Failures of Gears
The most common types of wear are Pitting, 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 driven gear showing also the tendency to ridging at the pitch line as the sliding action of the teeth tends to drag the surface material towards the pitch line
Pitch line pitting on a case hardened pinion which 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 short time in service b: The same teeth after one year in service c: Some improvement two years later d: Almost smooth after three years
Appearance and extent of scoring and wear sustained by the propellershaft rear thrust flange contact surface.
When gears mesh, the tooth surfaces roll and slide 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. If the oil film is thick enough, the asperities from the mating gear teeth do not contact one another. Based on elastohydrodynamic lubrication (EHL) theory, lubrication falls into one of three regimes. In Regimes 1 and 2 there is some contact between the mating gear teeth. Most high-speed gears fall into Regime 3, in which full EHL oil film develops and the asperities in the tooth surfaces do not contact one another.
Gear tooth distress includes scuffing, wear and surface fatigue (micropitting and macropitting). ANSI/AGMA 1010-E95 (1995) provides descriptions and pictures of these types 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), “The basic mechanism of scuffing is caused by intense frictional heat generated by a combination of high sliding velocity and high contact stress.” Scuffing will normally not start at the pitch line because the sliding velocity is zero. Thus, scuffing generally starts in either the top or bottom half, or root, of the teeth.
Wear Wear is defined as the removal or displacement of metal from gear tooth surfaces. Damage due to scuffing or pitting is not normally considered wear. Wear reduces tooth thickness and can change the contour of the teeth. Wear can result from mechanical, chemical or electrical action.
Micropitting Micropitting is a high rolling contact fatigue incident that occurs in the Hertzian contact band area. It is a function of combined rolling and sliding velocities, 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 to 20 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.
Figur e 5. Macro pitt ing 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 sınıflandır ılabilir. Dişli hatalar ı , hata türüne özgün belirgin titreşimler oluşturur. Bu yüzeyden diğer makine elemanlar ında olduğu gibi dişli hatalar ı titreşim analizi ile belirlenebilmektedir. Dişlilerdeki titreşimin birinci kaynağı üretim ve montaj hatalar ı kadar kavrama esnasında rijitliğin değişmesidir. Çalışan tüm makine elemanlar ı 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 ayr ıldığı ,yüzey yorulma hasar ıdır.Yükün diş üzerindeki bir bölgeye yoğun olarak etki etmesi sonucu oluşur.Kabarma , büyük yüzey gerilmeleri ve büyük kavrama hızlar ının beraber etki etmesi sonucu oluşur. Bir dişte oyuklaşmış bölge kavramaya girerse darbeli temastan dolayı gerilme dalgalar ı oluşur. Bu dalgalar düşük genliklidir, etkili bir şekilde kullanılırsa hasar ın erken belirlenmesinde bundan yararlanılabilir. Bir dişte oluşan kabarma veya çatlağın başlamasından, dişin tamamen ar ızalanmasına kadar önemli bir zaman geçer. Hasar ın ilk zamanlar ında hasar şiddetindeki artış yavaştır, fakat tamamen ar ızalanmadan hemen önceki aşamada bu artış aniden hızlanır. Bu yüzden hasar ilk aşamalarda belirlenebilirse teşhis anlamlı olmaktadır.
