UNIT-1 MACHINE ELEMENTS-1 Part – A Springs

SPRINGS A spring is an elastic object used to store mechanical energy. 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. The force can be a linear push or pull, or it can be radial, acting similarly to a rubber band around a roll of drawings.

APPLICATIONS OF SPRINGS To provide Cushioning, to absorb, or to control the energy due to shock and vibration. To Control motion To Measure forces To store the energy.

HELICAL SPRINGS HELICAL COMPRESSION SPRINGS

CHARACTERISTICS OF HELICAL COMPRESSION SPRINGS: 1) The gap between the successive coils is larger. 2) It is made of round wire and wrapped in cylindrical shape with a constant pitch between the coils. 3) By applying the load the spring contracts in action.

HELICAL TENSION (or) EXTENSION SPRING

Characteristics of tension helical spring (or) extension spring It has some means of transferring the load from the support to the body by means of some 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

HELICAL TORSION SPRINGS

TORSION SPRING It is also a form of helical spring, but it rotates about an axis to create load. It releases the load in an arc around the axis. Mainly used for torque transmission. (Torque is a measure of how much a force acting on an object causes that object to rotate.) 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.

SPIRAL SPRINGS It is made of a band of steel wrapped around itself 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.

Solid length When the compression spring is compressed until the coils come 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 the wire. Free length The free length of a compression spring, is the length of the spring in the free or unloaded condition. It is equal to the solid length plus the maximum deflection or compression of the spring and the clearance between the adjacent coils

Spring index The spring index is defined as the ratio of the mean diameter of the coil to the diameter of the wire. Mathematically, C = D / d where D = Mean diameter of the coil, and d = Diameter of the wire.

Spring rate. The spring rate (or stiffness or spring constant) is defined as the load required per unit deflection of the spring. Mathematically, k = W / δ Where W = Load, and δ = Deflection of the spring. Pitch The pitch of the coil is defined as the axial distance between adjacent coils in uncompressed state

MATERIAL FOR HELICAL SPRINGS The material of the spring should have high fatigue strength, high ductility, high resilience and it should be creep resistant. It largely depends upon the service for which they are used i.e. heavy service, average service or light service.

LEAF SPRING

LEAF SPRING Sometimes it is also called as a semi-elliptical spring, as it takes the form of a slender arc shaped length of spring steel of rectangular cross section. The center of the arc provides the location for the axle,while the tie holes are provided at either end for attaching to the vehicle body. Heavy vehicles,leaves are stacked one upon the other to ensure rigidity and strenth. It provides dampness and springing function.

NIPPING IN LEAF SPRING? Because of the difference in the leaf length,different stress will be there at each leaf. To compensate the stress level, prestressing is to be done. Prestressing is achieved by bending the leaves to different radius of curvature before they are assembled with the center clip. The radius of curvature decreases with shorter leaves. The extra intail gap found between the extra full length leaf and graduated length leaf is called as nip.Such prestressing achieved by a difference in the radius of curvature is known as nipping.

NOMENCLATURE OF A COMPRESSION HELICAL SPRING

NOMENCLATURE OF A COMPRESSION HELICAL SPRING

Equivalent-when springs are in series

Kequivalent-when springs are in parallel PARALLEL(SYMMETRIC DISPLACEMENTCASE) (Δ1= Δ2)

UNSYMMETRICAL DISPLACEMENT(Δ1, Δ2, ΔTOTAL) WHEN THE SPRINGS ARE IN PARALLEL (Δ1≠ Δ2)

COMBINED SYSTEM (BOTH SERIES AND PARALLEL)

ENDS IN HELICAL SPRING TYPES OF ENDS FOR COMPRESSION SPRING

Types of end used in helical extension spring

SPRING NOMENCLATURE Active Coils Those coils which are free to deflect under load. Angular relationship of ends The relative position of the plane of the hooks or loops of extension spring to each other. Buckling Bowing or lateral deflection of compression springs when compressed, related to the slenderness ration (L/D). 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 As with closed ends, except that the end is ground to provide a flat plane

Deflection Motion of the spring ends or arms under the application or removal of an external load. Elastic limit Maximum stress to which a material may be subjected without permanent set. Endurance limit Maximum stress at which any given material may operate indefinitely without failure for a given minimum stress. Free angle Angle between the arms of a torsion spring when the spring is not loaded. Hysteresis The mechanical energy loss that always occurs under cyclical loading and unloading of a spring, proportional to the arc between the loading and unloading load-deflection curves within the elastic range of a spring.

