Understanding Torque 245

assembly line

A production process in which products are mass-produced in stages. Assembly lines are a linear method of manufacturing in which the object being produced passes through different workstations until it is complete.

base components

A material or object being joined by fasteners. Base component materials affect the amount of torque required to properly install threaded fasteners.

bolt

A cylindrical threaded fastener with a head that usually mates with a nut. Bolts typically have blunt ends.

breakaway torque

Torque tested by turning a tightened fastener in the tightening direction until it begins to rotate. Breakaway torque is usually greater than the torque used for assembly.

calibration

The comparison and adjustment of a device with unknown accuracy to a device with a known, accurate standard. Calibration of torque tools should generally occur annually, but some industries set stricter guidelines.

clamping force

The compressive force that a fastener exerts on a joint. Clamping force is usually equal in magnitude and opposite in direction to preload.

click wrench

A torque wrench that indicates that a fastener has reached torque by making an audible click. Click wrenches can be used for both assembly and inspection.

coatings

A thin layer of material added to a component to improve its properties. Coatings can reduce friction for a fastener.

compression

A pushing or pressing force that is directed toward the center of an object. The compression that a tightened fastener applies to joint components is called clamping force.

dial wrench

A torque wrench that measures the amount of torque applied to a fastener and displays the torque value on a dial on the face of the wrench. Dial wrenches can be used for both assembly and inspection.

distance

d. The space between two points. Distance is the amount of space through which forces travel.

failure

A catastrophic malfunction that breaks a fastener or prevents it from operating correctly. Fastener failure commonly results when the fastener yields due to overtightening.

fatigue

A type of failure that occurs when a fastener is subjected to repetitive loading or stressing from undertightening. Fatigue can cause catastrophic failure.

force

F. A push or pull that changes an object's motion or state of rest. Forces have specific directions and magnitudes.

friction

A force that resists motion between components that are in contact with each other. Friction is necessary to secure a fastener in a joint, but too much friction can be problematic.

geometry

The measurement, properties, and relationships of the lines and points of an object that make up its shape. Geometry describes a shape using its components.

hard joint

A joint formed by base components made of harder, more resistive materials, such as metal. A hard joint requires a snug-tight fastener to turn less than 30° to reach torque.

joint

The meeting point of the two materials that are joined together. Most fasteners create a joint that can be disassembled and reassembled.

kilonewton

kN. A metric system unit for force that is equal to 1,000 Newtons. Kilonewtons are often used to measure large preloads.

load

The force applied to an object by another object. The load a fastener must withstand determines proper preload, clamping force, and torque.

loosening torque

Torque tested by turning a tightened fastener in the loosening direction until it begins to rotate. Loosening torque is usually equal to the torque needed for assembly.

lubricants

A substance used to reduce friction between two surfaces in relative motion. Common lubricants are oil, grease, and wax.

magnitude

The size, amount, or quantity of force. The magnitude of preload and clamping force is usually the same.

marking torque

Torque tested by drawing a mark from the center of a tightened fastener onto the joint, loosening the fastener, and retightening the fastener to realign the mark. Marking torque is roughly equal to the torque needed for assembly.

maximum service load

The greatest amount of force or pressure that a fastener must withstand during regular use. The maximum service load determines how much preload is necessary for a fastener.

Newton-meter

Nm. The metric unit used to measure torque. The Newton-meter's English equivalent is the pound-foot.

