CRANKSHAFT BALANCE

Most of the crankshaft balancing is done during manufacture. Holes for balance are drilled in the counter weight to lighten them. Sometimes, these holes are drilled after the crankshaft is installed in the engine. Some manufactures are able to control their casting quality so closely that counterweight machining for balancing is not necessary. Engines with cast crankshafts usually have some external balancing. External balance of these engines is accomplished by adding weights to the damper hub and to the flywheel or automatic transmission drive plate.
After the basic dimensions of the crankshaft have evolved, including length, engine stroke, number and size of bearings, bearing journal diameters, oil holes, and so on, the crankshaft balance then receives attention. This involves the determination of the size, weight and location of the crankshaft counterweights.
The counterweights are required to balance the reciprocating and rotating motions of the piston and connecting-rod assemblies and cranks. Thus, the weights of these assemblies, as well as the stroke and the crank radius, must be established at this time. The designer must know what these weights are as well as what they will be doing during the rotation of the crankshaft.
Once these factors are established, vector analysis can then be used to determine the resultant of the inertial and centrifugal forces from the reciprocating and rotating masses. Determining the position, shape and weight of these counterweights is called design balancing the crankshaft.
A limiting dimension is the radius of the counterweights. If the radius is too large, the counterweights strike other engine parts-the piston skirt, for example. In many engines, the piston skirts are cut away to provide room for the counterweights to swing around them at BDC as the crankshaft rotates. The counterweights cannot e too thick through from front to back either. There must be clearance between the counterweights and the connecting rods. Also, there must be clearance between the counterweights and the cylinder-block webs supporting the crankshaft. These dimensions and clearances determine the maximum radius and thickness of the counterweights.
Two basic factors in designing a counterweight are the amount of weight (It must balance the piston and rod weight) and the placement and the distribution of the weight (it must be so placed as to cancel out the opposing piston and rod weight). One procedure is to divide the counterweight into three parts for separate analysis: the arm, the left hand half, and the right hand half. Each of the three parts is then subjected to analytical routine that determines its volume (weight), center of gravity, and polar moment of inertia. The weight distribution is then determined. In effect, the distribution should be such that the unbalancing force of the piston and rod motion is countered exactly at any instant by a balancing mass from the counterweight, pulling in the opposite direction.