
• Spin-offs into similar, industrial applications such as vacuum applications (semiconductor industry, large tests stand such as DESY) and cryogenic application (which is a growing industrial sector) are clearly visible.
Project Results:
As planned, at beginning of the project a workshop was organised together with an end user group (European space industry). Following that, the requirements for Harmonic Drive® gears were established.
The project was setup in two lines running partly in parallel:
• development of coatings based on sample level
• development on component level
The development of coatings based on sample level was started with a survey of coatings and the selection of “Starters” (COTS and a space proven coating by TECNALIA) was done. After manufacturing of Lab-samples by HDAG, TECNALIA prepared first coatings and did structural characterisation like XRD and scratch testing. HDAG purchased reference coating from commercial suppliers. Testing of these coatings was performed by TECNALIA and AAC on tribometer level including post-inspection by SEM. AC2T investigated the surface chemistry on these coatings by XPS, as well as on components from previous projects as benchmark. The developed coatings were benchmarked with dry lubricant coatings based on MoS2 and DLC (commercial and prototype) and WC (commercial).
Main findings on sample level are that benchmark testing showed that the most promising solid lubricant coating is based on MoS2 being reinforced with WC, being referred to as “MoS2-WC”. Alternatives e.g. based on DLC, WS2 or WC did not offer reasonable lifetimes (<10.000 cycles). For the actual application based on high strength steels like SS17-4PH, even standard (pure) MoS2 coatings offer lower lifetimes (range <50.000 revs) than MoS2-WC coatings: repeatability by TECNALIA shows a “repeated lifetime” over 200.000 revs, but tests at AAC under vacuum achieved more than 700.000 revs. Among the variations in process parameters and layup, an optimum coating type (with reference “25988”) was identified as the most promising with regard to endurance. Also changes in the interface (thickness/gradient) did not show significant improvement of endurance. Besides that, an increased subsurface strength seems to further increase lifetime, which was shown by fretting and PoD-tests under vacuum.
Besides coating properties, also the substrate plays an important role. First tests on MoS2-WC coatings failed because of the high roughness of the substrates (Ra<0,6µm). This initiated a change the machining of the gear components using new machining parameters and revealing in lower “medium” roughness in range of Ra = 0,1-0,4µm. The achieved roughness on the gear components was the one applied to discs. It was finally proven to be the optimum range with regard to lifetime of the MoS2 –WC coatings. Tribological tests on coatings applied on samples with medium roughness showed significantly increased lifetimes in contact to pins relevant to Harmonic Drive® gears. When testing those coatings on polished gear steel life time decreased. Even further increase in lifetime of the MoS2-WC coatings on samples was obtained, after performing pre-treatment of the steel surface. This leads to improved mechanical support of the solid lubricant coating. The findings were transferred stepwise to the second line, i.e. onto the gears.
Second line was the development on component level, i.e. gear parts were coated with the new coating types and tested. This second line was split again in two paths: influence of coating types (thickness, compositions, pre-treatment) but also influence of gear design. Also these lines were followed in parallel, taking findings of the coating development into account. As first step (prior to testing of the gears itself), the test devices at HDAG and AAC were upgraded: it was needed to establish a kind of stiffness testing which can be done in-situ in the vacuum chamber, as the efficiency alone was not sufficient to assess the condition of the Harmonic Drive® gears. While upgrading the test devices, the re-design of the Harmonic Drive® gear was started with a detailed FEM-simulation. These findings were introduced step-by-step to the gear components. After testing in air and vacuum, wear was investigated by SEM (AAC) and XPS (AC2T).
Besides the gear development also the test devices needed upgrades, which were already used for testing the gear components. Especially, the ability to measure in-situ the stiffness was used to assess the integrity of the gears. In order to ease testing, a so-called “standard test box” was utilized. This was used to just change the main gear components (Flexspline, Circular Spline and Wave Generator bearing) without the need to build a complete gear with housing. Measurement of axial forces enabled to verify proper assembly of the test gear, as well as the assessment of the friction coefficient between FS and WGB outer ring.
The main outputs are prototypes of optimized solid lubricated Harmonic Drive® gears, which showed increasing lifetime with more development steps implemented.
First tests using the new coating failed very early, after few hundreds of revolutions. Increasing the thickness did not help. Then a first step of the re-design of the gears was finalised and inserted to the components. Then life times in the order of magnitude of few thousands of output revolutions were achieved with dry lubricated gears. The main conclusion was that changes in coating thickness do not improve life time but design changes (leading changes in geometry, contact pressure, internal preload) do lead to significant increases in life time. As a further step, the lubrication of the Wave Generator bearing was improved: the races were coated with MoS2, which lead to a significant improvement of the gears efficiency. In parallel, a next step in design was done. As final design changes a new tooth profile in combination with a decreased gear preload showed high influence on the life time. (Such a decrease of life time when increasing the contact pressure was also seen in pin-on-disc tests, and is well known for MoS2 based coatings.) With that, lifetimes in the range of the requirements were firstly achieved: gears were stopped at 17.500 and 22.000 revolutions without having failed. Changes in coating, e.g. thicker coating did not led to significant improvements. Also pre-treatment of the Circular Spline (although without the last step on design change) improved the performance, but not as significant as the final design change did. For WP 4, the joining of both developments (latest design plus pre-treatment) was introduced.
