Experimental Investigation of the Performance of SBR and RSS Materials as a Sliding Bearing Element

Abstract

Conventional bearing systems and materials, which are important components of machine elements, have often failed to fulfil requirements, especially in places where sealing is required. In such cases, sliding bearings with PTFE- and NBR-based polymer and elastomer materials operating in water or oil are preferred. In this study, the performance of two alternative rubber-based materials, synthetic styrene butadiene rubber (SBR) and natural rubber smoked sheet (RSS), as a sliding bearing was experimentally investigated in terms of frictional resistance using pin-on-disc and pin-on-cylinder devices. In the study, very low friction forces emerged at low sliding speeds compared to PTFE and NR operating in water. The friction force values and friction coefficients obtained are presented in diagrams and tables.

1. Introduction

Various types of composite components produced by coating materials such as natural rubber (NR), nitrile rubber (NBR), silicone rubber (MQ, VMQ, PVMQ), flora rubber viton (FKM), and polyurethane (TPU) on a metal body are used in the shipping, construction, textile, machinery, automotive, and construction machinery sectors. These types of machine elements are generally called rubber-reinforced metal products. In the coating process of these, industrial machine elements are produced by vulcanising rubber elastomer, steel, aluminium, brass, and similar hard metals. The most important manufacturing process for the vulcanisation of hard metal elastic materials involves sandblasting the metal to be bonded and correctly applying the appropriate chemical, which differs for each elastomer. Next, the hot vulcanisation process is performed. Parts manufactured using this method mostly serve as steel-reinforced gaskets with limited flexibility, vibration pads reinforced with hard steel, metal-reinforced silicone materials, metal-reinforced sealing elements, and filter gaskets with wire strainers and fluid-lubricated sliding bearings, as shown in Figure 1.

The aim of this study was to investigate the usability of carbon black-added SBR and RSS materials for metal-bodied rubber-coated sliding bearings. These materials, which are used in the production of automobile tyres due to their high wear resistance, are relatively easy to find and process. Since low friction force is required in sliding bearings, lubricated friction force tests of these materials were carried out in this study, which found very low coefficients of friction. It is thought that these materials could be a new and useful alternative for manufacturing bearings. This study makes a valuable contribution to the literature as these materials have not previously been sufficiently investigated as bearing materials.

Prominent studies on the subject in the literature are as follows: Liu et al. presented a new water-lubricated rubber bearing model for vibrating operations [1]. Ixiao et al. examined the tribological behaviour of water-lubricated rubber bearings sliding against stainless steel [2]. Zhou et al. investigated the straight-groove or spiral-channel Stribeck curve of WLRB with NR material in a comparison experiment using the rotational speed and external load as variables and found that the friction coefficient of straight-groove WLRB decreased with an increase in the load over the entire speed range [3]. Hu et al. developed a mechanical stress-resistant device for rubber bearings [4]. Ouyang et al. conducted an experimental study of the dynamic performance of water-lubricated rubber bearings in mixed lubrication [5]. Yuan et al. studied the wear behaviour of NBR/cast copper alloys [6]. Liu et al. investigated the lubricating properties of water-lubricated rubber bearings at high rotational speeds in an experimental study [7]. Cantournet et al. carried out an experimental investigation and modelling of compressibility due to damage in carbon black-reinforced natural rubber [8]. Pang et al. investigated the tribological properties of graphene oxide-reinforced ultra-high-molecular-weight polyethylene under seawater lubrication conditions [9]. Xiang et al. conducted a numerical study of the dynamic properties of water-lubricated rubber bearings under rough contact conditions [10]. Nakanishi et al. presented a new specially designed type of packing capable of insulating with low friction torque [11]. Bhuian et al. performed a seismic performance evaluation of highway bridges equipped with super-elastic shape memory alloy-based laminated rubber isolation bearings [12]. Sun et al. analysed the stability of a rubber-supported thrust bearing under instances of external disturbance in a rotor-bearing system used in marine thrusters [13]. Liu et al. performed stability and dynamic analyses of water-lubricated rubber bearings at two different Reynolds boundary conditions [14]. Hongling et al. conducted an experimental study of the friction-induced vibration of water-lubricated bearings in a submarine propulsion system [15]. Litwin conducted a comparative experimental study between a water-lubricated three-layer plain bearing with lubrication grooves and a rubber bearing [16]. Xie et al. studied the lubrication performance of a water-lubricated bearing under wall shear and inertial force [17]. Dong et al. conducted a study of the wear behaviour of NBR/stainless steel under sandy water-lubricated conditions [18]. Zhang et al. investigated the lubrication performance of a magneto-rheological fluid-lubricated rubber-bearing test ring [19]. Kuang et al. proposed a transparent glass cylinder and NBR rubber block model, capturing the frictional contact area using a high-speed camera, to study the tribological phenomenon in water-lubricated rubber stern bearings [20]. Zhou et al. investigated the relationships between the friction coefficient, specific pressure, temperature, and hardness of vibration noise produced by water-lubricated NBR rubber bearings (WLRB) by using temporal dynamic analysis (Abaqus/Standard). The results showed that proportionally increasing the coefficient of friction and specific pressure caused higher vibrations and the effect of specific pressure on vibration was more pronounced than the coefficient of friction [21]. Zhao and Zhang carried out theoretical analysis and experiments with the finite difference method to determine the hydrodynamic properties of water-lubricated plain bearings in their study and determined that when the speed is increased, the hydrodynamic effect is obvious, and the water film is discontinuous at speeds lower than 1800 rpm [22]. Wang et al. investigated the effects of both friction and wear of a water-lubricated rubber bearing on the bearing vibration using different rubber materials and different operating conditions of the bearing coating, both theoretically and experimentally. The research results showed that both the friction coefficient and wear of water-lubricated rubber bearings are affected by the rotational speed, load, and cooling water temperature, and as a result, the vibration state of the bearing changes significantly [23]. Smith presented a methodology for estimating the minimum film thickness between the cutlass-type WRLB shaft bearing used in ships and the heaviest loaded plate, finding that the theoretical predictions and the experimental results were compatible [24].

