the Stresses In Helical Spring Of Circular Wire
Introduction
A Spring Is Defined As An Elastic Whose Function Is To Disort When Loaded And To Recover Its Original Shape When The Load Is Removed.
Helical Spring
The Helical springs are made up of wire coiled in the form of helix and is primarily intended for compressive or tensile loads. The cross section of wire from which the spring is made it may be either circular or squared. The two forms of helical springs are compression and tension helical spring. These springs are said to be “Closely Coiled” when the spring is coiled so close that the plane containing each turn is nearly at right angle to the axis of helix and wire is subjected to tension. In closely coiled helical spring the helix angle is very small it is usually less than 10 degrees. The major stress produced in helical springs is shear stresses due to twisting. The load applied is either parallel or along spring. In “Open Coiled” helical springs the spring wire is coiled in such a way that there is gap between 2 consecutive turns, as a result of which the helix angle is large.
Advantages Of Springs:
- Easy to mfg.
- Available in wide range.
- Reliable.
- Constant spring constant.
- Performance is accurate.
- Characteristics can be varied by changing dimensions.
Applications Of Springs:
- Absorbs energy due to shock or vibration.
- To apply force in brake, clutches, spring loaded values.
- To measure forces in spring balance and engine indicators.
- To store energy.
Material For Helical Springs
The material of spring should have high fatigue strength, high ductility, high resistance and it should be creep resistance. It largely depends upon the service for which they are used.
Severe Service
Rapid continuous loading where ratio of minimum and maximum load is half an automotive valve spring.
Average Service
Includes same stresses range as in severe service but with only intermittent operation as in engine governor spring.
Light Service
It is subjected to loads that are static or very infrequently varied as in safety valve springs. Actually springs are made from oil tempered carbon steel wires containing 0.6% to 0.7% carbon and 0/6% to 1% manganese.
Soild Length
When the compression spring is compressed until the coils comes in contact with each other, then the spring is said to be solid. The solid length of a spring is the product of total number of coils and the diameter of wire.
Ls = n’d.
N’ = total number of coils.
d = dia of wire.
Free Length
The free length of compression spring is the maximum length of spring in free condition. It is equal to solid length. Plus the maximum deflection of spring and clearance adj to two coils.
Lf = solid+max compression+clr b/w adj coils
= n’d+$ max+0.15 $ max.
Spring Index
The spring index is defined as the ratio of the mean diameter of the coil to the dia of wire.
C = D/d.
D = Mean dia of the coil.
D = Dia of the wire.
Spring Rate
The spring rate is defined as load required per unit deflection of the spring.
K = W/£.
W = Load.
£ = Deflection of the spring.
Pitch
The pitch of the coil is defined as axial distance between adjacent coils in uncompressed state.
P = Free Length/ N’-1.
Stresses In Helical Spring Of Circular Wire
Consider a helical compression spring made of circular wire and subjected to an axial load “W”.
D = Mean dia of spring coil.
d = Dia of spring wire.
N = number of active coils.
G = Modulus of rigidity for spring material.
W = Axial load on spring.
T = Max shear stresses induced in wire.
P = Pitch of the coils.
£ = Deflection of the spring.
Now consider a part of the compression spring. The load “W” tends to rotate the wire due to the twisting moment (T) set up in the wire. Thus torsion shear stress is induced.
A little consideration will show that part of the spring is in equilibrium under the action of two forces “W” and twisting moment (T)
T = W * D/2 = π/16 * T1 * d3.
T1 = 8WD/Πd3.
The Torsional
