El- Bolted Joints. Effect of Bolt Head Size the Loading Conditions

Effect of Bolt Head Size on the Loading Conditions in Bolted Joints.

A.K. Abd El- Latif and H.H. Zlada··

• Professor. King A bdulaziz University. Saudi Arabia .

J. Eng. Sci., King Saud Univ., 8 (I),

pp.

39-45 (1982)

•• Assistant Professor. King A bdulaziz University, Saudi Arabia.

In bolt design, weight reduction is taking much attention. This is achieved primarily by reducing the material content of the bolt head or nut. The effect of bolt head size, height and diameter, on the loading conditions in bolted joints has been investigated using the finite element technique.

Computed results show that the load and stress distribution on the bearing area of the bolt head are neither constant nor uniform. The pattern tends to flatten up as the diameter to the height of bolt head ratio SI h increases.

Smoother distribution is obtained at mid plane and interface of each joint.

The end of loador opening position on interface increases with the increase in the ratio of diameter of bolt head to diameter of bolt Sid. However it has a maximum value at diameter to height of bolt head, Sih equal to 3.5.

Introduction

An optimum bolt design is one in which the desired mechanical properties are obtained with the minimum fastner weight and envelope. Light - weight bolts, extensively used in the aerospace industry, are used to increase product performane and conserve raw material. In bolt design, weight reduction is achieved primarily by reducing the material content of the bolt head [11. [2].

Designing a bolt to make the most efficient use of material without sacrificing product performance and reliability requires a thorough understanding of the aspects of the head design which influence bolt properties. An optimized bolt design is best suited for one set of joint parameters. As the description of joint conditions becomes broader, the optimum design becomes a compromise.

Not only the bolt design, but also the joint design is critical to the performance ofthe bolt [3], [4], and [5].

Numerous articles have been published Which discuss one or more aspects of bolt head design [1], [6]. The head volume for hexagonal flange bolts could be reduced for a cost and material saving without functional degradation. This comes from the fact that the head volume for hexagonal head bolts

and their width across flats is dictated by both wrenching and bearing stress requirements [1].

In the following the effect of bolt head size, diameter and height of bolt head upon the joint load and stress has been investigated using the finite element technique.

The Finite Element Analysis

The analytic solution of bolted joints is governed by many complex interactions. To take into account all the factors that enter into this process would be extremely difficult. Thus it is well advised to seek simplifications that retain the basic elements of the problem while reducing those related mainly to the configuration of the joint and boundary conditions used.

Figure la. shows a typical bolted joint. The true behaviour of an actual hexagonal head is more complex. Therefore, round bolt heads are more appropriate and they are used in this analysis. The modified model shown in Fig. lb is about as simple as one can device and still takes into account the major influences.

The finite element model shown in Fig.2, represents half of the joint for equal plate thickness.

o

A.K. Abd EI-Latif and H.H. Ziada

A. Bolted joint

B Simplified model

Fill. 1. Holled jolol.

The model was constructed from two - dimensional isoparametric axisymmetric ring elements for the plates, bolt and nut. This element type gives considerable improvement in accuracy over the constant strain element [7], [8].

The interfaces between bolt head, joint plates and nut were modeled by two dimensional interface element, which represents the physical contact between the two plane surfaces. It may maintain or break the physical contact and allow relative slide to each other. The interface elements are defined by two nodal points one on each surface of contact. The interface elements were given an initial gap of zero magnitude, which resembles the case of contact without preload. Then the load on the bolt and joint was developed by introducing axial displacement at the bolt end.

From several solutions with different fixed reference conditions the nodes along the inside surface of nut was selected as a fixed reference in the Y direction. This condition gives similar distributions for the bearing loads under both bolthead and nut, thus approaching to symmetry and.

simplifying the real conditions.

During solution, the interface elements will indicate a normal load if there is contact between the surfaces, and zero load for separation of the two plates. The solution yields a result exactly compatible with the physical modeL Computations are iterative and the convergence was obtained when the conditions of the interface elements remain unchanged for the last two successive iterations.

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Fill. 2. Fiolle elemeol model.

The material properties for the steel joint elements, bolt, joint plates and nut used in this analysis are as follows:

Material yield strength Modulus of elasticity Poissons ratio Density

Coefficient of friction for the interface element Result^c: and Discussion

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Although knowledge of load is slightly needed in routine work, there are instances where such information is more useful as in stiffness determination.

Computations were performed to determine the load distribution along and across two equal circular plates of bolted joints. The joint diameter is 125 mms and the thickness of each plate is 25 mms. Bolt head and nut sizes were taken as 1.25xO.5 to 3x I of bolt diameter.

The effect of bolt head size or nut on the loading conditions in bolted joints is.illustrated in Figs.3A to F.They show the·loading pattern on surface, mid and interface planes of bolted joint. For cases having Sih

~ 2.5, constant load with half value of the maximum load occurs across the joint at Sid

=

1.75.

Figures 4,5 and 6 show the load distribution along the joint surface, mid plane and interface plane respectively.

Journal of Eng. Sci., Vol. 8, No.1 (1982). College of Eng., King Saud Univ.