Newtons

N. A metric unit of measurement for force. One Newton is equivalent to the amount of force required to accelerate one kilogram of mass at a rate of one meter per second squared.

nominal diameter

D. The overall diameter of a fastener, which may be different from its actual, measurable size. Nominal diameter is used to indicate a fastener's nominal size in inches or millimeters.

nut

A fastener with a threaded hole that mates with a bolt. Nuts are often six-sided to accommodate a driving tool.

pneumatic

Powered by compressed air. Pneumatic tools are good for repetitive fastening tasks and for applications with high torque requirements.

pound-foot

Lb.-ft. An English unit used to measure torque. The pound-foot's metric equivalent is the Newton-meter.

pound-inch

Lb.-in. An English unit used to measure torque. The pound-inch is used instead of the pound-foot to measure torque for smaller fasteners.

preload

P. The amount of tension a fastener experiences after reaching the proper torque value. Preload tension creates the force that holds a joint together.

relaxation

The natural loosening of a joint shortly after assembly. Relaxation is caused by factors like extreme temperature changes or the compression of components in some soft joints.

residual torque

The amount of torque required to move a fastener that has already been tightened to its torque value. Residual torque is measured during torque audits by tightening or loosening the fastener.

screw

A cylindrical threaded fastener that either fits into a threaded hole or forms threads in a material. Screws may have blunt or pointed ends.

shearing force

A force that attempts to cause the internal structure of a material to slide against itself. Shearing forces can damage a fastener and break apart a joint.

snug tight

Assembled with mild manual pressure. Once snug tight, a fastener must be torqued for the joint to be properly assembled.

soft joint

A joint formed by base components made of less resistive materials, such as plastic. A soft joint requires a snug-tight fastener to turn more than 720&deg, or two full rotations, to reach torque.

strain gauges

A sensor that uses changing electrical resistance to measure the amount of strain within an object. Strain gauges are used when inspecting bolted joints, and some sophisticated torque tools contain strain gauges for measuring torque during assembly.

strip

A condition in which a fastener's threads become damaged and ineffective. Fasteners strip when excess torque is applied.

tension

A pulling force that is directed away from an object and attempts to move, stretch, or elongate it. The tension that a joint exerts on a tightened fastener is called preload.

test cribs

A designated area in which technicians test and calibrate torque tools. Test cribs are used for calibration at the worksite, but most torque tools need to be calibrated at off-site laboratories.

threaded fastener

A type of fastener that has threads to hold objects together or grip material. Threaded fasteners include bolts, screws, and nuts.

torque

T. A rotational force. Torque is the product of force and distance and is critical for proper fastener tightening.

torque and angle

A technique for applying proper torque that involves tightening a fastener until it is snug tight, then turning the fastener an additional amount, usually one half-turn, to reach torque. Torque and angle works with a variety of tools and reduces concerns about friction.

torque audits

The examination of a fastened joint to ensure that the correct torque value has been obtained. Torque audits can be conducted before, during, or after assembly, depending on the specific requirements of each part and worksite.

torque coefficient

K. A non-dimensional number between 0.1 and 0.3 that is used to calculate fastener torque. Many manufacturers specify the torque coefficients of their products, based on the fastener’s coating or lubricant, if used. kilonewton |kN. A metric system unit for force that is equal to 1,000 Newtons. Kilonewtons are often used to measure large preloads.

torque screwdriver

A torque tool that is used to apply specific amounts of torque to a screw. Torque screwdrivers typically include a display or sensor to indicate torque.

torque tables

A reference chart used to find recommended torque values for a variety of fasteners. Torque tables provide recommended values for different fasteners, lubricants, coatings, threads, and grades.

torque tool

A tool with specialized features that ensure greater torque accuracy. Torque tools can provide readings of torque or be programmed to stop rotating or signal when torque is reached.

torque wrench

A torque tool, typically in the form of a socket wrench, that is used to apply specific amounts of torque to a nut or bolt. Torque wrenches include click and dial wrenches.

ultrasonic devices

A sensor that uses high-frequency soundwaves to measure the amount of strain within an object. Ultrasonic devices are used when inspecting bolted joints, and some sophisticated torque tools contain ultrasonic devices for measuring torque during assembly.

yield

The permanent deformation of a material caused by excessive force. A fastener may yield if it is overtightened.

yield strength

A metal's ability to resist progressive force without permanent deformation. Yield strength of fastener materials is usually expressed in pounds per square inch or megapascals.