Eaton Balancing

“Connecting Rod Balancing”

By Ted Eaton

An engines connecting rods exhibits traits of both rotating and reciprocating mass and hence, must be match weighed end for end to insure these two masses are kept independent of each other. As a point of clarification, the reciprocating end is the small end of the rod or the portion of the rod that is representative of up and down motion in the cylinder while the rotating end of the rod is the bearing end which rotates with the movement of the crankshaft. Your balancing shop will have a rod weighing fixture that’s designed for separating these two masses and then being able to have all the rod small and big ends match in weights throughout the particular set of rods being balanced.

Simply finding the lightest rod in a set for total overall weight and then reducing the weight of all the other rods without any regard to which part of the rod the weight is being removed from to match the lightest does not make for a balanced set. This is because the weight being removed is most likely being taken from the wrong spot on the rod and thus actually making the rods even more out of balance than before attempting to weight match them. This method fails to take into account whether the mass being removed is reciprocating or rotating mass which is a major consideration in a dynamically balanced engine.

Connecting rod balancing requires a fixture that allows each end to be weighed independently. There are several different fixture designs available on the market but all utilize the same concept; each end of the connecting rod is isolated from the other for weighing purposes.

Similar in concept to the match weighing of the pistons, the ends of the rods must be weighed with the lightest small and big end of each rod within a set being found and isolated. Very rarely will the same connecting rod from a factory installed set have both the lightest small end and lightest big end on it. After finding the lightest ends, it is then just a matter of taking the remaining heavier rods and making the ends match the previously found lighter end weights.

Your balancing shop can employ one of several different methods in which to reduce the connecting rod end weights. Typical tools for this operation can vary from using a grinder, belt sander, or a milling machine. The design of the connecting rod in itself can dictate what machining or weight removal operation will be used. Most stock style connecting rods have a balancing pad on each end which is a convenient spot from which to remove material for balancing purposes. Many of the newer aftermarket rods and especially the H-Beam style do not have these balance pads on the ends and do require some forethought before attempting to remove any material from them. For many of these newer designed rods, material from the big end is removed at the rod bolt edge instead of the very bottom. The small ends for rods without balance pads are usually best done on a belt sander using a nice rounding motion in which to remove material evenly from around the pin end. Regardless of the method used for weight removal, it’s important that the metal not be unduly overheated. This may require repeated quenching if excessive grinding must be performed in which to remove the required amount of material. Excessive overheating of the big end can cause out-of-roundness to the big end bore which can prove disastrous to bearing clearances besides affecting the structural integrity of the metal itself on either end.

After all the connecting rods have been weight matched, the reciprocating and rotating end After all the connecting rods have been weight matched, the reciprocating and rotating weights are then recorded on a balance card or work sheet for the upcoming bobweight calculation.  All that remains at this point is to clean the connecting rods of any debris or grinding/sanding residues caused by this particular balancing step and rebox them until engine assembly takes place.

The next article in this series will cover the nuances involved within the bobweight calculation in preparation for spin balancing the crankshaft. Until then, Happy Motoring.

Originally published in Y-Block Magazine, Nov-Dec 2004, Vol 11, No. 6,  Issue #65

“Bobweight Calculation” By Ted Eaton

The previous articles in this series have expounded upon match weighing the pistons as well as the connecting rod small and big ends. Now it’s just time to start thinking about the crankshaft bobweight calculation. The bobweight will be a specifically weighted fixture that attaches to each of the connecting rod journals for electronic spin balancing purposes and will in turn simulate the rod and piston assembly weights for those mass characteristics necessary for a perfectly balanced engine. Like most V8 engines, the venerable Y-Block will require four of these bobweights, one on each rod journal. Each bobweight will take care of the rotating and reciprocating mass requirements for two connecting rod and piston assemblies along with their respective rod bearing and piston ring packages.

With the weights of the pistons and each end of the connecting rods already recorded on the balance job worksheet, there are still some miscellaneous weights required before calculating what the total weight requirement will be for the bobweights. At this time, the weights of the piston rings and connecting rod bearings for one cylinder are needed. This is a simple matter of weighing these pieces on a gram scale and recording their values on the same work sheet or balance card. Piston pin locks are also weighed and recorded if being required on the engine being balanced.

All parts continue to be weighed in grams due to the increased resolution garnered by this measurement system versus that of using ounces. As a for instance, there are 28.35 grams in an ounce and for a point of reference, a typical dollar bill weighs a gram. Saying a dollar bill weighs a gram is much simpler than saying it weighs 3½% of an ounce or 3/85th’s of an ounce. Thus it is grams as they can then be further broken down as fractions or tenths for additional detail or resolution.

