Sunday, June 5, 2011

Balance of reciprocating parts

The reciprocating parts of the engine are the piston assembly and the upper part of the connecting rod. Each time a piston reaches TDC or BDC it has to stop and change its direction. Each time this occurs an inertia force is produced which tries to keep the piston moving and this tries to move the engine up or down.
To reduce the effects of inertia forces, all the pistons and associated parts are of equal weight.

Fig 4.27

Reducing resonance
Everything has a natural frequency at which it will vibrate if caused to do so. The engine vibrations are related to speed so, at various times, the engine will be producing vibrations at the same frequency as the natural frequency of some other part. When this occurs, the part will commence to vibrate. This is known as resonance.
To prevent vibrations from being transmitted to the body and becoming noticeable, insulators are used. These include rubber mountings for the engine and sub-frame, and insulation and sound-deadening materials applied to body panels.

Engine mountings are designed to be flexible at idle, and more rigid at higher speeds. Figure 4.27 shows a liquid-filled engine mounting. The rubber molding is hollow and contains liquid. It has two chambers connected by an orifice. When lightly loaded, liquid can pass between the chambers through the hole, but when heavily loaded or subjected to large vibrations, the liquid has a dampening effect.
Another method of reducing vibrations is with a dynamic damper (Figure 4.28). This is a rubber- mounted weight attached by a metal bracket to the sub-frame. Its function is to absorb vibrations and prevent resonance noise being transferred to the interior of the vehicle. This damper is designed to come into operation in the 2000 engine rpm range.
Fig 4.28
The damper action depends on the inertia of the weight. If a vibration causes the damper to move, the inertia of the weight causes it to lag behind and oppose movement. In this way, the weight absorbs energy and acts as a dampening device. Vibration dampers act by temporarily absorbing energy and then releasing it.

Fig 4.29
Balance shafts
Balance shafts are used in some engines to reduce vibration. Some engines have a single balance shaft, while others have two balance shafts. Balance shafts are mounted in bearings in the crankcase, parallel to the crankshaft, and are driven by a chain or gears.
Figure 4.29 is one example, where a single balance shaft is driven by gears from the crankshaft at crankshaft speed.
The purpose of balance shafts is to reduce the up- and-down vibrations of the engine by producing their own inertia forces. The shaft is designed to counter the forces at both TDC and BDC. Balance shafts can be driven at engine speed, but they are often driven at twice engine speed to increase their effect.
Figure 4.30 shows how two balance shafts are used. Both shafts are driven at twice the engine speed. The chain rotates the left balance shaft in the same direction as the crankshaft, while the right shaft rotates in the opposite direction via an idler sprocket and gear.
Fig 4.30

See Cylinder block, crankshaft and bearing service 

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