ARTICLE IN PRESS

Tribology International 42 (2009) 1146–1153

Contents lists available at ScienceDirect

Tribology International

0301-67

doi:10.1

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journal homepage: www.elsevier.com/locate/triboint

Study on development of polyamide gears for improvementof load-carrying capacity

Hayrettin Duzcukoglu �

Technical Education Faculty, Selcuk University, Konya 42250, Turkey

a r t i c l e i n f o

Article history:

Received 7 May 2008

Received in revised form

3 March 2009

Accepted 16 March 2009Available online 27 March 2009

Keywords:

Wear

Thermal failure

Polyamide spur gears

Durability

9X/$ - see front matter & 2009 Elsevier Ltd. A

016/j.triboint.2009.03.009

+90 0332 223 3322.

ail address: hayduzcukoglu@hotmail.com

a b s t r a c t

In polyamide based gears, thermal damage of the gear tooth surfaces occurs during gear meshing due to

accumulated heat in the tooth body. In the experimental study reported in this paper, polyamide gear

teeth have been modified in order to distribute the generated heat on the tooth surface by means of

drilled cooling holes at different locations on the gear tooth body. The main aims of this paper were to

study the effect of cooling holes on the accumulated heat on the tooth surface and on the measured

wear. It was shown that the drilled cooling holes on the tooth body decreased the tooth surface

temperature and led to an increase in the load carry capacity and improved wear resistance.

Geometrically modified gears have showed an improved service life and a decreased surface

temperature.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Polyamide gears are now widely used because of low friction,noiseless performance, light weight, oil-less conditions, and lowcost [1]. Polyamide gears have been used with success in theautomotive industry, as well as a host of other areas. Polyamidegears are mostly used under un-lubricated conditions. However,polyamide gears have a number of drawbacks. For example,Plastic gears have a low load-carrying capacity, a short service life,and poor heat resistance. These drawbacks typically limitapplications of polyamide gears, particularly in high speed, heavyload or high ambient temperature conditions [2]. Generally,plastic gears show different damage mechanisms such as toothfatigue, creep, excessive wear, plastic deformation, the fatigue andthe plastic deformation, which are hardly separable. Over the pastfew decades, a considerable number of studies have been studiedon the performance of polyamide gears [3–15]. It was reportedthat degradation of the plastic gear materials originated from hightemperature caused by accumulated heat on the tooth surface,which decrease the service life. At the same time, the accumulatedheat on the gear tooth surface resulted in thermal failure, severewear, and early fracture [16–19]. The fracture of part or all of agear tooth has suddenly occurred with increase in the toothsurface temperature. Therefore, accumulated heat on plastic geartooth surface should not be permitted, and the accumulate heatmust be inspected. This accumulated heat should be expelled.

ll rights reserved.

Mao [5,6,20] carried out both measured gear tooth surfacetemperature and gear flash temperature prediction. Polyamidegears are known to have high thermal expansion coefficients. Thethermal conductivity of polyamide material is lower than metals.This leads to more heat accumulation in gears comparedpolyamide with metal gears during service. Some experimentalstudies have mating polymer gears with steel pinions [21–24],which have been mainly confined to running polymer gearsagainst steel. Because of the different thermal characteristics ofsteels and polymers the life and failure modes under theseconditions are very different from those found when runningpolymer against polymer. For example, when running acetalagainst steel, failure is generally due to fatigue at the root of thetooth, while when running acetal against acetal failure isinvariably due to excessive tooth wear. The failure mechanismsof polyamide 6 (PA 6) gears against polyamide gears are still notclear though in engineering polymer gears running againstpolymer gears are more common than using polymer againstmetals.

Studies conducted by the power transmission group from theUniversity of Birmingham were carried out to determine the flashtemperature and partition of generated heat between acetal gearsagainst acetal gear pairs by assuming the gear pair to be ‘‘tworubbing bodies,’’ and solve the problem numerically [20].

The main aim of this study, the effects of cooling holes formedon gear tooth, is to observe and investigate the diminish ofgenerated heat on the tooth surface and wear characteristic underhigh tooth loads and sliding velocities. Experiment results of toothsurface temperature in a gear tooth show that the drilled coolingholes on the gear tooth have increased the service life of the gear

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Fig. 1. (a) Heat source and temperature distribution [25], (b) heat accumulation area [26].

