Thread Torque Research Articles

Torsional and compressive properties of bolted joint under forward-reverse tightening

Obtaining the stress evolution law of bolted joint during the assembly process is of great significance for tightening process design and improving assembly performance. In this study, the bolt tightening analytical model is used to obtain the deformation coordination mechanism. Then, the evolution law of the stress during assembly process are obtained by FEA model, and the influence of different assembly parameters on the stress change is analyzed. Finally, the bolt tightening experiment is carried out, which verified the theoretical hypothesis and the FEA model. The results show that: in addition to the tensile normal stress, the thread torque also generates torsional shear stress that causes the torsional deformation of the screw during the tightening process. The torsional shear stress can be reduced by forward-reverse tightening process. Meanwhile, the utilization rate of bolted joint can be improved. This can increase the assembly design margin and improve the assembly performance.

Study on the Load Conversion of Bolt-Nut Fasteners between Tightening and Service Conditions

At present, the common method to realize the pre-tightening effect of threaded fasteners is by applying an accurate tightening torque on them. However, when removing the tightening torque, the load characteristics will have a transient conversion, so that the pre-tightening loads will be significantly different from the tightening condition. For the aim of understanding the pre-tightening effect under the service condition, it is important to model the load conversion between tightening and service conditions. Based on the most basic mechanical equilibrium analysis, the load characteristics of bolt-nut fasteners under a round of the tightening and service conditions were studied. The torque equilibrium equations were built respectively, the load conversion between two conditions was analyzed and an approximate relationship between the applied tightening torque under the tightening condition and the pre-tightening torque under the service condition was obtained. In the meanwhile, a dynamic strain measuring system was proposed to accurately determine the pre-tightening loads in real time. Results verified that the thread torque and the bearing friction torque balance each other and act as pre-tightening torques under the service condition. The final pre-tightening loads can be roughly estimated to be larger than half of the difference in tightening torques by wrenches, but smaller than half of the sum of tightening torques, or can be accurately measured through dynamic strain measurements. • We study on bolt-nut fasteners , load characteristics under tightening and service conditions. • We model the relationship between the pre-tightening torque and the applied tightening torque. • We verify that the pre-tightening torque is apparently determined by the friction on the contacting interface.

Research on Small Size Thread Torque Coefficient with Assembly Pressure

Threaded connection is one of the most commonly used methods in product assembly. The stability of the joint quality has a great impact on the performance of the product. At present, research on threaded connection rarely considers the influence of assembly pressure. This paper proposes a method about small size thread torque coefficient test that takes into account the assembly pressure. The small size threaded connection test device was developed by our lab. The law of dynamic torque coefficient under different assembly pressures can be got to guide the quantitative control of small-size thread pre-tightening force.

An Influence of Tightening Torque Stored in Tightening Process on Fatigue Strength of Aluminum Bolts

Fatigue strength of bolts has been generally investigated loading only axial cyclic loads on the bolts. However since the bolt is used to be tightened into a internal thread part or a nut, the bolt receives twisting torque due to friction torque between thread torque during tightening. Therefore an influence of the thread torque on the fatigue strength shoud be investigated. In this study, transverse fatigue tests have been conducted because the test can perform the fatigue tests in situations a bolt is tightened. And the transverse fatigue tests loosening the nut slightly to release the thread torque after tightening have been also conducted. Test bolts and nuts were hexagon head bolts M8 and the bolts and the nuts were made of aluminum alloy A5056. The bolt/nut assemblies were lubricated by Molybdenum disulfide grease in the transverse fatigue tests. Results showed that the transverse fatigue strength of bolt/nut assemblies for aluminum alloy bolt decreased by influence of the thread torque. Results also showd that the transverse fatigue strength of bolt/nut assemblies for aluminum alloy bolt could improve 15 % by means of releasing the thread torque.

Effects of tightening speed on torque coefficient in lag screw timber joints with steel side plates

To investigate the effects of tightening speed on the torque coefficient in lag screw timber joints with steel side plates, tightening tests were conducted on main timber members made from Cryptomeria japonica, Chamaecyparis obtusa and Pseudotsuga menziesii, under four tightening speed conditions (1, 4, 10, and 20 rpm). Major stick-slip behavior was observed in C. obtusa based on the relationship of tightening angle with clamp force, tightening torque, and thread torque at tightening speeds of 1 and 4 rpm. In addition, tightening speed’s effects on the torque coefficient (K) varied depending on the wood species of the main member. In P. menziesii, K was not affected by the tightening speed: the ratio of torque expended on tightening was 25% on average, and the ratio of torque expended on bearing surface friction was higher than the ratio of torque expended on thread friction.

