Stable Bolted Joints

Nov. 15, 2002
Some joint-design considerations are universal; others vary with the assembly methods used.

Some joint-design considerations are universal; others vary with the assembly methods used.

Each joining technique has particular design requirements, while certain joint requirements may suggest a particular joining technique. Design for assembly, automation, and fastener selection impose their own requirements.

Bolting is a common fastening method, for example, but welding may reduce the weight of an assembly. Naturally, joints designed for the two techniques would differ greatly. However, all joint designs must consider characteristics such as load conditions, assembly efficiency, operating environment, overhaul and maintenance, and the materials used.

Reliable joints require a firm, strong connection, materials that are stable in the environment, stable geometry, and appropriate stresses in the parts, including a correct clamping force.

Joint strength: Bolted joints should prevent slip, separation, vibration, misalignment, and wear of parts. Joints and bolts must deflect elastically but must be rigid enough to provide structural integrity. Rigid joints result from specifying enough bolts of adequate diameters, strong enough bolt and joint materials, sturdy joint members, and correct installation procedures.

Well-designed joints provide strength and integrity without being excessively large or heavy or requiring assemblers to use bulky tools. The specific geometry and materials for a joint depend on the application, but general guidelines can be given.

For instance, bolt size should be specified so that tensile stress is no greater than 60% of bolt yield strength, though this percentage may vary depending on the application. For example, concern about stress-corrosion cracking might dictate 20% preloads, while some slip-critical joints may be loaded beyond 100% of the bolt yield strength.

Stronger bolt materials can often decrease the size and number of bolts required, cut assembly costs, and permit use of smaller joint members. However, stronger is not always better. Increased strength is usually accompanied by an increase in hardness and resulting loss of ductility.

In relative terms, bolts should be flexible and joint members stiff. A favorable stiffness ratio can be produced by using bolts with the largest possible length-to-diameter (L/D) ratio. An extreme L/D ratio, such as 8:1, may make a bolt immune to such things as vibration loosening.

Bolt and joint materials should have similar coefficients of thermal expansion, so the bolts will not work against the joint when temperature changes. If identical coefficients are impossible, closer attention should be paid to a favorable L/D ratio, so bolt flexibility can offset differences in thermal expansion.

Stable materials: Strength is important when choosing a bolt material, but it is not the only factor to consider. Often, bolts should also resist corrosion, stress-corrosion cracking, and fatigue; maintain stiffness at high temperatures regardless of a loss in elastic modulus, and not lose tension because of stress relaxation or creep.

Common bolting materials may not be suited to extreme conditions. For example, a fairly exotic bolting alloy such as MP35N may be needed if a combination of unusual properties -- such as high strength and stress-corrosion resistance -- is required.

Stable geometry: Several refinements of bolt and joint geometry can improve clamp stability. Many of these refinements reduce stress concentrations, which can significantly shorten bolt fatigue life.

For example, if the joint will be loaded in shear, thread runout on the bolts should not coincide with the shear plane between joint members. This situation combines the normal stress concentrations at thread runout with additional concentrations located at the shear plane. Fillets between the head and body of the bolt can also reduce stress concentrations, and making fillet cross sections elliptical may offer further improvements.

Stress concentrations may also be reduced by rounding the roots of thread teeth, tapering thread runout instead of ending the thread abruptly, and using flanged-head instead of hex-head bolts. Rolling the bolt threads after heat treatment, instead of before, and shot peening bolt surfaces can also improve stability under fatigue or stress-corrosion conditions.

Clamping force: The main purpose of the bolt is to clamp parts together with enough force to prevent loosening in service. Clamping force depends on a combination of stress introduced at assembly and the service loads imposed on the joint. Selecting the right clamping force and producing it at assembly are critical.

Too high a clamping force causes excessive stress in bolts, joint members, and gaskets. High stress can damage parts, open leak paths, and encourage stress-corrosion cracking or fatigue failure. Also, overstressed gaskets can lose their ability to respond to a decrease in load, opening a leak path when the system is pressurized.

Insufficient clamping force can result in the same lack of joint strength or rigidity as using too few or too small bolts. Improperly tightened bolts may loosen under vibration, thermal cycles, or other in-service conditions. Such joints can also slip, putting excessive loads on bearings or causing wear between parts. Loose gasketed joints can leak. Insufficient tension can also lead to early fatigue failure under cyclic loads.

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