Dişli çarklarda yorulma olayının incelenmesi
Bending fatigue strength of gears
- Tez No: 21971
- Danışmanlar: PROF. DR. MUSTAFA AKKURT
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1992
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 78
Özet
Güç ve hareket iletiminde en çok kullanılan mekanizmalar dişli çark mekanizmalarıdır. Günümüzde makina tasarımlarında, boyutları küçük ve hafif yapıların tasarımına doğru bir eğilim vardır. Bu nedenle, küçük boyutlu ve hafif dişli çark mekanizmaları gerçekleştirmek için dişli çarkların yorulma dayanımlarının hangi faktörler ile nasıl değiştiğinin bilinmesinin önemi artmıştır. Bu çalışmada geometrik faktörlerin değişiminin dişli çarkların eğilme yorulma dayanımı üstüne etkisi incelenmiştir. Ayrıca eğilme yorulma dayanımını artıran yöntemler ve eğilme yorulma kırılmasının mekanizması da açıklanmıştır. Sırasıyla dişli çarkların geometrik faktörlerinin Cprofil kaydırma.kavrama açısı.helis açısı v.b) değişiminin düz dişli çarkların.helisel dişli çarkların ve iç düz dişli çarkların eğilme yorulma dayanımına etkileri ele alınmıştır. Dişli çarklar da uygun miktarda profil kaydırma yapıldığında, kavrama açısının derecesi standart değere göre daha büyük alındığında Cdüz dişli çarklar da>, diş dibine bi lya püskürtme işlemi yapıldığında ve çember kalınlığı uygun değerlerde Ciç düz dişli çarklar da> alındığında, dişli çarkların eğilme yorulma dayanımlarında önemli artışlar olmaktadır.
Özet (Çeviri)
The aim of this study is to make a synthesis of geometrical factors, namely, addendum modification coefficient, pressure angle, helix angle and rim thickness, which influence the bending fatigue strength of gears. Furthermore, to examine some methods which increase the bending fatigue strength of gears, such as shot-peening and case-hardening, and the mechanisms of the bending fatigue breakage of gears. When the applying load exceeds the fatigue limit, both sliplines and cracks appear first at the compressive side, then later at the tensile side and they grow with the successive load application. Since the crack appering later at the tensile side grows and propagets more rapidly than that at the compressive side, cylic life of the gears is governed by the crack initiated much later at the tensile side. When the applying load corresponds to the fatigue limit, a crack appears only at the compressive side and grow to some ex tent, t hen after a certain number of load cycles it ceases its growth, which is called a non propagating crack. When the applying load is below and closely approaching the fatigue limit, no sign of sliplines can be observed at the tensile side, but at the compressive side sliplines can be observed clearly. But no cracks initiate at both sides. -viii- >The root stress factor A decreases with an increasing addendum modification coefficient x for both the tooth tip loading and the highest point of single tooth loading. It is possible to raise the bending fatigue limit loads of cast iron and cast steel gears by more than %40, casehardened gears by more than % 25 over standart gears by selecting the proper amount of addendum modification. The bending fatigue strength of profile shifted gears can be estimated with fairly high accuracy, by introducing the addendum modification factor B =1+0. 5x into the bending strength of standart spur gears. The contact ratio of the profile shifted gears decreases with an increase in addendum modification coefficient x and x. 1 2 The root stress factor A of gears with a pressure angle 0=27 is smaller compared with the case of a=2Ö irrespective of the addendum modification coefficient x, and it decreases with an increasing addendum modification coefficient x. The bending fatigue limit loads of normalized steel, cast iron and cast steel gears with a=27 are higher by % IS than those of gears with a=20°. It is possible to raise the bending fatigue limit loads of spur gears by more than % 40 over standart gears with oc=20 by selecting a higher pressure angle Cot=27 > and a proper amount of addendum modification. The effect of case depth on the strength is not remarkable. The ratio of maximum root stress on the compressive side to that on the tensile side for helical gears isThe root stress factor A decreases with an increasing addendum modification coefficient x for both the tooth tip loading and the highest point of single tooth loading. It is possible to raise the bending fatigue limit loads of cast iron and cast steel gears by more than %40, casehardened gears by more than % 25 over standart gears by selecting the proper amount of addendum modification. The bending fatigue strength of profile shifted gears can be estimated with fairly high accuracy, by introducing the addendum modification factor B =1+0. Sx into the bending strength of