pin photo and diagram

Pins with Recess Nuts


Pin and Recess Nut Applications

Pins with recess nuts are used widely in structural steel applications for pin connections, hanger assemblies and bridge bearings. Smaller sizes are often used with clevises in place of standard straight pins with two cotters.

Pins with recess nuts offer several advantages over bolts for bearing (shear) connections. The reduced threaded diameter of the pin combined with the recess area of the nut allows grip adjustment while ensuring that the threads are excluded from the bearing area. Also, the reduced threaded diameter on the pin helps to keep the threads free from damage while the pin is being inserted into a close-fitting hole.

Relevant Standards

Pin and recess dimensions are shown in the American Institute of Steel Construction Inc. (AISC) Steel Construction Manual, 13th edition, page 15-18. Chemical and mechanical properties of the materials used will conform to any ASTM, AASHTO, AERMA and or state DOT specifications identified at the time the order is placed.

Materials

Standard pins are produced from cold finished steel bars conforming to ASTM A108 with 50 Ksi minimum yield strength. Other available carbon and alloy steels include ASTM A668 forged bars, ASTM A434 quenched and tempered alloy steel bars, and ASTM A572 grade 50/50W bars. Available stainless steels include gall resistant types such as ASTM A276 UNS S21800 (Nitronic 60)and 304, 316 and 410 grades.

Standard nuts are produced from ASTM A36 plate or 12L14 hex bar. Other available nut materials include ASTM A572 grade 50/50W plate, ASTM A668 forged bars and stainless steel. Pins are also occasionally specified with ASTM A563 heavy hex nuts nut in A, C or DH grades.

Finishes

Standard finish is plain with a cold drawn mill finish on the bearing diameter. Hot rolled and forged bars are machined to produce an ANSI 125 finish or better. Hot dip galvanized and zinc electroplating coatings are available.

Threads

Standard threads are 6UN 2A/2B, with other thread series and profiles available upon request.

Dimensions

Refer to the table for standard pin and recess nut dimensions. The grip length of the pin should be determined by the maximum thickness of the materials which make up the connection. The recess feature of the nuts compensates for members whose thickness is on the low side of the tolerance range. The recess also ensures that the chamfer on the pin’s bearing diameter (dimension 'C') is excluded from the grip and that the structural members have full bearing on the pin.

Pins over 10 inches in diameter are often secured with cap washers and a tie rod instead of recess nuts.

Accessories and Options

A wide range of washers are available, fabricated from carbon steel, stainless steel, PTFE, neoprene and high density polyethylene. Bushings can be provided in bronze, PTFE, or composite materials. Link plates can be supplied for bridge suspender applications.

Features can be provided for securing nuts in place. Cotter pins are often used where maintaining a precise grip dimension is not critical, while set bolts allow the nuts to be locked securely in the field after final grip adjustments are made.

Pilot and Driving Nuts

Pilot nuts and driving nuts are available to protect the pin threads during field installation. Driving nuts have a domed end and a shoulder which permits the pin to be driven to the proper depth. Pilot nuts have a tapered nose which serves to align the bearing members as the pin is driven home. Short pilots are generally sufficient unless there are many plies in the connection or assembly conditions are difficult.

Download the Pin and Recess Nut Data Sheet pdf.