Columbia MFG, LLC
Stainless Steel Fastener Assemblies
For Bolts Used in Electrical Connections
This document is intended for use by utility and consulting engineers in developing a specification for stainless steel fastener assemblies used in bolted Al-Al or Al-Cu electrical connections.
Reference is made to Columbia Mfg. Engineering report entitled Bolting Techniques and Practices, for a comparative discussion and specific description of component selection.
Either aluminum alloy 6061-T6 or Stainless Steel 18-8 series fastener components are acceptable and provide the desired level of mechanical integrity required in electrical joint connection, but this paper will deal only with stainless steel materials.
A. Hexagon Head Bolts. All hexagon head bolts shall be 1/2” – 13 UNC 2A 18-8 stainless steel, full thread, finished hex bolt. Standard assemblies are listed in the following chart:
Bolt Assembly Catalog No. Bolt Length Weight 100 pcs. Si Br Nut 18-8 Nut 150BSS-SML 150BSS-188 1.5" 22.5 175BSS-SML 175BSS-188 1.75" 23.5 200BSS-SML 200BSS-188 2.00" 25.0 225BSS-SML 225BSS-188 2.25" 26.0 250BSS-SML 250BSS-188 2.50" 27.0 275BSS-SML 275BSS-188 2.87" 28.0 300BSS-SML 300BSS-188 3.00" 29.5 325BSS-SML 325BSS-188 3.25" 30.5 350BSS-SML 350BSS-188 3.50" 31.5 400BSS-SML 400BSS-188 4.00" 34.0 450BSS-SML 450BSS-188 4.50" 36.0 500BSS-SML 500BSS-188 5.00" 38.0
B. Flat Round Washers. Flat round washers shall be stainless steel 18-8, Type B with minimum dimensions as follows:
Bolt Size. . . . . . . 1/2”
O.D. . . . . . . . . . . 1.25”
I.D. . . . . . . . . . . . 0.515”
Thickness . . . . . . 0.120”-0.140”
This section of the flat washer used in conjunction with the Belleville washer is critical to the integrity of the connection as it distributes the sustained compressive force of the Belleville. The alloy and thickness of this washer must be such to eliminate any deformation and resultant loss of compressive load to the electrical connection.
C. Belleville Washer. The Belleville washer shall be stainless steel alloy 17-7, with dimensions and load to flat rating after pre-set, as specified for 1/2” dia. bolts as follows:
Bolt Size. . . . . . . 1/2"
O.D. . . . . . . . . . . 1.063”
I.D . . . . . . . . . . . . 0.515”
Thickness. . . . . . . 0.109”
Dish . . . . . . . . . . . 0.017”
Load to flat. . . . . . 3500 lbs. after pre-set
Specifying alloy 17-7 Belleville washer is recommended for several reasons. 17-7 is tempered after forming. Series 300 Bellevilles are not tempered and rather are work hardened during fabrication, and not pre-set (pre-flattened) prior to dish forming.
The result is that 17-7 Bellevilles provide approximately twice the compressive load capability as an equivalent 300 series Belleville; and will also exert more sustained compressive force in a fully flattened mode than a 300 series washer due to the pre-set and tempering. The 300 series Belleville will lose a significant percentage of its load to flat rating after initial installation, owing to not having a load to flat rating after pre-set.
D. Hexagon Nut. The hexagon nut shall be:
Stainless steel, Class 18-8
Full Hard Silicon Bronze
Specifying a full hard silicon bronze nut will significantly reduce and possibly eliminate entirely the potential for thread galling (seizing) which can occur when using bolt and nut components of the same material alloy, i.e. 18-8. For this reason, Si Br nuts are recommended for mechanical integrity and only where severe corrosive environmental conditions exist, is the use of 18-8, or other stainless steel alloy nuts, recommended.
Figure 1 - Bolt Assemblies
Stainless steel 18-8 1/2" - 13 thread
Flat Washer (2)
Series 300 (T18-8), Type B, stainless steel
Series 17-7 full hard stainless steel load/flat 3500 lbs.
Stainless steel 18-8 or full hard silicon bronze
Components are also available on inquiry
The fastener assembly shall consist of:
1 ea. hexbolt (specify diameter and length)
2 ea. round flat washers
1 ea. Belleville washer
1 ea. hex nut (specify material)
Above 5 components to be fully assembled and packaged to customer specifications. Refer to Price Sheet for standard packaging.
Each carton shall be labeled with the following information:
Manufacturer’s Catalog Numbers
Purchaser Product Code No.
