Fastener Overview

Materials

Fasteners are available in a huge variety of materials, including steel, stainless steel, bronze, brass, and aluminum.

Alloy steel fasteners are made from high-strength, heat-treated steel. They are not plated and normally have a dull black finish.

Carbon steel fasteners are the most common and come with various surface treatments (zinc plating, galvanization, chrome plating). Carbon steel fasteners are sold in different grade ratings to indicate their strength.

Stainless steel fasteners are non-magnetic, but are otherwise more expensive (grades 18-8, 316 and 410).

Since steel is generally stronger and more widely available, there is very little use for bronze, brass, or aluminum fasteners in FRC. However, aluminum is a common material for rivets.

Fastener Drives

Fasteners are turned with a variety of different drives, each designed for use with a different tool. Each drive system has different advantages and disadvantages, ranging from simplicity to cost. Some of the more common varieties in FIRST are listed below:

Slotted Drives
Slot Drive	The slot screw drive has a single slot in the fastener head, and is driven by the “common blade” or flat-bladed screwdriver. It was the first type of screw drive to be developed, and for centuries was the cheapest and simplest to make. The slotted screw is common in simple woodworking applications, but is not often seen in applications where a power tool would be used because a power driver tends to slip out of the head.
Cross	A cross, or double-slot, screw drive has two slots oriented perpendicular to each other in the fastener head. A slotted screwdriver is still used to drive just one of the slots. This type is usually found in cheaply-made roofing bolts. The sole advantage is that they provide some measure of redundancy: should one slot be deformed in service, the second may still be used. A cross screw drive is not considered cruciform because the shape is not recessed and consists of only two superimposed simple milled slots.

Slotted Drives

Slot Drive

The slot screw drive has a single slot in the fastener head, and is driven by the “common blade” or flat-bladed screwdriver. It was the first type of screw drive to be developed, and for centuries was the cheapest and simplest to make. The slotted screw is common in simple woodworking applications, but is not often seen in applications where a power tool would be used because a power driver tends to slip out of the head.

Cross

A cross, or double-slot, screw drive has two slots oriented perpendicular to each other in the fastener head. A slotted screwdriver is still used to drive just one of the slots. This type is usually found in cheaply-made roofing bolts. The sole advantage is that they provide some measure of redundancy: should one slot be deformed in service, the second may still be used. A cross screw drive is not considered cruciform because the shape is not recessed and consists of only two superimposed simple milled slots.

Cruciform Drives
Phillips (PH)	The Phillips screw drive (ANSI Type I Cross Recess) was developed as a direct solution to a number of problems with slotted screws, namely cam-out, precise alignment to avoid damage, and difficulty of driving with power tools. The design is critizied for a tendency to cam out at lower torque levels than other cruciform designs (and it is popularly believed that this is deliberate, for the purpose of assembling aircraft without overtightening the fasteners). This type is usually found in woodworking screws, and generally should not be used in competition robotics (teams should prefer hex or hex socket).
Frearson	The Frearson screw drive (ANSI Type II Cross Recess), also known as the Reed and Prince screw drive, is similar to a Phillips but with a sharp tip and a larger angle in the V shape. One advantage over the Phillips drive is that one driver or bit fits all screw sizes, though it does require a specialized driver to work properly. The tool recess is a perfect, sharp cross, allowing for higher applied torque than Phillips head screws, which can cam out.

Cruciform Drives

Phillips (PH)

The Phillips screw drive (ANSI Type I Cross Recess) was developed as a direct solution to a number of problems with slotted screws, namely cam-out, precise alignment to avoid damage, and difficulty of driving with power tools. The design is critizied for a tendency to cam out at lower torque levels than other cruciform designs (and it is popularly believed that this is deliberate, for the purpose of assembling aircraft without overtightening the fasteners). This type is usually found in woodworking screws, and generally should not be used in competition robotics (teams should prefer hex or hex socket).

Frearson

The Frearson screw drive (ANSI Type II Cross Recess), also known as the Reed and Prince screw drive, is similar to a Phillips but with a sharp tip and a larger angle in the V shape. One advantage over the Phillips drive is that one driver or bit fits all screw sizes, though it does require a specialized driver to work properly. The tool recess is a perfect, sharp cross, allowing for higher applied torque than Phillips head screws, which can cam out.

Hex and Hexalobular Drives
Hex	A hex screw drive uses six-sided fastener heads, and the fastener is generally referred to as a hex head cap screw. This drive has the advantage that it can be turned with an adjustable wrench, combination wrench, and 6- or 12-point socket.
Hex Socket (Allen)	The hex socket screw drive has a hexagonal recess and may be driven by a hex wrench, also known as an Allen wrench, Allen key, hex key, or inbus, as well as by a hex screwdriver (also known as a hex driver), or bit.
Torx	The hexalobular socket screw drive, often referred to by the original proprietary brand name Torx or by the alternative generic name star drive, uses a star-shaped recess in the fastener with six rounded points. It was designed to permit increased torque transfer from the driver to the bit compared to other drive systems. Torx is very popular in the automotive and electronics industries beacuse of resistance to cam out, and extended bit life as well as reduced operator fatigue by minimizing the need to bear down on the drive tool.

Most heads come in a range of sizes, typically distinguished by a number, such as “Phillips #00” or “Torx T5”. These sizes do not necessarily describe a particular dimension of the drive shape, and are often arbitrary designations.