ESDU 68045-1968 轴向载荷下大规格钢制螺纹的疲劳强度

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68045

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FATIGUE STRENGTH OF LARGE STEEL SCREW THREADS UNDER AXIAL

1. NOTATION

Both SI and British units are quoted but any coherent system of units may be used.

2. NOTES

The results of about 160 fatigue tests on solid steel bolts and studs with nominal thread diameter equal to

or greater than 1.5 in (38 mm) have been analysed to give curves of Sa /ft against N.

The ranges of the experimental variables and other details are listed in Table 2.1.

tensile strength of material N/m2lbf/in2

endurance

alternating stress (that is, half the range of stress) based on core

area N/m2lbf/in2

mean stress based on core area N/m2lbf/in2

TABLE 2.1

Variable Figure 1Figure 2

ft for bolt or stud material 5 800 to 102 000 lbf/in2

(405 to 703 MN/m2)130 000 to 170 000 lbf/in2

(896 to 1170 MN/m2)

Yield stress / ft0.46 to 0.66 0.7 to 0.85

Sm / ft0.15 to 0.4 0.2 to 0.5

Nominal thread diameter 2.0 to 3.0 in

(51 to 76 mm) 1.5 to 5.06 in

(38 to 129 mm)

Thread form Whitworth type Unified type

Finished

root radius cut or ground

threads 0.137 of thread pitch 0.032–0.114 of thread pitch

rolled threads 0.149–0.173 of thread pitch 0.136–0.16 of thread pitch

Method of thread manufacture

(see Table 2.2)A, B, E, and F A, B and D

ft for nut material 58 000 to 70 000 lbf/in2

(400 to 483 MN/m2)Not known

Test frequency 4.5 to 50 c/s

(4.5 to 50 Hz) Not known

Derivation (see Section 3) 1, 2, 3, 4, 5, 8 6, 7

Issued November 1968

ESDU product release: 2006-01. For current status, contact ESDU. Observe Copyright.

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68045

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All tests were carried out at room temperature under a fluctuating tensile load. Care was taken to maintain

axiality of loading during the tests. All failures occurred in the threaded portion of the bolt or stud just

inside the nut, close to the loaded face. Steel nuts of standard thickness were used in all cases. The results

plotted are from unplated bolts.

The various methods of thread manufacture to which this Item refers are listed in Table 2.2.

Each figure shows a mean line and a scatter band including 90 per cent of the results; these bands have

been drawn so that 5 per cent of the results lie above the band and 5 per cent lie below. In Figure 1 the band

is divided, as dictated by differences in the method of thread manufacture. Figure 2 also shows scatter bands

for data from similar bolts not exceeding 1.0 in in diameter given in Item No. 84037, “Fatigue strength of

external and internal steel screw threads under axial loading. (Standard forms not greater than 1.0 inch

diameter.)”.

For the results plotted here, differences between the two thread forms are not of practical significance.

The advantage of rolling the thread from fully heat treated material can be seen in Figure 1. The majority

of rolled thread results shown in this figure are from bolts that had been root-rolled after machining of the

thread form (Method F) and show no appreciable difference from the threads that had been fully form-rolled

(Method E). Either process induces residual compressive stresses in the surface layers of the material at

the root of the thread. The rolling process also tends to reduce the thickness at the thread root of any

decarburised layer remaining after heat treatment. The surface finish is usually improved by rolling and,

in the case of the results plotted in Figure 1, an increased root radius is provided by using a roller of section

slightly larger than the standard root radius (see Table 2.1). All these effects and in addition the work

hardening that occurs with some materials tend to increase the fatigue strength of the threads at long

endurances.

In Figure 2 results are plotted from a limited number of tests on bolts ranging from 1.5 to 5.0 in diameter.

The threaded portions of these bolts were either cut or ground to give root radii smaller than standard or

rolled to give a range of root radii. Although there are no appreciable differences shown by these results

other data suggest that a proportionately larger root radius is beneficial, see Item No. 67020, “Fatigue

strength of steel screw threads with large root radii under axial loading”.

