SUBJECT:DESIGN OF MACHINE ELEMENTS TOPIC:DESIGN OF HELICAL SPRING SUMITTED TO:Mr. DHAVAL DARJI MADE BY: YASH SHAH(130010119110) SHIVANG PATEL(130010119111) SHIVANI SISODIA(130010119112) RAKESH SODHIA(130010119113) A.D.PATEL JAYDEEP SOLANKI(130010119114) INSTITUTE OF TECHNOLOGY

A coil spring, also known as a helical spring, is a mechanical device, which is typically used to store energy due to resilience and subsequently release it, to absorb shock, or to maintain a force between contacting surfaces.

Outline Spring Functions & Types Helical Springs Compression Extension Torsional

The Function(s) of Springs Most fundamentally: to STORE ENERGY Many springs can also: push pull twist

Some Review Parallel Series linear springs: k=F/y F k nonlinear springs: Parallel ktotal=k1+k2+k3 y

Types of Springs Helical: Compression Extension Torsion

More Springs Washer Springs:                                             Power springs: Beams:

Helical Compression Springs d diameter of wire D mean coil diameter Lf free length p pitch Nt Total coils

Length Terminology Lf La Lm Ls Free Length Assembled Length minimum of 10-15% clash allowance Free Length Assembled Length Max Working Load Bottomed Out Lf La Lm Ls

End Conditions Na= Active Coils Plain Plain Ground Square Ground

Stresses in Helical Springs F F Spring Index C=D/d T F

Curvature Stress Inner part of spring is a stress concentration (see Chapter 4) Kw includes both the direct shear factor and the stress concentration factor under static loading, local yielding eliminates stress concentration, so use Ks under dynamic loading, failure happens below Sy: use Ks for mean, Kw for alternating

Spring Deflection

Spring Rate k=F/y

From the free body diagram, we have found out the direction of the internal torsion T and internal shear force F at the section due to the external load F acting at the centre of the coil. The cut sections of the spring, subjected to tensile and compressive loads respectively, are shown separately in the Fig.1 and 2.

The broken arrows show the shear stresses ( τT ) arising due to the torsion T and solid arrows show the shear stresses ( τF )due to the force F. It is observed that for both tensile load as well as compressive load on the spring, maximum shear stress (τT + τF) always occurs at the inner side of the spring. Hence, failure of the spring, in the form of crake, is always initiated from the inner radius of the spring.

fig.1 fig.2 The radius of the spring is given by D/2. Note that D is the mean diameter of the spring.

The torque T acting on the spring is If d is the diameter of the coil wire and polar moment of inertia , the shear stress in the spring wire due to torsion is

Average shear stress in the spring wire due to force F is

where, C=D/d, is called the spring

The above equation gives maximum shear stress occurring in a spring The above equation gives maximum shear stress occurring in a spring. Ks is the shear stress correction factor. To take care of the curvature effect, the earlier equation for maximum shear stress in the spring wire is modified as,

Where, KW is Wahl correction factor, which takes care of both curvature effect and shear stress correction factor and is expressed as,

From simple geometry we will see that the deflection, δ, in a helical spring is given by the formula,

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