Educational Resources

Glossary

Active Coils

Those coils which are free to deflect under load. Angular relationship of ends relative position of hooks or loops of an extension spring (-Iegs of a torsion spring) to each other.

Angle of Thread

The angle included between the sides of the thread measured in an axial plane.

Angular Offset (Bellows)

The angle between the centerlines of the ends of the bellows.

Axis of a Screw

The longitudinal central line through the screw.

Baking

Heating of electroplated springs to relieve hydrogen embrittlement.

Bellows Free Length

The manufactured length of the bellows convolutions, with the bellows at a neutral at rest position with no applied forces acting on it.

Bellows ID, max./min

The bellows acceptable inside diameter range for the application. This helps to define the acceptable size envelope for the bellows.

Bellows OD, max./min

The bellows acceptable outside diameter range for the application. This helps to define the acceptable size envelope for the bellows. It is critical for tool selection and to optimize performance.

Block

Length of a compression spring when deflected under sufficient load to bring all adjacent coils into contact – no additional deflection is possible.

Buckling

Bowing or lateral displacement of a compression spring. This effect is related to slenderness ratio L/D.

Closed and Ground Ends

Same as Closed Ends, except the first and last coils are ground to provide a flat bearing surface.

Closed Length

Length of a compression spring when deflected under sufficient load to bring all adjacent coils into contact – no additional deflection is possible.

Close Wound

Adjacent coils are touching.

Coils Per Inch

Distance from center to center of wire in adjacent coils in an open wound spring.

Cycle Life

The number of cycles required for compression bending or offset movements. Loading is generally not a consideration here.

Deflection

Motion imparted to a spring by application or removal of an external load. Elastic Limit Maximum stress to which a material may be subjected without permanent set. Endurance Limit Maximum stress, at a given stress ratio, at which material will operate in a given environment for a stated number of cycles without failure.

Engagement Depth

The depth of thread contact of two mated parts, measured radially.

Engagement Length

The length of contact between two mated threaded parts measured axially.

External Thread

An external thread is a thread on the outside of a fastener or similar component.

Fixture Tempering

Restraining parts during tempering to improve dimensional control.

Free Angle

Angular relationship between arms of a helical torsion spring which is not under load.

Free Length

Overall length of a spring which is not under load.

Gradient

Spring gradient, or change in load per unit of deflection.

Heat Setting

A process to pre-relax a spring in order to improve stress relaxation resistance in service.

Helical Springs

Springs made of bar stock or wire coiled into a helical form. This category includes compression, extension and torsion springs.

Helix Angle (Fasteners)

The angle made by the helix, or conical spiral, of the thread at a pitch diameter with a plane perpendicular to the axis.

Hooks

Open loops or ends of extension springs.

Hysteresis

Mechanical energy loss occurring during loading and unloading of a spring within the elastic range. It is illustrated by the area between load deflection curves.

Initial Tension

A force that tends to keep coils of a close wound extension spring closed and which must be overcome before the coils start to open.

Internal Thread

An internal thread is a thread on the inside of a member.

Lateral Offset (Bellows)

The distance between the centerlines of the ends of the bellows that are parallel but not on the same line.

Leak Rate

If the part or assembly is required to be leak tested, if known, the required leak rate should be provided. The achievable leak rate will vary due to the design requirements (ID, OD, wall thickness, etc.)

Leak Test

Testing to determine whether the application requires the part to be leak tight (generally refers to bellows or bellows assembly).

Loops

Formed ends with minimal gaps at the ends of extension springs.

Major Thread Diameter

The largest diameter of the thread of the screw or nut. The term “major diameter” replaces the term “outside diameter” as applied to the thread of a screw and also the term “full diameter” as applied to the thread of a nut.

Maximum External Pressure (Bellows)

If the bellows needs to be leak-tight, this is the maximum differential applied externally to the bellows, in the application.

Maximum Internal Pressure (Bellows)

If the bellows needs to be leak tight, this is the maximum differential applied internally to the bellows, in the application.

Mean Area (of Screws)

The term “mean area of a screw,” when used in specifications and for other purposes, designates the cross-sectional area computed from the mean of the basic pitch and minor diameters.

Mean Diameter

The average diameter of the mass of spring material, equal to one half the sum of the outside and inside diameters. In a helical spring, this is the equivalent to the outside diameter minus one wire diameter.

Minor Thread Diameter

The smallest diameter of the thread of the screw or nut. The term “minor diameter” replaces the term “core diameter” as applied to the thread of a screw and also the term “inside diameter” as applied to the thread of a nut.

