McGuire Glossary of Fastener Terminology

U - 45 terms

U DRIVE - A U-Drive screw has a helical thread with an angle acute enough to be driven by a hammer, and thus has a domed head with no means of turning it.

UHMW (PLASTICS) - While ultrahigh-molecular-weight polyethylene retains the inherent qualities of low-density polyethylene, its increased toughness allows its use in a wide variety of rugged applications. It is commonly employed to provide rollers, cams, impellers and bumper guards. Because of its high lubricity, it is also used to coat conveyer-belts, ramps and hoppers. UHMW's imperviousness to attack by steam and chemicals, and its continued good performance at low temperatures, has also enabled it to gain FDA approval for a variety of applications within the food and drug industries.

ULTEM(R) (PLASTICS) - Unreinforced ULTEM(R) (polyetherimide) keeps its hardness, and mechanical properties from -40F up to temperatures of 356F. It is radiation-resistant, microwave transparent and is naturally flame-resistant. Reinforced grades have even higher mechanical strength. Because ot its unequalled properties, ULTEM(R) is the ideal replacement for steel and other metals. It also has a wide range of electronic and medical applications.

ULTIMATE DESIGN LOAD - The product of the ultimate factor of safety, the fitting factor, and the limit load. Also referred to as Design Ultimate Load.

ULTIMATE SHEAR STRENGTH - The failure load of the fastener in a shear situation. It may not reflect the strength of the joint material.

ULTIMATE TENSILE LOAD - The maximum tensile-applied load a fastener can support prior to fracture, and normally expressed in terms of pounds per square inch (psi) or pounds-force (lbf).

ULTIMATE TENSILE STRENGTH - The ultimate load which a fastener can withstand before breaking.

ULTIMATE THREAD STRENGTH - The force required to cause significant yielding or fracture of an insert when the head is supported and a load is applied to the threads in a pulling direction from the head side. It is equivalent to the insert being over-installed to failure. The difference between ultimate thread strength and installation load is significant in that it represents the size of the force window needed for tools that install to a constant force. Spin-spin tools install to a constant torque, but for consistent thread friction conditions, they in fact install to a constant force.

ULTIMATE TORQUE - The amount of force at which a threaded fastener begins to strip or otherwise fail in joint; or strip the threads of an insert or nut.

ULTRASONIC CLEANING - Cleaning by any chemical means aided by ultrasonic vibration

ULTRASONIC EXTENSOMETER - An instrument which can measure the change in length of a fastener ultrasonically as the fastener is tightened or measure a length before and after it is tightened.

ULTRASONIC TESTING - A method of nondestructive testing of solid metal for internal flaws utilizing high-frequency sound waves.

UNC (United Coarse) THREADS - The coarse thread series UNC/UNRC is the most used thread system for screws, bolts, and nuts. It is used for producing threads in low strength materials such as cast iron, mild steel, and softer copper alloys, aluminum, etc. The coarse thread is also used for raid assembly and disassembly.

UNCERTAINTY OF MEASUREMENT - Is a parameter associated with the result of a measurement that characterizes the dispersion of the values that could reasonably be attributed to the measurand

UNDER-SIZE BODY - The reduced body of a bolt or screw, the diameter of which may range from below the pitch diameter to the minimum major diameter of the thread. Such a body diameter is found on some bolts or screw having rolled threads.

UNDERCUT - In milling, when a cutter has to reach under a shelf, undercut refers to a surface that is not visible when the part is viewed from the spindle.

UNDERCUT - The triangular or rectangular shaped groove applied to the shank of a self-clinching fastener located between the displacer and the shank. The purpose of the undercut is to accept the panel material displaced by the displacer.

UNDERCUT ANCHOR - A post-installed anchor that develops its tensile strength from the mechanical interlock provide by undercutting of the concrete at the embedded end of the anchor.

UNDERCUT HEAD - For short lengths of flat and oval head machine screws, the heads are undercut to 70 percent of normal side height to afford greater length of the thread on the screws.

UNDERCUTS - Sections of a forging which, if driven into the impression while the metal is hot, would lock themselves into a die impression and prevent removal of the forging without distortion.

UNDERFILL - A portion of a forging that has insufficient metal to give it the true shape of the impression.

UNDERFILL - A portion of the formed part which has insufficient metal to give it the full shape of the tooling impression or cavity.

UNDERFILL - Primarily used in heading and forming work to donote a condition in which the work material did not competely fill the die cavity. Most common examples are on the head of type S nuts, and on step of studs.

