Coil Spring Design Basics 1

[smjmitchell]

Hi All,

I need to do some work designing steel coil springs (approx 2" dia x 12" long - loads approx 1500-2000 lb). I am familar with the basic theory of coil springs for calculating stresses etc however I have a couple of questions of a more practical nature:

1. What are the most common steel alloys used in the manufacture of coil springs.

2. What maximum working stress levels, considering fatigue etc, are used for the common alloys or is there a general rule of thumb that is applicable to all steels (i.e. some percentage of max shear stress).

3. What is the normal level of heat treatment for the common steels (i.e. Rockwell C etc). What heat treatment specs (MIL-SPEC, SAE etc) are usually used by the industry.

4. What is a typical deflection to length ratio at rated load ... I am assuming that you would want some margin against the spring bottoming out at maximum load.

5. Are all springs shot peened or are there some situations where you peen and others where you do not ?

6. Are there any good references (spring design bibles) that spring designers use that provide guidance on practical issues such as those above.

I am just trying to educate myself so I am not a total vacuum of knowledge when I go to talk to the spring manufacturer.

Thanks, for any help. It will be appreciated.

Stephen

 

[desertfox]

Hi smjmitchell

Material selection will depend on your operating enviroment,
theres's a complete list of spring materials and other information in the Design Handbook by Associated Springs
(Barnes Group).Materials such as grades of Stainless Steel,Phosphor Bronze, Carbon steel, alloy Steels etc are used to make springs.
Fatigue will depend on the frequency of spring operation and
in a situation of high cycle fatigue then shot peening would improve fatigue strength but is usually not worth doing for low cycle applications.
Heat treatment again would depend on service conditions and material but heat treatment is not always required.
Finally keep your spring operating between 15% and 85% of the full deflection range as outside these limits the spring rate is none linear.

regards desertfox

 

[jackpot]

I might recommend three books to consider:

"Handbook of Spring Design" put out by SMI

"Mechanical Springs" by A. M. Wahl

or the

"Spring Design Manual AE-11" by SAE

Most springs that operate at temperatures below 250°F are made from a high carbon steel with a good surface finish called Music Wire (ASTM A228). Springs that operate above this temperature are typically Chrome Silicon (ASTM A401). There are many other materials as pointed out by dfox.

Generally speaking, Fatigue life is proportional to tensile strength. Anything that can be done to reduce surface defects helps like electropolishing, peeling or surface inspection will also improve performance.

Jack

 

[israelkk]

smjmitchell

I took the liberty to make a preliminary check if a 2" dia x 12" long helical compression spring can be designed for approx 2000 lb and it seems quite impossible even when I am using 17-7PH CH900. More then that, the highet to diameter ratio is quite high so the spring will buckle unless it is guided but, then you will face a friction between the spring and its guide which may render its ability to resist fatigue.

http://www.webspawner.com/users/israelkk/

 

[smjmitchell]

Thanks you all for your advice.

The spring I am looking at replacing is of Danly manufacture. A look in their catalog shows that the existing 2" dia x 12" (50mm x 300mm) long heavy duty spring could carry a peak load of approx 750 kg (1650 lb) at 72mm deflection. The extra heavy duty spring would carry 1000 kg at 60mm deflection (k = 16.66 kgf/mm). 1000 kg is approx 2200 lb. These are from the columns in their tables designated 0.3 million .. which I can only assume means a life at this load of 0.3 million cycles (I need to clarify this). These springs are made from rectangular section wire.

I should clarify a couple of issues.

2000 lb is the peak or limit load that may only be seen a few times in the life of the spring. Normal operating loads are likely to be half this.

Yes the spring will be contrained inside a tube to prevent buckling.

I have some flexibility to change the spring length, diamter etc to get the required characteristics. The peak loads will ultimately be determined by the deflections of the spring and so I need to use an interative design process to refine the geometry.

