Transmission to shaft coupling bolts sheared '07 H38

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flightlead404

.
Apr 2, 2021
416
Hunter 38 On the move
Last weekend I pulled up the cabin sole it the aft cabin for routine maintenance and cleaning and found only 2 of the 4 bolts holding the transmission output flange to the prop shaft were still there. I found both bolts, one had sheared the other was undamaged so possibly backed out. Prior to this we had run from Key West to Hilton Head under sail, power, and combination without noticing any issues, including unusual vibration.

1) What transmission is on this boat? (can't find in the Hunter manual)
2) Any ideas what might cause this?
3) What is a suitable replacement bolt allow? 316 ok?
 
seadaddler

seadaddler

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Dec 19, 2006
5,824
Hunter 36 Punta Gorda
Flange bolts do loose some times and need to be check now and than,happen to me once.
 
JamesG161

JamesG161

SBO Weather and Forecasting Forum Jim & John
Feb 14, 2014
7,755
Hunter 430 Waveland, MS
Try to match the same type metal for your new bolts.
Jim...
 
JamesG161

JamesG161

SBO Weather and Forecasting Forum Jim & John
Feb 14, 2014
7,755
Hunter 430 Waveland, MS
Knowing a bit about Yanmar, my guess is Military spec Stainless Steel.

You can change later if corrosion seen.
Jim...
 
F

flightlead404

.
Apr 2, 2021
416
Hunter 38 On the move
heritage said:
Here is a tutorial on types of high strength bolts:

www.tfgusa.com

Using High Strength Bolts for Structural Bolting | The Federal Group USA

Learn about high strength bolts, or structural bolts, and how they are used to connect structural members in a variety of industrial applications.
www.tfgusa.com www.tfgusa.com
.

I have read that stainless steel bolts are typically not as strong as hardened steel ones.
Click to expand
Not a metallurgist, but I've built a few engines (and currently fly, upside down, at low level, in formation behind an engine I built) and know enough about bolts to be dangerous lol. "Strength" is a poorly defined term here I think. Ultimately a bolt with some ductility might be "better" than a harder bolt with possibly higher ultimate tensile strength but more brittle.

I just don't know.

I assume the loads are pulsing shearing loads, but I'm really just guessing.
 
F

flightlead404

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Apr 2, 2021
416
Hunter 38 On the move
Ralph Johnstone said:
Angular shaft misalignment would be a first guess. This is not a high stress area.
What is your angular misalignment and what is your flange diameter. ?
Click to expand
Unsure on alignment issues, have not tried to measure. Looking at the area I'm guessing the bolts are original, the remaining ones look undisturbed.

I recently (Dec, and probably ~100 hrs of motoring ago) had the prop removed, new cutlass bearing, prop "reconditioned" which included some balancing. Engine/drive train was smooth.
 
Ralph Johnstone

Ralph Johnstone

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Jan 4, 2006
7,230
Hunter 310 West Vancouver, B.C.
flightlead404 said:
I assume the loads are pulsing shearing loads, but I'm really just guessing.
Click to expand
Close but no cigar :biggrin:. The loads on the bolts are pulsing tensile loads.

With angular misalignment, one bolt will have only its initial tensile loading at, let's say 12:00 high. As this bolt rotates around 180° to the opposite side (6:00 low) it's tensile load increases due to the angular misalignment trying to pull the flange faces apart on that side. At 1000+ RPM, the pulses soon add up and .................... Houston, we have a failure.
 
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F

flightlead404

.
Apr 2, 2021
416
Hunter 38 On the move
Ralph Johnstone said:
Close but no cigar :biggrin:. The loads on the bolts are pulsing tensile loads.

With angular misalignment, one bolt will have only its initial tensile loading at, let's say 12:00 high. As this bolt rotates around 180° to the opposite side (6:00 low) it's tensile load increases due to the angular misalignment trying to pull the flange faces apart on that side. At 1000+ RPM, the pulses soon add up and .................... Houston, we have a failure.
Click to expand
Interesting. Without assuming misalignment (which I will check) would the load change to more of a shearing load as each cylinder fires and twists the drive shaft?
 
Ralph Johnstone

Ralph Johnstone

.
Jan 4, 2006
7,230
Hunter 310 West Vancouver, B.C.
flightlead404 said:
would the load change to more of a shearing load as each cylinder fires and twists the drive shaft?
Click to expand
If I were a guru with a black cape and a crystal ball, I would say it's unlikely. The engine flywheel keeps things sailing along smoothly and partially stores each piston stroke due to its inertia so little pulsing torque. The flanges are kept from rotating relative to each other due to the large friction force between the contacting flange faces.
 
smokey73

smokey73

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Oct 26, 2010
2,122
Hunter 40.5 Beaufort, SC
It's really hard for me imagine a "shear" of this bolt if others are not also sheared. If this one sheared, I would at least expect the others to have been compromised and all would need to be replaced. One would expect that if enough displacement occurred to shear one bolt it would have sheared all. I would also think that there is some sort of key in the shaft/flange joint to take the shear load.

Are you sure it is a shear failure? With the discussion of shaft misalignment as the cause or a contributing factor it would seem more likely to be a fatigue failure, possibly caused by over-torquing the bolt in the first place when installed, especially if that was the only bolt over torqued. With it over torqued, with misalignment, when the position of rotation is the point when the tensile load was at its max the stress on that over-torqued it could cause "elastic stretching" and when it reached the other point 180 degrees around it would have "relaxed" a little (although still under tension), thus causing a cycling load, in tension, on the bolt which could eventually lead to fatigue failure. Over torquing has been known to cause failure of head bolts on high compression engines where one or more of the head bolts has been over torqued and the cycling of the compression strokes, over time, causes fatigue failure.

Do you have a picture of the broken bolt, including a closeup of the surface at the point of failure (ie clearly showing the break surface)?
 
F

flightlead404

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Apr 2, 2021
416
Hunter 38 On the move
One looks sheared, one looks like it backed out, the other two are in place - did not check torque. I agree it seems odd. I'm replacing all of them, nuts, and washers also with 316.
 

Attachments

Ralph Johnstone

Ralph Johnstone

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Jan 4, 2006
7,230
Hunter 310 West Vancouver, B.C.
You photo shows two well defined "oyster lines" and is a textbook case of cyclic failure due to tensile stress.

1655404587104.png


Oyster lines do not usually show in shear stress as they tend to microscopically grind away at one another and produce smoother fracture faces.

That and $1.50 will get you a cup of coffee in any restaurant. The real question here is "what caused the cyclic tensile stress ?" I got two nickels, a dime, and a ball of pocket lint that say's it's an angular shaft misalignment problem. Only an alignment check will give us a few more "maybes" to work with.
 
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F

flightlead404

.
Apr 2, 2021
416
Hunter 38 On the move
Ralph Johnstone said:
You photo shows two well defined "oyster lines" and is a textbook case of cyclic failure due to tensile stress.

View attachment 206339

Oyster lines do not usually show in shear stress as they tend to microscopically grind away at one another and produce smoother fracture faces.

That and $1.50 will get you a cup of coffee in any restaurant. The real question here is "what caused the cyclic tensile stress ?" I got two nickels, a dime, and a ball of pocket lint that say's it's an angular shaft misalignment problem. Only an alignment check will give us a few more "maybes" to work with.
Click to expand
So possibly over torquing along with misalignment?
 
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