18-10-2016, 02:50 PM
DESIGN AND ANALYSIS OF MECHANICAL PROPERTIES USING HOLLOW HELICAL SPRING MADE OF NON-FERROUS MATERIAL
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ABSTRACT
This report is an in depth discussion on the comparison of mechanical
properties between hollow and solid helical springs made of non-ferrous
materials. It allows us to decide whether hollow springs have better structural
stability on compared to solid springs made of non-ferrous materials.
The first step to designing the helical springs was the selection of
materials. Copper and aluminium materials are selected due to their excellent
mechanical properties. Then the spring was designed and fabricated using
critical design parameters. Mechanical testing (compression testing) was done
on the spring under various loads until breaking point, to study how well the
springs can withstand loads under various deflection parameters. Analyzing
modeled spring on ANSYS helped to further study its mechanical properties
such as deformation, stress and strain under various load conditions.
Finally the results are calculated, tabulated and compared. It can be
concluded that hollow springs show better structural properties in both
aluminium and copper.
INTRODUCTION
Springs are elastic bodies (generally metal) that can be twisted,
pulled, or stretched by some force. They can return to their original shape
when the force is released. In other words it is also termed as a resilient
member. A spring is a flexible element used to exert a force or a torque
and, at the same time, to store energy. A force can be linear push or pull,
or it can be radial, acting similarly to a rubber band around a roll of
drawings. A lot of research has been done for improving the performance
of spring.
When a coil spring is compressed or stretched slightly from rest,
the force it exerts is approximately proportional to its change in length
(this approximation breaks down for larger deflections).
The rate or spring constant of a spring is the change in the force it exerts,
divided by the change in deflection of the spring. That is, it is
the gradient of the force versus deflection curve. The inverse of spring
rate is compliance. The stiffness (or rate) of springs in parallel is additive,
as is the compliance of springs in series. Depending on the design and
required operating environment, any material can be used to construct a
spring, so long as the material has the required combination of rigidity
and elasticity.
Helical springs are generally made from round wire. It is
completely rare for springs to be made from square or rectangular
sections. The strength of the steel used is one of the most important
criteria to consider in designing springs. For designing helical springs we
need to identify, describe and understand principles of several types of
springs.
HELICAL SPRINGS
These springs are Made from round wire and wrapped in
cylindrical form, with a fixed pitch. The wire of a helical compression
spring is loaded mainly in torsion and is therefore usually of circular
cross- section. This type of spring is the most common and we shall focus
on it.
TENSION HELICAL SPRINGS
A tension helical spring has some means of transferring the load from
the support to the body by means of an arrangement.
It stretches apart to create load.
The gap between the successive coils is small.
The wire is coiled in a sequence that the turn is at right angles to
the axis of the spring.
The spring is loaded along the axis.
By applying load the spring elongates in action as it mainly
depends upon the end hooks.
1.1.1.2 COMPRESSION HELICAL SPRING.
This type of springs are made of round wire and wrapped in
cylindrical shape with a constant pitch between the coils.
The gap between the successive coils is large.
By applying the load the spring contracts in action.
Types of compression helical springs;
1) Plain end
2) Plain and ground end
3) Squared end
4) Squared and ground end
1.1.1.3 TORSION SPRINGS
Torsion springs are helical springs that exert a torque or rotary force.
The ends of the torsion springs are attached to other components.
It releases the load in an arc around the axis.
The ends of the spring are attached to other application objects, so
that if the object rotates around the center of the spring, it tends to
push the spring to retrieve its normal position.
These springs are mainly used for torque transmission.
It helps to resist the twisting motion.
1.1.1.4 SPIRAL SPRINGS
This is a spring consisting of a wire coiled usually in a flat spiral or helix.
It is made of a band of steel wrapped around it a number of
times to create a geometric shape.
Its inner end is attached to an arbor and outer end is attached
to a retaining drum.
It has a few rotations and also contains a thicker band of
steel.
It releases power when it unwinds.
1.1.1.5 LEAF SPRINGS
A spring made of a number of strips of metal curved slightly
upwards and clamped together one above the other. Sometimes these
springs are also called as semi-elliptical springs, as it takes the form of a
slender arc shaped length of spring steel which’s of rectangular cross
section. These springs provide dampness and springing function. A leaf
spring can either be attached directly to the frame at both ends or attached
directly at one end, usually the front, with the other end attached through
a shackle, a short swinging arm. The shackle takes up the tendency of the
leaf spring to elongate when compressed and thus makes for softer
springiness. Some springs terminated in a concave end.
SPRING MATERIALS
Springs are elements designed to withstand great amounts of deflection
and return to their original shape and size on its release. To be capable of
such cyclical loading, spring materials must possess very high elastic
limits. Often materials not specifically made for the spring application are
utilized for that purpose because their elastic limits are within the above
requirements. Steels of medium-carbon and high-carbon content are
considered good spring materials. Where a copper based alloy is required,
beryllium copper and phosphor bronze are utilized. The surface quality of
the spring material has a considerable influence on the function of a
spring, namely, on its strength and fatigue. Where possible, the surface
finish has to be of the highest grade, preferably polished. Music wire, the
highest-quality spring material, is polished, and its surface is almost
defect-free. Of course, the higher quality the material, the more expensive
it is. The designer should strive to find the best combination of price
versus quality for each particular job.