H. Duzcukoglu / Tribology International 42 (2009) 1146–1153 1147

tooth with the aid of a jet centrifuge cooling method, and thesurface temperature of newly design gear is lower than thestandard gear tooth surface temperature.

Fig. 2. Gear heat transfer model [5,6].

2. Surface temperature measurement of plastic base gears

Terashima and colleagues carried out a study on heatgeneration on the tooth surface; Fig. 1 shows the changes intemperature with running time on the tooth surface when the testgears were rotated at a room temperature of 20 1C. The highesttemperature appears at first near the pitch point. This reportconfirmed that the inner temperature was approximately 10 1Chigher than that of the end of the tooth body surface, as measuredby setting a thermo-couple in the inner and middle parts of thetooth. This particular point was the focus of high pressure andfriction, and, consequently, the location where high temperatureprimarily occurs [4].

3. Body temperature prediction

Because of the very low thermal conductivity of polymers,virtually all the generated heat must be removed by convectionfrom the surface. The air temperature around the gears remainsclose to ambient and that there is only a small difference betweenthe temperatures of the contacting and non-contacting flanks ofthe gears suggests the possibility of a simplified model [5,6,20].The airflow into the gaps (module width) between the gear pairs’mating as the gears came out of mesh will be extremely turbulent.This turbulence will be balanced by temperatures on contactingboth tooth and non-contacting tooth of the gear. The temperatureof plastic gear tooth surface was researched by Mao in 2007 and amodel for gear heat transfer was developed (Fig. 2) [5,6,20]. Asshown in Fig. 2, the gear runs effectively as a gear pump. Airflowbetween the gear teeth gaps is pumped from one edge of contactregion to the other. Pockets of air are trapped between gear toothduring meshing and this warm air is moved around the gear. Thewarm air is expelled outside the gear during meshing operationand is replaced by fresh, cold air as the teeth pull apart in thisapproximation; it may be assumed that the warm air within eachtooth pocket will reach a temperature close to that of the gearsurface. Turbulence within each pocket will ensure that the non-contacting gear surface is heated to a temperature close to that ofthe contacting surfaces [6]. Therefore, the bulk temperature of

gear can be estimated by considering the heat capacity of thedisplaced air [5,6].

4. For newly designed polyamide gear tooth, heat dissipationmodel

Fig. 3 shows a graphical representation of drilled air-coolingholes on the tooth body. For drilled cooling hole, the gear axisdirection from number two determines the distance on the geartooth drilled, and the radial direction three cooling holes on thetip circle diameter two were drilled. The gear axis directioncooling hole and the radial direction cooling holes wereintersected. With this kind of a tooth gear configuration, theplastic gear tooth surface was protected from high temperature bydistributing the generated heat by means of the cooling holesformed on the gear tooth body. According to Bernoulli law, amoving fluid generates a lower pressure than a stationary one.Therefore, when the gear rotates it will rotate the surrounding airmolecules in accordance with ‘‘no slip’’ condition. When the gearrotates at a constant revolution speed (W), the points on the gearmove at different speeds according to their radial positions. So, a,

b and c points (Fig. 3) have different pressures and the speeds havea relationship such as (Va4Vb4Vc) and (Pc4Pb4Pa) with the helpof inertia forces on the air and pressure difference. It is obviousthat there will be a mass transfer between the point holes b, c and

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Fig. 3. Gear tooth-cooling model.

Table 1Specification of the test gears.

Pinion gears Driver Gears

Material PA 6 +oil AISI 8620

Module 4.5

Number of teeth 20

Pressure angle (deg) 20

Operation meshing angle (deg) 22.44

Profile shift coefficient 0.177

Diameter of pitch circle (mm) 90

Diameter of tip circle (mm) 100.593

Tooth width (mm) 18 22

Centre distance (mm) 91.5

Contact ratio 1.496

Table 2Properties of the plastic materials.

Polyamide 6 AISI 8620

Density (gr/cm3) 1.135 7.85

Tensile modulus of elasticity (N/mm2) 3000 205,000

Thermal conductivity W/(Km) 0.28 46.6

Poisson’s ratio 0.41 0.3

Hardness M80 56 HRC

Tensile strength, yield (N/mm2) 70 560

Fig. 4. The novel draft test spur gears.

H. Duzcukoglu / Tribology International 42 (2009) 1146–11531148

a by the effect of pressure difference and centrifugal forces.Therefore, the transferred air will expel heat by convection.

