Prop shaft slipped off transmission when in fwd gear

[LifesBetterWhenYoureBeating]

Hi folks,

A few days ago my prop shaft slipped off of the transmission coupling while I was motor sailing. The wind had picked up so I throttled down and immediately heard a loud bang. The set screws were still tied together with the locking wire, but after measuring the length of screws and amount of thread exposed, I'm pretty confident they were either not fully tightened or the shaft dimples were not properly aligned with the holes in the coupling. The marks around the dimples suggest the latter to me

I've now put it back in place and made sure the dimples line up with the coupling holes. Indeed now the screws have less thread exposed

However, the dimples have a conical shape (see photo) and the screws have a flat end, so only around 2mm of thread goes into the shaft. Is that good enough or should I get set screws with different ends?

Also, I'm wondering how the shaft slipped off when I was in forward gear? Could it be just the force incurred against the prop by the boat moving at around 4kts was much greater than that of the prop spinning at low revs? How come it didn't slip off a day before when I drove hard in reverse to set my anchor?

Any other ideas on how to make sure I fix this properly would be highly appreciated!

[SailingHarmonie]

In my experience, proper set screws have a tapered end, and a dimple in the flat spot at the end of the taper so they can “bite”. They look different than machine screws, because they do different things.

[LifesBetterWhenYoureBeating]

Originally Posted by SailingHarmonie

In my experience, proper set screws have a tapered end, and a dimple in the flat spot at the end of the taper so they can “bite”. They look different than machine screws, because they do different things.

Like this?

[markxengineerin]

How is the fit between the hub and the shaft? Ideally it is a very light press, on the order of ten thousandths of inch to .001", and requires heat to remove/install. If it is loose as-is, I would scrap that flange and buy a split style one from Buck Algonquin or similar that can be positively clamped on. Make sure the shaft outside diameter is in good shape before you install- rotate it by hand and take off any high spots with a file/sandpaper. Measure shaft diameter with a good caliper or micrometer before placing your order. This is just me, but I am not a fan of using set screws to take any significant torque such as what comes out of an engine. Same with using a keyway- much better to have a press or clamp fit to prevent fretting over time and early failure. The split type are nice because installation/removal using heat, required on a non-split type, is a risky business with little room for error ie very easy to get stuck halfway on/half cocked and damage trying to remove.

[Scubaseas]

Originally Posted by markxengineerin

How is the fit between the hub and the shaft? Ideally it is a very light press, on the order of ten thousandths of inch to .001", and requires heat to remove/install. If it is loose as-is, I would scrap that flange and buy a split style one from Buck Algonquin or similar that can be positively clamped on. Make sure the shaft outside diameter is in good shape before you install- rotate it by hand and take off any high spots with a file/sandpaper. Measure shaft diameter with a good caliper or micrometer before placing your order. This is just me, but I am not a fan of using set screws to take any significant torque such as what comes out of an engine. Same with using a keyway- much better to have a press or clamp fit to prevent fretting over time and early failure. The split type are nice because installation/removal using heat, required on a non-split type, is a risky business with little room for error ie very easy to get stuck halfway on/half cocked and damage trying to remove.

Good solid advice. The dimple in your shaft is what is known in the trades as "fuffnick" as in not what you call good. The shaft itself looks a bit beat up. A split collar would be a very good idea for what the picture shows.

[LifesBetterWhenYoureBeating]

Originally Posted by markxengineerin

How is the fit between the hub and the shaft? Ideally it is a very light press, on the order of ten thousandths of inch to .001", and requires heat to remove/install. If it is loose as-is, I would scrap that flange and buy a split style one from Buck Algonquin or similar that can be positively clamped on. Make sure the shaft outside diameter is in good shape before you install- rotate it by hand and take off any high spots with a file/sandpaper. Measure shaft diameter with a good caliper or micrometer before placing your order. This is just me, but I am not a fan of using set screws to take any significant torque such as what comes out of an engine. Same with using a keyway- much better to have a press or clamp fit to prevent fretting over time and early failure. The split type are nice because installation/removal using heat, required on a non-split type, is a risky business with little room for error ie very easy to get stuck halfway on/half cocked and damage trying to remove.

