Automotive Dictionary: Balancing

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Automotive Dictionary: Balancing


Throughout this site we use many technical terms, and given the breadth of readership our site enjoys, sometimes we are remiss and incorrectly assume everyone knows what we are referring to. For those that do not, here are some explanations of the technical terms use.
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Understanding The Need To Balance The Engine



To understand the need for balancing, let's examine the sources of the unbalancing forces and their effects. Imagine a disc one inch in diameter and onri inch wide, revolving on an axle. If the disc is precisely balanced, it will spin true and impart only centrifugal force to the axle bearings. However, if a weight (say one ounce) is added at one section of the disc, an unbalanced condition will be created. This unbalance will not allow the disc to revolve smoothly and forces the axle to follow an egg-shaped path within its bearings. Naturally, this will tend to cause vibration and premature bearing failure.

Two other factors now enter the field. If the disc is replaced with one of six inch diameter, the one ounce weight at its perimeter would force considerably more out-of-balance stresses at the axle. This condition would be aggravated even more if the disc was lengthened to six inches. Since the out of balance section is seldom, if ever, found at the exact centre of the disc lengthwise, the unbalanced section will try to make the disc wobble as well as spin untrue. Thus, the axle must absorb the forces by flexing slightly, or pass them on through the bearings. Neither condition is desirable, particularly when the components are revolving at high speeds.

In automobiles, these centrifugal forces apply to the crankshaft throws, journals, the lower half of the connecting rods, the flywheel and clutch assembly and the drive shaft. The centrifugal forces are constant throughout every revolution. However, the upper-section of the connecting rods and the piston assemblies set up an irregular reciprocating inertia force because they change direction twice for every revolution of the crankshaft. This causes the crankshaft to suffer periodic and varying amounts of unbalance every time the pistons "stop and start". These inertia forces can be effectively countered on multi-cylinder motors by arranging the crankshaft throws so that the forces on one journal will cancel out the forces on the journal next to it.

Inertia Forces



So it is that motors with more than four cylinders (in-line), need have only the rotating mass counterbalanced. Now that we have discussed why an engine does have unbalancing factors, let's examine the means by which they are alleviated. The inertia forces vary according to the weight of the piston assembly and upper half of the connecting rod. The amount of inertia force also varies throughout the length of the piston's travel. Maximum force is exerted as the change of direction takes place at both TOP and BOTTOM dead centre, while the force is at its minimum when the crankshaft journal is at 180 degrees to the cylinder bore. The INERTIA forces also produce torsional stresses at the crankshaft because they try to twist it harder at the time of maximum pressure. The positioning of the crankshaft throws actually provides a self-cancelling effect.

With four, six, or eight cylinder in-line motors, it is quite safe to alter the weight of the reciprocating parts, within reason, without ill effects, but this is not the case with V-type engines, as we shall show further on. Back to the in-line motors. When advanced stages of tune are brought about, the hot-up has usually required different pistons to be fitted. In some cases (high compression pistons), the new pistons will be considerably heavier than those they replace, while in other instances the new pistons will be lighter. Providing the difference in weight is not excessive, no worries should be encountered. It is essential, however, that each of the piston assemblies fitted are of exactly the same weight. The CENTRIFUGAL forces, on the other hand, are overcome by incorporating bob-weights on the crankshaft. The bob-weight is equal to the weight of the crankshaft throw, its journal, the bearings and the lower half of the connecting rod, all combined.

It will be seen that with a well balanced crankshaft the weight of the reciprocating mass in each cylinder (the complete piston assemblies and upper halves of the connecting rods) is exactly the same, and the bob-weights will correspond with their appropriate throw, journal etc. Unfortunately, this stage is almost impossible to achieve on motors that are mass-produced. An automobile manufacturer cannot ensure that every unit he produces will be correctly balanced because of the time and cost factors involved. Therefore, the assemblies are built to tolerances which, although satisfactory for every-day motoring, are not precise enough for high speed work. The person who wishes to have the task done correctly must, for obvious reasons, have it attended to by a specialist.