An Influence of Thread Torque during Tightening Process on Transverse Fatigue Strength for Non-ferrous Bolt

An Influence of Thread Torque during Tightening Process on Transverse Fatigue Strength for Non-ferrous Bolt

S113021 引張荷重によるねじのゆるみに関する研究 : ナットの不完全山がねじ部トルクに及ぼす影響([S113-02]機械要素の設計・製造・応用技術(2))

S113021 引張荷重によるねじのゆるみに関する研究 : ナットの不完全山がねじ部トルクに及ぼす影響([S113-02]機械要素の設計・製造・応用技術(2))

3154 ねじ締結性能評価のためのトルクセルの開発 : 誤差要因の解析と精度向上(S35-2 機械要素の設計・製造・応用技術(2) 部材の締付け・圧縮特性,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Tightening experiment for evaluating tightening characteristics is one of the most important issues to boost the reliability of bolted joints. However, the conventional testing machines have some problems in terms of the accuracy, the applicability and/or the rigidity to perform the experiment specified in ISO 16047. In the previous study, the load cell system using piezo electric sensors was proposed to resolve such problems. However, there is one problem left on the accuracy of separate measurement for thread torque and bearing surface friction torque. The aim of this study is to investigate the cause(s) of measurement error, and to develop the new torque cell system by eliminating such causes. Analysis and experiment have been performed to clarify the relationships among torques and clamp force, and to point out the probable causes. Finally, it is found that the error (change of the sensitivity of torque sensors) to be removed is not caused by the characteristics of the torque sensors themselves, but may be caused by the cross talk of torques acting on two preloaded joint bodies including preloading bolts, torque sensors, and upper and lower plates.

Effect of Tightening Speed on Thread and Under-Head Coefficient of Friction

Abstract Applying torque to a fastener creates friction as well as clamp-load. Friction and clamp-load are inversely proportional: as friction increases, the amount of clamp-load generated decreases. The speed at which a fastener is tightened has a pronounced affect on the magnitude of friction, and thereby clamp-load generated in a metal joint. This paper examines the relationship between the tightening speed with friction and clamp-load. The applied torque, clamp-load, and under-head torque were measured as the fastener was tightened within a torque/tension load cell. The corresponding thread torque, under-head and thread coefficients of friction values were calculated from equations given in both the DIN946 and ISO16047 standards. One bolt, one washer, and one nut (all of similar hardness) were used for all trials of this study. Two different anti-seize lubricants were used (one composed of molybdenum-disulfide and graphite and the other of mineral oil, nickel, and graphite). Lubricant was applied to both the thread and under-head regions of the bolt prior to torquing. Further trials were executed with the bolt, washer, and nut in the dry, or plain, condition (all of similar hardness). The results of this investigation indicated that the thread and under-head coefficients of friction decreased as speed increased. The programmed speeds ranged from 25 to 700 r/min, with a torque target of 28 N·m. Since the friction coefficients decreased, the amount of clamp-load generated at target torque increased with increasing speed. The average difference in clamp-load from minimum to maximum speed was determined to be 6.3 kN for both lubricants and 1.2 kN for the unlubricated condition.

5241 ねじ締結性能評価のためのトルクセルの開発(S64-2 ねじ(2),S64 機械要素の設計・製造・応用技術)

Tightening experiment for evaluating tightening characteristics is one of the most important issues to boost the reliability of bolted joints. However, the conventional testing machines have some problems in terms of the accuracy, the applicability and/or the rigidity to perform the experiment specified in ISO 16047. In the previous study, the load cell system using piezo electric sensors was proposed to resolve such problems. However, there is one problem left on the accuracy of separate measurement for thread torque and bearing surface friction torque. The aim of this study is to seek the cause(s) of measurement error, and to develop the new torque cells by eliminating such causes. Experiments have been done on the characteristics of sensor itself and of the torque cell system. Finally, it is concluded that the error might be caused by the cross talk of the axial force on the shear force sensor due to the crystalline direction error, the installation error and/or the surface stress distribution, and it can be minimized by using all-in-one type torque cells consist of a number of small quartz crystal elements.