It is evident from these studies that the behaviour of rubber-based materials has been examined mainly in water-lubricated conditions. In some studies, vibrations or seismic loads have been added to the water lubrication condition. In a small number of studies, the bearing condition of the rotating shafts lubricated with seawater has been investigated. Yet, SBR and RSS materials have not been presented in the literature as bearing materials. There has also been little research into oil-lubricated rubber, though a study was conducted on carbon-reinforced rubber. Meanwhile, commercial companies have been observed to be using PTFE and NR as bearing elements. Against that background, the need was noted for a study to compare SBR and RSS both with commercial plain bearing materials and with one another. Therefore, this study was conducted to experimentally investigate the frictional behaviour of carbon black-added SBR and RSS materials. The friction coefficient values were also compared under different surface friction conditions (pin-on-disc and pin-on-cylinder).

2. Origin of Frictional and Lubricating Forces

The forces are thought of as a force-distance law of force acting on an object in tribology. Motion or acceleration can be defined according to Newton’s second law,

$$F_x=m\ddot{x}$$

(1)

Frictional and lubricating forces act in different ways. There are no applicable laws of force, and they only occur in response to motion or some other force. Viscous and hydrodynamic forces also fall into this category. These are called non-conservative forces. They cause loss of energy or dissipation and the transfer of thermal, potential, or kinetic energy from one object to another. This phenomenon is known as lubrication and wear tribology (derived from the Greek word for friction) [25].

Figure 2 shows the realistic tribological approach, also known as Coulomb friction, in which energy is transferred. Accordingly, the friction force F_S or lateral force F_[] is written as:

$$F_s, F_{[]}=F_{\perp }· tan\theta =\mu ·F_{\perp }$$

(2)

where µ is the static friction coefficient and F_⟘ is the normal force or internal adhesion force. This equation can be written in relation to the bond number contact area A for molecularly smooth surfaces as follows:

$$F_{[]}=\mu · F_{\perp }+\sigma A$$

(3)

where σ is the shear stress. The first term is related to the structure or topography of the surfaces, and the second is related both to their structure and intermolecular forces [25].

3. Mechanical Properties of Materials

SEM (scanning electron microscope) micrographs, EDS (energy-dispersive X-ray spectroscopy) analyses, and tensile tests were carried out of the RSS and SBR materials used in this study in Sakarya University’s engineering faculty laboratories.

SEM micrographs of RSS are shown in Figure 3 and SEM micrographs of SBR are shown in Figure 4.

Figure 5 and Figure 6 show the EDS outputs of RSS and SBR, respectively.

The hardness values of the samples were measured between 59 and 60 Shore A.

The tensile test results for RSS and SBR are shown in Figure 7 and Figure 8, respectively.

