*5.1. Pin on Disk Tribometers*

The pin on disk (PoD) configuration is one of the most popular devices used to study the friction and wear. Introduced by the ASTM standards G133 [71] and G99 [72] for wear and erosion tests, both these devices are wide used also to study materials like tyre rubber.

Pin on disk (PoD) tribometers, in general, are designed and constructed to study the complex friction phenomena in many engineering applications such as railway wheels, automotive and aircraft brake systems, clutches, bearings, mechanical joints, tyre and biomedical materials. Conventional PoD machines provide a normal contact load between a stationary pin and a revolving disk and measure the resulting frictional force to evaluate the coefficient of friction [73–76]. PoD tests can use different contact geometries:


An example of the above equipment is reported in the studies of Carbone et al. [77], in which the authors have investigated the friction properties of styrene-butadiene rubber (SBR) copolymer, furnished by Pirelli Tyre, by means a ball-on-disk configuration. The friction measurement has been carried out using the CSM Instruments Tribometer available at the Tribology Lab at the Department of Mechanical and Industrial Engineering (Politecnico di Bari, Bari, Italy). The device, shown in Figure 22, is composed by a rotating SBR disk in contact with a sphere made in polytetrafluoroethylene (PFTE) fixed to the holder and unable to rotate.

**Figure 22.** The CSM- instruments HT-Tribometer [77].

The sphere is mounted on a stiff lever, designed as a frictionless force transducer, and is loaded onto the test sample by applying calibrated weights so as to allow adjustment of the normal force in the range 0–10 N. The friction coefficient is determined during the test by measuring the deflection of the highly linear and precise elastic arm, with a resolution of 5 mN while wear coefficients for the ball and disk materials are calculated from the volume of material lost during the test. The instrument allows one to control the velocity of the disk and the radial position of the ball.

The main technical specifications are shown in a Table 12.


**Table 12.** Technical specification of the CSM instruments tribometer.

With the aim to investigate the viscoelastic contribution to friction, the authors chose a PFTE sphere to reduce friction and wear between the SBR disk and the sphere. The tests were conducted at the sliding velocity of 6 mm/s and for different normal loads. Figure 23 shows the comparison between experimental and numeric increase ∆µ of the friction coefficient µ as the normal load is increased from 1 N to 5 N. Δμ μ

Δμ μ **Figure 23.** Comparison between experimental and numeric increase ∆µ of the friction coefficient µ as the normal load is increased from 1 N to 5 N, blue rhomboids refer to experiments, red squared to numerical calculations [77].

This device is also designed to study friction and wear behaviour, since it allows one to vary the interfacial friction by replacing test spheres characterized by different roughness. The device allows measurement of friction in a small normal load range, and for this reason and also for the small contact surface, the device is not able to simulate the tyre/road contact conditions.

#### *5.2. Dynamic Friction Tester*

The Dynamic Friction Tester, shortly called DFT, was born like an on-field tester to characterize the paved surface frictional properties and shows a layout conceptually similar to the PoD tribometer. The DFT is equipped with a rotating disc with three connected rubber sample and does not have the fixed part (road surface) since this tester is usually used for friction measurements on the field, but in any case, as shown by a study conducted by Do et al. [78], the device allows laboratory measurements. This device designed and manufactured in Japan by the Nippo Sangyo Co., Ltd. has being used widely not only in Japan but also in the EU, the United States and many other countries by government agencies, construction companies, independent consulting companies, automotive manufacturers, tyre manufacturers, research institutions and universities among many other organizations [79]. In EU the leading user of this device is the "Institut Français des Sciences et Technologies des Transports, de l'Aménagement et des Réseaux (IFSTTAR)". Also, DFT has been chosen as the standard reference by IFI (International Friction Index) ASTM international Standards [80]. The DFT is reported in the ASTM G115 in the "vehicle pavement system" and the standard test procedure used in the United States can

be found in the ASTM E-1911 standard [81]. The device, shown in Figure 24, looks like a box where the bottom side has three rubber sliders mounted on the lower surface of a disk that rotates with its plane parallel to the test surface. In the upper side there are the DC electric motor and the control unit.

**Figure 24.** Dynamic Friction Tester, (**a**) upper side, (**b**) bottom side, (**c**) rubber sliders [82].

The main technical specifications are shown in a Table 13.

**Table 13.** Technical specification.


As mentioned, the DFT is commonly used for degeminating the road surface frictional properties, in several states in the world. A detailed analysis of the relevant aspects including the experimental results of pavement friction studies, using the DFT, is given by Rado et al. [83], and Kane et al. [82,84]. In the study conducted by Kane et al. [82] the results of the friction coefficient measurements conducted using the DFT in wet condition are presented, with the goal to develop and validate by means of the DFT a dynamic frictional contact model. During the measurement, the disk is accelerated to reach the target speed, after which, prior to reaching the desired speed, the water is applied and maintained during the entire measurement process through an irrigation system of the device. Once the set speed of the rubber pads is reached, the motor is turned off and the disk with the measuring pads is lowered in contact with the surface with a constant normal load. Each pad is loaded at 11.8 N, chosen due to the weight of the device and the rigidity of the pad holders. The speed of the pads decreases until it stops completely due to the friction generated between the pads and the contact surface, recorded during the deceleration phase from the set speed up to the stop.

The main advantages in the use of the DFT lies in the possibility to operate both indoor and outdoor, in the ease of use and in the possibility of making measurements in a wide range of speeds. The main drawback associated with the use of a commercial machine such as the DFT lays in the fact that it is not possible to study the friction coefficient of tyre tread compounds or to use tread blocks obtained from a tyre, because the friction pads are provided by the manufacturer. In addition, the DFT does not allow to vary the normal load on the sample.
