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Arc- Part of the circumference of a curve. The curved portion of a bend.

Arc Length- The curved distance along a circumferential line. The length of the curved portion of a member.


Backwall or Pipe Heel- The outer half or the half of the pipe undergoing tension during bending (see Extrados).

Bend Radius- The arc of the bend itself, usually taken at the centerline. The distance from the center of curvature to the center-line (axis) of the pipe, expressed as a number multiplied by the pipe or tube size. For example, the bend radius of a 5 x D bend for a 2-inch nominal pipe size (NPS) pipe or a 2-inch tube is 10 inches (see Center-Line Radius).

Bender- 1. A mechanical device capable of forming a bend in a straight length of material. 2. A commercial or industrial company engaged in the business of making bends.

Bending Moment- The amount of force required to initiate a change of direction in a straight length of material about the center of curvature.

Bending Procedure- A written procedure containing all of the essential parameters and information necessary to guide the operator in producing bends.


Center to Center- The distance between the theoretical or calculated centers of two adjoining bends on the same plane. Also used for diametric measurement between the center lines of two tangent points of a bend (i.e. 180° bend, for which the Center to Center distance will be equal to twice the Center-Line Radius).

Center-Line Radius- The distance from the center of the curvature to the center-line (axis) of the pipe.

Chord- The straight distance between two points on a curve.

Cold Bending- Any bending process where curvature is induced by load application at room temperature.

Compound Bend- A curve made up of two or more arcs in the same plane, joined tangentially without reverse curvature.

Compression- The forces which thicken the inside wall of the bend.


Degree of Bend- The angle to which the bend is formed.

Diameter- A straight line passing through the center of a circle.

Distance Between Bends- The actual length of the straight section between the tangent points of two adjoining bends (see Tangent).

Distortion- A deviation from the original cross section shape.

Ductility- The ability of the material to deform without fracture. This is measured by elongation of reduction of area in a tensile test.


Easy-Way- The orientation of a member relative to the plane of curvature where bending occurs about the weak principal axis.

Elongation- The increase in length of material fibers during bending, expressed as a percentage of the original length.

Extrados- The outside arc of the bend (see Backwall).


Flattening- The distortion of the cross section of pipe or tube from its normal (round) shape.


Gag Pressing- A cold bending method that uses hydraulic rams to simultaneously apply bending forces at discrete, widely-spaced, locations along the member. Also known as point-bending.


Hard-Way- The orientation of a member relative to the plane of curvature where bending occurs about the strong principal axis.

Hot Bending- Any bending process where curvature is induced by load application at an elevated temperature.

Hump- A rounded bulge on the backwall or heel of the material.


I.D.- Inside Diameter of the material.

Incremental Step Bending- A cold-bending method that uses hydraulic rams to apply bending forces at several discrete, closely-spaced, locations along the member.

Induction Bending- A hot-bending method that utilizes an electric induction coil to heat a short section of the member before it is curved by force.

In-Plane Flexure- Bending a curved member where moment is applied about the axis of curvature. The primary flexural stresses and deflections are in the plane of curvature.

Intrados- The inside arc of the bend.


Local Buckling- A type of potential cross-sectional distortion that is caused by compression stresses in the member induced during the bending operation. Local buckling can be in the form of a single half-wave or a series of wrinkles along the entire bend length.


Mandrel- A tool that can be inserted into a HSS member to support the walls and minimize cross-sectional distortion during the bending process.

Minimum Wall Thickness- The wall thickness specified on the fabrication drawing or computed in accordance with the applicable specification as the minimum acceptable for temperature and pressure application.

Multi-Axis Bend- A bend with curvature about more than one axis. Also known as a multi-plane bend.


Neutral Axis- The area where there is neither compression nor tension during bending.

Nominal- Usually refers to pipe sizes up to and including 12 inch. Also used in reference to wall thickness, generally as a "mean" or average measurement.

Normalizing- A thermal treatment where the member is heated to a suitable temperature above the upper transformation temperature, followed by cooling in still air room temperature.


O.D.- Outside Diameter of the material.

Off-Axis Bend- A bending orientation where the member is curved about a non-principal or non-geometric axis. Also known as conical rolling.

Out of Plane Flexure- Bending of a curved member where moment is applied in the plane of curvature. The primary flexural stresses and deflections are in perpendicular to the plane of curvature.

Ovality- The distortion of the cross section of pipe or tube from its normal (round) shape usually expressed as a percentage of the difference between major and minor axis (Pipe Fabrication Institute (PFI) Standard is 8%).


Plane of Bend (POB)- Is referred as the rotation from a neutral axis. It is specifically used for changes of plane in successive bends.

Pyramid Roll Bending- A cold-bending operation where a member is bent progressively by repeatedly passing it through a set of three adjustable rolls in pyramid arrangement.


Radius- A line segment that joins the center of a circle with any point on the materials circumference (see Center-Line Radius).

Reverse Compound Bend- A curve made up of two or more arcs in the same plane, joined tangentially with reversal of curvature. Also know as an s-curve or an offset bend.

Rise- The distance, perpendicular to the chord, between the mid-point of a chord and an arc. Also known as the mid-ordinate.

Rotary Draw Bending- A bending method where the member is clamped to a form and bent by rotating it around a bend die with a given radius.


Slope- The angle of an inclined member designated using the vertical (rise) and the horizontal (run) distances between two points. Also known as bevel or pitch.

Spiral- A three-dimensional curve with an arc in one plane and a constant slope in a perpendicular plane. Also known as helix, or helical curve. The curving process is often called sloped rolling or pitched rolling.

Springback- The deformation of a bent member immediately after a bending load is released, where a portion of the curvature is partially lost.

Synchronized Incremental Cold Bending- A bending process where pressure is applied in a highly synchronized fashion at several locations along the steel member.


Tangent- Straight section of material on either side of the bend or arc. May be any length. Notes: In order to make the bend itself, there has to be sufficient material at each end during the bending process. Excess may be trimmed after.

Tangent Point- The point at which the bend is started or ended.

Tensile Strength- The strength of material stretched to the point at which it will rupture.

Throat- The inner half of the pipe or the half undergoing compression during bending (see Intrados).


Variable-Radius Bend- Parabolic, elliptical and other non-circular bends with variable radii. Also known as multi-radius and non-circular bends.


Wall- The thickness of tubular material, usually expressed as "nominal" or "minimum".

Wall Thickness- The thickness of the material usually stated in "schedule" for pipe or "guage" for tube.

Wall Thinning- The amount of reduction from original wall thickness of pipe or tube to the amount of wall thickness remaining in the extrados of a bend after forming (see Extrados).

Wrinkles- Definite folds, creases or ripples formed on the surface of the pipe during bending operation.


Yield Point- The point at which material will deform permanently during bending.

Yield Strength- The stress at which a material exhibits a specified deviation.

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