AceBird
Elite Veteran
Location: Utica, NY USA
| Hi all,
I am new to helis. So new, I haven’t even flown one yet but my first choice is the Predator gasser! That being said, there are probably a lot of you that don’t want to hear what I am going to say about mating the fan to the engine shaft. James, please don’t take offense at this.
First of all the purpose of a tapered hub connection is to force concentricity of the two parts and remove any play between a shaft and it’s mating hub regardless of what that hub is used for. That means that you cannot influence the “machined in” concentricity of the mating parts with a hammer blow. There can be only two reasons why a tapered hub connection will yield an off center TIR (total indicator run out). The first is machining tolerance of the mating pieces. Unfortunately, in this case one part is made by Century and the other is made by Zenoah and it is unlikely that either will admit that their parts are out of tolerance. So if this is the case, you are pretty much burned.
So that brings us to the second reason why you could get out of spec TIR, that being assembly technique. What I recommend with any taper loc hub for any purpose is to clean both parts immaculately. This removes dirt and in some cases left over machining chips that will kick the hub off center. Next add what they call “never seize” which is a grease with either a copper, carbon, or silicone additive to the shaft or hub (super lube will also work).
Now you are going to ring the two together. This is called lapping, And what is does is remove all the high spots from the shaft or hub. In this case you can engage the pull starter and ring the fan hub on in the opposite direction. Start with light pressure and increase as you keep twisting in the same direction. Now remove the hub and clean both again immaculately. You should see a burnishing shine on both the shaft and the hub. If both tapers were exactly the same you would see the shine for the whole length of the taper and that means you got a beauty. The likely hood is it will only go half way. It doesn’t take much for this to happen. If you don’t put the hub on tight enough it is possible to cock the hub because of the unmatched tapers with a hammer blow. However, as you continue tightening it will go back to where it was.
So you are now ready to assemble for real. Use the same never seize as before (it makes it easier to remove the hub) and again ring the hub (fan) on to the shaft until you have evenly coated the surfaces with grease or oil. All that is left is tightening the screw and it must be tight. I think Century wants you to put locktite on the threads. I’ll just say this. If the screw comes loose on a taper lock draw bolt it wasn’t tight enough to begin with. On most taper lock hubs there are multiple screws and in those cases it is extremely important to tighten the screws evenly like the head bolts on a manifold, or the hub could cock and then the screws will come loose in time.
The second issue I have with James’s technique is the jack bolts. Although it will tend to remove the fan, Zenoah advises against this technique. The reason is it puts a concentrated load on the crank connections which are press fit! Jacking the four bolts unevenly can bend the crank and very few people will take the initiative to jack two of the opposing bolts at the same time to prevent this from happening. For obvious reasons, a wheel puller is recommended but in a pinch you can use a impact tool similar to a dent puller that will thread into the draw screw hole to tap on the end of the crank shaft while supporting the fan hub all around. Some people would just pound on the head of the loosened screw with a brass drift. It does the same thing, but if you are over zealous with either method you will destroy the fan.
James, please accept my apologies if I have offended you. Your build thread was excellent except for the two issues I have just stated. I wished I had seen earlier that RunRyder had a separate topic for Century. The other forum I browse is not to active on the Century topic.
For all those that are thinking of building a Predator Gasser, don’t hesitate, it is an awesome machine. Someday I hope to be able to fly it.
Ace |
