anyone know about harmonic vibrations

[wishinIwasFishin]

g'day mate's<br /><br />got this 73 80hp merc thunderbolt that broke a bolt that holds the reeds onto the crank and proceeded to eat it (not good)<br /><br />My mech said that this was probly harmonic vibrations <br /><br />My Q is what do you do to stop it or avoid it<br /><br />cheers daryl

 

[Joe Reeves]

Re: anyone know about harmonic vibrations

Wishin..... Mercurys are not my line of expertise but that problem could be associated with any outboard. If it were a harmonic viberation problem, there would be more than one broken bolt/screw.<br /><br />That type problem is usually caused by over torquing a screw, bolt, nut, whatever. That is not actually torquing the item with a torque wrench, but rather simply tightening it to an extreme, possibly with a manual or air driven impact driver setup. The metal stretches slowly, possibly over a lengthly period of time, and when it reaches a crucial point, it breaks/seperates.<br /><br />The only time I have ever experienced a harmonic viberation was with a 1964 40hp Johnson. Nobody, including myself, could trace where it was originating from. It finally came to light when the top of the crankshaft (with the flywheel) tore off at full throttle..... not a fun experience! The harmonic viberation was caused by a very slight crack just under the flywheel taper. I've always wondered if that was caused by over torquing also.

 

[wishinIwasFishin]

Re: anyone know about harmonic vibrations

thanks Joe<br /><br />so there is no way of knowing if it will happen again and is there a rev range that it would be worse<br /><br />cheers daryl

 

[djunk121]

Re: anyone know about harmonic vibrations

Harmonic vibrations are for real. However, if HV was the cause, it would be common across that model.<br /><br />No doubt that an out of balance rig, a crack, etc could be the starting point for a failure.<br /><br />My experience with HV relates to aircraft engines. I flew one years ago that had a RPM range which was a limitation - the pilot was required to move thru that range , as in do not run the engine in the range because HV was known to cause problems. Faster or slower was OK. I only flew that plane a few hours & I complied with the limitation so I will never know if the issue was real, which suits me fine.<br /><br />The use of term HV in your scenario may be valid - I would ask that mechanic if he has seen it before specific to your model engine.

 

[G DANE]

Re: anyone know about harmonic vibrations

Joe <br /><br />I bought a 35 HP 1979 motor once which tore the cranckshaft exactly the same place. Reused the block which was new. Dealer told med he experienced several big twins doing this when they were underpropped og moved to an inflatable or light boat without changing prop. Now I have second to no experience compared to your. Do you know this problem at bigtwins .

 

[DJ]

Re: anyone know about harmonic vibrations

Wishin,<br /><br />The Merc. model that you have has a flex plate between the flywheel hub and the ring gear. If that is cracked, it will set up a vibration and/or a knock.

 

[wishinIwasFishin]

Re: anyone know about harmonic vibrations

thanks djohns19<br /><br />Will check that flex plate and see if there is a crack .can't say i've herd a knock thought.<br />will the crack be evident from on top or will I have to remove it<br /><br />cheers daryl