Mine is definitely not a tight fit, but in my naive/inexperienced mind I was assuming the torque would be taken by the key. However I've just noticed the key is not a perfect fit in the keyway either and there's a bit of play when the screws are not tightened so I guess the screws are taking the torque? If that's the case it probably won't take long for the ends of those screws to get worn and let the shaft slip off again?

[SailingHarmonie]

Originally Posted by LifesBetterWhenYoureBeating

Like this? Attachment 271805

Bingo! Like those!

[Scubaseas]

Originally Posted by LifesBetterWhenYoureBeating

Mine is definitely not a tight fit, but in my naive/inexperienced mind I was assuming the torque would be taken by the key. However I've just noticed the key is not a perfect fit in the keyway either and there's a bit of play when the screws are not tightened so I guess the screws are taking the torque? If that's the case it probably won't take long for the ends of those screws to get worn and let the shaft slip off again?

You need a tight fit regardless of having set screws or keyways. Loose anything will make it come loose again.

https://www.generalpropeller.com/inb...plit-couplings

https://www.generalpropeller.com/inb...plit-Couplings

[LifesBetterWhenYoureBeating]

Originally Posted by Scubaseas

You need a tight fit regardless of having set screws or keyways. Loose anything will make it come loose again.



https://www.generalpropeller.com/inb...plit-couplings



https://www.generalpropeller.com/inb...plit-Couplings

Makes sense, I'm adding that to the list of things I'll address in the next haul out, but I wonder if there's anything I can do now to make it last 3 more months? I've just read about this loctite for cylindrical fitting parts...

[Scubaseas]

Loctite will help but it'll be tough removing it if your using the red bearing mount stuff. You need to heat it to about 600*F.
Blue or green bearing mount is a lot easier to remove but don't hold as well.

[dcondit]

Originally Posted by LifesBetterWhenYoureBeating

Hi folks,
wondering how the shaft slipped off when I was in forward gear Attachment 271803

Had that happen on a delivery once and I attributed it to the slow speed and rolling motion that likely caused a momentary lack of shaft pressure. My problem was a worn/broken key. The whole thing was wearing (set screws and key) because the collar had too much play. Since I was becalmed 1000 miles off shore and needed to motor, I managed to make a temporary key by using the house batteries to weld some fork tines together. Once in a marina I replaced the collar, key and screws. The real issue was the collar -- the other issues were symptoms.

[LifesBetterWhenYoureBeating]

Originally Posted by dcondit

Had that happen on a delivery once and I attributed it to the slow speed and rolling motion that likely caused a momentary lack of shaft pressure. My problem was a worn/broken key. The whole thing was wearing (set screws and key) because the collar had too much play. Since I was becalmed 1000 miles off shore and needed to motor, I managed to make a temporary key by using the house batteries to weld some fork tines together. Once in a marina I replaced the collar, key and screws. The real issue was the collar -- the other issues were symptoms.

What is too much play in this case? I can push the shaft into the collar manually but I don't see significant play between them

[dcondit]

I'm not a mechanic (although I play one on the open ocean), but with the correct set screws/torque, proper key and correct/unworn coupling, I wouldn't expect any play at all. It should feel solid.

[motion30]

The shafts on my boat use a split coupling, A key way, grubs screws And an a press fit pin that goes completely through the shaft and engages the coupler on both sides

[Scubaseas]

Originally Posted by LifesBetterWhenYoureBeating

What is too much play in this case? I can push the shaft into the collar manually but I don't see significant play between them

If you can put the shaft into the coupler by hand without heating the coupler or judicious use of a hammer it's probably too loose. It should have pretty close to zero clearance. Looking at the shaft in the picture it looks a bit beat up. You could fit the shaft back in with Loctite bearing mount and a new grub screw but it will likely come loose again.

Look up transition fit and press fit. You need a good micrometer and or vernier caliper to measure both the shaft and hole in the coupler properly though.