5. Experimental

5.1. Gear materials and processing

The specification and material properties of the gear sets usedin this study are summarized in Tables 1 and 2, respectively. In theexperiment studied, Polyamide (PA 6+Oil) was used as drivinggear, and AISI 8620 steel was used as driven gears. The drivengears were AISI 8620 steel with a carburized and quenched andoil annealed, measured using a Rockwell hardness tester with150 da N load, and was 56 HRC at the spur gear surface. Polyamidegears were produced with hobbing machine. All manufacturingoperations were performed under coolant (90% water and 10% boroil), in order to avoid excessive heat due to both poor thermalconductivity of the PA 6 and the heat generated by the turningchisel tip of the CNC machine. During all the manufacturingoperations, the materials have not been allowed to reach excessivetemperature.

The gears, tooth surface smoothness value for both polyamidegears and AISI 8620 was between Ra 0.6 and 0.8mm. KlingernbergPFS-600 Gear Lead/Profile tester apparatus was used to checkcontact errors for all the samples experiments. Initially, the gearswere run for 15 min at a pinion rotation speed of 200 rpm and alow contact load (2 N/mm), in order to smooth the originalmachine-finished teeth surfaces.

The manufactured gears were divided into two groups. Onegroup has the standard tooth body without any variation and theother group was formed by drilling cooling holes of a differentdimension and direction on the gear tooth body, as seen in Fig. 3.All kinds of PA 6 gears were run against AISI 8620 steel gears. Thesize and locations of cooling holes are shown in Fig. 4.

5.2. Gear test

All gear experiments were performed using FZG test machine(Fig. 5). The FZG test machine is a power-circulating test machinetest gear tooth failure apparatus [27,28]. The closed loop waschanged with 7.5 kW DC electric motor driving vehicle. Gearloading was generated by FZG closed loop geared system. In thisclosed loop, number 5 shaft was fixed with a pin and a twistingmoment was generated in number 6 shaft by applying a gearloading with an arm. Each PA 6 pinion specimen was operateduntil it arrived at a total revolution of 4.2�105 or until it revealedbreakage, then, variations of the tooth surface temperature and

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Fig. 5. Schematic view of the gear test rig [27,28]. 1: DC Motor, 2,3: Gears box, 4,5,6: Shaft, 7: Circulating gears, 8: Pinion and gears (Test gears), 9: Coupling 10,11: Load

coupling 12: Washer, 13. Support tube.

Table 3Test conditions.

Applied load (Nm) 6.12, 10.32, 16.53 and 23.3

Rotation speed (rpm) 1750

Pitch line velocity (m/s) 8.24 (Polyamide 6 gears)

Revolution 4.2�105

Environment, temperature Air, 20–23 1C

Humidity 30–45%

H. Duzcukoglu / Tribology International 42 (2009) 1146–1153 1149

the service life of the newly PA 6 gear pinion and original pinionwere evaluated.

The test conditions are shown in Table 3. The experimentalset was not stopped unless a tooth was broken or there wassudden thermal damage, because the gear tooth flash tempe-rature decreased immediately. The tooth surface temperature wasmeasured and investigated using a non-contact type temperaturesensor that was installed [3], as shown in Fig. 6. Non-contactinfrared temperature sensors were mounted at a distance of about6 mm from the contact surface of the test gear.

After each experiment, the cooperating AISI 8620 gears werereplaced with the second substitute gears and cleaned with asolvent. For each experimental condition, eight gears were used.The experiments were carried out under dry conditions. The netsurface temperature of the test gears were monitored continu-ously using a computer-based data acquisition system. The toothsurface temperature and the service life of the polyamide gearswere observed and evaluated after the test ended. The wear depthconsisted at tooth profile of polyamide gear and the wear due toweight loss are difficult to measure. For this reason, the thermalfailure in the tooth profile duration test was at the surface, and thewear particles were not exactly separated from the tooth surface.

However, the wear depth in the profile of newly designed gearsand original gears were measured and the change in profile wasdetermined. For measuring the wear of the gear contact profilesurface after experiment, to allow for more sensitive measure-ments, the tooth height (y axis direction) was divided into a totalof nine spaces (between from tip to root)(Fig. 7). Thus, Initialprofile of the spur gear tooth was measured before the experi-ment and the profile (x axis direction) after the experiment wascompared by measuring the wear depths to determine the profilechanges. Measurement of tooth profile wear requires the highestaccuracy possible. The SM350 Vertical Profile Optical Projectorapparatus was used to measure the profiles. The traditional

methods with a SM350 Vertical Profile Optical Projector proved tobe of the highest accuracy of the existing equipments. The opticalprojector works by producing a magnified silhouette of the objectthat can be measured with an accuracy of 0.001 mm [27].