Diameter-Thread-Length of Assembly (i.e. ½” – 13 x 2” UNC)
Terminal Connector Fastener Assembly
NOTE: Comments regarding the reference of specific stainless steel alloys versus 18-8 (300 Series Steel) for bolts utilized in electrical connections follows, as a means of providing both the technical and commercial factors in the ultimate selection of specific wording of the purchasers’ specification.
Many customers specify a stainless alloy (i.e. 304) for their materials often with no technical reason other than that stainless steel has provided satisfactory performance. Generically, all that is required is a stainless for corrosion resistance with Grade 1 mechanical properties. But, due to unfamiliarity with the nomenclature for the family of stainless steel, an alloy is commonly selected that is well known.
The most commonly specified stainless steel alloys are 304 and 316. The 304 is in what is generically referred to as the 18-8 family of stainless steels. The 18-8 indicates a group of stainless steels having nominally 18% chromium and 8% nickel content. The 18-8 family includes alloys; 302, 302HQ, 303, 303Se, 304 and 304L. Many suppliers also include 305 in the listing of 18-8, but 305 has 10.5% minimum nickel content and technically, should not be included as an 18-8. Similarly, 316 is not an 18-8, but is generally accepted as an 18-8 substitute since it has improved corrosion resistance over the 18-8 family. Industrial Fasteners Institute (IFI) rates each of the above stainless steels in the 18-8 family as equivalent in corrosion resistance.
Unless a specific alloy is required for defined purposes, and where Grade 1 mechanical properties are adequate, selection of an 18-8 or 300 series stainless will result in a more economic fastener with improved availability. The most common material used in the commercially mass produced 18-8 cold-headed stainless steels for stock is 302HQ, the HQ signifying heading quality. What is done in the HQ variety is to lower the allowable carbon and add 3 – 4% copper to the chemistry of a 304 to provide better cold-working properties to prevent cracking and reduce the tendency to work harden. The product supplied normally has a better appearance as formed, is free of forging cracks and generally has a better finish in the cold formed condition than do the straight 202, 302 or 304 stainless steel alloys.
ASTM Standard F593 and F594, Alloy Group 1 cover the 18-8 family of stainless steels and places the same minimum physical property by requirements on the finished product, reflecting the ability to interchange the alloys within the family. Alloy Group 2 is the 316 stainless which has the same physical properties as Group 1, but has improved corrosion resistance.
Another reason for specifying an 18-8 rather than an alloy such as 303 or 304 is that these stainless steels are not as readily available in a finished, stamped hex bolt, as are 18-8 designations.
As noted previously, most manufacturers use a 302HQ rather than 304, due to its better cold-forming properties and better tool life while supplying a functional part with nominally the same strength.
Regarding the proposed legislation concerning counterfeit bolts being supplied to the United States from foreign suppliers, Grade 1 general purpose fasteners are not affected since Grade 1 bolts have the lowest or minimum mechanical property requirements. Counterfeit bolts generally have low grade mechanical properties and are represented, and even marked, as a higher grade product. This can be a precarious situation if high strength bolts are required.
The following sources provided pertinent supporting data for the preceding discussion: American Society for Testing and Materials (ASTM standards and ISO recommendations); American National Standards Institute (ANSI); Industrial Fasteners Institute (IFI Documents).
We provide and recommend that users specify full length threading of all bolt lengths through 5 inches. Standard thread length for 1/2" bolts is 1 ¼” and is more readily available in longer length bolts than the special full threaded bolts.
Please send a description of your requirements and we will provide you with a proposal for providing fastener assemblies to you based on your specification and packaging requirements.
Stainless Steel Chemical Composition
% Max unless shown as Min or Max/Min limits given.
C Mn P S SI Cu Mo Ni Cr 302 0.15 2.00 0.045 0.03 1.00 - - 8.00 to 10.00 17.00 to 19.00 302HQ 0.10 2.00 0.045 0.030 1.00 - - 8.00 to 10.00 17.00 to 19.00 303 0.15 2.00 0.200 0.150 Min 1.00 3.0 to 4.0 .0600 8.00 to 10.00 17.00 to 19.00 303Se 0.12 2.00 0.120 0.150 1.00 - - 8.00 to 10.00 17.00 to 19.00 304 0.08 2.00 0.045 0.030 1.00 - - 8.00 to 12.00 18.00 to 20.00 305 0.12 2.00 0.045 0.030 1.00 - - 10.00 to 13.00 16.00 to 18.00 316 0.08 2.00 0.045 0.030 1.00 - 2.00 - 3.00 10.00 to 14.00 15.00 to 18.00
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This page revised 11/24/08