Do I Have Enough Tension?
Posted by Safety Officer, 18 September 2011 · 145 views
Do I Have Enough Tension?
(Tension in this article doesn’t apply to work or home )Let’s talk about the tensionon your muffler exhaust springs. Too little?---Too much? Although there several types of exhaust and muffler setups out there, I’ll limit this article to the most common and that is where the exhaust spring holds the exhaust male outlet pipe into the female socket on top of the Rotax stock muffler. If you don’t have a stock Rotax muffler, but do use springs, then this article will still have significance for you.
What happens if there is not enough tension on our exhaust springs? The muffler will have excessive vibration and pulsation from the exhaust and cause the exhaust tube to hammer in its female socket. This will cause cracks, broken out pieces and the chaffing will eat away the edges of the exhaust pipe and socket and too little tension will cause hot exhaust gas leaks that might impinge on your hoses or wiring.
I just performed maintenance on a 912ULS with 375 hours where the female socket on top of the muffler was destroyed from too little spring tension which allowed the exhaust pulsations to hammer on the socket. Since we are talking about this exhaust socket, or joint, I should bring up that this needs periodic lubrication. Use copper anti seize on this joint to help prevent early failure.This is the same anti seize used in the gearbox assembly. (You may have even seen what you thought was brass metal particles in the oil the first time the engine oil was changed. It was, most likely, just this copper anti seize.) When there is too little spring tension the spring hooks will vibrate excessively and wear at the spring hook where it comes in contact with the loop welded on the exhaust tube and muffler. (See attached pictures)
Conversely, when there is too much tension; although usually less of a problem because you have to be Godzilla to put them on, it can cause the spring to break and wear excessively at the hooks. Breaking will be the most common problem with too much tension. Either too little, or too much tension can cause the metal loops that the exhaust spring attaches to wear out prematurely. These can be replaced by cutting them off and welding on another metal spring attachment loop.
So, let’s examine the solutions There are a few different types of springs available not to mention all the ones owners may buy at the local hardware store. I would recommend Rotax approved springs. So it may be hard toget the perfect tension addressed in this article so again, I will limit this discussion to just two types. (Pictures below are the two most common.) One isthe Rotax spring part number, 938-795. The other one commonly seen is astainless steel spring and is much heavier in construction. It isn’tnecessarily better, just different and the correct tension will prevent premature wear. The wrong tension can make any spring wear out or break in as little as 200 hours. If you use the heavy stainless steel spring an effective quick evaluation for the best tension is obtained when you can see a little daylight between the coils or you have enough room to insert your fingernail betweencoils easily. If you can’t see any space,then the spring may not have enough tension. At least you will know that there is some tension on the spring if you see a little daylight and it won’t be over stretched. The Rotax springs will have more daylight between coils, around 1/8”– 3/16”. The loops welded on the exhaust tube and on top of the muffler can be adjusted by bending to help obtain your correct spring tension.
Get back in your seats; we aren’t finished yet. We need to apply some high temperature RTV silicone. This helps prevent vibration from wearing on the spring and helps give it a little extra support. There are two ways to apply the RTV silicone. The least common and least desirable way is to fill the spring’s interior with the silicone. I don’t particularly like this because you need to safety wire the spring and remove it at times and then it becomes a problem trying to get all the RTV out.The best, and most common way, is to lay a bead of the silicone from top to bottom on the outside of the spring. Here is where it was important to see a little daylight between coils because you want to work the silicone in between the coils as you apply your bead down the side. The bead should be at least 3/8”-1/2 inch wide and 3/16” – 1/4 inches tall. This gives the silicone some body and strength. Applying a pencil thin bead, or a line down a fully collapsed shut spring is close to worthless. One last thing you should do is safety wire those springs. Too many have gone through props on pushers. You can run the wire down through the center from welded loop to loop, or through the spring and back around the outside, but still through the welded loops. Do not make the safety wire tight. Leave a little play in it so it can move. It isn’t there to hold the exhaust together, but only keep the spring in place in case it comes off or breaks. If you make the wire tight the vibration will just wear through the wire quickly. My personal choice is to use .041 wire and not .032.
I have attached a couple of pictures to show what excessive wear will do to a heavy, stainless steel spring hook and a muffler socket. The red RTV silicone was applied well, but isn’t between the coils.There is a picture of the Rotax spring connected to the exhaust with the RTV silicone and safety wire already applied.