UNDERHEAD FILLET - The fillet at the junction of the head and shank of a headed fastener.

UNDERSLUNG FINGERS - Special transfer fingers that are used when the fingers must snap down from a larger diameter to a smaller diameter during the die kickout motion. They give added control to the part during the snap down process.

UNEF (United Extra Fine) THREADS - This is used when more threads are desired to be engaged that the fine thread series would allow. Works well in applications where thin materials are used or find adjustments must be made.

UNF (Unified Fine) THREADS - Have a larger minor diameter than UNC thread, which gives UNF fasteners slightly higher load carrying and better torque locking capabilities than UNC fasteners of the same identical material and outside diameter. The fine threads have tighter manufacturing tolerances than UNC threads, and the smaller lead angle allows for finer tension adjustment. More susceptible to thread nicking and thread stripping.

UNFINISHED FASTENER - A fastener made to the same basic dimensions as a finished fastener but having relatively wider tolerances than a finished fastener and having all surface in their formed condition.

UNIDIRECTIONAL BENDING - Fracture marks left on the surface of a fastener failure (breakage).

UNIFIED THREAD STANDARDS - The basic American standards for fastening screw threads as agreed upon by standards bodies of Canada, the United States of American and the United Kingdom. They are a complete and integrated system of threads for fastening purposes. Their outstanding characteristic is general interchangeability of threads achieved through the standardization of thread form, diameter-pitch combinations, and limits of size. This standard is currently controlled by ASME/ANSI in the United States.

UNIT PRESSURE - Load divided by area.

UNIVERSAL TRANSFER - A transfer design that combined features from the straight across transfer and the CNF transfer. The fingers units could be setup to take parts straight across between stations or to rotate the part 180 degrees between stations with fingers that did not open. The units could also be changed to have cam opened and closed fingers that could also transfer parts straight across or rotate them between forming stations.

UNJ - Unified thread form with a more heavily rounded root than UNR, causing the minor diameter of the external threads to increase. Therefore, UNJ specifies thread form for both internal and external threads. Commonly used in aerospace applications.

UNR - Unified Radiused root threads. Most common unified thread form. Not to be confused with the thread series UNC, UNF, UNEF, or 8-UN which designated pitch diameters as they relate to threads per inch rather than the form of the root and crests of the thread.

UNS - The Unified Numbering System. A system that provides a means of correlating many nationally used numbering systems currently administered by societies, trade associations, and individual users and producers of metals and alloys, thereby avoiding confusion caused by use of more than one identification number for the same material. It also avoids having the same number assigned to two or more entirely different materials.

UNS (Unified Special) THREADS - Unified threads of special diameters, pitches, and lengths of engagement.

UNSUPPORTED STOCK - The length of material during an upsetting operation not controlled buy the tooling. Usually expressed by Number of Diameters (length, Diameter).

UNTHREADED LENGTH - That portion of the axial length of a stud, at its head end, that is unthreaded.

UNTIMED HOLD - The second portion of the TKO motion in the heading slide. During this part of the stroke, the dimension form the face of the die to the face of the TKO lever or rod starts to increase meaning that the lever or rod is starting to move away from the face of the die. However, that movement is less than the movement of the heading slide during the same time. Consequentially, the kickout parts inside the tooling see more kickout stroke.

UPSET - When metal is reduced in height and the starting diameter is increased. A process typically used to form a head on a part. The upset material may be in the open between the tooling components or it may be totally enclosed within the tooling at the end of the forming stroke.

UPSET - A term sometimes used as a verb to describe the act of creating a the bulb when installing a blind threaded fastener. It is commonly used in cold heading and solid riveting and has been applied to inserts because forming the bulb is similar to forming a fastener head by upsetting. In heading work the operation of signicantly increasing the diameter of the work by striking it from the end.

UPSET FORGING - (1) A forging made by upsetting an appropriate length of bar, biller or bloom. (2) Working metal to increase the cross-sectional area of a portion or all of the stock. (3) A forging formed by heading or gathering the material by pressure upon hot or cold metal between dies operated in a horizontal plane.

UPSET RATIO - The ratio of the finished height of the upset versus the finished outside diameter of the upset. It is used to help determine the load required to do the upset.

UPSETTER (FORGING MACHINE) - A horizontal forging machine where the workpiece is gripped between two grooved dies and deformed by a punch that exerts force on the end of the stock.

UPSETTING - The process of increasing the cross-sectional area by displacement of material longitudinally and radially.