The final work package had the objective to merge the findings from WP 3 which means a combination of the latest design with the pre-treatment of the surfaces. However, initial tests with pre - treated, uncoated parts at HDAG showed some anomalies. Even a second trial did not lead to successful initial testing on gear level, leading finally to the decision to omit the pre-treatment. On the other hand, the pure MoS2 in the WG-bearing was replaced by a reinforced MoS2-WC. At last, also the design was slightly changed (i.e. the engagement of the toothing). Two demonstrators were finally assembled for testing under vacuum. The first failed before achieving 17.500 OPRs. As main reason, the WG-bearing was identified. Therefore, on short hand in the second demonstrator the WG-bearing was replaced by a pure-MoS2 coated one. Testing was extended also to low temperature (-150°C) which would never be possible by grease lubrication.
Main output was that 2 demonstrators were finally tested under thermal vacuum. Some more gears were manufactured, but they had to be ruled out, after the initial testing under air showed no proper operation. It had to be concluded that pre-treatment of the new toothing could not be successfully introduced. Hence, for both gears no pre-treatment was selected. Further improvement of this process would be necessary before applying it. Secondly, in the WG-bearing the use of the (pure) MoS2 was found to be the better choice, as the MoS2-WC failed before 10.300 output revolutions. The second gear, showed promising high efficiency (~80%). This did not decrease too much when running at -150°C to ~50% (grease lubricated gears cannot go below a range of -50 °C, as the greases turn into solid-type condition that makes relative motion between parts impossible as it acts as a sort of glue. A gear would get stuck). Finally, it failed around 5700 OPR with decrease in efficiency and stiffness. This leads to the conclusion that the last design change was self-defeating with regards to the endurance and that in order to further increase the lifetime alternative solutions need to be considered.
In brief, the project claims to have been successful. It could be shown that the selected coating concept (reinforced MoS2) enabled the best reasonable lifetimes on the steels used in Harmonic Drive® gears. Based on simulation several changes in the design were evaluated step-by-step to optimise the Harmonic Drive® gears towards solid lubrication. Finally, for a set of prototypes, the lifetime could be increased from a few hundred output revolutions to the required 17.500 and even more. Two gears were stopped without failure, indicating that even higher endurance might be feasible.
Potential Impact:
For space activities one of the main European strategies is “non-dependence”. In the past several missions were in danger, because products which were not available from European suppliers had to be bought from US-suppliers, but export was blocked by ITAR-regulations. ITAR is a permanent danger to European space activities. Hence, it is one of the top-priorities in ESA-Programs. Moreover, if the project is successful, Europe would reach world-wide leadership in this specific product.
Economic impact is seen in two ways: in sense of entities like industry and researchers, but also in terms of “public money”. European space programs are financed by public bodies (e.g. member states of ESA, EU). 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. This means, that space programs can be made more cost efficient and the tax money can be re-directed to social issues.
Second economic impact, is of course the improvement of European industry. Space market is a world-wide market. Leadership or USP have to be gained in world-wide competition. This project would definitely enable such a world-wide-USP. HDAG would become the only supplier world-wide of these high-end dry lubricated Harmonic Drive® gears gears. Other low-technology solutions with much, much lower performance (and therefore not usable for most of the applications described in the proposal) are existent. The project will ensure that HDAG is able to protect these high-end solution against attempts from outside Europe, which do enter into the low-demanding applications. So it will protect European competitiveness in a high-value market.
This will also be a sustainable strengthening and be held for several years close to a decade, since space industry is strongly relying on so-called “space heritage”: the first successful product will be bought for several years, therefore future competitors will not easily enter the market. Besides industry (HDAG as supplier of the product), also researcher will benefit from the project: TECNALIA as supplier of a new coating (in terms of reputation as developer but also in terms of selling this coating in terms of spin-offs, licensing, ..), also AAC benefits from this project as being proven partner for validation of mechanisms with Harmonic Drive® gears and will gain its position as space materials testhouse in contract to ESA/ESTEC.
The results of this project will in second level also strengthen the competitiveness of European space industry. Companies like Astrium, Sener, Kongsberg … will be able to offer new mechanisms to satellite manufacturers. Moreover, spin-offs into similar, industrial applications such as vacuum applications (semiconductor industry, large tests stand such as DESY) and cryogenic application (which is a growing industrial sector) will follow. Further fields could even be the machining sector, where attempts are made to reduce of cutting lubricants. By improving the competitiveness of industry, employment is secured in Europe and dependence on USA or Japan is reduced.
List of Websites:
www.harmles.eu
Contact:
Project Coordinator:
Ernst-Dieter JANOTKA
email: janotka@ac2t.at
Scientific Coordinator:
Dr. Andreas MERSTALLINGER
andreas.merstallinger@aac-research.at