standart spur gears. The contact ratio of the profile shifted gears decreases with an increase in addendum modification coefficient x and x. 1 2 The root stress factor A of gears with a pressure angle ck-2.7 is smaller compared with the case of ct=20° irrespective of the addendum modification coefficient x, and it decreases with an increasing addendum modification coefficient x. The bending fatigue limit loads of normalized steel, cast iron and cast steel gears with et=27 are higher by % IS than those of gears with a=20. It is possible to raise the bending fatigue limit loads of spur gears by more than % 40 over standart gears with oc=20 by selecting a higher pressure angle Cot=27°) and a proper amount of addendum modification. The effect of case depth on the strength is not remarkable. The ratio of maximum root stress on the compressive side to that on the tensile side for helical gears is -ix-smaller than for spur gears. A crack initiates at the position of maximum root stress on the tensile side, Which is at a certain distance from the tooth end, and propagates toward both ends with an increasing number of load cycles. In the case of helical gear, the position of crack initiation in each normal section lie in the neighborhood of critical section determined by Hofer's method, but the directions of crack propagation differ in each normal section. The maximum root stresses are almost the same regardless of the degree of helix angle and the amount of backlash under a constant circumferential load. The bending fatigue strength (circumferential loads) for casehardened helical gears and normalized steel helical gears is almost constant regardless of degree of helix angle C/5 . Accordingly the transmitted torque increases with an increasing degree of helix angle. The effect of partial loading (maldistribution of load) on the root stress distribution and the bending fatigue strength of helical gears is considerably large. The maximum root stress ratio in the case of partial loading factor c =0.6 in helical gear C/3 =20°) is 1.43 and the bending fatigue limit load decreases by about %16 The bending fatigue limit loads of helical gears show a tendency to increase with an increasing addendum modification coefficient but the increase due to positive addendum modification is much smaller than that in the case of spur gears. The root stress factor ratio and bending fatigue limit load ratio of helical gears increase at the same rate with an increasing addendum modification coefficient The effects of addendum modification on the root stress -x-factor and bending fatigue limit load increase with an increasing addendum modification coefficient of mating gear and decrease with increasing number of teeth and helix angle. The bending strength of spur and helical gears of total contact ratio s 2 can not. An increase in bending strength of helical gears is not expected even if the helix angle becomes larger when the effect of dynamic additional load is discounted. The maximum root stress of a thin rimmed internal gear increases with a decrease in rim thickness on both the tensile and compressive side. The maximum root stress of a thin rimmed internal gear occurs near the position of tangential angle 0=45 for 1 >3m. But as the rim thickness decrease still further, the position of maximum root stress moves toward the tooth bottom from the position 0=45. For thin-rimmed internal gears Cdriven), the tensile stress occurs at the root fillet c>f loaded side of the gear tooth before the begining of engagement and the compressive stress does after the end of engagement, due to the effects of elastic deformation of rim part. Hence the stress condition in the root fillet of thin-rimmed internal gears under normal running condition come to be partly reversed. The bending fatigue-limit-loads of the thin-rimmed internal gears remain almost constant for 1 >3m, those for 1 =2m, 1.5m are about % 5, % 24 lower than in the w case of 1 >3m respectively. Hence it migth be considered reasonable to take 1 =2m as a minimum rim thickness. V -*i-The root stresses of straight bevel gears at the toe heel and middle of tooth trace become larger in the order mentioned in the whole range of engagement. The root stress at the heel increase more sharply with the progress of meshing near the outer point of single tooth pair contact than those at the toe and middle. A crack initiates at the position of maximum root stress on the tensile side and propagates toward both ends with an increasing number of load cycles. The positions of crack initiation in each normal section lie in the neighborhood of critical section determined by Hofer's method, but the direction of crack propagation differs in each normal section. -xii-
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