Failures of Gears

The most common types of wear arePitting, Scuffing and Scoring:

Pitting is caused by localized metal fatigue on the surface of the gear. Microscopic cracks are forced to propagate which causes small metal chunks to break off. The best ways to avoid this are:

- reduce contact stress- use hardened gears- use proper lubrication

Scuffing is caused by small surface irregularities that can rub each other as gear teeth come into and out of mesh. It is caused by the plastic deformation of microscopic surface protrusion.The best ways to avoid scuffing are:

- use gears that are cut to a high quality surface finish

- “run in” with1/2 load for the first 10 hours- use proper lubrication at all times

Scoring is caused by small particles in the lubrication that get caught in the meshing teeth. This causes scratches that can extend from the roof to the tip. These can be quite deep. The best ways to avoid scoring are:

- proper high quality lubrication- change the lubrication after the “run in” period- change or filter the lubrication as needed

Scuffing damage on a gear tooth

Severe scuffing of an unhardened gear

Scuffing damage on the tooth faces of a drivengear showing also the tendency to ridging at thepitch line as the sliding action of the teeth tendsto drag the surface material towards the pitchline

Pitch line pitting on a case hardened pinionwhich has operated at high load and low speed

Pitting in the dedendum of a helical gear

a: Gear teeth showing signs of pitting after a shorttime in service

b: The same teeth after one year in service c: Some improvement two years laterd: Almost smooth after three years

Appearance and extent of scoring and wearsustained by the propellershaft rear thrust flange contact surface.

When gears mesh, the tooth surfaces roll andslide against each other

Figure 1. Helical Gear Terminology Figure 2. Direction of Sliding

This meshing creates enormous contact and shear stresses. Oil is used to lubricate the mating gear teeth and prevent scuffing,

wear and pitting damage to gear tooth surfaces due to metal-to-metal contact.

Gear Surface Distress

The mating surfaces of gear teeth are not perfectly smooth. The surfaces are full of small asperities, such as machining marks. This is referred to as surface roughness. Ifthe oil film is thick enough, the asperities fromthe mating gear teeth do not contact oneanother. Based on elastohydrodynamiclubrication (EHL) theory, lubrication falls intoone of three regimes. In Regimes 1 and 2there is some contact between the matinggear teeth. Most high-speed gears fall intoRegime 3, in which full EHL oil film developsand the asperities in the tooth surfaces do not contact one another.

Gear tooth distress includes scuffing, wearand surface fatigue (micropitting andmacropitting). ANSI/AGMA 1010-E95 (1995) provides descriptions and pictures of thesetypes of damage.

Scuffing

Scuffing is damage to a tooth surface due to welding and tearing of the tooth surface by the flank of the mating tooth.

Figure 3. Scuffing (Courtesy of ANSI/AGMA 1010-E95)Mild (Left), Severe (Right)

According to AGMA 925-A03 (2003), “Thebasic mechanism of scuffing is caused byintense frictional heat generated by a combination of high sliding velocity andhigh contact stress.” Scuffing will normallynot start at the pitch line because the slidingvelocity is zero. Thus, scuffing generallystarts in either the top or bottom half, or

root, of the teeth.

Wear

Wear is defined as the removal ordisplacement of metal from gear toothsurfaces. Damage due to scuffing or pitting is not normally considered wear. Wear reducestooth thickness and can change the contourof the teeth. Wear can result frommechanical, chemical or electrical action.

Micropitting

Micropitting is a high rolling contact fatigueincidentthat occurs in the Hertzian contact band area. Itis a function of combined rolling and slidingvelocities, load, temperature, specific film thickness and the lubricant itself. AGMA 925-A03 (2003) states: the pits are typically 10 to 20 µm deep by about 25 to 100 µm long and 10 to20 µm wide

Figure 4. Micropitting Courtesy of ANSI/AGMA 1010-E95

Macropitting

Macropitting is also a surface fatigue phenomenon. These pits are typically on the order of 0.5 to 1.0 mm in diameter and are large enough to be seen by the unaided eye.

Figure 5. Macropitting Courtesy of ANSI/AGMA 1010-E95)

Dişli Hataları

Dişlilerde tüm hatalar mil ( eksen kaçıklığı, dengesizlik,gevşeklik) ve diş(aşınma,çizilme,çatlak ) ile ilgili problemler olarak sınıflandırılabilir.Dişli hataları , hata türüne özgün belirgin titreşimler oluşturur. Bu yüzeyden diğer makine elemanlarında olduğu gibi dişli hataları titreşim analizi ile belirlenebilmektedir.Dişlilerdeki titreşimin birinci kaynağı üretim ve montaj hataları kadar kavrama esnasında rijitliğin değişmesidir.Çalışan tüm makine elemanları yorulmaya maruz kalırlar.Yorulma tüm çalışan elemanlarda olduğu gibi dişlilerde hasarlar oluşturmaktadır.

Dişlilerde yorulmadan kaynaklanan iki çeşit hasar vardır: Oyuklaşma ve kabarma. Oyuklaşma , diş yüzeyinden çok az miktarda malzemenin ayrıldığı ,yüzey yorulma hasarıdır.Yükün diş üzerindeki bir bölgeye yoğun olarak etki etmesi sonucu oluşur.Kabarma , büyük yüzey gerilmeleri ve büyük kavrama hızlarının beraber etki etmesi sonucu oluşur.Bir dişte oyuklaşmış bölge kavramaya girerse darbeli temastan dolayı gerilme dalgalarıoluşur. Bu dalgalar düşük genliklidir, etkili bir şekilde kullanılırsa hasarın erken belirlenmesinde bundan yararlanılabilir.Bir dişte oluşan kabarma veya çatlağın başlamasından, dişin tamamen arızalanmasına kadar önemli bir zaman geçer. Hasarın ilk zamanlarında hasar şiddetindeki artış yavaştır, fakat tamamen arızalanmadan hemen önceki aşamada bu artış aniden hızlanır. Bu yüzden hasar ilk aşamalarda belirlenebilirse teşhis anlamlı olmaktadır.