Choosing the Right Fastener

By Waymon P. Goch

With proper attention to the fastener assembly, the best choice is almost always fasteners of the same material as the components being joined. The primary reason is it eliminates differential thermal expansion and contraction, and just as important, since we are talking about electrical connectors, is that it will be conductive, at least more conductive than steel fasteners.  More discussion on this subject in a following article.

A common misconception is that stainless steel fasteners do not rust, but they do rust under certain conditions. Stainless steel remains stainless in unpolluted atmospheres and when immersed in moving or flowing fresh and seawater. In humid marine environments or stagnant moisture conditions, type 304 stainless, for example, may rust locally in pitting, crevice, stress, concentration cell, knife edge, and galvanic corrosion. The exposure environment and chemical composition of the stainless steel will dictate whether or not rusting will occur.

Stainless steel owes its corrosion resistance to a very thin, compact passive surface layer that forms upon exposure to oxygen. If the surface is scratched and exposed to oxygen, the scratches will again become passive. It is primarily this passive layer that is also responsible for galling and seizure of stainless threaded fasteners. The layer may flake with small particles becoming trapped in the thread bearing surfaces where the pressure is sufficient to cold weld the internal and external threads together. The use of anti-seize compounds and different grades of stainless are often recommended to prevent or minimize galling and seizure but these options may only be partly successful. By far, the best solution I have found over the years is to mechanically zinc plate either the stainless bolt or nut (normally the nut).

Particular attention should be given to this “galling” or seizing tendency.  The purpose of the bolt is to provide a clamping force between two or more components.  During assembly, the tensile stress which provides this clamping force is obtained by torque applied, less the friction between the threads of the bolt and nut, and the friction of the face of the bolt against the mating surface.  The most common means of determining that the appropriate tensile stress and elongation of the bolt is achieved during assembly is calculated against torque.  “Galling” or seizing of these threads consumes the force provided by the torque applied, giving a false indication of clamping force achieved.

There are many applications in which stainless steel fasteners are appropriate but they should not be considered universal.

How does a bolted joint work?

A properly tightened bolt is essentially a spring; achieved through elastic elongation of the bolt. Under constant static loading at a constant temperature, that alone would be entirely satisfactory without the need for lock washers, Belleville spring washers, or other devices to maintain clamping force. However, most applications are dynamic and require additional consideration.

The well-accepted equation for computing torque-tension relationships in bolted joints is:

T = kDW/12

Where T = torque in lb-ft, k = friction factor or torque coefficient, D = bolt diameter in inches, and W = bolt tension in lbf.

The static friction constant k varies with the fastener material and material condition (lubricated or dry) and ranges from 0.11 for lubricated steel to around 0.30 for lubricated aluminum and stainless steel. The overall torque coefficient depends upon the materials being joined, thread clearance, and the torque coefficient of the bearing surface against which the bolt head or nut is being turned. Friction is an important consideration because it represents torque that is lost in overcoming friction and not applied to create bolt tension.

Recommended fastener torques for common sizes, grades, and materials are provided by manufacturers as well as industry standards such as the Industrial Fasteners Institute, ANSI C119.4, and others. Recommended torque is usually based on final fastener tension within the elastic limit and is typically 60 – 70% of the proportional limit, yield point, or proof load. The bolt will continue to stretch if loaded beyond yield but will be unable to return to its original length, thereby reducing the clamping force.

The absence of torque wrenches in tool belts and bags of line personnel and installers frequently results in a policy of tightening “until tight” then applying another half or full turn.

ClampStar® avoids this potential problem, assures proper installation torque, and eases installation by providing torque-limiting nuts with an outer section that shears off at the proper torque, leaving a permanent hex nut in place, regardless of the type of wrench employed. This facilitates the use of pneumatic, electric, hydraulic, or battery operated nut runners, wrenches, and rattle guns.

Conventional spring or split lock washers are frequently recommended for maintaining fastener tension under dynamic loading. However, a good definition of a spring lock washer is a flat washer with a split. They are ineffective in maintaining live spring follow up because they completely flatten under relatively low compressive loading; typically around 350 lbf for ½” lock washers.

The most effective means of assuring live spring follow up is the use of Belleville type spring washers, properly sized, that will remain within its working range and not flatten or reverse, under any anticipated thermal or mechanical load excursions.

So, how do we choose the right fasteners?

The majority of electrical connectors are aluminum-to-aluminum, copper-to-copper, aluminum-to-copper, galvanized steel-to-galvanized steel and galvanized steel-to-copper or aluminum.

Proper surface preparation and the use of the correct inhibitors and joint compounds are critical steps in the creation of a low resistance electrical connection to assure long service life, but those are subjects for another time.

The preferred and recommended fastener materials for joining like and unlike metals are shown in the following chart. Although the chart references flat bar connections, the same applies to pad-to-pad as well as connections of other shapes.

Aluminum bolts are typically 2024-T4 with a #205 Alumilite finish, washers are 7075-T6 and nuts are 6061-T6 with a wax finish.

Silicon Bronze alloy bolts, washers and nuts are preferred for copper to copper connections.

Stainless steel bolts, washers and nuts are primarily type 304, 304L or 316 austenitic stainless. Type 316 has better corrosion resistance and greater creep strength than 304 or 304L due to its slightly higher nickel content.

Hot dip galvanized steel bolts are normally ASTM A307 grade 2 low carbon steel or ASTM A325 grade 5 medium carbon or low alloy steel. Grade 2 bolts do not have a grade marking on the head whereas grade 5 is marked with 3 radial lines. Both may contain the bolt manufacturer’s identification and both are galvanized according to ASTM A153. Galvanized or stainless steel flat and Belleville spring washers may be supplied and used with galvanized steel bolts. Galvanized steel nuts are tapped oversize for a class 2 fit on galvanized bolts.

We’re sure that readers will find this brief discussion worthwhile.

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So, what do you think ?