4. Experimental study4.1. Experimental setupThe experiments were conducted on an Amsler disk-on-disk tribo tester, shown in Fig. 5 to investigate the performance of lubricants used for lubricating gear contacts of the Exciter Sieve gear box. Amsler disk-on-disk tribo tester utilizes a disk on disk contact geometry, where the lower disk is mounted on the drive shaft of motor and rotated at the predetermined motor speed. The contacting disk i.e., the upper disk in this case, is mounted on the driven shaft and is allowed to make contact with bottom disk circumferentially. Thus, forming a line contact under no load condition. The load on the contact is applied using a compression spring pressing the upper disk onto the lower disk. When the two disks are in contact the driver disk rotates the driven disk in the opposite direction under pure rolling condition due to the contact friction. In case of lubricated con- tacts the bottom disk is partially submerged in the lubricant reservoir and if required the temperature of the lubricant can be increased with the help of heaters. The bottom disk, while rotating, carries the lubricant on the surface thus forming a thin protective lubricating film on the surface. The friction within the contact is continuously monitored and the load on the disks is varied in stages. However, with the increase in load, the lubricant film within the contact diminishes causing metal to me- tal contact and subsequently resulting in an excessive rise in friction.The Amsler disk-on-disk tribo-tester simulates the gear geometry in contact. The actual conditions occurring inside the gear box are far severe to be actually replicated on the disk-on-disk tester. Further, the restriction of maximum load and speed, limits the experimental procedure on the Amsler tester. As per the prior experimental experience, it has been observed that the lubricants passing FZG 12th load stage shows a load carrying capacity >1400 N on Amsler disk-on-disk tribo-tester. With this prior understanding, the load carrying capacity of the lubricants was assessed on the Amsler disk- on-disk tribo-tester. The objective of the experiments was to compare the load carrying capacity of the lubricants under identical conditions. However, keeping in view the operating conditions of the test rig, the replication of real operating con- ditions of the gearbox was not possible. Therefore, one to one comparison between the Amsler tests to real conditions in the gear box cannot be made.4.2. Materials4.2.1. LubricantsIn the present investigation, lubricants used for lubricating gears in Exciter Sieve gearbox were extensively studied. The lubricants SAE 10, SAE 90 and SAE 10W40 were tested along with the Base oil Neutral 150. All of the selected lubricants are mineral based products procured from the local Indian market. The physico-chemical characteristics have a great influence in determining the lubrication capabilities of lubricants. Hence, the selected lubricants were investigated for their physico- chemical characteristics in order to have a thorough knowledge of the characteristics of these lubricants. The lubricants were tested for their viscosity, density, CHN (Carbon, Hydrogen and Nitrogen) percentage and the trace metals. The viscosity and density of lubricants were determined using standard ASTM D procedures. The elemental analysis (CHN analysis) wasTable 2Geometric and the contact configuration of the test specimens.Specimen Value Lower disk £35 mm, 200 rpmUpper disk £40 mm, 200 rpmContact length 5.8 mm