塑性域締結におけるねじ締結体の挙動 剛塑性体モデルによる解析

The axial tension of the bolt in the plastic region tightening method depends on the yield point load of the threaded portion under the combined loads of the axial tension and the thread torque induced by the tightening. A new estimation method for the yield point load of the threaded portion is proposed taking account of the general yield criterion based on the rigid-plastic solution for combined loads. Considering the coefficient of friction at the flank of the thread and the residual thread torque after tightening, simple formulas are derived for the yield clamping force and the maximum additional axial tension under externally loading. It is confirmed from the experimental results obtained by a combined loading test that the proposed estimation method is superior to the conventional one taking into account only the local yield criterion. Upon applying the local or general yield criterion, the range of the initial clamping force is bounded between the elastic region tightening, the yield point tightening and the plastic region tightening.

塑性域締結におけるねじ締結体の挙動 降伏軸力の推定の際のねじ部の仮想直径

The yield clamping force of the bolt in plastic region tightening depends on the yield condition at the threaded portion under combined load consisted of axial tension and thread torque induced by tightening, namely the interaction curve for the yield. A new estimation method for the interaction curve at the threaded portion based on the flow theory, proposed in the previous report, is superior to the traditional one based on the local yield condition ;however the proposed method has a some estimation error in comparison with the experiment. Experimental results obtained for the combined load on cylindrical test-pieces show that the estimation error is caused by idealizing the thread portion as a smooth cylinder having stress area. For the exact estimation of the interaction curve, it is recommended to idealize the thread portion as a smooth cylinder of minor diameter of thread.

The influence of lubrication on the design of yield tightened joints

This paper presents the results of a series of tests in which high strength fasteners, socket head set screws (SHCS), were tightened to yield as a means of generating maximum clamping force. The tightening tests were performed with the aid of a microprocessor controlled torque wrench and a torque–tension testing rig. Load cells fitted to the test rig provided an accurate measurement of the joint clamp load and the faster thread and underhead torque distribution under a range of lubrication conditions. Measurements were made during tightening to yield, under static load condition after yield tightening, and during loosening. The results obtained established that the lubrication conditions dramatically affect the magnitude of the maximum clamp load achieved on the joint and the torque distribution on the fastener. The results also enabled an estimate to be made of the coefficient of friction on the various mating surfaces during the tightening and loosening of the fastener. It was shown that the design of bolted joints must take account of the influence of friction if the joints are to operate effectively in service.

塑性域締結におけるねじ締結体の挙動 ねじ部の降伏軸力の推定方法

The axial tension of the bolt in plastic region tightening is influenced by the yield point load of the threaded portion under combined loads of axial tension and thread torque induced by tightening, namely interaction curve for yield. However the traditional estimation method for the interaction curve of the threaded portion has some problems. Firstly, a new estimation method for the interaction curve of the threaded portion is proposed based on the flow theory. Secondly, a combined load testing machine is developed in order to apply the tensile and torsional combined load on the threaded portion of the bolt, and the interaction curves of the threaded portion are obtained experimentally. In conclusion, it is shown that the new method is superior to the traditional one considering only the local yield condition.

塑性域締結におけるねじ締結体の挙動 締付け停止時のねじ部トルクの影響

This paper describes the effect of residual thread torque at the termination of tightening on elasto-plastic deformation of bolts under external load after bolted joints are tightened in plastic region. Firstly, the effect of residual torque is analyzed by use of the flow theory of plasticity, and then the relationship between axial tension and thread torque of the bolt is obtained experimentally. The obtained conclusions are as follows : (1) At the beginning of the external loading, after tightening there is the region regarded as elastic deformation on the relation between axial tension and elongation of the bolt, which is extended with the decrease of residual thread torque. (2) It is recommended to slightly loosen the bolt or the nut after tightening so as to reduce residual thread torque to zero, in order to decrease the additional plastic elongation of the bolt.

弾・塑性域締結ねじ性能試験機の試作

In this study, a micro-computer-aided testing machine is designed and produced by way of experiment, which can realize various fastener tightening methods in elastic or plastic region and continuously apply an external force on the fastener. It consists of a DC-servo motor, a hydraulic cylinder, a new-type load cell, a new-type extensometer and a micro-computer, and can detect following nine factors : tightening torque, thread torque, bearing surface torque, axial tension, external force, elongation of bolt, torsional angle of bolt, rotational angle of nut, time. They are processed in a micro-computer and recorded on an X-Y plotter. Fundamental test was made with the bolts and the nuts of M 10 in elastic or plastic region of bolt material, and it is confirmed that the testing machine satisfies the specifications needed.