Initial tension The force that tends to keep the coils of an extension spring closed and which must be overcome before the coil starts to open. Loops Coil-like wire shapes at the ends of extension springs that provide for attachment and force application. Mean coil diameter Outside wire diameter minus one wire diameter. Modulus in shear or torsion Coefficient of stiffness for extension and compression springs. Modulus in tension or bending Coefficient of stiffness used for torsion and flat springs. (Young's modulus Open ends, not ground End of a compression spring with a constant pitch for each coil. Permanent set A material that is deflected so far that its elastic properties have been exceeded and it does not return to its original condition upon release of load is said to have taken a "permanent set".

CAM A cam is a rotating machine element which gives reciprocating or oscillating motion to another element known as follower.

Cams Cylindrical Cam Radial Cam Cam Principle A cam produces a simple means to obtain irregular or specified predictable designed motion. The simple operation of a cam has a rotating shaft with an irregularly shaped disc (Cam) attached. A follower with a small roller attached to it pushes against the cam. As the shaft rotates, the roller follows the irregular surface of the cam causing the follower to rise or fall according to the profile of the cam. There are two major kinds of cams, the radial ram and the cylindrical design. The radial arm design changes a rotary motion into either an up and down motion or a rocking action. The cylindrical design causes the follower to operate perpendicular to the cam shaft.

Cam Terms Cam Terms Working Circle The working circle is considered the distance equal to the distance from the center of the cam shaft to the highest point on the cam. Base Circle The base circle is a circle with a radius equal to the distance from the center of the shaft to the center of the follower wheel at its lowest position. Follower Displacement The follower displacement is the position of the follower from a specific zero or rest position in relation to time or distance measured in inches or degrees. Rotation The rotation of the cam is either clockwise or counter clockwise. Cam profile The cam profile is the actual working surface contour of the cam. .

Cam Terms Displacement Diagram The displacement diagram is a designed layout of the required motion of the cam. It is laid out on a grid with the length of the grid equal to one revolution of the cam. Dwell The dwell is a period of time during which the follower does not move. Time Interval The time interval is the time it takes the cam to move the follower to the designed height.

Cam Motion Uniform Velocity The cam follower rises and falls at a constant speed. The start and stop of the follower is very abrupt and rough. Modified Uniform Velocity The modified uniform velocity motion smoothes out the roughness of the uniform velocity motion by adding a radius at the high and low points of the cam motion.

Cam Motion Harmonic Motion The follower motion has a smooth start and stop but the speed is not uniform. This type of motion is used where the cam rotates at a very fast speed. Uniform Acceleration The uniform acceleration is the smoothest motion of all of the cams and causes the follower to maintain a constant speed throughout the cam travel.

CAM TERMS Pitch curve : The path generated by the trace point as the follower is rotated about a stationery cam. Prime circle: The smallest circle from the cam center through the pitch curve

Pressure angle: The angle between the direction of the follower movement and the normal to the pitch curve. Pitch point: Pitch point corresponds to the point of maximum pressure angle.

CLASSIFICATION OF CAMS According to shape of cam Radial or disc cam Cylindrical cam

According to shape of follower

According to Follower movement Rise-return-rise Dwell-Rise-Return-Rise Dwell-Rise-Dwell-return-Dwell

Radial or Disc cam In radial cams, the follower reciprocates or oscillates in a direction perpendicular to the cam axis.

Cylindrical cams In cylindrical cams, the follower reciprocates or oscillates in a direction parallel to the cams axis.

TYPES OF CAM MOTION Uniform motion ( constant velocity) Simple harmonic motion Uniform acceleration and retardation motion Cycloidal motion

UNIFORM MOTION

Uniform motion (constant velocity) Displacement diagram Since the follower moves with uniform velocity during its rise and fall, the slope of the displacement curve must be constant as shown in fig

b) Simple Harmonic motion

Simple harmonic motion Since the follower moves with a simple harmonic motion, therefore velocity diagram consists of a sine curve and the acceleration diagram consists of a cosine curve.

Uniform acceleration and retardation Since the acceleration and retardation are uniform, therefore the velocity varies directly with time.

d) Cycloidal motion

CAM Profile