LO 7-6 Identify and explain the purpose, function, and construction of piston pins

1. Piston Pins

Piston Pins

Piston pins, also called wrist pins or gudgeon pins in service literature from the United Kingdom and Japan, are the connecting joint between the connecting rod and the piston. Piston pins may be either hollow or solid and are fabricated from high-alloy steels. The area of the piston pin bore of a piston is referred to as the pin boss . This area is reinforced, since the thrust forces of combustion are transmitted through the piston to the pin through the pin boss.

Semi-Floating Piston Pins

There are a variety of piston pin designs. One design is the semi-floating wrist pin. This design requires the piston pin to be pressed into the connecting rod and piston pin bore. In this arrangement, only the piston rotates on the pin while the pin is fixed by an interference fit with the rod. Assembling and disassembling the pin requires several tons of force using a special fixture to prevent damage to the piston. The advantage of this pin design is that it does not have the problem of pin-retaining clips loosening and falling out. This configuration is not commonly used on diesel engines, since wear is concentrated only on the piston pin bore and not evenly distributed across the connecting rod bushing.

1.1. Full-Floating Piston Pins

Full-Floating Piston Pins

The piston pin design common to most diesel engines is a full-floating piston pin. Both the piston and the connecting rod rotate on the pin. Although this requires a bushing in the small end of the rod, more surfaces are rotating with less load, and they have an oil film to better minimize wear. This type of pin is held in place by spring clips or, in the instances of some newer engines, Teflon caps that prevent the pin from coming out of the bore and scratching the cylinder Figure 39. Some wrist pins may have an interference fit when at room temperature, but the pin bore will increase in diameter with temperature in a running engine to allow the pin to turn freely in both the piston and the connecting rod. This type of pin needs to be heated when removing and replacing it Figure 40. A trapezoidal pin bushing is most commonly used with this design because the bearing surfaces that absorb combustion forces are wider on the connecting rod and piston; this improves piston and connecting rod load-bearing capabilities Figure 41.

FIGURE 07-39: Ends of the piston pin clip must be installed vertically to prevent dislocation by inertia forces when the engine is operating at high speed. Note the paint mark on the rod indicating service weight.

FIGURE 07-40: Hollow and solid wrist pins. Lower cylinder pressures mean that hollow pins can be used.

FIGURE 07-41: A. The trapezoidal style of connecting rod has a wider load-bearing surface on top of the pin than B. the parallelogram style of connecting rod.

1.2. Wrist Pin Offset

Wrist Pin Offset Although the piston moves linearly in the cylinder, the angle formed between the connecting rod and the crank throw pushes the piston sideways as it converts the reciprocating piston motion to crankshaft rotational motion. This push is known as side thrust and is divided into a major and minor thrust. The amount of thrust is a function of the angle formed between the connecting rod and the piston and cylinder pressure. Major thrust is produced during the power stroke and minor thrust during the compression stroke. Long stroke engines, like diesels, have greater thrust because of the angles formed between the long crank throw and the longer connecting rods. Side thrust combined with higher cylinder pressures can cause three problems in diesels engines: Higher side thrust forces can lead to excessive cylinder wall wear. Diesel piston speeds are fastest during mid-stroke where the greatest cylinder wall wear takes place, particularly in engines that are constantly heavily loaded engines. Cylinder wall and liner cavitation may appear on the coolant side of the cylinder wall due to rapid flexing of the cylinder wall or liner. This is a problem unique to diesel engines, where pitting on the coolant side of the cylinder wall can penetrate the cylinder. This phenomenon is explained in more detail in Chapter 11: Diesel Engine Cooling Systems. Noise can be heard as the piston crosses over from pressing against one side of the cylinder to the other near top dead center (TDC). Clearances between the sides of the piston instantly switch from one side to the other as the piston moves through TDC Figure 42.

FIGURE 07-42: The piston pin axis offsets from the cylinder centerline to eliminate noises caused by the piston rocking in the piston-to-cylinder wall clearances at TDC. Doing this allows the piston clearances to change sides before the piston reaches TDC, minimizing noise.

To minimize the degree of side thrust and minimize noise caused by piston slap, engine manufacturers often offset the centerline of the wrist pin with the centerline of the piston Figure 43. Because the centrelines of both the piston and the wrist pin are not the same, the piston must have a specific orientation in the engine. This means there is a side of the piston that must be oriented to the left or right side of the engine. The purpose of wrist pin offset explains why pistons have markings to ensure that they are installed in the correct direction in the engine. Markings are usually present on the piston to indicate its orientation toward the front, rear, or side of the engine.

FIGURE 07-43: Note the directional arrow on this piston crown. The offset centerline of the piston pins requires the pistons to have a specific orientation in the engine.
		FIGURE 07-44: When crossing TDC, the clearance between the piston and the cylinder wall changes sides to the opposite side of the piston. When pistons share the same centerline as the wrist pin, noise is created by the abrupt movement of the piston. To reduce piston crossover noise, offsetting the pin and piston centerline will allow the piston to rock near TDC instead of abruptly crossing over.

Another strategy to reduce the thrust angles in the engine is to offset the centerline of the crankshaft with the engine centerline. By moving the centerline of the crankshaft and wrist pin slightly to one side, the thrust angle is reduced Figure 44. What this means for the technician is that connecting rods and pistons have a specific front/back and left/right orientation in an engine. For example, pistons must be oriented typically with a reference to the camshaft or front of the engine. Similarly, connecting rods often have directional indicators or stamping marks facing the front of the engine.