Bolt tension

Figure 2.19 Failure of a connecting rod bolt in a diesel engine. In (a), the failed bolt is the upper one, having necked down in a nominally larger cross-sectional area. The lower bolt is another removed from the engine in unstretched condition. In (b), the stretched region of the bolt is shown, with cracks that have opened up during bolt tensioning and subsequent operation. (Reprinted with permission from ASM International. All rights reserved www.asminternational.org)...

Several publications have described the use of molybdenum disulphide pastes for the lubrication of screw threads. This is an application in which the paste assists assembly and subsequently acts as an anti-seize. The critical factors in screw or bolt assembly are the friction between male and female thread and between bolt-head and workpiece, and their effect on the relationship between assembly torque and bolt tension (clamping force). The stiffness and integrity of a bolted assembly are determined by the bolt tensions, but since it is impracticable to measure bolt tension directly, the required bolt tension is controlled by the torque applied to the bolt or nut. However, the relationship between assembly torque and bolt tension is determined by head friction and thread friction. If the friction is reduced, the required bolt tension will be obtained at a lower assembly torque. 

After the arresting action has stopped, the conveyance can immediately be released from this over-travel position by releasing the bolt tensions on the cover plate (Item No. 7) and thus releasing the friction elements. 

Design Boit Tension (KiPS) Torque Bolt Tension (KiPS) ... 

Bolt Dia (in) Tensile Area, Ra Design Bolt Tension (KIPS) (1) (2) Torque Bolt Tension (KIPS) (4) Torque (Ft-Lbs)... 

Torque bolt tension allows a % increase over bolt tension to allow for loss of pretension due to creep of concrete and bolt material. All torque values result in a tension stress less than. 8 Fy. 

Note that the initial anchor bolt tension does not increase the maximum bolt tension caused by wind or earthquake. This initial tension will only clamp the base ring to the concrete. Both are in equal compression until the external load is applied. The external load reduces the compression in the concrete before additional load is applied to the bolts. After the external load overcomes aU the compression in the concrete, the stress in the bolt wiU increase to the value it would have been, had there been no initial tension. 

Anaerobics are ideal for locking and sealing threaded parts and, with few exceptions (discussed separately in Section 2.3.2.9), are normally the only adhesives to consider. A variety of vibration and impact tests have shown that even medium-strength grades out-perform all conventional means of maintaining bolt tension (see Table 2.3). Frequently the test specimens fail due to metal fatigue with the nut still firmly attached to the bolt shank. 

The slight reduction in bolt tension illustrated in Table 2.4 is not due to any failure of the adhesive - used here as a locking medium - but to deformation of the metallic peaks, which transfers the load to the much greater surface area of the load-bearing polymer. 

Figure 2.23 The bolt tension generates friction which is manifest in the torque required to remove the nut (Figure 2.22). This is additional to the combined effects of adhesion and crushing load induced by anaerobic adhesives (Figure 2.21). Here, with the two sets of curves combined, it may be seen that very high torques can be required to initiate movement.

Cooper C and Turvey G J (1995), Effects of joint geometry and bolt torque on the structural performance of single bolt tension joints in pultruded GRP sheet material. Composite Structures, Vol. 32, Nos. 1, pp. 217-226. 

Turvey G J and Wang P (2007), Failure of PFRP single-bolt tension joints under hot-wet conditions. Composite Structures, Vol. 77, No. 4, pp. 514-520. 

Clearance is required to allow installation of bolt-tensioning equipment at equipment nozzles and pipe flanges. 

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