AUG

12

Design and Manufacturing of Leaf Spring

INTRODUCTION TO LEAF SPRING

Originally Leaf  spring  called  laminated or  carriage  spring,  a  leaf  spring  is  a  simple form of spring, commonly used for  the suspension  in wheeled vehicles. It  is also one of the  oldest forms of springing, dating back to medieval times.Sometimes referred to as a semi-elliptical spring or cart spring, it takes the form of a slender arc-shaped  length of spring steel of rectangular cross-section. The center of the arc provides location  for  the  axle, while  tie holes are provided at  either end  for attaching  to  the vehicle body. For very heavy vehicles, a leaf spring can be made from several leaves stacked on top of each other in several layers, often with progressively shorter leaves. Leaf springs can serve locating and to some extent damping as well as springing functions.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.

 

DESIGN OF LEAF SPRING  

      Materials for leaf spring:                        The material used for leaf springs is usually a plain carbon steel having 0.90 to 1.0%  carbon. The  leaves  are  heat  treated  after  the  forming  process. The  heat  treatment  of spring  steel  produces  greater  strength  and  therefore  greater  load  capacity,  greater  range  of deflection and better fatigue properties. 

According to Indian standards, the recommended materials are : 

1. For  automobiles  : 50 Cr 1, 50 Cr 1 V 23,  and 55 Si 2 Mn 90  all used  in hardened  and tempered state. 2.  For  rail  road  springs  :  C  55  (water-hardened),  C  75  (oil-hardened),  40  Si  2  Mn  90 (waterhardened) and 55 Si 2 Mn 90 (oil-hardened). 3. The  physical  properties  of  some  of  these materials  are  given  in  the  following  table. All values are for oil quenched condition and for single heat only

Design of leaf spring

                    Leaf  springs  (also  known  as  flat  springs)  are made  out  of  flat  plates.  The advantage of leaf spring over helical spring is that the ends of the spring may be guided along a  definite  path  as  it  deflects  to  act  as  a  structural member  in  addition  to  energy  absorbing device. Thus  the  leaf  springs may  carry  lateral  loads,  brake  torque,  driving  torque  etc.,  in addition to shocks. Consider a single plate fixed at one end and  loaded at  the other end as shown in Fig This plate may be used as a flat spring. Let t = Thickness of plate, b = Width of plate, and L = Length of plate or distancenof the load W from the cantilever end.

We know that the maximum deflection for a cantilever with concentrated load at the free end is given by

 It may be noted that due to bending moment, top fibres will be in tension and the bottom  fibres 

are  in  compression, but  the  shear  stress  is  zero  at  the  extreme  fibres  and maximum  at  the  centre,  as  shown  in  Fig. Hence  for  analysis,  both  stresses  need  not  to  be taken into

account simultaneously. We shall consider the bending stress only.

the centre, as If  the  spring  is not of cantilever  type but  it  is  like a  simply  supported beam, with length 2L and load 2W in shown in Fig., then Maximum bending moment in the centre,as shown in fig, then    Maximum bending moment in the center,                         M=W.L 

 Flat spring (simply supported beam type)

The above relations give the stress and deflection of a leaf spring of uniform cross-sectionThe stress at such a spring is maximum at the support.  If a triangular plate is used as shown in Fig , the stress will be uniform throughout. If thistriangular plate is cut into strips of uniform width and placed one below the other, as shown in  Fig to form a graduated or laminated leaf spring, then

A little consideration will show that by the above arrangement, the spring becomes compact so that the space occupied by the spring is considerably reduced.   When  bending  stress  alone  is  considered,  the  graduated  leaves may have zero width at the loaded end. But sufficient metal must be provided to support the shear. Therefore,  it becomes necessary  to have one or more  leaves of uniform cross-section  extending clear to  the end. We see from equations (iv) and (vi)  that for  the same deflection, the  stress  in  the uniform cross-section  leaves  (i.e.  full  length  leaves)  is 50% greater  than  in the graduated leaves, assuming that each spring element deflects according to its own elastic curve.  If  the suffixes F and G are used  to  indicate  the full  length  (or uniform cross section) and graduated leaves, then

Length of Leaf Spring Leaves The length of the leaf spring leaves may be obtained as discussed below : Let                        2L1 = Length of span or overall length of the spring,                                     =  Width  of  band  or  distance  between  centres  of  U-bolts.  It  is  the ineffective length of the spring,                                 nF = Number of full length leaves,                                 nG = Number of graduated leaves, and                                 n = Total number of leaves = nF + nG. We have already discussed that the effective length of the spring,                                                   …….. ...(When band is used)

MANUFACTURING  OF  LEAF SPRING

Raw material                                Generally leaf springs are made of various fine grade alloy steel. The most commonly used grades of steel are 55 Si 7,60 Si Cr7,50 Cr V4. The others are En 45 A, 65 Si 7,55 Si Cr 7,65 Si cr7,En 42 60 s 87.                          In  our  project  we  are  going  to  use  En  42  60  S87  grade  of  steel  alloy.Generally the width of the raw material varies from 40-100 mm and thickness varies from 4 to 20mm.

