Published by tedeaton on 16 Nov 2007 at 10:53 pm
“Balancing the Crankshaft” By Ted Eaton
In getting to the point in which the crankshaft from a V8 or other V style engine can be spin balanced, several different operations had to be already completed. Had this been an inline or opposed cylinder engine, then the crankshaft could have been balanced at any point in the operation due to not requiring any bobweight fixtures to be installed on it. However, when the cylinders are orientated in a V style, bobweights are required due to the rotating and reciprocating masses not being equally opposing. Throughout the previous articles in this series, the pistons have been matched to each other by weight, the connecting rod ends have also been appropriately lightened and matched by weight on each end as a set, and the miscellaneous components such as the bearings, rings, and piston locks have also been weighed. With all these values known and the estimated oil along with any heavy balancing values also added, the calculation for the amount of weight required for the bobweight fixture is ready to be put to use.
Next on the agenda is to build up the bobweight fixtures so that they match the calculated value. Bobweights vary in style and design from the different manufacturers but all have the same function in that they are simulating the rotating and reciprocating masses seen on a crankshaft as installed within an engine. Some bobweight designs have a small vessel on each half that are filled with bb’s or shot in which to achieve the desired weight while other designs simply take appropriately sized weights that are fastened on each half. Regardless of the design used, it’s imperative that each half weigh the same while the two halves being attached to each other also match the predetermined weight calculation.
Once the fixtures have been assembled to match the calculated weight value, it’s time to put them on the crankshaft. At this point it’s not only important that the two halves are spaced equally apart when placed on the crankshaft rod journal but that they are centered on the journal as well. The newer bobweight fixtures can now be purchased with built in micrometers in which to exactly split the halves while the earlier models require a manual or physical measurement in which to do this same function. Regardless of the style being used, it’s still important that the halves be split equally during the mounting process. In theory, the bobweights can be pointed in any direction when mounted on the journals and still give the same high degree of balance that’s being targeted for. General accepted practice though dictates that they be installed with the longest parts of these fixtures pointing at 90° angles to the stroke when placed on the journals. This keeps outward protrusions at a minimum while spinning the crankshaft at speed in the balancing machine. Once the bobweights have been installed on each journal, the crankshaft is ready to be placed in the machine to be spun. If the assembly is an external balanced design, then the flywheel and damper will also need to be installed prior to making the initial spin of the crank.
With placement of the crankshaft assembly in the machine, the balancing machine is then configured for the total static mass of the crankshaft so the electronics can give appropriate feedback on the amount of imbalance that’s present. The machine will spin the crankshaft at a predetermined speed in which to measure the dynamic and static imbalance of the crankshaft assembly. Dynamic imbalance or the state of balance from end to end can only be determined with the crankshaft being run at speed. Static balance on the other hand, if bad enough, can be determined by merely finding the heavy side of the crank while it’s sitting in a pair of rollers but without dynamically balancing the crank, it would be quite difficult to determine which end of the crankshaft would be responsible for the static state of imbalance.
The electronics can isolate the imbalances at each end of the crank and if done correctly, the crankshaft will not only be perfectly dynamically balanced when finished, it will also be statically balanced.
Without getting into detail on the mechanics of the spin balance machine itself, I’ll simply say that the electronics will indicate where the crankshaft is out of balance and by how much. Throughout the rpm range that the crankshaft goes through while in the machine, the electronics will proceed through 180° of phase angles while accelerating from low speed to high speed. Typically, high speed readings are desirable but there are those instances where low speed readings must be taken mainly due to the crankshaft being so far out of balance that it will not safely spin up to speed necessary for a high speed reading. After initial weight adjustments are made to the crankshaft counterweights in these instances, then high speed balancing can be performed which allows for the remaining weight adjustments to be performed to the crankshaft. These weight adjustments can be either by adding weight or taking weight away.
If the readings indicate that the crankshaft counterweights are heavy, then weight reduction at the counterweights is required. Removing weight is reasonably straight forward in that it’s usually removed using a drill press in which a predetermined amount of material is removed. The drilling works best when drilling two holes instead of one that are equally spaced from the indicated location of out of balance. This allows subtle weight adjustments by going to either of the two drilled holes instead of concentrating all efforts on a single hole that could be off just a few thousandths fore or aft of the actual out of balance position. Besides drilling, there are those instances where the weight will be removed through some form of grinding or lathe operation depending upon the final result being achieved.
On the other end of the spectrum are those crankshaft counterweights that are too light and thus requires material to be added in which to make the counterweights heavier. The simplest weight addition is where some of the existing balance holes can be refilled either with machined pieces of steel or a given amount of weld. The more extreme cases require the use of a heavy metal such as Tungsten or Mallory metal. I’ve even seen lead used. Anytime these metals are added, it’s desirable that they be installed parallel to the crankshaft centerline so that the tendency to be dislodged from the crankshaft by way of centrifugal force is minimized. Again, because of crankshaft design, heavy metal may be required to be installed within a vertically drilled hole in the crankshaft and secured in such a manner that it will not be centrifugally dislodged. Because of the costs involved with heavy metal and its installation, the customer can opt for a external balance which forces the damper and/or flywheel to become an integral part of the balanced assembly. This can create a major difficulty later when replacing either externally balanced part without a complete teardown and rebalance especially if the piece being replaced was destroyed and cannot be used as a reference.
Another option in lieu of adding heavy metal to a crankshafts counterweights are lightening holes within the crankpins themselves. This artificially adds mass to the counterweights as it makes the crankpin side lighter and is a desirable way in which to move the mass around. If having a crankshaft built, this is a typical option as it also allows the overall weight of the crankshaft to be reduced by also reducing the size of the counterweights. When building stroker cranks, lightening holes in the crankpins are a normal course of action.
And then there’s centrifugal mass reduction. This is where the bobweight calculation for a particular engine is considerably lighter than the standard values and hence, the crankshaft is much heavier than it needs to be. Rather than drill a pair of deep holes in the outermost crankshaft counterweights to balance the crank, a greater number or series of shallow holes are drilled or the counterweights themselves are machined in a lathe across the length of the counterweights effectively reducing the centrifugal mass of the crankshaft. For a drag car, this allows the engine to accelerate at a faster rate by lieu of a reduced crankshaft mass in which to get it up to speed. This will be at the expense of having less stored energy within the crankshaft for launching purposes. For a circle track or road race vehicle, the benefit is two fold. It not only allows for a faster accelerating engine but also an engine that can slow down at a quicker rate. This allows the driver to run further into the corners under throttle knowing that the engine will slow down at a quicker rate before having to apply the brakes or just applying less braking to achieve the same result. For centrifugal mass reduction, the rotating mass is reduced at the outer edge of the crankshaft. This minimizes the amount of stored energy that would have normally been present in the crankshaft which in turn would have inhibited the crankshaft to slow down and subsequently continued to push the vehicle forward.
Regardless of the method employed in removing or adding material to a crankshaft in which to balance it, the degree of accuracy is critical as the final state of balance will still dictate how much power or torque is being potentially lost at the flywheel due to imbalances.
I hope you’ve enjoyed this series of articles and found that it wasn’t too detailed. The intent was to give a much better understanding of what’s involved in getting your engine balanced and possibly what operations some of the more experienced readers may want to undertake on their own. Until next time, happy smooth motoring.
Originally published in Y-Block Magazine, Mar-Apr 2005, Vol 12, No. 2, Issue #67