Balance shaft


  • Preface
  • Balance shaft operation

Vibrations are caused by the mass forces in an engine. The more cylinders an engine has, the less vibrations there will be. This is because with a 3-cylinder engine a power stroke takes place every 240 degrees, with a 4-cylinder engine every 180 degrees of crankshaft, with a 6-cylinder every 120 degrees, with an 8-cylinder every 90 degrees and with a 12-cylinder engine. cylinder every 60 degrees. If an engine has more cylinders, there are therefore more power strokes in a short time and the engine is virtually vibration-free. Most passenger cars use 4-cylinder engines. This engine generates a lot of vibrations that are passed on to the interior. The counterweights on the crankshaft mainly limit engine vibrations.

To further limit engine vibrations, manufacturers of a number of car brands have applied the “balance shaft” principle. Each brand has its own construction (a single balancer shaft, 2 balancer shafts at the same height, 2 balancer shafts, 1 of which is lower and 1 higher in the block, etc.) The balancer shaft drive takes place via the timing (gears, belt or chain) and must also be set “on time” when working. A balance shaft that is not in time will amplify engine vibrations even more, resulting in component failure.

Balance shaft operation:
The balance shaft is an axis that is itself unbalanced and thus compensates for the mass forces mainly caused by the secondary piston movement to arise. Over the entire length there are thickenings, ridges or deformations that cause the required imbalance when rotating (turning around). Both the primary forces (up-and-down piston movement) and the secondary forces (the lateral forces due to the connecting rod being pushed obliquely downwards) are absorbed by the balance shafts. To accomplish this, the balancer shafts rotate at double the speed of the crankshaft and in the opposite direction.

1: Piston is in TDC. The balance shafts point downwards. The lower balance shaft rotates counterclockwise and the upper balance shaft rotates clockwise. The balance shafts both rotate 2x as fast as the crankshaft.

2: The crankshaft rotated 45 degrees and the piston moves from TDC to ODP. In this position, the mass forces exerted by the secondary piston movement arise, the greatest. During the secondary piston movement, mass forces are created that are directed downwards. To compensate for this, the balance shafts point upwards in this position.

3: The crankshaft turns again 45 degrees and is in ODP. The balance shafts are pointing downwards.

4: The crankshaft moves from ODP to TDC. After 45 degrees of crankshaft, the balance shafts are pointing upwards again. Again, in this position the greatest (downward directed) mass forces are created, which come from the secondary piston movement. The upwardly directed balance shafts compensate for these mass forces.

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