load”. The normal load is applied via an air piston that acts through a load cell to move the drive motor  plate that is pivot-mounted relative to the test chamber. Pressurizing the piston moves the input shaft in  an arc motion toward the test roller. The arc motion is measured by an LVDT. Once the rollers are in  contact, additional pressure to the air piston increases the normal load between the test rollers. Careful calibration processes allow calculating the test roller load using the sensor outputs from the load cell and the LVDT that measures the input shaft angular orientation [4]. Shaft speeds and total number of shaft revolutions were measured using encoders on each shaft. The encoder pulses were counted and recorded via a digital pulse counter. The encoders provide 6,000 pulses for each shaft revolution. (a)      (b) Figure 4. Simplified schematic view including some  of the important sen sed data.(a) Schematic,  front view. (b) Schematic, side view. When rollers operate in a misaligned condition, a force will develop in the direction of the shaft axis [7-10]. In such a condition points on the two rollers in intimate contact and within a “stick” zone of the contact  patch are constrained to move in unison. If the points were not in contact the kinematic constraints would  provide a slightly different path of motion. The difference in the actual path of motion and the motion that would occur if the points were not in contact gives rise to surface strains and a resultant axial force. A sensor to measure this force is labeled as the “axial force” sensor in Figure 4. The axial force sensor is co-located on the output shaft with the torquemeter.  The visual appearance of the test rollers were documented by digital photographs obtained through a  viewport using a single-lens reflex camera with a 150-mm lens and a 12-million pixel image sensor. Wear debris was captured onto a flat rectangular pan located below the rollers. The analysis of such captured debris is beyond the scope of this article. Contact Analyses and Test Conditions Hertz contact analyses were completed to guide use of the VRR to best mimic the contact conditions of  the NIRSpec mechanism. Results of the analyses were also used to guide selections of materials for evaluation. Although the actual contact conditions of rough surfaces differ from Hertz models that assume perfectly-smooth surfaces, the Hertz model solutions provide useful design indices. The Hertz contact  solutions herein were produced using the approximate solution technique of Antoine, et al [11]. The mechanism makes use of steel rollers contacting a Ti6Al4V plane surface while the VRR tests a pair of rollers. The contact solutions for the mechanism are for roller geometry having a circular crown with 510-millimeter radius and with a 38-N (8.5-lbf) normal load pressing the steel roller to the Ti6Al4V plane. Contact solutions for VRR are for a normal load of 130 N (30 lbf) pressing the two rollers together. The  VRR rollers are 35.6-mm outer diameter, the upper roller has a circular crown of 400 mm, and the lower  roller has a flat profile. The Hertz contact condition solutions are summarized in Table 1. The contact ellipses are short in the rolling direction (labeled “b’ in the diagram contained in the table) and have similar aspect ratios, 14.3 for the mechanism and 11.3 for the VRR.  The contact ellipse is somewhat  larger for the VRR because of the larger diameters of the VRR rollers. The contact pressure magnitudes  are somewhat larger in the VRR, approximately 40% larger comparing the maximum contact pressures.  Use of a lower normal load in the VRR would have provided a closer match of the contact conditions, but because of VRR constraints the 130-N load was used for the experiments.normal load braking torquetangential force normal loadnormal load braking torquetangential force normal loadnormal load braking torquetangential force normal loadnormal load braking torquetangential force normal loadaxial forceaxial force normal loadinput shaftinput shaft The image cannot be displayed. Your computer may not output shaft axial force normal load normal loadinput shaft The image cannot be displayed. Your computer may not output shaft normal load normal load normal loadinput shaft The image cannot be displayed. Your computer may not output shaft normal load axial force normal loadinput shaftinput shaft The image cannot be displayed. Your computer may not output shaft 90