Sunday, June 19, 2011

Connecting rods

Fig 6.23
The connecting rod must be very strong and rigid, but it must also be as light as possible. Connecting rods are highly stressed, being subjected to stretching, compression and bending. They are made as a forging of H-section, because this shape provides greater strength than a solid rod of the same mass (Figure 6.23).

The crank-pin end is the larger end of the connecting rod and is often referred to as the big end. The piston end is much smaller and so is referred to as the small end, or sometimes as the little end. The big end of the connecting rod has a bearing cap that enables it to be installed on the crank-pin. The big end also carries the two halves of the bearing.

Fig 6.24
Connecting-rod features
To maintain engine balance, all the connecting rods in an engine are matched for mass. The cylinder number is stamped on most rods and caps so that they will not become mixed up when the engine is dismantled (Figure 6.24).
During the manufacture of most connecting rods, the parting faces of both the bearing cap and the connecting rod are machined as flat surfaces. The cap is bolted to the rod, and the
bore for the bearing is then finished to an accurate size.
Another process uses a fracture method. The connecting rod is made as a single piece and the cap is then broken away from the rod. The cap is reassembled to the rod and the bore for the bearing is machined. With this process, the parting faces of the cap and the connecting rod are irregular, but they fit together so closely that the parting line can barely be seen.

Connecting-rod bolts
Some connecting rods have cap bolts with nuts, similar to those in Figure 6.23. The bolts are a neat fit in the connecting rod and are not usually removed unless they are to be replaced. Other connecting rods have cap bolts that are threaded into the connecting rod.
Some bolts have their shank reduced in diameter so that it is less than the thread and this actually increases their strength. In a normal bolt, the root of the thread is the smallest diameter and so takes the greatest stress. When the shank of a bolt is reduced in size, the stress is distributed over a different part of the bolt and so prevents failure at the thread.

·           The cap bolts are subjected to a tensile stress (stretching) caused by the piston inertia
at the top of the stroke.

Forces in a connecting-rod assembly
Fig 6.24
Forces in a connecting-rod assembly
The connecting-rod and piston assembly are subjected to various forces. These are caused by combustion pressure, piston inertia and centrifugal force.
An example of the way in which inertia forces act on the various parts of the assembly can be seen in Figure 6.25. The piston in the illustration is almost at the end of an upstroke (exhaust stroke) and at TDC will produce an inertia force that will be transmitted to other
parts of the assembly.
On the down-stroke, when the piston reaches BDC, the direction of most of the forces due to inertia will be the reverse of those shown.
The piston provides an inertia force at TDC and BDC where the piston changes its stroke. In effect, the piston tries to keep moving but is held by the connecting rod. This places the connecting rod in tension when the piston is at TDC and in
compression when the piston is at BDC.  

Figure 6.25 Effects of inertia of the piston at the end of the exhaust stroke the assembly is for a diesel engine
1 piston inertia, 2 toad on upper half of small-end bush,
3 connecting rod in tension, 4 bolts in tension, 5 toad on
tower half of big-end bearing, 6 toad on upper half of main

There are many forces acting at different times. On the compression stroke, compression pressure will have an influence on the forces, as will combustion pressure during the power stroke. The effects of centrifugal force, due to crankshaft rotation, will increase with engine speed.
The assembly shown in Figure 6.25 is for a larger diesel engine. The parts are bigger and have a greater mass than those of a petrol engine, so greater stress can be created. However, the engine rotational speeds (and piston speeds) will be much lower than those of a petrol engine.

Connecting-rod bearings
Connecting-rod bearings are split-sleeve bearings of the precision-insert type. One half of the bearing is carried in the connecting rod and the other half in the connecting-rod cap.
Various points relating to crankshaft bearings were discussed in last posts, and there is more information related to bearings in the posts that follows.

See piston, Connecting rod and bearing service>>>>>>>>>>>>>>>>>>

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