design of connecting rod(basics)

DESIGN OF CONNECTING ROD

Connecting Rods:

            It is an intermediate link between the piston and the crank shaft of an IC engine. The basic purpose of it is to transmit motion and force from piston to the crank pin. It consists of a long shank, a small and a big end. The cross section of shank may be rectangular, circular tubular, I-section or H-section. Length of connecting rod (l) depends up on the ratio , where ‘r’ is the radius of crank.

Design of Connecting Rod:

            In designing a connecting rod, the following dimensions are required;

1.         Dimensions of cross-section of the connecting rod:

             A connecting rod is to subjected to alternating direct compressive and tensile forces; Hence the cross section of the connecting rod is designed as a strut and the Rankine’s formulae is used.

A connecting rod as in figure-A, is subjected to an axial load W may buckle with X-axis as neutral axis or Y-axis as neutral axis. The connecting rod is considered like both ends hinged for buckling about X-axis and both ends fixed for buckling about Y-axis.

             A             =    Cross sectional area of the connecting.

             l               =    Length of connecting rod.

                        =    Compressive yield stress

             W_B          =    Buckling load

             I_xx, I_yy      =    Moment of Inertia of section about X-axis and Y-axis respectively.

             K_xx, K_yy   =    Radius of gyration of the section about X-axis and Y-axis respectively.

Rankine’s formulae

             W_B about X-axis =

             W_B about Y-axis =

             a = 1/7500 for mild steel

             = 1/9000 for Wrought Iron.

             = 1/1600 for Cast Iron

In order to have a connecting rod equally strong in buckling about both the axis, the buckling loads must be equal.

              =

              =

              =

             I_xx = 4I_yy

This shows that connecting rod is four times strong in buckling about Y-axis than about X-axis. In actual practice, I_xx is kept slightly less than 4I_yy. It is usually taken between 3 and 3.5.

The most suitable section for the connecting rod is I-section with the proportions as in figure.

Thickness of the flange and web          =    t

Width of the section                             =    B = 4t

Depth or Height of the section             =    H = 5t

Area of the section                                =    2(4t*t) + 3 txt

                                                              =    11 t²       

Moment of Inertia of the section about X-axis

            

            

            

After determining the proportions for I-sections of the connecting rod, its dimensions are determined by considering the buckling of the rod about X-axis and applying the Rankine’s formulae.

             Buckling load

OR

             W_B = Max. gas force × Factor of safety

             FOS may be taken as 5 to 6.

2.          DIMENSIONS OF THE CRANK PIN AT THE BIG END AND THE PISTON PIN AT THE SMALL END:

              Since dimensions of the crank pin at the big end and the piston pin at the small end are limited. The crank pin at the big end has removable precision bearing shells of brass or bronze or steel with a thin lining of bearing metal; on the inner surface of the shell.

The allowable bearing pressure on the crankpin depends upon many factors such as material of the bearing, viscosity of lubricating coil, method of lubrication and the space limitation.

             The value of bearing pressure may be taken as 7 N/mm² to 12 N/mm² depending up on the material and method of lubrication used.

             The piston pin bearing is usually a phosphor bronze bush of about 3mm thickness and the allowable bearing pressure may be taken as 10.5 N/mm² – 15 N/mm².

Since the maximum load to be carried by the crank pin and piston pin bearing is the maximum force in the connecting rod (F_c). Therefore dimensions for these two pins are determined for the maximum force in the connecting rod (F_c) which is taken equal to maximum force on piston due to gas pressure (F_L) neglecting inertia forces.

Maximum gas force

             F_L            =               .................. (i)

             D             Cylinder bore or piston dia in mm

             P             Maximum gas pressure in N/mm².

To find out dimensions of crankpin and piston pin;

             d_c            Dia of the crankpin

             l_c              Length of crankpin

             P_bc           Allowable bearing pressure in N/mm².

             d_p, l_p, P_bp Corresponding values for the piston pins.

             Load on crankpin:

                            = d_c.l_c.P_bc     .................. (ii)

             Load on piston pin:

                            = d_p.l_p.P_bp                .................. (iii)

             Equating (i) and (ii).

                            F_L = d_c.l_c.P_bc

             Take l_c = 1.25 d_c to 1.5 d_c

Equating (i) and (iii)

             F_L = d_p.l_p.P_bp                    (l_p = 1.5 d_p to 2 d_p)

3.          SIZE OF BOLTS FOR SECURING BIG END CAP:

             Inertia force on reciprocating parts;

                           

             m_R           Man of the reciprocating part

             W            Angular speed of the engine rad/s

             r              Radius of crank in meters

             l               Length of connecting rod.

             The both may be made of high carbon steel or nickel alloy steel.

             Factor of safety may be taken as 6.

              Force on the bolts;

             =

             d_cb           Core dia of bolt in mm.           

Equating the inertia force to force on bolt;

            

                        No. of bolts

From this expression d_cb is obtained

            

                        Nominal or major dia.

4.          Thickness of the big end cap:

              Thickness of big end cap (t_c) may be determined by treating the cap as a beam freely supported at the cap bolt centres and loaded by the inertia forces at the top dead centre on the exhaust stroke. This load i assumed to be act in between the uniformly distributed load and centrally concentrated load.

             Max. bending moment acting on the cap

                  

References

1.      Mechanical Engineering Design – Joseph Shigley

2.      Machine Design – Mubeen

3.      Machine Design – Black

4.      Principles of Lubrication – Cameron A.