4. Methods

The studies were performed in the mechanical engineering laboratory of the Sakarya University of Applied Sciences. In the experimental tests carried out at 0.25, 0.5, 0.75, and 1 m/s speeds and with 0.2–15 kg loads, using SBR- and RSS-based samples, the friction forces of the materials and the variation of friction forces depending on speed and load were determined by using a wear test device. Two minutes of frictional work was generated for each sample in order to establish stable conditions in the test run.

For the test, a specially manufactured precision-ground pin-on-disc device was used to form the friction surface. The drawings of the disc and pin parts used in the experiments are shown in the Figure 9 and Figure 10.

In order to get accurate results from the experiment, the sample must first be properly prepared. The sample preparation process includes adjusting the samples to the appropriate geometric dimensions after they are supplied.

Furthermore, the rubber test specimens must be cleaned and any foreign materials on the surface must be removed before the test. Otherwise, results obtained from the data collected may not be reliable.

4.1. Test Sample Preparation Steps

• Sourcing of samples in rough sizes;
• The samples to be used in the test device are adjusted to the appropriate diameter on the drill bench with a hollow cylindrical drill (D = 8mm);
• The sample, which has been adjusted to the required diameter, is cut to a length that allows it to be connected to the clamping apparatus in the test device (L = 22 mm);
• In order to absorb the oscillation and stretching of the soft rubbers that will be exposed to axial and perpendicular forces, a steel shaft is inserted into the sample to increase its strength;
• The piece of wire passed through the axis of the sample should be shorter than the sample and should not touch the friction surface of the drilling with a resulting effect on the axial force.

4.2. Test Device and Operational Steps

A precision-manufactured wear and friction test device is used in the experimental studies.

• The friction disc, the surface roughness of which has been removed by precision grinding, is connected to the bearing unit in a way that allows it to rotate without any run-out;
• The disc is connected to the sample holder with the help of a screw (pin), which will cause friction. Then, the sample holder is placed between clamping shoes, and the fixing screws are tightened;
• The specimen (pin) is placed so that it is fully in contact with the disc, and the fixing bolts of the load unit and motor unit are tightened;
• The frequency value corresponding to the cycle to be applied is selected from

  Table 1. Frequency cycle table.

  Frequency	Transfer	Frequency	Transfer	Frequency	Transfer	Frequency	Transfer
  1	4	15	56	29	108	42	156
  2	8	16	60	30	112	43	160
  3	11	17	63	31	115	44	164
  4	15	18	67	32	119	45	167
  5	19	19	71	33	123	46	171
  6	22	20	74	34	126	47	175
  7	26	21	78	35	130	48	179
  8	30	22	82	36	134	49	182
  9	33	23	86	37	138	50	186
  10	37	24	89	38	141
  11	41	25	93	39	146
  12	45	26	97	40	149
  13	48	27	100	41	153
  14	53	28	104

  ;
• Indicators showing the friction force and applied load are activated. One end of the data transfer cables is connected to the indicator (electronic scale) and the other end to the appropriate computer port (VGA input);
• After the computer is turned on, the A&D win CT programme is run;
• The indicator is started and runs until the desired speed is reached. Then, the desired load is applied by tightening the pulling nut on the load arm. The applied load should be displayed on the indicator;
• The program is run by clicking the start link in the programme to be used from the A&D win CT programmes, and then the test device and programme are run for the coded duration;
• After the test is finished, the data are saved.

The test device is shown in Figure 11 and Figure 12.

5. Experimental Results

The experimental tests examined the friction behaviour of SBR and RSS materials, the physical and chemical properties of which were being investigated.

Table 2. Classification table designed for the friction experiments to be applied.

Sample Type	Experiment Type	Friction Status	Sliding RPM	Sliding Velocity (m/s)	Friction Load (kgf)	Coefficient of Friction (µ)
SBR	Pin-On-Disc	Dry	48	0.25	0.23	0.38/0.23 = 1.65
			98	0.50	0.23	0.35/0.23 = 1.52
			142	0.75	0.23	0.31/0.23 = 1.34
			186	1.00	0.23	0.23/0.27 = 1.18
		Oiled	48	0.25	15	0.70/15 = 0.0466
			98	0.50	15	0.26/15 = 0.0173
			142	0.75	15	0.26/15 = 0.0173
			186	1.00	15	0.25/15 = 0.016
RSS		Oiled	48	0.25	15	0.29/15 = 0.0193
			98	0.50	15	0.22/15 = 0.0146
			142	0.75	15	0.21/15 = 0.014
			186	1.00	15	0.20/15 = 0.0133
	Pin-On-Cylinder	Oiled	48	0.25	6	0.48/6 = 0.08
			98	0.50	6	0.45/6 = 0.075
			142	0.75	6	0.40/6 = 0.066
			186	1.00	6	0.35/6 = 0.058

provides a reference to be used when examining friction behaviour.