Modulus in Tension or Bending

(Young’s Modulus E) Coefficient of stiffness used for torsion or flat springs. Moment A product of the distance from the spring axis to the point of load application, and the force component normal to the distance line.

Modulus in Shear or Torsion

(Modulus of Rigidity G) Coefficient of stiffness used for compression and extension springs.

Natural Frequency

Lowest inherent rate of free vibration of a spring vibrating between its own ends.

Operating Pressure

If the part needs to be leak-tight, this is the differential pressure the part will be subjected to in the application. It is important to specify if the pressure will be applied internally or externally to the bellows or part.

Operating Temperature

The temperature that the part or assembly will experience during normal operation.

Patenting

The process of heating carbon steel above its critical temperature and cooling at a controlled rate to achieve a fine pearlitic microstructure.

Pitch Diameter (Fasteners)

On a straight screw thread, the diameter of an imaginary cylinder, the surface of which would pass through the threads at such points as to make equal the width, the threads, and the width of the spaces cut by the surface of the cylinder. On a taper screw thread, the diameter, at a given distance from a reference plane perpendicular to the axis of an imaginary cone, the surface of which would pass through the threads at such points as to make equal the width, the threads, and the width of the spaces cut by the surface of the cone.

Pitch Line (Fasteners)

An element of the imaginary cylinder or cone specified under “Pitch Diameter”.

Pitch (Fasteners)

The distance from a point on a screw thread to a corresponding point on the next thread measured parallel to the axis.

Pitch (Springs)

Distance from center to center of wire in adjacent coils in an open wound spring.

Plain Ends

End coils of a helical spring having a constant pitch and ends not squared.

Plain Ends, Ground

Same as Plain Ends, except wire ends are ground square with the axis.

Pressure Responsive

What is the desired response with pressure changes?

Rate

Spring gradient, or change in load per unit of deflection.

Rear-End Locker Springs

Rear-end locker springs maintain a critical constant force on the ratcheting mechanism to ensure a smooth and consistent transfer of power to the rear drive line.

Residual Stress

Stress mechanically induced by such means as set removal, shot peening, cold working, or forming. It may be beneficial or not, depending on the spring application.

Screw Crest

The surface of the thread corresponding to the major diameter of the screw and the minor diameter of the nut.

Screw Lead

The distance a screw thread advances axially in one turn. On a single-threaded screw, the lead and pitch are identical; on a double-threaded screw the lead is two times the pitch; on a triple-headed screw, the lead is three times the pitch, etc.

Screw Root

The surface of the thread corresponding to the minor diameter of the screw and the major diameter of the nut.

Screw Side (or Screw Flank)

The surface of the thread which connects the crest with the root.

Screw Thread

A ridge or uniform section in the form of a helix on the external or internal surface of a cylinder, or in the form of a conical spiral on the external or internal surface of a cone.

Set

Permanent change of length, height or position after a spring is stressed beyond material’s elastic limit.

Set Point

Stress at which some arbitrarily chosen amount of set (usually 2%) occurs. Set percentage is the set divided by the deflection which produced it.

Set Removal

An operation which causes a permanent loss of length or height due to spring deflection.

Shot Peening

Blasting the surfaces of spring material with steel or glass pellets to induce compressive stresses that improve fatigue life.

Slenderness Ratio

Ratio of spring length to mean diameter L/D in helical springs.

Solid Height

Length of a compression spring when deflected under sufficient load to bring all adjacent coils into contact – no additional deflection is possible.

Spiral Springs

Springs formed from flat strip or wire wound in the form of a spiral, loaded by torque about an axis normal to the plane of the spiral.

Spring Index

Ratio of mean diameter to wire diameter.

Spring Rate

The force in pounds applied to a part and measure of stiffness under pressure.

Squared and Ground Ends

See Closed and Ground Ends.

Squared Ends

See Closed and Ground Ends.

Squareness

Angular deviation, between the axis of a compression spring in a free state and a line normal to the end planes.

Stress Range

Difference in operating stresses at minimum and maximum loads.

Stress Ratio

Minimum stress divided by maximum stress.

Stress Relief

A low temperature heat treatment given springs to relieve residual stresses produced by prior cold forming.

Thread Base

The bottom section of the thread; the greatest section between the two adjacent roots.

Thread Depth

The distance between the crest and the base of the thread measured normal to the axis.

Thread Thickness

The distance between the adjacent sides of the thread measured along or parallel to the pitch line.

Torque

A product of the distance from the spring axis to the point of load application, and the force component normal to the distance line.

Total Number of Coils

The sum of the number of active and inactive coils in a spring body.