3.16.
Piston and Connecting-rod Gudgeon-pin Joints
The gudgeon-pin (piston pin) connects the piston and connecting-rod. It is supported in holes
bored in the piston at right angles to the piston axis at about mid-height position, and the centre
portion of the gudgeon-pin passes through the connecting-rod small-end eye. This hinged joint
transfers directly the gas thrust from the piston to the connecting-rod and allows the rod to pivot
relative to the cylinder axis with an oscillating motion.
The gudgeon-pin bosses in the piston experiences a temperature of about 393 to 423 K for
both petrol and diesel engines. Also the temperature rise due to friction between the pin and
the bosses is in the order of 20 to 30 K. Therefore the gudgeon-pin has to withstand a temperature
of about 431 to 453 K. The connecting-rod during its oscillating movement squeezes the oil film
alternatively from one side of the pin to the other under semi-boundary-lubrication conditions.
In contrast the rotating crankshaft journals operate under full fluid lubrication.
The gudgeon-pins are in tubular shape, which provides adequate strength with minimum
weight. They are usually made from low-carbon case-hardened steel of composition 0.15%
carbon, 0.3% silicon, 0.55% manganese, and the balance 99% iron. This steel is carburized at a
temperature of 1153 to 1203 K, refined at 1143 to 1173 K and then hardened by oil quenching
from 1033 to 1053 K. Finally it is tempered at a temperature below 433 K.
The finish and size of gudgeon-pins are very closely controlled. A loose gudgeon-pin in the
piston or in the connecting rod, causes a rattle during the engine operation. If the pin is too tight
in the piston, it restricts piston expansion along the pin diameter which produces piston scuffing.
Gudgeon-pin operating clearances are usually about 0.0075 mm, which is critical for quiet
running and long life.
The gudgeon-pins are generally lapped to a surface finish of 0.08 to 0.16 \xm for longer service
life. A coarser finish produces stress-raisers, which may cause fatigue failure and pick up the
softer bearing metal from the pin’s rubbing surface. But a smoother finish avoids the oil clinging
and wetting the cylindrical working face of the pin.
Piston pin holes located in the piston have an offset of approximately 1.57 mm from the
piston centre line. Pin offset reduces piston slap and noise, which is created due to crossover
action as the large end of the connecting rod swings past both upper and lower dead centres.
The piston pins must stay centered in the piston; otherwise, they can move endwise and gouge
the cylinder wall. The method of locating and securing the gudgeon pin in position can be
achieved in two ways; (i) semi-floating, and (ii) fully-floating.
Semi-floating Pinch-bolt Small-end-clamped Gudgeon-pin.
In this method of fasten-
ing the rod to the pin (Fig. 3.95A), the central portion of the pin incorporates a full or partially
formed circumferential groove. When the connecting-rod small-end is centrally aligned to this
groove, the relative movement takes place only between the gudgeon-pin and the piston bosses.
This method allows the use of a narrow small-end due to which the width of the rubbing surface
between the piston and the gudgeon-pin boss can be large.
Semi-floating Force-fit Small-end-clamped Gudgeon-pin.
In this arrangement (Fig.
3.95B), the connecting-rod small-end faces are polished with emery cloth and heated evenly
using an oxyacetylene torch at about 503 to 593 K, until a pale-straw to dark-blue oxide color
appears on the bright surface around the eye. Then the gudgeon-pin is forced through both the
piston and the small-end eye so that it is centrally positioned. Subsequently the small end cools
the shrinks tight over the pin. In this case also the relative rubbing movement is only between
the pin and the piston bosses.
Semi-floating Piston-boss-clamped Gudgeon-pin.
In this method (Fig. 3.95C), the
gudgeon-pin is clamped to one of the piston bosses. The connecting-rod small-end is lined with
an interference-fit phosphor-bronze plain bush bearing. This bush locates the gudgeon-pin and
provides it with a low-friction surface. Care must be taken not to strip the thread in the relatively
soft alloy while tightening the tapered locking bolt. This approach is adopted when the bearing
properties of the piston material are not suitable for heavy-duty continuous oscillatory rubbing.
Fully Floating Gudgeon-pin End-pads.
If the gudgeon-pins are allowed to float (Fig.
3.95D) both in their piston bosses and in the small-end eye, they must not touch the cylinder
directly to avoid scoring of the walls by their very hard outer edges. In one of the methods of
preventing scuffing, spherical end-pads made from aluminium, brass, or bronze are used to act
as buffers between the walls and the pin. During operation, the gudgeon-pin freely revolves both
in the small-end and in the piston boss, which has a tendency to improve lubrication.
Fully Floating Gudgeon-pin with Circlip Location.
In this design, the fully floating
gudgeonpins (Fig. 3.95E) provide the bearing-surface area to the piston-boss bores as well as
the small-end bronze bush bearing. Engines with small connecting-rod-to-crank throw ratios
and large bore-to-stroke ratios have both large pivoting angular movement and heavy thrust
loads on the piston skirt. The double swivel action of the fully floating pin reduce this tendency
under heavy-duty conditions. Circlips are used to restrain the gudgeon-pin from sliding from
side to side. The clips are positively located in internal circumferential grooves formed near the
outer end of each gudgeon-pin-boss bore. Two types of circlip in use are the heavy rectangular-
section Seeger circlip, and the circular-section wire circlip, which is lighter and cheaper but not
so secure.
Fig. 3.95. Piston and connecting-rod joints.