6. Result and discussion

6.1. Service life of original spur gears

The extreme thermal damage and the excessive wear failureoccurred in standard polyamide gear tooth under high tooth load.In standard gear tooth, the accumulated heat on the tooth surfacecan be partially removed by convection from the surface.However, as shown in Fig. 8, there has been less expelsion ofgenerated heat to the outside environment as shown with hiddenlines. The mechanical properties of PA 6 material were reducedwith the increase of accumulated heat in this region, and themechanical properties of polyamide material degraded due tohigh temperature. Therefore, the service life of these gears wasshortened, especially under heavy load. For increasing the servicelife of polyamide gears, they were run with AISI 8620 pairing gearswhich have a high heat conductivity coefficient. However, the factthat the polyamide gears were run against steel gears could aidthe convection of heat only up to certain levels of tooth load. Aftera certain PV value, the generated heat on the tooth surfacesuddenly rose during mating of the spur gear pair due to frictionalheat, leading to thermal damage.

As shown in Fig. 8(a), there was no thermal damage on thetooth surface throughout the operation in which PA 6 gears andAISI 8620 pair gears with a gear torque of 6.12 Nm were used, butwear did occur in the tooth profile. In this case, it means that thetooth load is appropriate and the accumulated heat on the toothsurface is expelled by the convection heat transfer of against steelgear pair. Thus, the increasing of accumulated heat on the toothsurface wasn’t permitted against gear.

As seen in Fig. 8(b–d), under applied high tooth load, becauseof the tooth surface temperature (especially around the pitchpoint in the middle of the tooth), the sides of the tooth exposed tothe load were bent and the middle part is exposed to wear. Duringgear running, because of the friction and hysteresis effect, somesoftening and some pitting holes occurred around the pitch zoneshown as an ellipse. The damage effect of the accumulated heat onthe tooth surface was formed in the middle of the tooth surface

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Fig. 6. Measurement of a tooth surface temperature.

Fig. 7. Meshing point and positions of the intermediate points on the gear flank.

H. Duzcukoglu / Tribology International 42 (2009) 1146–11531150

because of material softening (Fig. 8b–d). The thermal damage atthe tooth surface was formed at high tooth load (Fig. 8d).

It is observed that the newly designed spur gears, thanks to thedrilled cooling hole protected the gear materials from the hightemperature by increasing the heat transfer. The newly designedspur gears have a longer service life than the standard spurpolyamide gears even under all loading conditions. It was seenthat while the tooth surface of standard gear formed surfacesoftening at around the pitch point region, while original gearsrevealed the highest tooth surface temperature at the center areaof the tooth width, this tooth damage at the newly designedcooling holes gears wasn’t observed. Though, the standard geartooth has showed residual deformation at pitch points, thisdamage has not been formed on modified gears due to increasedheat transfer which decreased the tooth temperature.

(Fig. 9a–d). However, wear was observed at the root and toothtip. With the effect of the centrifuge air, which is the result ofcooling by the rotating of the drilled holes, air was blown throughthe first hole outside. The second hole was opened to expel theheat accumulated in the center of the tooth contact surface andthe third hole was used to expel the heat accumulated at thesecond hole (assisted by the cooling effect of air turbulence) andto provide fresh air to the first hole.

When the gear rotates at a constant revolution speed (W), thepoints on the gear move at different speeds according to theirradial positions. The holes have different pressures and the speedshave a relationship as Pc4Pb4Pa and Va4Vb4Vc with the help ofinertia forces on the air and pressure difference. It was obvious

that there was a mass transfer between the holes. Therefore, thetransferred air expelled heat by convection, with the toothtemperature especially at pitch point.

As shown in Fig. 9(d), tooth breakage occurred under 23.3 Nmthe tooth torque for some specimens. It was observed that gearmaterials, mechanical properties were decreased by the drilledcooling holes in the tooth body. So, the geometric positions anddimension of the drilled cooling holes in the tooth body must becarefully selected.

