Spring specifications

A frequent cause of confusion between the spring supplier and the spring user is lack of precision in specifying the spring. This often results in high costs due to the manufacturer taking considerable trouble to meet dimensions which are not necessary for the correct functioning of the spring.

It is recommended that, while all relevant data regarding the design should be entered on the spring detail drawing, some indication should be given as to which of the particular design points must be adhered to for the satisfactory operation of the component; this is necessary to allow for variations in wire diameter and elastic modulus of the material. For example, a compression spring of a given diameter may, if the number of coils is specified, have a spring rate which varies between ±15% of the calculated value. In view of this, it is desirable to leave at least one variable for adjustment by the manufacturer, and the common convenient variable for general use is the number of coils.

A method of spring specification which has worked well in practice is to insert a table of design data, such as that shown below, on the drawing. All design data is entered, and the items needed for the correct functioning of the spring are marked with an asterisk. With this method the manufacturer is permitted to vary any unmarked items, as only the asterisked data is checked by the spring user's inspector. The following are specifications typical for compression, tension, and torsion springs.

Compression spring Total turns Active turns Wire diameter *Free length *Solid length *Outside coil diameter *Inside coil diameter Rate

*Load at 9 mm Solid stress *Ends Wound *Material

*Protective treatment

Tension spring Mean diameter *O.D. max. *Free length

Total coils on barrel Wire diameter *Loops

Initial tension

1 mm

5 mm

7850 N/m

881 N/mm2

Closed and ground

Right-hand or left-hand



1.62 mm

Full eye, in line with each other and central with barrel of spring None


2697 N/m


53 ± 4.5 N

*At loaded length

73 mm

Stress at 53 N

438 N/mm2


Right-hand or left-hand


BS 1408 B

*Protective treatment


Torsion spring

Total turns on barrel


Wire diameter

2.6 mm


Left-hand close coils

Mean diameter

12.7 mm

*To work on

9.5 mm dia bar

*Length of legs

28 mm

*Load applied at

25.4 mm from centre of



41 ± 2 N



Stress at 41 N

595 N/mm2

*Both legs straight and

tangential to barrel


BS 5216

*Protective treatment


Tin Phosphate
Fig. 25.22

tin. Phosphate coatings are frequently specified. Organic coatings, originally confined to stove enamels, now include many plastics materials such as nylon and polythene, as well as many types of epoxy resins.

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