"Preloading" drive shaft

[fishndvm]

My manual says that when returning the top end of the drive shaft to the powerhead after dropping the lower unit, I should not grease the top of the shaft...only the splines, in order to avoid "preloading" the drive shaft which may cause failure of the powerhead. What does this term mean, and why would it cause damage to the powerhead? Also, the manual says that when removing the lower unit, the manual shift lever should be in the forward gear position, but when reassembling it, everything (shift rod/shift lever) should be in the reverse position. Is this a standard procedure for all manual shift engines? Thanks for your help.

 

[angus63]

Re: "Preloading" drive shaft

Which motor?<br />Preloading means that an assembly has an inherent load on it without applying an external load. In other words, having a large amount of grease compressed in the small space between the crankshaft and the driveshaft will cause an internal force acting on the crankshaft trying to "push" it up towards the flywheel. This load will be absorbed by the crankshaft main bearings and may cause premature wear and possibly enough to damage internal close tolerance parts.<br />As far as the shift rod goes, each model has its own shift rod position that eases reassembly. Post the year and model and someone familiar with that one will be sure to chime in.<br />Good luck

 

[R.Johnson]

Re: "Preloading" drive shaft

I alwayds' wondered where the grease lock " Air lock" between crankshaft, and drive shaft came from. Some engineer saying! " I really don't know", These parts are not that close of a fit. Take a grease gun, and see how close of a fit you can force grease through. To put a pre-load on a hardened steel bearing, or thrust washer with something that you can push together by hand is beyond my understanding. If the grease lock, air lock is that tight, you are not going to push it together. A film of grease, and I don't care how much you smear on, is measured in but .0001"s 0f an inch when in a running condition. Push a crankshaft up, or a pinion gear down, and wear it out, " I don't think so"

 

[DHPMARINE]

Re: "Preloading" drive shaft

angus63 very well stated.<br />this is what the manuals warn about.I've always thought nobody could put that much lube on top of a driveshaft.Since it would be grease between metal to metal it should squeeze out even past the driveshaft o-ring.<br /><br />DHP

 

[R.Johnson]

Re: "Preloading" drive shaft

DHPMARINE: That o-ring is nothing but a dam, it is'nt acting as a seal at all.

 

[lawyertob]

Re: "Preloading" drive shaft

Originally posted by R.Johnson:<br /> ... These parts are not that close of a fit. Take a grease gun, and see how close of a fit you can force grease through. To put a pre-load on a hardened steel bearing, or thrust washer with something that you can push together by hand is beyond my understanding. If the grease lock, air lock is that tight, you are not going to push it together.

R.Johnson,<br /><br />I am glad I am not the only person who laughed when I read that in my manual. I can imagine a fit tight enough to (very) temporarily create some (slight)pressure...but it would have undoubtedly leaked down by the time you get the rest of the motor assembled...and even if it expanded when it heated it would have to leak down in short order. There are mechanical seals and such with surfaces smooth enough to not leak by...but like you said, you sure aren't going to assemble them one into the other by hand. Whoever wrote that the first time must have been an engineer AND a lawyer to cover his butt to that extent. <br /><br />The funny part is that once it gets written once, people write it over and over and take it as the gospel. Kind of like saying that poly resin doesn't stick to wood...but that is a different thread. <br /><br />Just my $0.02 worth,<br />Joe

 

[Silvertip]

Re: "Preloading" drive shaft

I also feel is not an issue unless an air wrench is used in the assembly process. However, I've seen it over an over again where a person thinks if a little grease is good, alot must be better. In this case, its not a long-term hydraulic lock that can cause the problem as in a very short time excess grease would ooze out of the substantial clearance on these parts. But rapid compression at the time of assembly could initiate future damage by momentarily forcing the crankshaft, even though slightly, out of position. Perhaps enough to misalign things such that in the long term, damage can be done. A remote chance, but nevertheless, a possibility.

 

[fishndvm]

Re: "Preloading" drive shaft

Hey Upinsmoke...I'm not an engineer, but let's say that were true...you could then carry the argument as far as saying that when greasing the splines (which seems to be allowed and encouraged), one would have to be careful to place the grease perfectly symmetrically around the shaft lest you run into that same malposition problem. Seems unlikely, if not impossible.

