We received this from Davis DA-2A builder Carl Luhning in Canada:
A recent comment regarding the a Harmonic Balancer coming loose leads me to wonder if I have installed mine correctly.
I used the called for Loctite for shafts and tightened it to the proper torque using the correct bolt for the purpose. I did not however use Loctite on the threads of the bold. Should I have and if so which one.
Thank you for your reply.
Good to hear from you. Follow through these photos and descriptions, and you can verify that your balancer is correctly installed. I would like to emphasize that having a properly installed balancer come loose is unheard of in the land of Corvairs. GM made 1.7 million Corvairs, and about 600,000 of them left the factory with a balancer installed. Over a lot of years, the original rubber elastomer could rot, and allow the outer inertia ring to slip or wobble. If you read stories from car guys, this is the type of problem they are talking about. This is why we tell people to get a rebuilt or new balancer with modern synthetic elastomer. But the point is that the hub itself, where it fits to the crank, is not a common issue that builders need be overly concerned about, with 2 exceptions that I will bring up further down.
The above photo shows the stock washer and bolt that hold the balancer on a Corvair. The bolt is a grade five 1/2″ -20. The washer is a very special part made by GM. It is made of heat treated steel, and it has a very slight concave surface on the side that touches the balancer. When it is tightened, this washer goes flat, and spring loads the bolt tight. This is why in its stock form on the car, it doesn’t have a lock washer nor Loctite applied to it. The “bevel” washer is doing the locking task. On cars, this system works flawlessly. On aircraft, it works great also. However, it is important to note that some people who choose to use a rear starter assembly do not have this washer incorporated in the system. Our Pietenpol had a rear starter from 1996-2001. It had an aluminum Puck that held the ring gear onto the balancer. This Puck had a pocket on the rear face of it so that I could use the same washer with a longer bolt. In 2009, we built several rear alternator set ups for engines that had our standard front starters. These had a Puck like part that also had the v-belt pulley. On these systems, I made sure that we had a provision to safety wire the mounting bolts in lieu of being able to use this washer. If a builder uses our standard front starter/front alternator, he just used the GM washer and bolt on his balancer, and he will have the same reliability that the cars have experienced.
The above photo shows that the GM washer is nearly 5/16″ thick. It takes this thickness to do the job of only going flat when the full torque is applied to the mounting bolt.
Above is a photo showing where the washer fits in the balancer. The balancer shown is a rebuilt DaleMfg.com unit.
Above is the bolt and washer going into its pocket on the back of the balancer. When I install a balancer I do not use Loctite on the threads of the bolt. I use oil on them. This will not affect the ability of the bolt to stay put, because the special washer is doing the job, not friction on the threads. Rebuilt balancers come with a special tube of Loctite that goes on the inside surface of the balancer, where it would contact the crankshaft itself. This takes up any space that may have developed over years of use. There are also new balancers sold by Clark’s that have a precision fit, but I would still consider using the Loctite because the crank itself may have experienced slight wear having the balancer removed and replaced in the past.
When the balancer is correctly installed on the engine, there is a gap of .150″ or so between the balancer and the pad with the timing marks cast in it.
Above, the part in my hand is the oil slinger that goes on the back of the crank, aft of the brass timing gear. This part is installed before the rear case goes on the engine. It is loosely floating on the rear of the crank until the balancer is installed. When the balancer is mounted all the way, this part is firmly clamped down. After the balancer is installed, this part can be seen by looking down the distributor hole.
Above, the last step in the process is to reach your finger down the distributor hole and touch the oil slinger. If the balancer is down all the way, it will be tightly clamped in place. If the balancer isn’t all the way down, even slightly, the slinger will have a detectable wobble in it.
Above is the other issue about balancers. If you look closely, the cross sectional area between the keyway and the outside seal surface of the balancer is only .125″ (1/8″). On cars, this has never been an issue. However, on aircraft there are two things that can cause a problem with this. First is a prop strike. If your engine experiences a prop strike, this keyway is going to get stressed. The balancer has a fairly high rotating moment of inertia. In laymens’ terms, it wants to keep rotating, and the fit between it and the crank, and the shear value of the woodruff key is all that is stopping it. Experience has shown me that in an overload between the crank, the key and the slot in the balancer, the loser is most likely to be the balancer cracking through the slot.
The second thing that some planes have that cars don’t is rear starters. The rear starter ring gear weighs less than the 5 pound balancer, but it is larger in diameter. It roughly doubles the rotating moment of inertia. In a prop strike, it goes after the keyway savagely. At Corvair College #13 in California, Pat Panzera brought out a rear starter engine that he purchased from a guy online. It happened to come from a builder I had visited in Florida, right after he had a prop strike ground running his Baby Ace without the tail tied down and without the wings on it. When Pat brought the engine to the College to run it, the engine showed very poor starter engagement. At first glance someone thought the crank was broken at the back, but upon a little digging the issue showed that the keyway on the balancer had split to the seal surface, allowing the balancer to become loose and the starter ring gear mounted to it to wobble.
In the above photo I have a hub from a GM balancer that I hit with a 24 ounce ball peen hammer about 5 times. What I would like people to look at it is the surface of the material where it broke. The photo isn’t great, but the material is clearly porous. It is cast material. It you put it on a grinder, it throws yellow-orange sparks, indicating that it is some sort of cast steel. (Cast iron on a grinder throws a more reddish spark.) Material like this works, but it isn’t as strong as if it were a billet part or a forging. GM engineers knew what they were doing and there is nothing wrong with this part for the task it was designed to do. The only issue that comes into play is when this type of cast material in a section only 1/8″ thick is subjected to an excessive load asking the part to absorb a shock load, be part of the starter system, or both. Rear starters work, but when they experience an overload, or if the mounting hardware is not safetied with the stock washer or wire tied, the balancer keyway stands a good chance of being damaged. This is the primary reason why a balancer on a plane would ever come loose. Excluding the issue of prop strikes, there is no issue for a person with a Front Starter to be concerned about.
Rear starters have been used for years, and our Pietenpol flew several hundred hours with one. They work. However, basic observation shows that they work the keyway much harder than a standard front starter installation. We are not just speaking of prop strikes. All the force of starting the engine is transmitted through the crank to balancer fit and the keyway. If the timing isn’t set correctly or the engine kicks back on cranking, there is a tremendous amount of force going through this area. If you have some doubt about how strong a starter is, ask any mechanic if they are powerful enough to bend a connecting rod in an engine that is liquid locked or has a piston stopped by mechanical interference. The three Corvair flight engines that I have seen that have broken a balancer through the keyway have all been rear starters.
This is not an across the board condemnation of rear starters. It is just a frank discussion to remind builders using them that they have things to stay observant of. It is also to remind all builders that when people discuss an issue, it may or may not apply to your aircraft. Today, more than 90% of the flying Corvairs use front starters. These engines have a good track record just like cars because they use the balancer and its hardware just as it is used in the car. Rear starter installations have different hardware, more weight, starter forces and a higher moment of inertia to contend with.