TABLE 2.2

Method Description of method Location of data

A Thread cut from fully heat treated material Figures 1 and 2

B Thread ground from fully heat treated material Figures 1 and 2

D Material of near nominal diameter fully heat treated

before rolling of thread Figure 2

E Material fully heat treated, then surface ground

before form-rolling of thread Figure 1

F Material fully heat treated then threads cut before

root rolling Figure 1

ESDU product release: 2006-01. For current status, contact ESDU. Observe Copyright.

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68045

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To ensure consistent improvements in fatigue strength due to root rolling it is important to use a roller load

sufficiently large to cause some plastic deformation of the thread root. Investigations have shown that for

a given bolt diameter the required roller loads increase with pitch, typical values for the 2.0 in diameter

bolts of Figure 1, made from 45 ton steel, being 600 lbf for 8 t.p.i. and 1100 lbf for 4 t.p.i. The rollers used

in this case had profiles 0.003 in larger than the standard root radius for Whitworth threads. Excessive roller

loads should be avoided.

To obtain a symmetrically rolled root with a good surface finish several roller passes both up and down the

thread are recommended. A minimum of 4 roller passes has been demonstrated as sufficient and the

additional benefits to be obtained from more than 8 passes have been shown to be insignificant.

The curves in both figures may be assumed independent of mean stress, except when the maximum load

in the cycle is sufficient to cause yielding over the whole core area. In practice, when calculating the mean

and alternating stresses in a bolt or stud, any pre-tension applied during assembly of the joint must be taken

into consideration.

This Item applies only to axially loaded threads, Any bending, for example that induced by a slightly

out-of-square seating for the nut, can produce a severe reduction in the endurance of a screw thread, as is

shown in Item No. 67034, “Effect of inclined nut seatings on the fatigue strength of steel screw threads”.

Further information on the strength of screw threads may be located by reference to the Engineering

Sciences Data Index.

3. DERIVATION

1. TAYLOR, B.

NEWBY, D. The accuracy of various screw-cutting methods and their influence on

the fatigue strength of large bolts. B.S.R.A. Report No. 175, 1955.

2. TAYLOR, B.

WILSON, G.J. An investigation of the effect of thread rolling on the fatigue strength of

bolts. B.S.R.A. Report No. 186, 1955.

3. DOWIE, W.F. The effect of thread rolling on the fatigue strength of bolts. B.S.R.A.

Report No. 283, 1959.

4. COOK, R.

McCLIMONT, W. Influence of screw forming methods on fatigue strength of bolts. Trans.

Inst. Mar. Engrs, Vol. 73, pp. 417-432, 1961.

5. DOWIE, W.F. A comparison of the fatigue strength of bolts with Whitworth and

Sulzer Power Acme threads in both cut and rolled thread conditions.

B.S.R.A. Report No. 390, 1962.

6. SNOW, A.L.

LANGER, B.F. Low-cycle fatigue of large-diameter bolts. J. Engng Ind., Vol. 89,

Series B, No. 1, pp. 53-61, February 1967.

7. FRITZ, R.J. Cyclic stress for bolts and studs. Am. Soc. Mech. Engrs, Preprint

67-Met-23, April 1967.

8. MORRISON, J.

DOWIE, W.F. Further investigations into the fatigue strength of root-rolled bolts.

B.S.R.A. Report No. NS 190, 1968.

ESDU product release: 2006-01. For current status, contact ESDU. Observe Copyright.

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摘要：

ESDU 68045-1968 是一项针对轴向载荷下大规格钢制螺纹疲劳强度的权威工程标准，发布于1968年。该标准主要适用于承受循环轴向负荷的大尺寸钢制螺纹连接件（如螺栓、螺柱或紧固件），提供基于实验数据的疲劳强度评估方法。内容涵盖螺纹几何参数、材料等级、应力集中系数以及疲劳寿命预测公式，帮助工程师在航空航天、重型机械和结构工程中准确计算螺纹接头的抗疲劳性能。该标准的独特价值在于其针对大规格螺纹（通常直径超过1英寸或M24以上）的专门分析，弥补了通用疲劳设计手册在该领域的不足。

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ESDU 68045-1968 轴向载荷下大规格钢制螺纹的疲劳强度

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