Helical Machined Springs

A Brief Intro to Machined Springs

Background

The first question generally asked about Machined Springs is how they compare with Wire Wound Springs.  Commencing with this question, this review of Machined Springs will proceed.

Wire Springs appeared preceding the Industrial Revolution.  They established their value immediately, and have not wavered from that most useful course.  Certainly, enhancements in materials and manufacturing have been forthcoming, but the basic concept has not changed much.  Spring wire coiled hot or cold with ends configured within the limits of coil wire has proven to be a very cost effective, industrial tool that exhibits elasticity within the bounds of known engineering understanding.  Uses range from deep ocean applications to man’s reach into the universe.  Finding a modern day device large or small, that does not benefit from elasticity, and particularly that provided by Wire Wound Springs, is a rare find.

Machined Springs are similar in function to Wire Wound Springs, but they are manufactured in a different way.  Although any machinable material including plastics can be used, metal in the form of bar stock is the most common starting point for Machined Springs.  The bar stock is first machined into a thick wall tube form, attachment features are added and then a helical slot is cut revealing multiple coils.  When deflected, these coils provide the desired elasticity.

Small Wire Spring and Machined Spring examples Configuration Differences

Small Wire Spring and Machined Spring examples Configuration Differences

The cost to manufacture Machined Springs exceeds that of winding Wire Springs.  Wire Wound Springs can be created with just a few seconds of process time, where a Machined Spring requires minutes at a minimum.  The machines used to create both forms are highly specialized and benefit from modern day CNC controls.

Each of the subsequent comparisons will provide descriptions and, if applicable, a decision matrix that can be useful when deciding which of the two spring forms will be most appropriate for a given application.

Coils

The coils found on Wire Wound Springs are typically round with sometimes rectangular and/or rectangular with rounded OD and ID surfaces.  The two latter forms are less common due to cost, but when used, they provide increased stiffness and compactness of design.  The rectangular coils are typically used so that the long leg is radial, but making the long leg longitudinal is possible.  Rectangular wire comes in set sizes; venturing away from those sizes can be done but at an increased cost and lead time.

Coils used on Machined Springs are square, rectangular (radial or longitudinal) and trapezoidal.  Trapezoidal coils are common to springs used in lateral bending and lateral translation.  This shape allows for additional lateral motion without coil contact.  The size of the coil is easily changed to fit the spring’s needs.  No standard sizes apply.

Slots

Exaggerated Comparison of Slot Widths of Wire and Machined Springs with the same Length, OD and Compression Rate

Exaggerated Comparison of Slot Widths of Wire and Machined Springs with the same Length, OD and Compression Rate

 

On Wire Wound Springs the space between the coils (slots) is typically uniform for Torsional Springs.  For Compression Springs, they are uniform also, but the end slots usually taper to zero.  This process is called “closing” the ends, and is created by an additional forming process.  Optional grinding then makes the ends nearly flat.  Extension Springs can have a uniform slot width from zero to most any size.  If desired, the coils can be pre-stressed so that an Extension Spring exhibits a zero slot that furthermore requires a force threshold which needs to be overcome before the coils start to separate.

Currently, Machined Springs come with minimum slot of about 0.020 inch (0.51 mm).  Wider slots, but generally not exceeding 0.250 inch (6.35mm), are possible.  The slot width can be closed to near zero using a stress relieving process, but no pre-stressing common to Wire Springs is currently available.

 If a compression spring application requires the absolutely best repeatability to support calibration and/or high precision uses, it is best that the coils never touch.  Even better, the minimum slot width needs to be wide enough to not permit any contamination between the coils from restricting and/or changing the compression motion.  Machined Springs are ideal for calibration and precision usages from this standpoint. The closed aspect of Wire Springs ends can result in elastic differences in the presents of common contamination.

Number of Coils

Wire Wound Springs can be made very long. A good example of a long Wire Wound Spring is exhibited by garter springs. The general length limitation is governed by the quantity of continuous wire available on the feed spool.

Machined Springs are limited to about 30 coils depending upon size, but Machined Springs with coil numbers above 20 are rare.

 

Feture Section

Wire Wound

Springs

Machined

Springs

Coils Round yes no
Square yes yes
Rectangular, radial yes yes
Rectangular, radial

with high ratio

no yes
Rectangular, longitudinal yes yes
Rectangular, longitudinal

with high ratio

yes yes
Slots Zero width yes yes, with

special process

Zero width w/prestress yes no
0.100 inch yes yes
Over 0.250 inch yes maybe
Number of coils 1 coil minimum yes yes
20 coils yes yes
50 coils yes maybe

 

Length

In a Wire Spring, the entire length of the wire contributes to the elasticity of the spring because the forces and moments are distributed end to end with the ends providing the interface with adjoining equipment.

Machined Springs are different.  The Flexure, the section providing the desired elasticity is captive between the end sections that provide structure and attachment features.  The structure and attachment features have infinite stiffness when compared to the Flexure.  Furthermore, the slots on Machined Springs do not taper to zero at the ends; they do remain at the full or initial width, as seen at free length.  As a result, to accomplish the same elastic performance, Machined Springs likely need to be longer than wire ones.

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