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Flange Screws

Hex flange screws have a large bearing surface that distributes clamping force over an extended area, which allows them to be used with irregular and oversized holes. A common application includes vehicle frames and especially truck frames. Hex flange screws and Grade G Flange Style Lock Nuts create a screw/nut system that spans a wide area and resists loosening. Hex flange screws are also called: 

hex flange bolts, hex frame flange bolts, hex washer frame bolts.

 Teeth under the head of serrated hex flange screws grip the mating surface to resist loosening. 12 point flange screws, also known as 

counterbore cap screws,

 are easily installed using a 12 point socket and pneumatic impact wrench. They are popular in the aircraft and aftermarket automotive industries, while the externally wrenched small decorative head is favored by motorcycle enthusiasts.

Hex and 12 point are available in Unified inch coarse series (UNC, Unified National Coarse) and Unified inch fine (UNF, Unified National Fine); serrated hex in Unified inch coarse. Flange screws are partially and fully threaded depending on size and length. Threads are standard right-hand. The point for hex heads is chamfered (beveled); for 12 point types, it is flat and chamfered.

Typical sizes for hex range from 3/8" to 3/4"; #10 to 1/2" for serrated hex; and 1/4" to 1" in diameter for 12 point. Lengths for hex span from about 1" to 5"; 3/8" to 2 1/2" for serrated hex; and 1/2" to 7" for 12 point. Length is measured from the underhead flange bearing surface to the threaded end of the screw.

To ensure full thread engagement with the nut, flange screws should be long enough to allow at least two full threads to extend beyond the nut face after tightening. Conversely, two full threads should remain exposed on the head side of the nut to make sure the nut can be properly tightened. A "springier" screw will result if there are more exposed threads within the grip (the area between the head and nut). It is also recommended that threads not be in the shear plane(s).

The size of the head varies with the size of the flange screw; the flange diameter of hex types is over 2.1 times the screw size whereas 12 point heads are smaller at just 1.5 times. Tables 1 and 2 list head width across flats, which is wrench size, and across corners, flange diameter and head height.

Common types of flange screws are hex serrated, Grade 5, steel, zinc plated; hex, 150,000 PSI, steel, plain finish; and 12 point, alloy steel, plain finish. 150,000 is sometimes abbreviated 150M—"M" is the Roman numeral for 1,000. You may also see 150K—"K" refers to kilo, also an abbreviation for 1,000, which is based on a French modification of the Greek word chilioi. Zinc plating offers moderate corrosion resistance but there is no plating or surface treatment of plain finish screws to inhibit rust.

It is very important that nuts used with graded flange screws be the same grade as the screw; one grade higher is also acceptable as one source states. Use hardened flat washers with screw Grades 5 and 8. Use regular lock washers, which are often split (helical spring) type, for Grades 2 and 5 screws and high strength alloy steel for Grade 8.

Always follow the manufacturer's recommendations regarding screw reuse, thread lubricants/locking compounds and torque values.

Refer to Industrial Fasteners Institute Standards IFI-111, Hex Flange Screws, and IFI-115, Flange 12-Point Screws, for specifications relating to flange screws.

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