6.2. Tooth surface temperature

For the standard spur gears, as seen in Fig. 10, under a 6.12 Nmtooth torque, the tooth temperature steadily increased at thebeginning of the experiment and stabilized at around 66 1C asthe steel gear pair came to a heat balance. At the beginning of theexperiment, under 10.32 Nm torque, the tooth temperatureincreased faster as compared to the previous load, and the toothsurface temperature was stabled around 83 1C.

The increase of the tooth load on the PA 6 material rapidlylowers the heat-deformation temperature, which leads to asoftening of the material. This can be explained by the observationthat PA 6 materials suffer polyamide deformation and severe wearthat is relatively high. This leads to a decrease in the mechanicalstrength of the PA 6.

With the increasing amount of tooth load, it was observed thatrapid increases in the tooth surface temperature occurred under16.53 and 23.3 Nm torque (Fig. 10). Particularly under a 23.3 Nmtooth load, the amount of the tooth temperature increased veryquickly up to 120 1C. This leads to a decrease in the mechanicalstrength of polyamide, and finally, tooth breakage occurred onthe tension side of the standard gear tooth. With regard to the16.53 Nm tooth torque, the tooth temperature reached almost thesame levels. However, under a 16.53 Nm tooth torque, the surfacetemperature rose to 120 1C but over a longer time than it didunder higher loads.

Fig. 10 shows that the tooth surface temperature under a23.3 Nm tooth torque was lower than that under 16.53 Nm torque.This decrease in the tooth surface temperature was thought tohave resulted from less friction as a result of tooth damage, andwas not produced as heat during contact. At the same time, thisdecrease also indicated that the gears were damaged. As an effectof the tooth temperature, the material endurance and rigiditydecreases. Because the temperature that occurs within thematerial was higher than the heat expelled, the accumulatedheat of the material body increased continually. After a certain

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Fig. 8. The standard damage under a certain tooth load; (a) 6.12 Nm, (b) 10.32 Nm, (c) 16.53 Nm, (d) 23.3 Nm.

Fig. 9. The damage in newly designed gears under certain tooth load, (a) 6.12 Nm, (b) 10.32 Nm, (c) 16.53 Nm, (d) 23.3 Nm..

H. Duzcukoglu / Tribology International 42 (2009) 1146–1153 1151

level of temperature, the features of the materials decreasedresulting in diminished load-carrying capacity and damage.

This means that the polyamide material that experienceda high temperature exceeding its glass-transition temperature,was then degraded, and rapidly lost its mechanical properties.Subsequently, mass plastic flow and wear take place. Abovecritical loads, the thermal failure and the tooth breakage is veryrapid and becomes excessive until the teeth eventually melt,causing softening and failure by gross tooth bending. The proper-ties of polyamide materials are sensitive to temperature, and an

increase in temperature therefore affects the mechanical andtribology properties of the gear material. Most of the wear isobserved at the tooth root and tooth tip.

When using the newly designed spur gears, at all tested toothload conditionals, the new spur gears have shown a lower surfacetemperature as compared with the standard gear tooth surface.The tooth surface temperature was decreased by the effect of thedrilled cooling holes in the tooth body. Under 6.12 Nm toothtorque, the gears reached a balance of heat at around 57 1C due tothe centrifuged cooling and AISI 8620 pair gears (Fig. 11). No

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Fig. 10. The original spur gears tooth outer surface heat under various loads.

Fig. 11. Variation in tooth surface temperature of modified polyamide gears at

different tooth load (PA 6/AISI 8620 gear pairs).

Fig. 12. The results of the experiment PA 6 +oil spur gear wear.

Fig. 13. SEM image of a tested plastic pinion surface: (a) Original gears and (b)

modified gears.

H. Duzcukoglu / Tribology International 42 (2009) 1146–11531152

thermal damage was seen on the gear surfaces. The toothtemperature occurred with standard gears under the sameconditions, with a maximum of 66 1C.

As the tooth temperature under 10.32 and 16.53 Nm toothtorques increases, at the beginning of the experiment, the toothtemperature increased more slowly in the experimental gearsthan in the standard tooth. Later in the experiment, the toothtemperature reached a thermal balance and this temperature wasthen stabled. This can be explained by the fact that the hystereticheat loss of the PA 6 was decreased by the hole in the tooth, whichabsorbed the deformation of the material.