Shearing or cutting process                             Shearing is a process for cutting sheet metal to size out of a larger stock such as roll stock. The raw material is cut into different sizes with the help of the 100 ton cutting press machine. 1)  At first the raw material is placed on the roller bed so that it will be easy to move the material towards the machine. 2)  Required length of the material to be cut is measured with a tape and marking is done on the raw material. 3)  Now move the material in to the cutting area of the machine so that the mark is placed exactly at the cutting edge of the blade. 4)  Now lock the material with the help of the lock nut provided.5)  Allow the lubricant to flow for free action of cutting and for reducing friction. 6)  Now apply the load on the material by pressing the brake provided. 7)  Now the required length of the material piece can be obtained.Drilling operation                            Necessary holes are provided on the strips of leaf springs to hold all the plates together. So drilling operation  is performed. Generally vertical drilling machine  is used  for this operation. 1.) The given material piece is placed on the table of the drilling machine.   2.) The diameter of  the required hole is choosen and  the required drill bit  is connected to the spindle. 3.) The  workpiece  is  placed  in  the  required  position  for  drilling  and  the  coolent  oil  is sprayed over the work piece. 4.) Now by applying the hand lever the required hole diameter is drilled. Required number of holes are drilled.

Eye rolling of main blade                                                                                       The master blade  is heated at  its  two ends for eye formation,  these are done to attach with the frame of the vehicle. The heating is done in a end heating furnace  at a temperature of 1000 degree centigrade. The heating is done only at the ends so that it will be easy to bend at the ends. 

  Furnace oil and the air are used for heating the furnace. The furnace is first allowed to heat freely for 45 mins. Now the master blades are placed in the furnace such that only ends are heated.                                      After heating is done for one end it is bend to form the eye and again it is placed in the furnace to heat the other end. After heating the main blade is bent slightly to form curve at the end with the help of 50 ton punching machine so that it will be easy to roll to form eye formation. In the eye rolling machine the master blade end is placed between the circular wheel and the die. after placing the hand lever is moved so that the end of the master blade rolls over the die thus forming eye shape

Hardening (Quenching)                          Hardening is carried out to achieve the maximum hardness.                 The main blades after  the eye  formation are heated  to  a  temperature of 800-1000 degree centigrade in a furnace to increase the hardness of the material.   The other blades along with main blades are heated in  the furnace. The furnace  is heated by using air and  furnace oil  through conventional air  flow system. The conventional air  flow  system  is  used  to  mix  both  air  and  furnace  oil  for  heating  purpose.  A  pump  is provided for the air to go out.                 After heating the blades in the furnace for 45 minutes they are taken out and bent to the required angle on the hydraulic bending machine. The required angle can be obtained by using required angle dies.   The blades after making  the required angle  they are  immersed  in  the Quenchngon oil to increase the hardness. The hardness at the end of this stage is about 50 to 60.

Tempering     Quenched  steel,  while  very  hard  and  strong,  is  too  brittle  to  be  useful  for  most applications.  A  method  for  alleviating  this  problem  is  called  tempering.  For  most  steels, tempering  involves heating  to between 250 and 500 °C, holding  that  temperature  (soaking) for  an  appropriate  amount  of  time  (on  the  order  of  seconds  or  hours),  then  cooling  slowly over an appropriate  length of  time  (minutes or hours). This heat  treatment  results  in higher toughness  and  ductility, without  sacrificing  all  of  the  hardness  and  tensile  strength  gained from rapid quenching. Tempering balances the amount of hard martensite with ductile ferrite and pearlite. In  some  applications,  different  areas  of  a  single  object  are  given  different  heat  treatments. This is called differential hardening. It is common in high quality knives and swords.

Clamps Clamps are the devices which are used for holding the leaf springs together. Clamps are made of Mild Steel. Procedure for making clamps                 At first the material is cut into the required length on the 50 ton cutting machine by  using blades. Next  the cutted material  is punched on  the 5o  ton punching machine to obtain

required circular shape at the edge by using suitable die.                  Now  after measuring  the  correct  size  the  other  end  is  also  punched  on  50  ton punching machine  to obtain circular shape by using  the same die. This circular shape at  the ends is provided for clamps of width greater than 20 mm.                After making  the  reqired  size  the  clamp materials  are  heated  in  the  end heating  furnace  at 1000 degree centigrade by using furnace oil and air. The clamps are heated for 15 minutes in the furnace. After heating  they  are  taken out  and placed on  the   50  ton punching machine  to make  the  clamp  of  „U‟  shape  by  using  clamp  dies.    Different  dies  are  used  for  making  clamps  of  different widths.  Generally 4 clamps are used for 12 plates and 2 clamps are used for 7 leaf plates.the clamps are arranged at equal distance from the centre.

 

Stock ready to supplyAfter  inspection  the  leaf  springs with good quality and performance are allowed  to stock storage. Now the stock is are ready to supply.