Proceso de Evaluación del Desempeño

The total wear volume increases with sliding distance during sliding between two contacting bodies. In general, there are three stages, each with the typical trends. First is the running in (run-in or break-in) stage, where the period is short for dry sliding and the wear rate is usually quite high because surfaces in contact are wearing to remove the asperity peaks. Then, it enters a steady state stage, is the main stage of the tribological process. The wear rate is usually lower than during run-in and it lasts up to the end of the useful component life. However, in some situations a wear transition may take place after some sliding distance when the wear rate may increase or decrease. This transition is led by a change in the wear mechanisms, often accompanied by a change in temperature, friction coefficient or both. Examples of wear curves are shown in Figure 2.1.

16.10Nm 14.36Nm 13.05Nm 10.79Nm 10.00Nm 9.50Nm 9.00Nm 8.50Nm 8.00Nm 7.00Nm 0.0 0.0 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0

Test Duration (cycles × 105)

W e a r (mm )

18 35 Nm 25 Nm 30 Nm 20 Nm 15 Nm 10 Nm 5 Nm 2.0 1.5 1.0 0.5 0.0 0.0 0.3 0.6 0.9 1.2

Test Duration (cycles × 106)

W e a r (mm )

(b) Composite against Composite gears [24] Figure 2.1 Wear curves of non-metallic gears

Failure modes of polymeric gears

For polymeric gears, the failure modes are mainly wear, pitting, fracture, scuffing and plastic flow [7, 18, 25].

Pitting as shown in Figure 2.2, is a fatigue phenomenon arising in the motion that combines sliding and rolling actions. It is very common for bearing and gears. The formation mechanism of pitting is cyclic surface contact stresses that exceed the material endurance limit.

Helical gear, 35 HRC Spur gear, 60 HRC (b)

(a)

Figure 2.2 Pittting: (a) Pitting form of plastic gears (after [26]), (b) Photographs of pitting on iron gear teeth (after [2])

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Fracture as shown in Figure 2.3, may take place at tooth root or pitch line due to fatigue and overload. Fracture at the tooth root or pitch line will depend on material properties, which for polymeric gears can vary quite widely and are importantly temperature dependent.

(b) Cracking at root (a) Cracking at pitch circle

Figure 2.3 Fracture forms of plastic gears (after [26])

Scuffing is normally caused by local excessive heat, as the surface material is removed rapidly by welding and tearing. It is associated with flash temperatures. With polymer gears, high loads are likely to induce scuffing. An example of scuffing wear on metal gear tooth is shown in Figure 2.4.

Area where the coating has spelled off

Scuffing damage

Relatively low scuffing damage in area where coating had remained

Figure 2.4 Photograph of a case carburised metal gear with Me-DLC coating following scuffing (after [27])

Plastic flow results from high contact stress and relative sliding motion, where the contact area material has been thermally softened. Peening,

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rippling and ridging are also forms of plastic flow. The examples of plastic flow of polyacetal gears are described in detail in Section 4.2.2.3 (see Figure 4.17: SEM micrograph of plastic flow on polyacetal gear)

Marshek and Chan revealed that pitting and ridging are major contact surface damage for phenolic and polyacetal worm gears [28]. Terashima et al. found that abnormal wear frequently resulted in tooth fracture near the pitch point of (maching, Hobbed, improved) 6-nylon gear (S45C steel gear against 6-nylon pinion) [29]. Senthilvelan and Gnanamoorthy found that the failure of injection molded unreinforced, carbon reinforced and glass reinforced Nylon 66 gears was usually due to fracture near the tooth root [30]. Fracture near the tooth root also occurs to composite gears (55% nylon, 30% glass fiber and 15% PTFE) [7]. Tooth breakage under fatigue mostly arises from bending stress [31]. Pogacnik and Tavacr found that acetal (Acetal 500P, DuPont) and Nylon 6 gears failed by fatigue and sudden melting [32]. The melting failure took place within a few hours of the gear test being initiated. Mao proposed solutions to reduce gear fatigue wear through a micro-geometry modification approach, such as face-width crowning, lead correction and tip relief [33].

Thermal-mechanical behaviour

Surface temperature can indicate or reveal critical information on the conditions of operating gears. Thermal cycling causes microscopic changes in the internal structures of materials and the cyclic cumulative changes consequently result in cracks on gear surfaces. Many thermal-mechanical

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wear behaviours of metallic gears have been investigated [34-46], in terms of the temperature of gears or their lubricant oil affect life, the quality of the lubrication film, pitting, cracking, spalling, scoring, scuffing and so on. Polymeric gears are significantly more susceptible to surface temperature than are metallic gears. Therefore, the thermal behaviour of non-metallic gears is being studied by numerous researchers via experiments and theories. For instance, Yousef and Burns built test rig and IR microscope to assess the running temperature and fatigue strength of lubricated acetal and polycarbonate gears [47]. Gauvin, Patrick and Henry used an infrared radiometer to measure the temperature distribution along the tooth contact profile of hob cut Nylon 66, acetal and UHMWPE (Ultra High Molecular Weight Polyethylene) gears and adopted statistical analysis of test results to predict the maximum operating temperature [12, 48]. Hooke, Mao, Walton Breeds and Kukureka measured the operating temperature of plastic gears (such as acetal, Nylon 66, PEEK and glass reinforced Nylon 66 composite gears) by using infrared camera video camera [7, 21, 24, 49-53]. They reported that for acetal gears there was a sharp rise in wear as transmitted torque was increased and proposed that the wear transition was effectively associated with acetal elting point. Based on the experimental results, they predicted the body and flash temperature by using a finite element method. Melick and Dijk studied the fatigue lifetime of Stanyl polymide (Nylon 46) gears running in oil lubricant at 140°C and made correcting for tooth bending and calculating actual root stresses [54]. Terashima et al. measured the operating temperature of pairs of a hob improved 6-nylon gear running against a S45C steel gear by using three thermos-paints and estimated the

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temperature distribution on tooth flank [29]. Karimpour, Dearn and Walton investigated into the contact behaviour of Delrin® _{500 (Du Pont) gear}

transmissions using numerical finite element and considering the operating temperature effects on the properties of polymeric materials [55]. A rise in operating temperature results in a reduction in the mesh stiffness, which affects load sharing ratio due to tooth bending. Hoskins, Dearn, Chen and Kukureka studied the wear mechanism of poly-ether-ether-ketone (PEEK) running against itself in non-conformal, unlubricated rolling-sliding contact over a range of loads and slip-ratios by employing a twin-disc test rig [56]. Test results indicats that a rapid increase in the coefficient of friction is due to the temperature exceeding the glass transition temperature of the material, which affects the friction and wear. The test results were suggested to be in conjunction with the designs of more effective and highly loaded polymeric gear systems. Walton and Shi compared ratings of plastic gears and stressed the influence of body and surface temperatures on the properties of polymeric material [26]. Singh and Singh measured the wear and surface temperature of polymeric gears (i.e. ABS, HDPE and POM gears) running against metal (AISI 1040) gears and presented specific wear rate and surface temperature rise at various loads and speeds [57]. However no dynamic relationships between wear rate and surface temperature of polymeric gears are presented. The surface temperature prediction and the relationships between operating temperatures, wear, wear rate, load and speed still need to be explored deeply in the hope of establishing reliable and predicted design standards or references.

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dissipations, temperature estimations and known achievements on thermal experimental investigations will be outlined in following sections.