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Effect of Bolt Head Size on the Loading Conditions in Bolted Joints

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Figure 4 illustrates the dependence of load pattern applied to the joint (at surface) on the size of bolt head, and shows that decreasing the bolt head size localized the load efforts to a small area. Although, most of the present use of load efforts concept

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assumes it at the end of bolt head, however, it is quite clear from the fIgure that the effort area of loading is not localized to the bearing area of the bolt head but occurs on a larger area which increases as the bolt head size increases.

·2 A.K. Abd EI-Latif and H.H. Ziada

From the standpoint of practical application, the diameter and height of the bolt head are more interesting than one longitudinal diemsnion. On the other hand it seems that the height of bolt head alone has a slight effect on the separation diameter.

It is convenient to compare the present investigation results with those obtained by Gould and Mikic [6] as indicated in Fig. 7. It appears now that as

SI

d increases, the separation diameter increases but there is large scatter. However, considering the effect of joint thickness, it is more appropriate to bound the results with a band which represents d/L = 0.5 to d/L = I.

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Figure 8 provides a convenient additional means of assessing a possible relation between bolt head size S/h and separation diameter Old, The presentation shown in the figure indicates a systematic relation whch depends on

SI

d. Assessing, presenting and comparing results with Gould and Mikic [6] has been attempted in cases having SI d = 3 to Sid = 1.25.

There is an indication to have maximum separation diameter at S/h = 3.5 or generally between 3 and 4.

The load distribution on the surface demonstrates a high value at the bolt hole - joint interface which decreases sharply to a moderate value at a step near the boIt head end. This is followed by another steep load gradient. At different levels below the surface, the load distribution maximum shifts according to the size of bolt head and nut.

The effect of bolt head size on the maximum load developed in bolted joint is readily obtained from

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Figs. 4,5 and 6 and is presented in Fig. 9. These resu Its show that the maximum load decreases with the increase in both height and diameter of bolt head or generally the cross - sectional area of bolt head.

Journal of Eng. Sci., Vol. 8, No. I (1982). College of Eng., King Saud Univ.

Effect of Bolt Head Size on the Loading Conditions in Bolted Joints

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F1a, 9. Effect of bolt bead ,Ize on the maxImum load.

From these results and those of earlier work [6] it is clear that the most appropriate height of nut is 0,5 the bolt dia(l1eter (h f d = 0,5) and bolt head diameter is 1.75 the bolt diameter(Sf d), thus giving Sf h= 3,5,

Recent review on fasteners recommends similar bolt head ratios, only based on fastener standards theoretical analysis,

design however,

without [9]

it is any

Figures 10,11 and 12 also give some information on the efTect of bolt head size on the stress distribution across the joint. The behaviour appeared

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4 ^A.K. Abd EI-Latif and H.H. Ziada

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to be related to the load distribution. However smoother stress pattern occurred at mid and interface planes. The maximum stress increases as the ratio Slh increases, thus inducing good contact stress. In summing up the results in Fig. 13, it is clear that maximum effective stress occurs at Slh

=

^{3 to 4.}

From these results and those given in earlier work [6]

it is clear that the most appropriate height of nut is 0.5 the bolt diameter (hId

=

0.5) and bolt head diameter is 1.7 5 of the bolt diameter, thus giving Slh

=

^3.5,

Conclusion

Computed results emphasized the dependence of load pattern applied to the joint on the size of bolt head and showed that decreasing the bolt head size localized the load effects to a smaIl area. The effort area is not localized to the bearing area of the bolt head but is always greater.

The opening position. or separation diameter increases as SId ratio increases. The height of bolt head alone provides a slight effect on the separation diameter. From the standpoint of practical applicaticnt the diameter and height of the bolt head are more interesting, the maximum separation

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diameter occurs at Slh

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The effect of bolt head size on the load across the joint indicates that the most appropriate height of bolt head or nut is 0.5 of the bolt diameter and the bolt head diameter is 1.75 of the bolt diameter thus giving Slh ratio as 3.5.

References

1. Landt, R.C., "Criteria for Evaluating Bolt Head Design", J. Eng. Ind. Nov. (1976)

2. Nagy, J.F., "Hexagonal Flange Bolt Head Weight Reduc- tion Design Criteria", SAE International Automotive Engineering Congress and Exposition, Feb.-March (1977).

3. Mick, H.R., "Analysis of Bolt Spacing for Flange Sealing", Tran. ASME(1969).

4. Ziada, H.H., and Abel El Latif, A.K., "Load, Pressure . Distribution and Contact Area in Bolted Joints", J. Indian

Eng. 61, 1, Nov. (1980)

5. Gould, H.H., and Mikic 8.8., "Areas of Contact and

Pre~sure Distribution in Bolted Joints", Trans. ASME, August (1972).

6. Zienkiewiez, O.c., "The Finite Element Method in Engi- neering Science", McGraw-Hili, London (1977).

7. Pnemieniecki, J.S., "Theory of Matrix Structural Analysis"

McGraw-Hili, New York (1968).

8. Mallen, S.E., (Metric Fastener Overview), SAE, Inter- national Automotive Engineering Congress and Exposition.

Feb.-March (1977).

Jf)urnal of Eng. Sci., Vol. 8, No.1 (1982). College of Eng., King Saud Univ.

Effect of Bolt Head Size on the Loading Conditions in Bolted Joints

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