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Thread: Vibration Problems on a Homemade SPecial

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    Default Vibration Problems on a Homemade SPecial

    Spent all winter (and many $$) building this beast and so far itís a total loser. To wit:

    Merc 850 (Big Four) power head mounted on a Mark 55 type tower and LU. Made an adapter that lines up within a 1/16 inch, and my buddy made the 8 spline to 11 spline drive shaft adapter. Though the motor runs beautiful at low speeds, the combination develops a HORRENDOUS vibration starting at 4,000 rpms. Itís smooth as silk below that, and a bit less nasty above that (though way too harsh for parts to survive).

    I thought the driveshaft may be bent, or the drive shaft adapter running out. To see if that's it, I powered the prop shaft with a 3/8 drill (spinning the drive shaft at least 800 rpms) and, no run out whatsoever. So, what/ where/ why/ how am I getting that destructive vibration?

    Iím asking here since many of you have adapted various power heads to racing towers and MAY have experienced something like this.

    Any advice will be heavily appreciated, for the project is dead in the water (so to speak) at this point.

    Jeff
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    Check Alingment. Sometimes when you adapt a powerhead to a different tower with an adapter plate, you might be slightly off center. Same for the gearbox. This will casue a vibration. Also, if you are using a coupler for the driveshaft, it could be off center a little. For example, we use a driveshaft squared to 12MM. That 12MM square end may be off .005 from center in relation to center of the driveshaft diameter. Just a thought for you to check. Another thought is perhaps the crank is not balanced correctly ? Sometimes the factory could put out a crank that was not real balanced. It would work fine for a fishing version engine, but wind it up to race RPM's and she shakes.

    Just food for thought, and it might not even be close to the real issue you are having.

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    Quote Originally Posted by David Mason View Post
    Check Alingment. Sometimes when you adapt a powerhead to a different tower with an adapter plate, you might be slightly off center. Same for the gearbox. This will casue a vibration. Also, if you are using a coupler for the driveshaft, it could be off center a little. For example, we use a driveshaft squared to 12MM. That 12MM square end may be off .005 from center in relation to center of the driveshaft diameter. Just a thought for you to check. Another thought is perhaps the crank is not balanced correctly ? Sometimes the factory could put out a crank that was not real balanced. It would work fine for a fishing version engine, but wind it up to race RPM's and she shakes.

    Just food for thought, and it might not even be close to the real issue you are having.
    I ran a 650 on a 55H unit with no problems. I would build a simple engine stand using the mid section and water tube without the lower unit and drive shaft. Run the engine up and check for vibration. At this point you should be able to isolate the vibration to the engine or the drive train. I'll bet your problem is at the crankshaft to driveshaft connection. Rod

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    Thanks for th replies!

    The alignment is very close--within a 1/16 of an inch. And the drive shaft adapter (below) is very well made.

    Also, I DID run the power head with the LU removed and the vibration was about the same ON THE STAND. The vibration that's killing the project only occurs only in gear and on the boat.

    That your 650 on a mark 55H tower was not a problem is very encouraging, however.

    Jeff
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    Rod:

    Just had a thought: What did your drive shaft set up look like? I assume you used the stock Merc 650 drive shaft, connecting to the 55H lower unit near the top bearing (on the LU), using a square shaft adapter deal? Is that correct?

    Thanks a zillion! There is hope yet.

    Jeff

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    First of all, .062" (1/16th) is a lot of offset from straight. If you are doing this properly you need to make sure you have a machined adaptor that is more like within .020 if you are using splined couplings on the shaft. You can get away with more if the shaft is flexible at both ends (using a coupling near the LU and one near the engine), but if you are using a long stiff shaft you need to maintain close alignment or you are going to have troubles.

    Secondly, if it is vibrating without the LU and drive shaft installed then you have a system vibration problem. That is, the natural frequency of the powerhead and tower combination is within the operating range and that isn't really a shaft or balance issue, although balance would help you reduce the amount of vibration. To fix that you need to understand how the system is moving and stiffen the system or add damping to reduce the vibration. I suggest you look at it with the timing light in a relatively dark space and see if the powerhead is moving and which way it is moving. Is the powerhead you are using heavier than the original Mk55 powerhead? If so you might need to stiffen the tower to get this under control.

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    The Merc 850 power head is MUCH heavier than a Mark 55--possibly twice as heavy--but the tower and adapter section is very ridged (and heavy).

    I was thinking the vibration is torsional (wind/ unwind of the drive shaft). Mercís Big Fours, starting with the 650, use the in-line six's tower and LU. By contrast, the 65/ 70 hp triples got away with the smaller, 44 cube/ 50 hp tower and LU (similar in shaft/ gearing size as my Mark 55 type set up). It's logical, therefore, that the Merc 850ówith its massive rotating bulk compared to a 44 cubeócaused a torsional vibration; and, compared to the 3 cylinder, it has 4 power pulses per revolution.

    However--Rod's 650/ 55H conversion should have experienced the same problem (which it did not). His drive shaft was far more ridged, being extremely short, where mine is a normal length long shaft. Then there's the way it was coupled--I assume he used a shaft from the PH to a coupler right at the LU (far less "floppy" than my set up with the short drive shaft adapter right below the PH). My two, splined connections only 54 inches apart with their "sloppiness", may have allowed the drive shaft to whip at a resonance speed of 4,000 rpms. AArrnnnngh!

    Thanks again for your help!

    Jeff

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    Is that a rubber hubbed prop? If the vibration is only in gear at speed, it could be the prop hub has spun. Just a thought.

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