Various classifications can be made for friction test applications, such as surface states, different speeds, time conditions, motion states such as linear-circular-translation, etc.

The results of the experiments are presented via friction force diagrams (Figure 13, Figure 14, Figure 15, Figure 16, Figure 17 and Figure 18 and Table 2,

Table 3. Data table of dry friction test with SBR rubber sample.

Speed—V (m/s)	Load—N (kg)	Friction Force—Fs (kgf)	Coefficient of Friction
0.25	0.23	0.38	1.64
0.50	0.23	0.35	1.52
0.75	0.23	0.31	1.34
1.00	0.23	0.27	1.18

,

Table 4. Oiled friction test data table with SBR rubber sample.

Speed—V (m/s)	Load—N (kg)	Friction Force—Fs (kgf)	Coefficient of Friction
0.25	15	0.70	0.0466
0.50	15	0.26	0.0173
0.75	15	0.25	0.0166
1.00	15	0.24	0.0160

and

Table 5. Oiled friction test data table with RSS sample.

Speed—V (m/s)	Load—N (kg)	Friction Force—Fs (kgf)	Coefficient of Friction
0.25	15	0.29	0.0193
0.50	15	0.22	0.0146
0.75	15	0.21	0.0140
1.00	15	0.20	0.0133

).

In the experimental study, the first tests were run with the SBR sample under dry friction conditions, and a 0.23 kg load was selected, taking into account the length of the part of the sample outside the clamping element. If a load higher than the selected value had been used, a tensile reaction would have occurred due to friction in the free part of the rubber (between the disc and the fastener), that is, in the plane perpendicular to the sample axis and in the horizontal axis.

As can be seen in the diagrams, a stable friction force curve was formed under dry friction conditions. However, in oily friction conditions, an unstable and increasing friction force curve occurred at the beginning (first 60 s). The most obvious reason for this is that dry friction occurred in the initial conditions, as in many lubricated bearings. Evidently, the friction force curves formed a more stable curve sequence during the second 60 s period under when full lubrication conditions.

In order for the tests to give reliable results, it is necessary for them to be protected from unwanted load and surface tension factors.

A smoother friction force curve was formed in the two-minute oily friction tests of RSS rubber samples. The data obtained from the experiments performed at four different speeds are given in Table 5.

In Figure 19, the experimental results are compared to the values of commercial products. In the range up to a 1 m/s sliding speed, RSS and SBR seem to give values well below commercial PTFE and rubber bearing (MRB) [26]. In this range, the friction forces of all materials gradually decrease as the speed increases. After a speed limit is reached, the friction force in all materials reaches horizontal stability. Since the speed limit of the friction device is 1 m/s, it is estimated that the friction force values between 1 and 2 m/s of the RSS and SBR materials will continue horizontally due to the horizontal equilibrium in the previous speed range. For this reason, value series in the range of 1–2 m/s are emphasized with the suffix (est).

6. Conclusions

As can be understood from the experimental results, the data obtained for RSS and SBR materials were good in terms of friction resistance for rubber bearing applications. The values obtained with the SBR material at low speeds were slightly higher, and at high speeds, they were close to the values obtained for the RSS material. Bearing applications operating at low speeds with SBR material may be inefficient compared to RSS material, but their behaviours at higher speeds are almost the same. As expected, the RSS and SBR materials used in this study gave much better results than commercial water-lubricated rubber and PTFE bearings. The low coefficients of friction observed with RSS even at low speeds put this material one step ahead of SBR as a bearing material. SBR and RSS are both considered good material choices primarily for low-speed (up to 1m/s) sliding bearings. It is estimated that they represent a good alternative up to the 2 m/s speed limit. In this study, the friction coefficients obtained on the pin-on-cylinder surface with RSS were also compared to the values on the pin-on-disc surface and other materials. When the values of the RSS material were examined for two types of friction surfaces, great differences became apparent. These arose because the contact area of the material on the cylindrical surface was smaller than the contact area on the disc surface, resulting in an increase in surface pressure per unit area. In future studies, it will be beneficial to determine the wear rate and depth for RSS and SBR.