Citation: Abdul Kadir, K.A.; Nazmi, N.; Mohamad, N.; Shabdin, M.K.; Adiputra, D.; Mazlan, S.A.; Nordin, N.A.; Mohd Yusuf, S.; Ubaidillah. Effect of Magnetorheological Grease’s Viscosity to the Torque Performance in Magnetorheological Brake. Materials 2022, 15, 5717. https:// doi.org/10.3390/ma15165717 Academic Editors: Jianbo Yin and Yancheng Li Received: 11 June 2022 Accepted: 11 August 2022 Published: 19 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Effect of Magnetorheological Grease’s Viscosity to the Torque Performance in Magnetorheological Brake Khairul Anwar Abdul Kadir 1 , Nurhazimah Nazmi 1, * , Norzilawati Mohamad 2 , Muhammad Kashfi Shabdin 3, *, Dimas Adiputra 4 , Saiful Amri Mazlan 1 , Nur Azmah Nordin 1 , Shahir Mohd Yusuf 1 and Ubaidillah 5 1 Engineering Materials and Structures (eMast) iKohza, Malaysia-Japan International Institute of Technology (MJIIT), Universiti Teknologi Malaysia, Jalan Sultan Yahya Petra, Kuala Lumpur 54100, Malaysia 2 Faculty of Engineering, Universiti Malaysia Sabah, Jalan UMS, Kota Kinabalu 88400, Malaysia 3 Department of Physics, Faculty of Science, Universiti Putra Malaysia, UPM Serdang, Selangor 43400, Malaysia 4 Electrical Engineering Department, Institut Teknologi Telkom Surabaya, Surabaya 6023, Indonesia 5 Mechanical Engineering Department, Universitas Sebelas Maret, J1. Ir. Sutami 36A, Kentingan, Surakarta 57126, Indonesia * Correspondence: nurhazimah@utm.my (N.N.); kashfi.shabdin@upm.edu.my (M.K.S.) Abstract: Recently, magnetorheological grease (MRG) has been utilized in magnetorheological (MR) brakes to generate a braking torque based on the current applied. However, the high initial viscosity of MRG has increased the off-state torque that led to the viscous drag of the brake. Therefore, in this study, the off-state viscosity of MRG can be reduced by the introduction of dilution oil as an additive. Three samples consist of pure MRG (MRG 1) and MRG with different types of dilution oil; hydraulic (MRG 2) and kerosene (MRG 3) were prepared by mixing grease and spherical carbonyl iron particles (CIP) using a mechanical stirrer. The rheological properties in the rotational mode were examined using a rheometer and the torque performances in MR brake were evaluated by changing the current of 0 A, 0.4 A, 0.8 A, and 1.2 A with fixed angular speed. The result shows that MRG 3 has the lowest viscosity which is almost 93% reduction while the viscosity of MRG 2 has lowered to 25%. However, the torque performances generated by MRG 3 were highest, 1.44 Nm, when 1.2 A of current was applied and followed by MRG 2 and MRG 1. This phenomenon indicated that the improvement of torque performances was dependent on the viscosity of MRG. By reducing the viscosity of MRG, the restriction on CIP to form chain formation has also decreased and strengthen the torque of MRG brake. Consequently, the utilization of dilution oil in MRG could be considered in MR brake in near future. Keywords: magnetorheological brake; braking torque; magnetorheological grease; additive; rheological properties 1. Introduction Magnetorheological (MR) brakes have several potential applications in a variety of areas and machineries, including automotive, construction, fitness equipment, and computer numerical control machine tools. MR brakes have a simple design, a smooth and dependable functioning mechanism, a rapid and reversible reaction, and are easy to control in the presence of a magnetic field [1]. Carlson et al. [2] presented the first MR brake actuator in 1998, using MR fluid (MRF) as a medium, and the highest produced torque was up to 4 Nm with an applied current of 1 A. This outcome was sufficient to be applied to low torque devices, such as ankle foot orthoses [3]. MRF has been utilized in dampers [4,5] and brakes [6] due to its unique ability to modify rheological properties by adjusting the strength of the magnetic field. As a result, the proposal of using MRF in car braking systems [7–9] has been presented, with a target torque of at least 500 Nm for the usage of 1000 kg automobile [10]. As a result, numerous ways to improving the torque Materials 2022, 15, 5717. https://doi.org/10.3390/ma15165717 https://www.mdpi.com/journal/materials