Wear rate velocity

PV (MPa-m/s) Pressure, P (MPa) Velocity, V (m/s) Coefficient of friction Wear rate after 1000b (mm)... 

Figure 8.12 illustrates a solid particle impinging on a surface. It has been found that the erosive wear rate depends upon the impingement angle, a, the particle velocity, vq, and the size and density of the particle, as well as the properties of the surface material. It has also been found that there is a difference in erosive wear properties of brittle and ductile materials. The maximum erosive wear of ductile materials occurs at a = 20°, whereas the maximum erosive wear for brittle materials occurs near a = 90°. Since the impingement angle is probably lower than 90° for these type of flow situations, we might consider only brittle materials, such as ceramics for this application. Let us examine brittle erosive wear in a little more detail first. 

The dependence of erosive wear rate, W, on particle velocity, vq, has been observed to follow the general form... 

At high particle velocity (above 50 m/s), the wear rate increases more than what would be expected from straight kinetic modeling (Arnold and Hutchings, 1990). This is believed to be from higher formation and propagation of microcracks. 

The rate and amount (flux rate) of material impinging on the elastomer have an effect on the wear rate of the elastomer (Arnold and Hutchings, 1989). With flux rates of between 500 and 5000 kg nr2 sec-1, the erosive wear decreased with flux rate at low velocities and low angles of incidence. At higher flux rates (1000 to 10000 kg nr2 sec-1), the wear rate decreased when the velocity was lower and the angle of impact was more normal to the surface. 

Walker and Bodkin (1993) discuss the advantages and limitations of a number of the commonly used erosive wear testers. The influence of particle size and shape are very important as well as the impingement angle and concentration of the slurry. They found that the wear rate increases with the jet velocity to the power of 2.2 (Mens and de Gee (1986) gives 2.8-3.2.). Wear rate is at a maximum at 30° impingement angle. The mechanism is mainly cutting. The rate increases with the size of the particle. 

The PV (pressure x velocity) convention is utilized to define the maximum combinations of pressure and velocity at which a given material will operate continuously without lubrication. The values are usually given for operation in air at temperatures of 21°C-27°C. PV limits do not always define the actual combinations of pressure and velocity where the material can be practically used, because wear is not considered in the determination of PV values. In other words, the application must not exceed PV limit and wear limits of a material. Such a limit can be determined by finding the pressure and velocity combination at which wear rate accelerates or exceeds the expected life of a part. 

Coefficient of friction is inversely proportional to pressure and proportional to velocity. Wear rate of fluoropolymers is proportional to load (/ ) and velocity (V). Combinations of pressure and velocity are defined where the material can be used, thus a FV limit is defined. Above this PV limit, the wear increases exponentially because of the heat that is generated as a result of motion. Generally, a polymer or its compounds can be characterized by PV limit, deformation under load, and wear factor. Wear factor or specific wear rate is defined as the volume of material worn away per unit of sliding distance and per unit of load. 

Figure 5. The wear of the tissue with time (For all experiments the plate was made to oscillate at an average velocity of 0.52 M/min. The abscissas can therefore be converted to distance slid.) Curve A pressure on the tissue alternated equally between 0.69 and 6.9 MN/m at a frequency of 0.02 Hz. Curve B the pressure was alternated equally between 0.69 and 4.14 MN/m at a frequency of 0.02 Hz. Note the initial higher wear rates and the two steady state rates of Curve B.

The search for complete understanding of friction properties led to the methods (17), (18) accounting for the combined effects of the main factors. Prom Ref. (l ) relations are found for the friction coefficient, temperature, wear rate versus sliding velocities and loads. Then by the data obtained, a set of curves is drawn in P — V coordinates, having the same values of the friction coefficient, temperature, and wear rate. It is clear that great difficulties arise in obtaining and using this volume of information. Crease (j ) finds only... 

The set of loads and velocities at which the wear rate reaches 0.25 yum/h for the polymeric material is determined from the curve of the [PV] limiting values. From Table I a minimum velocity is selected and a load applied that provides pressure on the friction unit of o.5 [PVl. After 100 h operation the wear is measured. Should its value be over 25 yUm, another testing is conducted at a lower loading level. Should the wear be below 25 yum, the load is increased up to a next level. 

Similar tests are conducted for all the rest ranges of sliding velocities. The test results are presented as curves from which a complex of loads and velocities is selected that corresponds to wear rate of 25 after 100 h opera-... 

Figure 3. Wear rate dependence on load and sliding velocity.

Figure 4. Card for friction characteristics of materials. The conditions and friction test results from Figures 2 and 3 are filled into cards. [PV] and PV23 curves in P - V coordinates are a complex of loads and velocities at which limiting operation regime [PV] and wear rate 0.25 /im/h (PV25) are reached.

Fig. 4.1. Wear rate of steel with 0.52% carbon content depending on loading. Velocity - 0.1 m/s [10]...

The friction joint has been tested using a shaft on a bush friction machine with 2cm friction area, 0.35 MPa load and 2.4 m/s sliding velocity. A 40-mm-diameter shaft has been made of a carbon steel of 40-45 HRC hardness and 0.8-1.0 xm surface roughness. The outer bush material was aluminum, the inner was copper, the polymer layer was 200-p.m-thick PVB. A 0.1 N solution of NaCl was fed into the friction zone, the wear rate was determined by weighing. The test results are presented in Table 4.7. 

Worn-out dies constitute a substantial cost factor in the extrusion of ceramics. Their wear rate is extensively determined by the column s advance velocity, the extrusion pressure and the composition of the body. 

Friction and Wear. Graphite, molybdenum di-sulfide, and PTFE filled PABMs exhibit peirticularly low wear rates in dry friction vs metal at high PV s and, in particular, at high pressures applied at low velocities. 

Wear tests were carried out at room temperature imder dry condition. Normal load values of 0.98, 1.96, 2.94, 3.92, and 4.9 N were used. Sliding velocity was fixed at 0.02 m/s, and the sliding distance was 120 mm. Wear of the pin was measured by a gravimetric method using an electronic balance at a 0.0001 g precision. Each worn surface was measured with a profilometer after the wear test to obtain profiles normal to the direction of friction. The profiles were used to calculate the wear rate. The wear rate, w, is defined as w = V/L, where V is the wear volume and L is the sliding distance. Each point of the diagrams from the experimental results is an average of five tests and measiu-ements. 

Quinn, 1980 dA, exp[-Q/(R,TJ] 3eyvH Wcorr = wear rate p = density of material Aj, = Arrhenius constant Q = activation energy Rg = gas constant Tj, = contact temperature d = asperity contact diameter V = sliding velocity e = critical thickness of reaction layer asperity layers formed tribochemically are... 

At time erit> which corresponds to the sliding length = ferit i si> a volume of oxide Ar Lcrit breaks off from the surface. Here Vji designates the shding velocity and the real contact area. If the specific volume of the oxide is approximately equal to that of the metal (if not, it would be simple to introduce a correction factor), the wear rate (in m per m) is equal to ... 

In this expression is the film growth current and kp = M/pnF is a constant taking into account Faradays law (c.f. Section 6.2.2). The second term represents the wear rate of the film in m/s. It is equal to the wear rate per unit length, given by equation (10.19) multiplied by the sliding velocity Vsp... 

Velocity This is the combination of all the planar motions of the pad and the conditioner. Pad wear rate increases with conditioning velocity. On the other hand, surface roughness would be more mgged because of higher conditioning velocity. Wafer MRR increases with pad speed. 

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