The final value required for the bobweight calculation will be a nominal value in grams for the estimated amount of residual oil that resides at any given time within the crankshaft and on any given pair of piston and rod assemblies. Although industry standard for this oil is 2-4 grams, different shops will add an additional amount based on their experience or preference. Some engine designs will even mandate a much higher value due to its engineering attributes that has the crankshaft or its attached components holding more oil than the standard amount within them. An example would be hollow crankshaft rod journals that hold additional oil either by function or machining ease during the crankshafts manufacture. The Flathead Ford V8 crank would be a good example for simplifying the manufacturing process by using oil reservoirs in the crank pins while the 427 Ford steel crank would have even larger crank pin oil reservoirs designed specifically for stored oil in the event of momentary oil pump starvation. The Ford Y-Block crankshaft design is such that the industry standard could be used but an increase in the oil value may be required to simulate some of the other weight variables that can work their way into the mix.

Adding a specific amount of weight for a given bobweight in excess of what is initially called for would be referred to as heavy balancing or being over-balanced. This is done in instances where anticipated weights or forces will be changing either during the course of an engines life or if the rotating and reciprocating mass characteristics are expected to change at a given rpm range or condition.

If a carbon build-up on the piston top was anticipated over the long haul, then this could be also added to the oil value at this point. If you have a preference for a different oil value to be used on your rotating assembly upon getting it balanced, then talk this over with your shop and get their input on this. Most shops will be agreeable to sutle changes in the bobweight values if you have specific preferences.

There are a variety of other conditions which would require “overbalancing” as part of the balancing process. A change in rod lengths or crankshaft stroke can benefit from a given amount of overbalance depending upon the amount of change in rod/stroke ratio. The use of nitrous oxide, superchargers, or turbo chargers typically also requires a certain amount of overbalance. Using nitro methane in conjunction with a blower is likely the worse case scenario as cylinder pressures are extremely high under detonation which artificially increases the piston weight by a more than a normal amount. Any form of blown engine will benefit from a given amount of overbalance simply due to the weight of the piston averaging artificially heavier not only from the increase in cylinder pressure at ignition, but the increase in cylinder pressure taking place while the cylinder is also filling during the intake stroke. In this instance, the piston is averaging an overall heavier weight when running at speed. A normally aspirated engine has a given amount of pressure counterbalance in that the piston is subjected to negative pressure when the cylinder is filling but is under increased pressure during compression and ignition. If an aspirated engine is working with an extremely well designed induction system and is benefiting from a ramming effect to fill the cylinders at the upper rpm ranges, then overbalancing also helps here. And then there’s the rpm factor. Balancing is linear up to a point throughout the rpm range but depending upon the masses at work within your particular assembly, there is a point in which the crankshaft rpm starts to out run the dynamics of the existing state of balance. Overbalance allows these dynamics to stay in tune or “caught up” to the rpm’s of the crankshaft. There are proprietary formulas that calculate these amounts of overbalance for all the different variables and will vary somewhat from shop to shop. Again, talk with your balance shop regarding overbalancing and determine if this would be best applied to your application.

Now that all the rotating assembly’s component pieces have been weighed, it’s time to calculate the amount each bobweight will weigh before building them and attaching them to the crankshaft. To repeat what was stated in an earlier article, a 90° V8 engine will normally require a bobweight that simulates 100% of the rotating mass and 50% of the reciprocating mass. Because a single bobweight is being used for each V8 journal and represents a pair of connecting rod and piston assemblies, the weight of one piston with its pin, ring set, and a single rod small end will be added to the weight of two connecting rod big end weights along with the weight of two complete rod bearings. This in effect will give the required 50% reciprocating (that which goes up and down) and 100% of the rotating mass. The appropriate amount of oil and desired overbalancing is also added at this point.

With the bobweight calculation now being complete, it’s then just a matter of assembling the bobweights on a grams scale to replicate the calculated weights and then attaching these bobweights to the crankshaft in preparation for spin balancing. The next article in this series will cover exactly this. Special thanks goes to Ernie “Bounty Hunter” Phillips in allowing the use of his balance card for his racing Y as an example. Until then, happy motoring.

Originally published in Y-Block Magazine, Jan-Feb 2005, Vol 12, No. 1,  Issue #66

“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