This means that the drilled cooling hole on the tooth body willabsorb the deformation energy, resulting in the decrease intemperature. These low temperatures might reduce the possibilityof heat-deformation and maintain the mechanical strength of PA66 materials. When compared with the standard gear tooth under16.53 Nm torque, lower run temperatures were observed andthere was no thermal damage. It might be inferred from thesedata that the heat transfer by the drilled cooling holes, asmentioned before, led to a decrease in the tooth surfacetemperature, and these effects were enough to maintain thetemperature at a constant range.

When the tooth torque was increased to 23.3 Nm, the toothsurface temperature increased for a certain period of time (100 1C)and then reached a thermal balance at around 84 1C. However, underthis tooth load, two of the eight-piece experimental gear teeth werethought to have broken as a result of the fact that their second holeshad larger diameters, which decreased the endurance of the toothand led to its breakage because of bending tension (Fig. 9d).

7. The tooth profile change due to wear

For the PA 6 gear, above the pitch point the wear was smootheras the tooth roll and slide in the same direction. With the

increasing amount of the tooth load, the tooth profile wear of gearincreased under the pitch point especially in the region of toothroot and tooth tip (Fig. 12). In this region, as the slide direction iscontrary to the rolling direction, there is more friction. The weardepth on every point along the profile of a newly designed gearwas measured at relatively lower levels as compared to that of astandard tooth profile. However, the standard tooth profile hasobserved thermal failure at the pitch region vicinity, even if wear

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H. Duzcukoglu / Tribology International 42 (2009) 1146–1153 1153

depth of the gear tooth profile under high load conditional (16.53and 23.3 Nm) was observed small (Fig. 8c and d). The root wearwas greater in the driving gears because of the friction force.The tip of the driven gears AISI 8620 caused wear in the root ofthe polyamide gears. The wear depth profile was not uniformalong the tooth face and wear at the root and tip was considerablymore than at the pitch point. However, the pitch point vicinity ofstandard gear tooth profile was formed on the gear thermal wear.The tip of the AISI 8620 driven gears caused wear in the root of thepolyamide gears.

Scanning electron microscope (SEM) observation of a PA 6tooth surface in the present tests (Fig. 13, after a run of 4.2�105

revolutions) reveals evidence of the surface failure. In Fig. 13(a),some amount of particles can be seen detached from the surfaceof the standard gears because of thermal damage. The particledetachment from the surface causes pitting in the shape of thegears.

Many transverse cracks can be seen on the worn surface.As shown in Fig. 13(a), severe spalling can be seen between eachpair of cracks and it has been suggested that debris collectedduring the tests was mainly produced in this surface area. Somecracks propagated to join their neighbors, and, as a result, thematerial between the two cracks was fractured, severe spallingoccurred, and debris was formed. It was noted that the crackpropagated and fractured gradually rather than suddenly. It wasalso seen that the particles tended to cling to the surface of thegears. In Fig. 13(b), there were no particles observed detachedfrom the material but the surface of the tooth was still softenedand there was a large amount of flake-like material. It appearedthat there is a layer of film and some flakes on the surface.

8. Conclusions

The following results were obtained;

1.

The thermal conductivity of polyamide gears is generally notgood; softening and the detachment of large particles from thepitch area of standard polyamide gears occur at certain levelsof load and velocity.

2.

Standard gears revealed the highest tooth surface temperatureat the center area of the tooth width, and polyamide materialslost their mechanical properties due to high temperature andcaused the vast wearing of the original polyamide gears.

3.

When the standard polyamide gear was run against steel gear,under certain PV values, the gear pairs reached a thermalequilibrium after a few cycles and no more temperatureincrease was observed. In case of high PV values, a thermalequilibrium was not observed and the gear pair showedcontinued temperature increase until tooth failure.

4.

An unexpected failure was observed of standard polyamidegear when the surface temperature showed a sudden increase.

5.

The cooling holes machined on the tooth body delayed thedamage by increasing the heat transfer and avoided the surfacedamage near the pitch point.

6.

Especially at high loads, tooth breakage has been observed onnewly designed gears. But it is concluded that this breakagehas not arise due to the softening at the tooth body but hasoccurred due the stress concentration around the coolingholes.

7.

One of the most important problems within polyamide gears isthe limited load-carrying capacity due to surface damagebecause of accumulated heat especially at high loads andsliding velocities.

8.

It is concluded that the newly designed gears have a longerservice life than that of original gears.

Acknowledgment

This work is supported by the Coordinatorship of SelcukUniversity’s Scientific Research Projects.

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