Compression Springs

A compression spring is an elastic open-coil helical spring that is primarily used to absorb force or provide resistance. It comes in various shapes, viz. conical, barrel (concave) or hourglass (convex) other than the very common cylindrical shape. The diameter varies from 20-80mm and possesses the hardness of 40-50 HRC or so. It is available in black, blue, red, yellow and few more colours. It can be used as an energy accumulator, shock absorber, force generator or vibration damper.

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Detailed Description for Compression Springs

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Having engineered differently from other version of springs, these springs are very effective for building up energy and do serve countless potential applications. These springs are designed to keep things from coming together having coils of it kept opened and lightly wound as compared to other type of springs. Compression springs are at rest when they are extended.

As per the temperature, stress, and corrosion resistance requirements, the springs are made up from a variety of materials. These springs are being used in different types of machines and products and are commonly found in electronic devices such as door locks, watches, mattresses, compressors, switches, writing pens etc. Precisely saying, compression springs work to resist force and store energy in a wide variety of applications.

Nevertheless, it is recommended to not to overload the springs to order to get the maximum service life from a compression spring. Make sure that you are using the least possible force and travel on it to achieve the longest life. The optimum solution is to use only 30% of the maximum force and travel rather than 90% or 95% of the same. Because the fact is, either you use 30% or either you use 90%, the effect will be the same under both the scenarios.

So it is to be concluded that a compression spring must never be compressed in such a way that the coils touch one another. This does nothing but shortens the service life of the spring considerably.

General Consideration:

Following guidelines could be used for all the compression spring designs.

1)      Select the appropriate material and note down the shear modulus (G) and tensile strength (TS).

2)  Calculate the diameter (Dm) and Inner Diameter (ID) or OD for the spring. Then compare the ID of the spring with any work over rod requirements.

Dm = OD – d

ID = OD – 2d

3)  Diameter of the spring will increase when it gets compressed. Now calculate the OD and compare this to any work in hole requirements. Make sure you incorporate the high side of the OD tolerance while examining the work.

        ODexpansion = [ {D2m + p2-d2/(pi)2}1/2+ d] – OD

 

4) Calculate the Pitch and the Spring Index. Ensure that the pitch of the spring is not greater than the OD. Pitch is the distance measured between two coils. Spring Index depicts the correlation between the mean diameter of a spring and the wire diameter of a spring.

 

End Type

Open

Open/Ground

Closed

Closed/Ground

(FL-d)/NA

FL/NT

(FL-3d)/NA

(FL-2d)/Na

Coils per Inch (1/in) = 1/p

Spring Index = C = Dm/d

5)  In case the spring index doesn’t range between 4-10, further examinations are required like:

·         Special tooling (for tight index springs)

·         Special packaging (for high springs)

·         Stress relieving

·         Tangling issues

·         No grinding (grinding high index springs worsens the situation more)

·         No additional operation, viz. plating or tumbling.

 

 6)    Establish the corresponding design criteria using one of the following five methods

a)      Design based on physical dimensions

b)      Design based on spring rate.

c)      Design based on two loads.

d)      Design based on one load and spring rate.

e)      Design based on one load and free length.

 

7) After the spring rate (R) and number of active coils (NA) are determined, calculate the number of total coils (NT).

End Type

Open

Open/Ground

Closed

Closed/Ground

NA

NA+1

NA+2

NA+2

 

8)    Calculate a solid height (SH). A variation of 3% in nominal SH value is allowed to calculate the maximum solid height.

End Type

Open

Open/Ground

Closed

Closed/Ground

(NT+1)/d

NT/d

(NT+1)/d

NT/d

 

9)    If the percent stress at any load > 40%, set operation should be considered.

     If the percent stress at any load > 60%, a re-design must be considered.

 

Stress (psi) = rho = (8PDm)/ (pi) d3

Corrected stress (psi) = rho = {(8PDm)/ (pi) d3} K

Wahl Correction Factor = K = {(4C – 1)/ (4C – 4)} + 0.165/C

                                                                       Percent Stress (%) = 100.rho/TS

           10)   In case of an over-stressed situation, following measurements shall be considered.

 

Load at Solid Height (lb) = Ps = R (FL – SH)

Stress at Solid Height (psi) = rhos = 8PsDm/ (pi) d3

Percent Stress at solid height (%) = 100. Rhos/TS

Diameter Tolerances

Commercial tolerance chart (OD)

.025” to  .050” O.D. ± .001”

.851” to 1.125” O.D. ± .020”

.051” to  .100” O.D. ± .003”

1.126” to 1.250” O.D. ± .025”

 .101” to  .250” O.D. + .003” - .005”

1.251” to 1.480” O.D. ± .030”

.251” to  .500” O.D. ± .008”

1.481” to 1.750” O.D. ± .040”

 .501” to  .850” O.D. ± .015”

1.751” to 2.000” O.D. ± .055”

CPC Values:

ODcpc = (2/3) OD Tolc

CPC = Calculated Process Ability

Free Length Tolerances

FL Tolc = FL [1/p (6.10.107)]0.25 or FL[C1.53]/p (5.18.108)]0.25 if C>4

Note: If Free Length (FL) Value is < 0.500inch, use 0.500 as the FL value in the above calculation.

CPC values:

FLCPC = (2/3) FL Tolc

Rate Tolerances

Tolerance having variation of +/-10% is the standard. Anything smaller than this must consider the factors influencing the spring rate! Following formula should be used to calculate the commercial rate tolerance and rate CPC.

Rate TolC = 2.R. (OD TolC)/Dm

RateCPC = (2/3) Rate TolC

Load Tolerances

Consider the following formula to calculate the commercial load tolerance and load CPC.

Rate TOLC = R (FL TOLC) + 2.P. (OD TolC)/Dm

LoadCPC = (2/3) Load TOLC

Squareness

Standard measurement of this dimension is a maximum of 3degree tolerance. If, whatsoever, there is a tighter square requirement then particular attention must be given to coiling and grinding setup hours.

 

Shapes of springs:

There could be variety of shapes of compression springs. It is to be noted that custom designs may have any number of shapes depending upon the application. Nevertheless, there are some common shapes and designs which are in use.

1)      Battery Spring: It is a cone shaped spring where the spring radius decreases.

2)      Hour Glass: Dimensionally, it is convex-shaped. Tighter towards the centre, and outer coils have a larger diameter.

3)      Barrel: Concave in shape. Reduced at the end and wider in the centre.

4)      Reduced Ends: Spring is totally straight across the centre coils and tapers towards the end coils only.

 


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Adriana Grey

Detailed information Posted on 9/30/2016

While ordering Compression Springs, give the following information as completely as possible- #Free Length, Maximum, Minimum. #Controlling diameter, Outside diameter maximum, Inside diameter minimum. #Number of coils. #Wire size (Decimal size if possible). #Loads at deflected positions. #Maximum solid length. #Frequency of Compression

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Elasticity Posted on 8/1/2016

To have an adequate amount of elasticity in a spring has always been a matter of concern for the customers. Well, compression springs serves the very same; with honesty!

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