Your location:  Home  >  Bearing News  >  World Bearing News
<<  Back

Fatigue Strength Verification

Resource from:  https://www.powertransmission.com Likes:121
Sep 03,2024

2.png

For the fatigue strength, the stress amplitudes (Equation 3) of all bins are relevant, contributing to the damage accumulation. The equivalent amplitude Sequ used in FKM is a constant stress amplitude with an assigned cycle number equal to the knee point of the S-N curve, which is damage-equivalent to the related stress spectrum under consideration of the shape of the S-N curve, the required total number of cycles and the maximum amplitude in the spectrum. The description of the verification proceeding with all formulas would exceed the purpose of this paper. So only some of the important steps will be presented. For every stress component (axial, bending, torsional, and shear) the same procedure is applied. In the first step, the mean stress value with amplitude (stress Sm.i and Sa,i) of every bin is converted in an equivalent alternating stress with amplitude SaW.i (with Sm.i = 0 and SaW.i). Then the range of bins must be resorted, such that the first bin has the biggest amplitude (SaW.1). In the third step, the variable amplitude factor must be found, using Equation 4 and Equation 5.

This permits obtaining the equivalent stress amplitude at the knee point of the SN-curve, Sequ, FKM Equation 2.4.27 (Ref. 5). (6) For the fatigue strength assessment, depending on the position of the considered shaft section, at locations with shaft shoulders, keyways, etc., the stresses are increased due to local stress concentration. In the FKM (Ref. 5), as in AGMA 6001 (Ref. 13) or DIN 743 (Ref. 8), for the verification the nominal stresses are used (not increased), but the fatigue strength is modified (reduced) by stress concentration factors. As mentioned in AGMA, “since the fatigue strength is largely influenced by physical conditions, environmental conditions, and application conditions as well as material conditions, the basic fatigue strength must be modified (Ref. 13)”. In FKM (Ref. 5) all these effects are documented and combined in the so-called “design factor” KWK. So, the component fatigue limit for axial, bending, torsional and shear are reduced by the design factor per component to obtain the effective nominal values of the component fatigue limit SWK, TWK. In ISO or AGMA ratings, the final result of a verification is the safety factor obtained by division of the permissible stress by the effective stress. The resulting safety S must then be equal to or higher than the minimum requested safety Smin. The result obtained by FKM is not a safety factor, but the “degree of utilization.” The degree of utilization is just the inverted value of the safety factor. The advantage of the utilization concept is that formulas for the verification with combined stresses are simpler. For this paper, the safety concept is used, and the FKM formulas are adapted accordingly. As a symbol for safety, we use here SY, as the symbol S in the FKM guideline is used for nominal stress. The basic equation for component safety SY, equal to the inverted value of the degree of utilization aBK according to FKM Equation 2.6.3 (Ref. 5), for axial, bending, torsional, and shear are: (7) So, based on the applied stress and the permissible stress per component, the safety factors obtained are SYBK,zd, SYBK,b, SYBK,t, and SYBK,s. For the consideration of the combined stresses in a shaft, these factors are then combined according to the von-Mises criterion (Equation 2.6.7, FKM [Ref. 5]) to obtain the final result. (8) In a shaft section, there are two locations where the combined stress may be highest, position W1 and W2 in Figure 5. In point W1, bending and torsion stresses are high, but shear is zero. In W2, shear and torsion stresses are high, but bending is zero. Thus, both positions must be checked to find the lowest safety factor. The verification of whether the shaft section fulfills the request is then given by: (9) with the requested minimum safety factor jD for fatigue strength assessment. For cases where the consequences of shaft failure are high, this factor can be assumed to be 1.5.

(https://www.powertransmission.com)
【CBCC News Statement】
1.The news above mentioned with detailed source are from internet.We are trying our best to assure they are accurate ,timely and safe so as to let bearing users and sellers read more related info.However, it doesn't mean we agree with any point of view referred in above contents and we are not responsible for the authenticity. If you want to publish the news,please note the source and you will be legally responsible for the news published.
2.All news edited and translated by us are specially noted the source"CBCC".
3.For investors,please be cautious for all news.We don't bear any damage brought by late and inaccurate news.
4.If the news we published involves copyright of yours,just let us know.

BRIEF INTRODUCTION

Cnbearing is the No.1 bearing inquiry system and information service in China, dedicated to helping all bearing users and sellers throughout the world.

Cnbearing is supported by China National Bearing Industry Association, whose operation online is charged by China Bearing Unisun Tech. Co., Ltd.

China Bearing Unisun Tech. Co., Ltd owns all the rights. Since 2000, over 3,000 companies have been registered and enjoyed the company' s complete skillful service, which ranking many aspects in bearing industry at home and abroad with the most authority practical devices in China.