Set Screws

Set screws are usually headless but are sometimes headed fasteners with external machine screw threads and a point that is application specific. Despite being heralded early on as "an invention of the devil," they are the de facto method used to locate and fasten collars, drive wheels, gears, knobs, pulleys and other parts to shafts. Aside from their popularity in collar-type applications where headless set screws are fully recessed, they can also fasten components together. When tightened against a shaft—or when their tip mates with a hole in the shaft—they prevent rotational (torsional) and lateral (axial) movement and resist dynamic loads like vibration. For better point contact and to prevent a burr from interfering with part removal, the shaft may have a milled flat. Unlike socket head cap screws, which create clamping force by tension, set screws are considered a compression fastener because they hold parts in place using point penetration and friction as a result of the clamping action. When their point is spotted, shear comes into play. Set screws are also known as:

Allen screws, grub screws, hexagon socket set screws.

The invention of "flush head set screws, that is, set screws so shaped and threaded that when put to use their heads are sunk even with the outer surfaces of the parts into which they are driven" is perhaps best documented in the patent application Manufacture of Screws, filed by William G. Allen in January 1909 and subsequently patented in June 1910. Marketed as The Allen Safety Set Screw, it promised a 30% increase in strength and, compared to a "projecting set screw," "Its use eliminates scratching of hands or tearing of clothing, because the safety screw is beneath the surface." Invented by Mr. Allen and originally manufactured by The Allen Manufacturing Co., Inc., explains why set screws are referred to as Allen screws and hex keys as Allen wrenches.

When choosing a set screw, it's important to consider material hardness, how often the assembly is adjusted or disassembled, desired drive type, point style or tip type, required holding power and any other factors that may affect the selection process.

The application largely determines which point or tip is appropriate. Point styles include cone, cup, flat, half dog, knurled cup and oval points and common tip types are nylon and brass. Full dog point is non-standard but is available in limited sizes and lengths. The set screw's point penetration can contribute as much as 15% to the total holding power. Cone point, because of its deep penetration, provides the largest holding power increase while oval point, with its minimal penetration, offers the least. In between are cup, flat and dog points. Up to a 15% loss in holding power can result if the relative hardness differential between the set screw and shaft is less than 10 points.

Below is a summary of point styles and tip types along with advantages and common applications of each…

When working with soft materials and hardened shafts and where high tightening torques are not practical, consider using cup point, which is the most widely used type of set screw for permanent and semi-permanent assemblies. The sharp perimeter of the cup-shaped recess digs into the mating surface to prevent slippage.
A sharp point gives cone point the greatest axial and torsional holding power because of the deepest penetration. For this reason, they are used for permanent location of parts, as well as pivots and as a hanger point especially when fine adjustment is needed. The point angle is 90° ±2° for nominal and longer lengths and 118° ±2° for lengths shorter than nominal. A slight flat or rounding of the cone's apex is allowed during manufacturing. When used with hardened shafts, the point may be spotted to half its length.
Flat point is used where parts are adjusted or relocated often because its point causes minimal damage to the mating surface. It can be used against hardened shafts—especially shafts with flats for greater contact area—and is excellent with thin-walled materials and brass and soft metal plugs.
Half dog point often replaces dowel pins and is used for the permanent or fixed location of components. The point, which resembles a cylinder, locks in a matching diameter hole drilled in the shaft or presses against a ground flat. It works well with hollow tubing and hardened members. The non-standard full dog point is used when a longer point length is needed.
For permanent location of parts and in similar applications as cup point, use knurled cup point when mechanical locking is desired. It resists loosening and can tolerate severe vibration due to its counterclockwise knurls, which create a self locking ratchet action. It is said to perform well even in poorly tapped holes.
Oval point, or round or crowned point as it is sometimes called, is used with angled surfaces and in applications requiring frequent adjustment and where little or no damage can occur to the part it bears against. Use with a circular "U" shaft groove when rotational adjustment is needed or with an axial "V" groove for longitudinal adjustment. It may be manufactured with a slight flat at its tip.
The soft tip of nylon tip set screws will compress and conform to the mating surface without marring making them ideal for use with unhardened shafts. They also insulate, lock without damage against screw threads and adjust shaft end play. The nylon tip is often mated with alloy steel and stainless steel bodies. Set screw length does not include the nylon tip.
Brass tip set screws won't mar the mating surface but will lock the components securely in place. The body is typically alloy steel. The brass tip is not included in the length of the set screw.

Threads, specified as the number of threads per inch, are Class 3A, standard right-hand, and Unified inch coarse series (UNC, Unified National Coarse) and Unified inch fine (UNF, Unified National Fine); Class 3A "threads provide for applications where closeness of fit and/or accuracy of thread elements are important." Set screws are fully threaded but protruding points like cone and dog, and tips like nylon and brass, are not threaded.

Common sizes range from #0 to #12 and 1/4" to 1 1/2"—small sizes are expressed as whole numbers and large sizes as fractional inches—and lengths span from 1/16" to 6" for socket set screws. For square head set screws, available sizes are from #8 to #12 and 1/4" to 1 1/2", while lengths range from 7/32" to 12". The length of a headless socket set screw is overall length and includes its point but excludes nylon and brass tips; measure square head set screws from under the head to the point.

The most common set screw is headless and the drive style is hex socket although slotted drive is also available. Drive hex sockets with a hex key or hex bit. Square heads, which are externally wrenched and require sufficient clearance for wrenching, allow higher installation torque.

Alloy steel and austenitic stainless steel (18-8 and 316) are common materials for socket set screws along with brass and nickel-copper (NI-CU) alloy; square heads are available in steel, alloy steel and austenitic stainless steel. Black oxide (thermal or chemical), which offers minimal rust protection, and zinc plating are typical finishes for alloy steel. The pitch diameter of zinc plated screws is manufactured undersized since Class 3A threads do not allow for the added thickness of plating.

A nylon patch or nylon pellet provides prevailing torque to resist vibration and allows the set screw to lock in position without seating. Patches and pellets are commonly found on alloy steel and stainless steel set screws.

Factors relating to set screw holding power and resistance to vibration are summarized below…

Screw size affects holding power so it's recommended that screw diameter be about half the diameter of the shaft.
Resistance to vibration is also affected by set screw size in additional to seating torque.
Increased seating torque increases holding power and reduces the number of screws needed, which reduces cost because fewer drilled and tapped holes are required.
For cup, flat and oval point set screws, holding power is almost directly proportional to seating torque.
Two set screws—axially inline or on the same circumferential line at an angle of perhaps 60° or more—may be used to increase holding power. However, depending on their position relative to each other when circumferentially aligned, the increase in holding power can be as little as 30% or as much as 100%.

Refer to American Society of Mechanical Engineers Standard ASME B18.3, Socket Cap, Shoulder, Set Screws, and Hex Keys (Inch Series), for specifications relating to set screws.

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