 

[DHPMARINE]

Re: "Preloading" drive shaft

Hi Guys,I've read your posts and reread mine.I've rethought my post.<br />The oring is in a groove on the drive shaft.The shaft engages the crank,which has a beveled area for the oring to press into.While the splines NEED lube,if too much lube is on top of the slines,it wil be trapped there.<br />This results in hydraulic pressure up against the crankshaft,and down against the driveshaft.<br />I've never seen it be a problem,but now I realize why the manuals put this in.<br /><br />DHP

 

[umblecumbuz]

Re: "Preloading" drive shaft

I'm with DHP - in theory.<br /><br />Think - What's the grease for? Both parts revolve at the same speed, and do not move in relation to each other once assembled, so the grease is just for ease of assembly and - possibly - ease of disassembly at some future date.<br /><br />This is a classic case of a smear of grease being better than a dollop.<br /><br />Ciao

 

[lark2004]

Re: "Preloading" drive shaft

mainly for disassembly, The driveshaft can be Very stubborn if not greased properly. Just do a search on stuck driveshafts, you'll get the idea.

 

[angus63]

Re: "Preloading" drive shaft

Let's say you lightly lubricate the assembly. As the drive shaft o-ring engages the crakshaft cavity, you effectively now have a piston and cylinder. Assuming the o-ring and gland recess provide an adaquate seal, the act of inserting the drive shaft will develop an internal pressure increase until full insertion is complete. For arguments sake, let's say 20 psi is developed and for ease of calculation let's also say the ID of the crank cavity is one inch (estimations on my part). 20 psig X pi (3.14) X radius squared (0.25) yields 15.7 lbs of force acting up on the crank assembly. Now the crank assembly force downward due to gravity (let's estimate at 15-20 lbs) is being neutralized to minimize bearing load.<br />Now I am a Coast Guard licensed, Navy commisioned, and federal academy degreed marine engineer, but I am not in any way an outboard designer. However, what I proposed above seems very logical to me. What I believe the warning is referencing is if the cavity were to be filled with a volume of grease that exceeded the volume of the void remaining after assembly, the crank would be theoretically seeing the force of the torque of the bolts when the lower unit is driven home. This assumes a perfect seal under high pressure would be maintained which is very unlikely due to finishes and surface conditions of the mating parts, but still possible.<br />Basically what I'm saying is use a light coating of grease and go catch some fish without a worry.<br />Good luck

 

[R.Johnson]

Re: "Preloading" drive shaft

I certainly was not suggesting that you not grease the drive shaft splines, as not doing so can make removal of the shaft vey difficult. When I have run into a stuck drive shaft, upon removal, the shaft will appear rusted. The first thought is a lack of grease. If you look at one of these shafts closely, you will in some cases see that the splines are forced over, or the groove widened. This can some times be done to the extent that they act as an interupped fit. This can be caused by a high impact, or many times, the result of a sprung gearcase or exhaust housing. If sprung, this sets up a high stress point at the splines createing heat, and friction which flakes the metal away. That is the rust you see. In machine work, this condition is called fretting. You will sometimes see this same thing on the taper of a flywheel, and crankshaft if it has been running loose. I have seen this to the point where it can hardly be lapped away. The Machinery Handbook explains this much better than I.

 

[fishndvm]

Re: "Preloading" drive shaft

I think Angus may have have hit it on the head. In fact, when I reread his initial post, he hit it then. Assuming close tolerances of fit between the top of the drive shaft and the crankshaft, and assuming a minimal route for escape of excess grease at the top of the drive shaft, bolting the lower unit into place would create a possible enormous amount of undue pressure longitudinally on the crankshaft. Think of how much pressure your master brake cylinder can generate.

 

[R.Johnson]

Re: "Preloading" drive shaft

Your master brake cylinder is a small cylinder acting on a large cylinder, Think of a lever here. " time, distance, force". The drive shaft would be 1 to 1., Force in, force out. You are not multiplying anything here, unless you take in consideration the gearcase screws when bolting up. This force would be dissapated because the o-ring is not acting as a perfect seal. Grease does not melt, but at very high temperature. The 0-ring simply holds it in place

 

[umblecumbuz]

Re: "Preloading" drive shaft

Just wipe it over with grease, fella's, and move on!

 

[angus63]

Re: "Preloading" drive shaft

Spoke with a Bombardier service tech while shootin' the breeze down at the local yard at lunch time. Mentioned this topic to him. He believes the oring was added to the assembly not to keep grease in, but keep moisture out. When I brought up the possibility of preloading if the seal was effective, he said " yeah, I guess that could happen!". Goes to show ya! Sometimes fixin' one issue can lead to creatin' another. His advise was just smear it and forget it. I tend to agree.