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New technique in Exhaust Spring installation

in General Tech Discussion 7 years ago
I just returned from Aero, where I visited the ROTAX booth.
The display engines had the exhaust pipe/muffler springs installed with safety wire around each spring holding it against the exhaust pipe.
The representative said this is the preferred method but silicone can be used instead of this method if preferred.
It appears that this will reduce the vibration and wear at the junction of the spring and the attachment.
IMG_20180420_125719.jpg (You do not have access to download this file.)
IMG_20180420_125712.jpg (You do not have access to download this file.)

  • Re: New technique in Exhaust Spring installation

    by » 7 years ago

    This is one way and is fully acceptable, but my concern this way is two fold. One it's right up against the hot exhaust pipe, if the spring breaks at least half of it is falling on a taxi way, runway over over some terrain or people.
    I personally prefer the safety wire down through the center of the spring and attached to the spring attachment loops. This way no matter where the spring breaks it stays on the aircraft. I also prefer silicone on the springs and worked into the spring coil openings to help prevent some of the vibration. Vibration and exhaust pulsation cause premature spring hook wear.
    You can see in one picture where this exhaust spring is all stretched out. That was from too much heat. You can see in two others that had old springs that should have been replaced long ago that broke, but they stayed on the plane because of the wire down the center. You can see in another photo how I like the springs to be. Away from the hot exhaust. Air space between spring and pipe is good. No direct transfer of heat. The spring is down the center to keep all spring breakage on board and silicone worked in between thew coils for dampening.
    Don't put silicone down through the center of the spring and fill up. The spring now retains too much heat and could possibly fail sooner than it should.
    FailedExhaustspring1.jpg (You do not have access to download this file.)
    Failedexhaustspring2.jpg (You do not have access to download this file.)
    Stretchedspringfromheat.jpg (You do not have access to download this file.)
    Exhaustspring1-Copy_2018-04-26.jpg (You do not have access to download this file.)

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: New technique in Exhaust Spring installation

    by » 7 years ago

    I must have been doing the safety wiring wrong because I have yet to have both ends captured when the spring breaks. I used the smallest wire, and find that the small diameter wire eventually breaks and lets both ends of the spring fall out before I find that the spring has broken. Also, I find that filling the inside of the spring with silicone renders the spring unable to contract.
    Partially filling the spring or just appling the silicone on one side of the spring still does not keep it from vibrating. I find that there is "witness" marks on the exhaust pipes indicating that the spring vibrates at least 1/4" against the exhaust pipe.
    I only have about 220 hours on my engine and find that the spring attach lugs have significant wear grooves. I have begun to wrap the lugs with safety wire as a sacrificial metal to keep the lugs from additional wear.
    Time will tell if this method is better than the silicone method.
    I have replaced about 6 broken springs since putting the engine in service.
    Depending on where the spring breaks, I suspect that the new method will probably keep the major portion of the spring from falling off. Obviously the unsecured portion from the brake will fall off, but that probably will be the "hook" end.
      
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  • Re: New technique in Exhaust Spring installation

    by » 7 years ago

    I use .041 safety wire. Not the thinner stuff. Plus you don't want the wire tight. That's what causes it to break. Leave it just a tad lose. It isn't there to hold the knuckle together, but just keep the spring from leaving the plane. I see tight wire break all the time. I haven't had any of mine break. If you are wearing springs out in 200 hours then you have a vibration to get rid of or need to put more load on the springs to keep the knuckles from pulsating and wearing the hooks out.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
    Report

  • Re: New technique in Exhaust Spring installation

    by » 6 years ago

    I just completed some engine work and inspected my exhaust springs. Not a single broken spring. It has been about 55 hours since I wired them to the exhaust pipe. I would have had at least 2 or 3 broken springs by now as they were previously installed.
      
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  • Re: New technique in Exhaust Spring installation

    by » 3 years ago

    What type of silicone do you prefer to use on the springs to help dampen vibrations?

      
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