LO 7-7 Identify and explain the purpose, function, and construction features of connecting rods

1. Connecting Rods

Connecting Rods

A connecting rod links a piston to the crankshaft, which enables the conversion of the piston’s reciprocating motion into rotational movement of the crankshaft. Connecting rods are the most stressed component of the engine because they experience either tensional loading or compression loading every time the piston changes direction Figure 45.

Tensional loading occurs at the end of the upstroke at TDC, when the weight of the piston assembly would naturally continue in a straight line through the cylinder head but is stopped by the connecting rod. Without the cushioning effect of pushing exhaust gases out or compressing the gases in the cylinder, the tensional forces would be even greater. Compression loading occurs during the power stroke when the combustion forces push the connecting rod down with total cylinder forces in excess of 30,000 lb (13,608 kg). Because of these forces, connecting rods are built to typically withstand forces more than 16 times greater than those expected during normal operation. These forces could be from abnormal combustion conditions or engine over-speeding. Engines that use heavier articulating pistons have larger connecting rods to handle the increased tensional forces Figure 46.

FIGURE 07-45: Connecting rod features and nomenclature.

FIGURE 07-46: The cross-sectional area of these connecting rods is much larger because they must resist tensional loading from heavier articulating pistons.

1.1. Video: Connecting Rod Construction

1.2. Connecting Rod Materials

Connecting Rod Materials Because connecting rods are highly stressed, they are commonly forged from high-alloy steel. Rods are then shot-peened to relieve stress risers (points where the rod can easily break) and add additional fatigue strength. This technique involves shooting thousands of small steel balls at the rod using compressed air. The balls make small dents in the rod surface and smooth, or peen, over any microscopic cracks or pores in the rod that could form stress risers. Bolts used on the caps are made from proprietary alloys of grade 8 or higher metals. Rolled threads are used on these bolts to increase their strength Figure 47. Rolled threads have a wider root diameter and are pressed into the bolt, not machine cut. The machine cut of a die making threads leaves stress risers. Rolling threads does not remove metal or disturb the metal’s grain structure. Bolts with rolled threads can withstand more stretching or preload than machine-cut threads.

FIGURE 07-1: TEXT

1.3. Connecting Rod Features

Connecting Rod Features

Connecting rods use an I-beam cross section, which has good weight-to-strength characteristics and resists bending. The smaller end of the rod will have a bushing pressed into it if it is used with a full-floating piston. When the small end of the connecting rod is shaped like a triangle, it is called a trapezoidal rod . The individual weight of a set of connecting rods is critical to engine balance. If the connecting rods vary in their mass, engine vibration will result. To adjust the weight of connecting rods, balance pads are located at each end for grinding to balance the rod’s weight between the rod ends and around its center point. Often, select fit rods are used together in sets in an engine. In select fit rod sets, rods are weighed after fabrication; those closest in weight are used together in sets. These sets can be identified by the paint marks that uniquely identify the weight variation of the rod (a group of connecting rods may have white, yellow, red, or other paint markings to identify them as belonging to a particular weight classification). Such rod sets should be kept together when assembling an engine. If a service rod is required, it will often have a different paint marking that is in the middle of the weight distributions. Because the final finish bore dimensions of the big end of the rod are made after the parting halves of the rod and cap are completed, the two parts must be assembled in the same way, as they were finished. If the cap is fastened to the rod 180 degrees away from the original position, the bore will not be concentric, even though the two pieces can be bolted together. Matching marks on a connecting rod identify the correct assembled position. Failure to align these marks will result in improper bearing clearances, and the engine may not rotate after the pieces are assembled. If the engine is operated when the components are assembled incorrectly, a tight connecting rod bearing will result in an insufficient oil film on the connecting rod journal, which will produce catastrophic engine damage Figure 48. To further ensure a concentric bore in the big end of the connecting rod, the parting halves of the cap and rod will have various machined features, serrations, or alignment dowels Figure 49.

It is also critical that rods be installed in an engine with the correct front-to-back orientation. Connecting rod caps will strike the block and severely damage an engine the first time it is rotated if the rod is not installed in the engine correctly Figure 50.

FIGURE 07-48: Proper alignment of the connecting rod and cap is made by matching up the markings on the cap and rod.

FIGURE 07-49: Machine features on the parting halves of connecting rods ensure proper alignment and bore concentricity.

FIGURE 07-50: Parting halves of these rods are serrated. Note the directional marking on the rod.

Diesel engines can have a passageway drilled through the connecting rod to the piston pin bushing to supply oil to the piston pin. In V6 and V8 engines, connecting rods also share a common crankpin journal, which means rods must also be correctly oriented. A chamfered edge in the rod cap is installed toward the crankshaft fillet radius, the edge between the journal and the crank cheek. The flat sides of each rod match each other, providing a surface for each rod to glide over one another. Even in in-line engines, the connecting rod must be correctly oriented front and back due to piston pin and rod cap split offsets.

1.4. Fracture Split Connecting Rods

Fracture Split Connecting Rods

One common feature of connecting rods is that the rod ends can have a 45–60 degree offset or split of the rod cap. This is to accommodate larger connecting rod journal sizes while still allowing the connecting rod to pass through the cylinder during assembly Figure 51. If the rod were not split at an offset angle, the big end would not pass through the cylinder due to the dimensions of the rod. The mass of the big end can also be somewhat minimized by using this feature. A split fracture connecting rod is the most common type of connecting rod manufactured today. These connecting rods have had the halves of the big end of the connecting rod and cap parting surfaces split by fracturing, rather than using extensive machining of the mating surfaces between the rod and the bearing cap Figure 52.

FIGURE 07-51: An offset split rod permits a larger crankpin diameter and a stronger rod cap retention.

FIGURE 07-52: Split fracture rods give a coarse-grainy appearance to both rod cap surfaces.

Using a split fracture connecting rod offers the following advantages: More precise alignment of the rod and cap than a machined rod and cap Improved control of oil clearances and oil film thickness Reduced production cost because machining steps are eliminated Greater retention strength between the cap and the rod Split fracture connecting rods allow the rod and cap to be separated along a predetermined fracture line. Instead of cutting the rod cap away from the rod and then machining and reboring the rod and cap, a hydraulically operated wedge pushes the two pieces away from one another. A line, scribed where the split should take place, provides a weakened point to control the split Figure 53. Because the big end of the connecting rod is fracture-split, the rough-edged big end and cap will resist movement better and are aligned more precisely than a conventional flat machined surface. Split fracture rods can be made about 17% lighter, which reduces engine vibration while maintaining the strength of traditionally manufactured rods. Split fracture rods are produced more economically and with less energy.

FIGURE 07-53: The appearance of the parting lines of a fracture split rod.