Static and Dynamic Balancing



Most enthusiasts are familiar with the names of the two types of balancing operations – Static and Dynamic - but we will explain the differences and suitable applications of them. Static balancing is just what the name implies. The object is balanced while stationary. Going back to our imaginary disc, you will remember that it was out of balance due to the extra weight at one section. Assuming that the disc could spin freely, it would always stop with the heavy section downwards. The condition could be remedied by either removing the appropriate amount of weight at the unbalanced point, or adding weight to a point diagonally opposite it. This method of balancing will only suffice if the disc's diameter is substantially greater than its length. The longer the disc is, in relation to its diameter, the less reliable static balancing becomes.

Both these dimensions are found in the usual car engine: The long, relatively small diameter crankshaft, and the thin, large diameter flywheel. The flywheel could be statically balanced quite satisfactorily, but the same method would not be accurate where the crankshaft was concerned. If the crankshaft was spun on its axis, it too would stop with the heavy section facing downward. By removing weight at the heavy point, or adding weight diagonally opposite, the crankshaft could be perfectly balanced — statically. But, being what they are, crankshafts "must revolve, and it is highly probable that static balancing would only worsen the out-of-balance problem. This is due to the fact that, although the shaft was balanced as a whole, there is no way of determining at what point along its length it was unbalanced.

Dynamic Balancing



Dynamic balancing is the only means of accurately balancing any component that revolves, for the out-of-balance area (or areas) is easily identified. A balancer has much the same lay out as a lathe. Facing the machine, the very large control cabinet will likely be on your left. A long "bed" will run parallel to it and mount the two uprights which will support the pick-up heads. On the end of the control panel that faces the bed, there is usually a large diameter disc, clearly marked off from 0360 degrees, and a small shaft which will spin the component(s) being balanced.

The component will sit in the semi-circular section of the pick-up heads, which in turn will transmit any vibrations to the diagnostic monitor and the Unbalance meter. Normally the inbalance of an engine will be dampened by the main bearings, but when each componenet is on the balancer and with nothing to restrain its movement, you will often (particularly with older engines) find the crankshaft and other components to be all over the place. When you find the cause of the inbalance, you need to drill at the point corresponding to degree of inbalance. Repeat the spinning process until it is perfectly balanced. Dynamic balancing will produce a far better result thant static balancing, which in most cases will not be able to remedy the fault.

Balancing The Assembly With The Flywheel



Balancing the assembly with the flywheel in place is one of the last steps in the engine balancing process. Do not be surprised if your old engine had a fair amount of unbalance evident. When this is corrected the pressure plate will be bolted onto the flywheel. The actual clutch plate is usually not included in this phase of the operation because its size and weight have a negligible influence on the assembly's balance. After the final balancing has been accomplished, the complete assembly should be able to be spun without a tremor. With in-line engines that are not intended for absolute competition work, a crankshaft/flywheel/clutch job is sufficient for your older classic car - and in particular either older Holden Grey or Holden Red engines or Ford straight six engines. V8 engines are more expensive to balance because of the extra work involved.

Naturally an engine balancing machine is capable of balancing anything that revolves, ie drive shafts, generator armatures, etc. Because V8's are the most common engine to be balanced, because they are of course found in higher performance classic cars, they do deserve special attention. Almost every older V8 has its crankshaft throws arranged at 90 degree intervals, with two connecting rods per throw. Thus, the inertia forces cannot be cancelled out as they are divided between the two connecting rods on any particular throw. As number one piston assembly is forced down, for example, its inertia is interrupted (at a point determined by the angle of the V) by the inertia of number eight piston which uses the same throw. By the 1960s V8 engines relied on torsional dampers in the crankshaft pulley and their deep rigid blocks for much of the inertia absorption.

Many early users of these V8 motors, whether in speed boating or car racing, found that they had to replace engine bearings with disconcerting regularity, particularly if high-top pistons have been fitted. The high-top pistons may each be only four ounces heavier than the original items, but when multiplied by eight, that four ounces became "two pounds of practically unbalanced mass". To realise just what this represented in the terms of force, a typical American V8 motor from 1960, in optimum balance, developed about 4000 pounds of inertia force every time one of its pistons changed direction. That was the figure at 5000 revolutions per minute. We leave it to your imagination to work out how much inertia force the same motor would develop when the crankshaft was spinning in excess of 7000 revolutions, and with another two pounds of unbalance added for good measure.
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