NORDTRIB 2016 Case study: The effect of an amorphous hydrogenated carbon-coated gear-wheel on a hydraulic orbital motor’s efficiency over time CA Bates 1 , HW Broe-Richter 2 , CR Bendlin 1 and P Ennemark 1 Abstract This case study examines how the application of an amorphous hydrogenated carbon coating to gear-wheels affects the volumetric and mechanical efficiencies of the hydraulic orbital motors into which the gear-wheels are assembled. The efficiencies over time of these orbital motors are compared with the efficiencies of orbital motors, which were assembled and tested with standard (uncoated) gear-wheels. Comparisons are based on experimental data, accumulated over 3000 h of steady-state tests, divided over six motors with a duration of 500 h each. The paper examines the surface roughness characteristics of amorphous hydrogenated carbon-coated gear-wheels before and after coating, the mea- sured magnitudes and positions of wear on the contact surfaces of the coated and uncoated gear-wheels after testing, and the observed differences in said wear utilizing scanning electron microscopic analyses. Finally, changes in the measured efficiencies of the orbital motors are examined, over the duration of the experimental tests, in order to consider the potential energy savings associated with hydraulic orbital motors assembled with coated gear-wheels. Keywords Amorphous hydrogenated carbon coating, energy savings, gears, hydraulic orbital motors, in situ testing Date received: 3 October 2016; accepted: 11 December 2017 Introduction As a world leader in the off-road mobile hydraulics industry, Danfoss Power Solutions (DPS) has an inherent interest in improving the efficiencies of their orbital motors over time. 1 This paper explores how the low-friction and wear-resistant characteristics of amorphous hydrogenated carbon-coated components can contribute to this endeavor. Amorphous hydrogenated carbon coatings (or films) have been characterized as, ‘‘very hard and hence resistant to scratching, and also possess[ing] a high dielectric constant, high index of refraction, excellent optical transparency, and high resistance to corrosion in strongly acidic solutions’’. 2 The coating under investigation is of the type a-C:H:W (tungsten- containing diamond-like carbon (W-DLC)). The coat- ing was applied through a reactive sputtering process, with a finished thickness of approximately 0.002 mm and a surface hardness of approximately 14 GPa. While others have investigated how amorphous hydrogenated carbon coatings can contribute to improved performance in sliding conditions, 3,4 or uti- lized scanning electron microscopy (SEM) analyses to demonstrate how the wear properties of boundary-lubricated surfaces can be improved by pairing W-DLC-coated elements with uncoated steel elements, 5,6 this is the first such investigation to include in situ testing of amorphous hydrogenated carbon-coated components in the hydraulic orbital motors. Hydraulic motors convert hydraulic energy, in the form of pressure and oil flow, into mechanical energy, in the form of torque and speed. An orbital motor is a type of hydraulic motor, where energy conversion is achieved through a mechanism consisting of a gear- wheel and a gear-rim. The orbital motor’s energy con- version occurs as the gear-wheel rotates about its own center, while the center simultaneously orbits on a circle, with the same center as the gear-rim. Because the gear-rim teeth are constantly in contact with (or in 1 Work Functions Division, Danfoss Power Solutions ApS, Nordborg, Denmark 2 Danfoss Technology Centre, Danfoss A/S, Nordborg, Denmark Corresponding author: CA Bates, Work Functions Division, Danfoss Power Solutions ApS, Nordborgvej 81, DK-6430, Nordborg, Denmark. Email: cabates@danfoss.com Proc IMechE Part J: J Engineering Tribology 0(0) 1–14 ! IMechE 2018 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1350650117752610 journals.sagepub.com/home/pij