Download - Spring Rating
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S P R I N G C A L C U L A T I O N S
To Calculate Coil Spring Rate-
RATE= (9909 x material dia. x material dia. x material dia. x material dia.)
divided by
(no. of active coils x mean dia. x mean dia. x mean dia.)
Mean dia. = I.D. + material dia. All measurements are in millimetres Result is in N/mm To convert answer to Kg/mm = N/mm x 0.10192
To Calculate Coil Spring Stress-
STRESS= (2.545 x rate(N/mm) x required travel x mean dia. x Wahl factor)
divided by
(material dia. x material dia. x material dia.)
Mean dia. = I.D. + material dia. C: (spring index) = mean dia. / material dia. Wahl factor = (C+0.2) / (C-1) All measurements are in millimetres Results is in N/mm / MPa Stress not to exceed 827 N/mm / MPa
Please note these formulas are provided as a guide only. Industrial Engineers & Spring Makers (Sales)
Pty. Ltd provides no guarantee to their accuracy.
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Testing a spring
For a constant rate spring, that is exactly how you test the rate.
You must check the rate between 20 and 80 % of the total possible deflection range. Thedeflection range is the range from the free length to the solid length.
Example:Free Height: 10 inchesSolid Height: 5 inchesDeflection range would be from 10-5 = 5 inches.Rate must therefore be checked at heights between 9 inches and 6 inches.
Load at 8.5 inch height: 15 LbsLoad at 7.5 inch height: 25 Lbs
Rate = Difference in Load / Difference in Length= (25 15) / (8.5 7.5)
= 10 / 1= 10 Lbs / Inch
Hope this explains it to you. Any further questions, just mail them to us.
Kind Regard s,
Daniel Shepard .Managing Director.W H Rooke & Co.
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Techn i ca l I n fo rm a t i on
Techn ica l I n fo r m a t ion
The num ber o f co i l s
The number of coils in tensionsprings varies with commercialproduction as follows:
Number of Coils Tolerance
3 to 5 20
6 to 8 30
9 to 12 40
For each additional coil, add 1 per coil. Springs can be built to closer tolerances by trimming aftercoiling, with additional cost.
Exam ple Load To lerances :
The load tolerances for tension springs are listed in the above table. For example of how to use these,let us take a spring of the following dimensions: Free Length 4.050"; Mean Dia. .390"; Wire Dia. .048"to give a load of 3.5lb when extended .900".
First we calculate the index orD
d
=.390"
.048"
= 8
Secondly we calculate thath
f
=4.050"
.900"
= 4.5
FindD
d
= 8 on the table, take the adjacenth
f
= 4.5.
Where the line 4.5 intersects with the column of wire dia. of .048" we get a figure of 10.1%.
This is the load tolerance of the above example.
Convert 10.1% to 3.5 lb. which gives a tolerance of 5.6 oz. at .900" deflection.
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A-DActive coils (na):Coils that are free to deflect under load.
Angular relationship of ends:
Relative position (in degrees) of the plane of the hooks or loops of extension springs to each other.
Baking:
Heating of electroplated springs to relieve hydrogen embrittlement.
Closed ends:Ends of compression springs where the pitch of the end coils is reduced so that the end coils touch.
Closed and ground ends:
Same as closed ends except the end coils are ground to provide a flat surface.
Close-wound:
Coiled so that adjacent coils are touching.
Deflection (F):Motion of spring ends or arms under external load (P).
E-L
Free angle:
Angle between the arms of a torsion spring when no load is applied.
Free length (L):
Overall length of a spring when no load is applied.
Helix:
The spiral shape of compression, extension and torsion springs.
Hooks:Open loops at the end of extension springs.
Hydrogen embrittlement:
Hydrogen absorbed in electroplating of carbon steels causing the spring material to become brittle, leading
to cracking and spring failure.
Initial tension (Pi):The force that causes the coils of extension springs to stay together and which must be overcome before the
coils begin to open.
Load (P):The force applied to a spring that causes deflection.
Loops:
Closed hooks at the end of extension springs.
M-P
Mean coil diameter (D):Outside diameter (O.D.) of a spring minus one wire diameter (d).
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Open ends, not ground:
End coils of a compression spring having the same pitch as the body of the spring.
Open ends ground:
Same as open ends except the end coils are ground to provide a flat surface.
Passivating:The treatment of stainless steel springs with acid to remove contaminants and improve resistance to
corrosion.
Permanent set:
A condition that comes from deflecting a spring beyond the elastic limits of the material; the spring does not
return to its original condition when the load is released.
Pitch (p):
The distance between two adjacent active coils measured from the center of the wire of each coil.
Preset:To remove permanent set prior to application installation.
R-T
Rate (R):
Change in load per unit of deflection, usually in pounds per inch.
Remove set:The process of deflecting to solid height a spring which has been coiled longer than the requested finished
length, so as to increase the apparent elastic limit.
Solid height:
Length of a compression spring when it has been deflected enough to allow each adjacent coil to touch each
other.
Spring index:
Ratio of the mean coil diameter (D) of a spring to its wire diameter (d).
Squared and ground ends:See Closed and ground ends.
Squared ends:See Closed ends.
Stress relieve:
The heat treating of springs at a temperature so as to relieve stresses.
Torque (M):A twisting action in torsion springs equal to the load multiplied by the distance (or moment arm) from the
load to the axis of the spring body.
Total coils (Nt):The active coils (Na) plus the number of coils forming the ends.