Steve

 

[TVP]

smjmitchell,

In addition to the info already provided, here is some more food for thought:

1. Most common alloys are Cr-Si alloys like SAE 9254 or 9260 (ASTM A 401) or plain-carbon grades like 1080-1095 for music wire (ASTM A 228). Alloy grades are quenched and tempered to a martensitic microstructure. Carbon steels are patented to pearlitic microstructures (extremely fine pearlite).

2. Stress levels should be maintained below ~ 1100 MPa for fatigue applications using commercial quality wire (ASTM A 401) that is shot peened. This works out to ~ 60% of the UTS. Higher stresses can be tolerated with improved spring steels like ASTM A 877 (valve quality), which has reduced limits for decarburization, non-metallic inclusions, and surface defects. Other improvements in fatigue strength are produced by variations to the shot peening process like double shot peening (high intensity followed by low intensity).

3. Hardness depends a great deal on the grades used and how they are fabricated (hot winding vs. cold winding). A general range is ~ 47-53 HRC for hot winding, which means the springs are wound hot, then quenched and tempered to the desired hardness of 47-53 HRC. Cold winding uses wire that has already been quenched & tempered (or patented in the case of music wire) to very high strengths. This wire is purchased to tensile strength ranges, not hardness. The tensile strength for ASTM A 401 ranges from 1530-2260 MPa depending on wire size. This translates into ~ 47-58 HRC, with the highest levels only attainable in very small diameters. A good source of heat treating info is MIL-H-6875.

4. If at possible, try to limit the travel so that spring is not completely closed (compressed from some nominal length to the solid length). Stresses increase greatly as you approach the solid length. However, properly processed springs can be taken to solid with fatigue lives in excess of 100,000 cycles.

5. Any spring that requires good fatigue life should be shot peened. Usually only small diameter springs or springs that are very lightly stressed are not shot peened (think springs in ink pens).

6. Jackpot and the fox provided information on good resources for spring design, calculations, etc. Another excellent resource is the Euronorm standard EN 13906-1 Cylindrical helical springs made from round wire and bars – Calculation and design - Part 1: Compression springs. It is available in English (BS or DIN) or German (DIN) versions.

 

[Wobulator]

I need to design a compression spring to diplace 50 mm for each kilo of load and an overall displacement of 300 mm. Any help would be well apreciated.

Wobulator

 

[NickE]

wobulator- start a new thread.

 

[israelkk]

Wobulator

First I agree with NickE.

However, your info is not enough.
How much space you have for the spring?
What is the maximum outside diameter?
What is the maximum free and compressed length?
How much the maximum force the spring will see?
How many times the spring will see the force, is it for static or cyclic application?
Is the wire material need to be corrosion resistant?
If it need not to be from stainless steel is it to plated and if so what type of plating?
Is the spring guided on a rod or inside a hole?
How about bucking of the spring and friction with the rod or hole?
etc.

A spring design is a job for an experienced mechanical engineer. Spring design is not just plugging the numbers into the formulas, and as you see from my previous questions there are many issues that the spring designer has to take into consideration.

http://www.webspawner.com/users/israelkk/

 

[suneelnewaskar]

Dear Sir,
I am looking for springs that each spring shall support
200 kg load , deflection is 30 mm , but these shall operate
at 400 cycles per minute . Thsese springs shall be used in power press operation . Me i do not have any dimensional constraint . Because i am going to use them as ha hangers.
I shall be thankfull if you can upgrade me on this
With best regards
suneel

 

[israelkk]

suneelnewaskar

Firstly you should start a new posting instead of entering a new design problem in an existing post.

To your specific design problem:

1. Are you looking for a catalog spring or a custom designed spring?
2. Is the spring load changing from zero load to 200kg or from another load to 200kg?
3. To correctly design and avoid resonance phenomena, what is the mass that the springs "see" when they are working?

http://www.webspawner.com/users/israelkk/