1.2.1 HIGH CARBON SPRING STEEL WIRE
This group of spring materials is lowest in cost, which may account
for its widespread use. It does not take impact loading or shock treatment
well. Also it should not be used in extreme temperatures, high or low.
1.2.1.1 MUSIC WIRE ASTM A228
Good for high stresses caused by cyclic repeated loading. A hightensile-strength
material, available as (cadmium or tin) pre plated. It’s a
common high-carbon steel alloy used for spring manufacture. It is cold
drawn and offers uniform tensile strength. Music wire springs are not
recommended for applications where the temperature exceeds 121° C
(250° F).
1.2.1.2 OIL-TEMPERED MB GRADE, ASTM A229
A general purpose spring steel, frequently used in coiled form. It is
not good with shock or impact loading. Can be formed in annealed
condition and hardened by heat treatment. Forms a scale, which must be
removed if the material is plated.
1.2.1.3 HARD DRAWN MB GRADE ASTM A227
Used where cost is essential. Not to be used where long life and
accuracy of loads and deflections are important. It can be readily plated.
It is used for more precise work, where a long life, high fatigue, and high
endurance properties are needed. If such aspects are not required, alloy
spring steel should be used in replacement.
1.2.2 HIGH CARBON SPRING STEEL STRIP
The main type of spring steel in this group is used with an absolute
majority of all flat spring. However, both are susceptible to hydrogen
embrittlement even when plated and baked afterward.
1.2.2.3 COLD ROLLED BLUE TEMPERED SPRING STEEL
The two main types of spring’s steel in this group are used with an
absolute majority of all flat spring. However, both are susceptible to
hydrogen embrittlement even when plated and baked afterward.
1.2.3 ALLOY SPRING STEEL
It’s a good spring steel for a high-stress application, with impact
loading and shock application involved.
1.2.3.1 CHROMIUM SILICON STEEL, ASTM A401
This material can be groomed to high tensile stress through heat
treatment. Applicable where long life is required in combination with
shock loading. Withstands higher stresses than high carbon steel. It also
has a good fatigue strength and endurance.
1.2.4 STAINLESS SPRING STEEL
A corrosion-resistant material. With the exception of the 18-8
type, none of these steels should be used for lower-than-zero temperature
applications. High-temperature tolerance is up to 550°F.
STAINLESS SPRING STEEL 302, ASTM A313
This material has quite uniform properties and the highest tensile
strength of the group. The slight magnetic properties are due to cold
working, as in annealed form it is nonmagnetic.
1.2.4.2 STAINLESS SPRING STEEL 304, ASTM A313
Because of its slightly lower carbon content, this material is easier
to draw its tensile strength is somewhat lower than that of type 302, even
though their other properties coincide.
1.2.4.3 STAINLESS SPRING STEEL 316, ASTM A313
Less corrosion-prone than the 302 type stainless, with its tensile
strength about 12 percent lower. Otherwise it is quite similar to the 302
type.
1.2.4.4 STAINLESS SPRING STEEL 17-7 PH, ASTM A313
With trace amounts of aluminum and titanium.. The tensile
strength of this material is almost as high as that of music wire. This is
achieved through forming in a medium hard condition and precipitation
hardening at low temperatures.
1.2.4.5 STAINLESS SPRING STEEL 414, SAE 51414
Its tensile strength is approximately the same as that of type 316
(above), and it may be hardened through heat treatment. In a highpolished
condition this material resists corrosion quite well.
STAINLESS SPRING STEEL 420, SAE 51420
May be obtained in the annealed state, hardened and tempered. Its
corrosion-resistant properties emerge only after hardening. Clear bright
surface finish enhances its corrosion resistance.
1.2.4.7 STAINLESS SPRING STEEL 431, SAE 51431
This material has very high tensile properties, almost on a par with
music wire. Such a characteristic is achieved through a combination of
heat treatment, followed by cold working.
1.2.5 COPPER BASE SPRING ALLOYS
This group of spring materials is more expensive than alloy steels
or high carbon materials. They are, however, very useful for their good
corrosion resistance and superb electrical properties. An additional
advantage is their usefulness in lower-than-zero temperatures.
1.2.5.1 SPRING BRASS ASTM B134
It cannot be hardened by heat treatment and has generally quite
poor spring qualities. Even though it does not tolerate temperatures
higher than 150°F, it performs well at subzero. It is the least expensive
copper based spring material, with the highest electrical conductivity,
out-weighed by its low tensile strength.
1.2.5.2 PHOSPHOR BRONZE ASTMB159
This is the most popular copper-based spring material. Its
popularity is due to its favorable combination of electrical conductivity,
corrosion resistance, good tensile strength, hardness, and low cost.
BERILLIYUM COPPER ASTM B197
It is the most expensive material of this group. It is better formed in
its annealed condition and precipitation hardened afterward. The
hardened material turns brittle and does not take additional forming. The
material has a high hardness and tensile strength. It is used where
electrical conductivity is of importance.