Seal and UJoint Advice

[DuckHunterJon]

Ok, so my first caveat is this isn't 100% related to my tractor, but I'm going to use tractor parts to build, so I'm hoping I can ask this here. I'm building a long tail mud motor for my duck boat and I was hoping to get some advice on seals. The arrangement is basically a 6.5 hp Honda motor, coupled to a U Joint, coupled to a 3/4" x 60" driveshaft, coupled directly to a prop.

The driveshaft will sit in a 2" DOM housing, suspended by tapered roller bearings on either end (to take thrust load of the prop). Outboard of the bearings, I plan on using double lip seals to seal out water, in grease/oil/atf (haven't decided on lubricant yet). My question - is there any hope of the seal lasting as the prop will be run 1:1 off the motor (3200 RPM's). I've contemplated having the OD of the shaft ground to improve surface finish, but it of course will add cost. This is really only important at the prop end, as it will reside in the water - but I will also have a seal at the motor/ujoint end to keep lubricant in the bearings.

Second question, any reccomendations on U-Joints? The angle will be 12 - 14 degrees from the driveshaft to the motor, and again, WOT is 3200 rpm's (possibly a bit more if everything works well, and I can turn down the governor a bit). I know I need to go with a needle bearing unit, but beyond that - can a standard PTO shaft UJoint from TSC hold up to that RPM and loading?

Any other info/advice on this project will be welcome. I have all the tools to cut, weld, form, etc - the only thing I plan on farming out will be the turning. On the odd chance there is anyone on here with a lathe in the Western NY area that would be interested in helping out, I'd be happy to hear from them.

Thanks.

 

[ovrszd]

The ujoint is no worries. 3200rpm is barely off idle for a ujoint. 12-14 degree angle isn't too sharp for a single needle bearing ujoint. Over 15 degrees and lifespan drops pretty fast. Your road vehicle with a stick shift in direct drive, not overdrive is turning the driveshaft ujoints at the same rpm as the engine. For example I drive my Jeep back and forth to Colorado/Utah every summer turning 3000rpm in direct drive all day long at 12 degree angle.

As for the seal, I'm concerned about that. I'm sure you can buy an aquatic seal designed to stand the water. I don't think a normal automotive seal will do it. An automotive seal has oil against it giving it lubrication. In your case your seal will be running dry on the inside and water on the outside.

 

[jonyyuma]

A Strut bearing, like on the bottom of a inboard boat. It has to be emersed in water most of it's life.I am sure some of the louisiana persons can shine a lite on supplies. Jy.m

 

[Hooked_on_HP]

Insted of tapered bearings use a sealed roller bearing. We use them on augers with no problems and that is basicly what a prop is.
Bill

 

[brokenot]

There are a couple of threads over on the boat design forum that may help....a couple of the posters have made their own longtails:

Thai Longtail/mudmotor With 6hp Lifan/honda - Boat Design Forums

 

[pat32rf]

Have you checked your prop rotation? Left or right hand? Lefties are harder to find.

 

[Raspy]

Waterless packing systems for boats are available, but the one I've seen that might be good for you was a custom setup with lip seals at each end of a pipe with the shaft going through. The outside end was in the sea and the tube went through the hull. The volume inside the pipe was filled with motor oil from a small tube (attached to a fitting that was threaded into the side of the pipe) going up to a reservoir. The reservoir was a small bottle with a vented cap, above waterline. If any oil leaked out, or water came in, the level would change. This setup had lasted for many thousands of miles at sea. You could have your thrust bearing mounted inside the tube and running in oil. But, do you need a thrust bearing? Could the thrust be sent to the outboard? It would be nice if the shaft was monel or stainless. If it's mild steel, corrosion will be a problem. Zincs may help to keep it alive.

 

[DuckHunterJon]

Wow - great info so far - thanks for the insight. To answer a few of the suggestions - the shaft will be stainless to prevent corrosion. The prop will be left hand rotation - I know they are difficult to find, but it's what all the mud props are - and the design is quite abit different from a normal prop. Has to do with the shape of the OD on the prop allowing it to ride up on obstacles (stumps, rocks) rather than hitting them square.

I want to look at the roller bearing idea a bit more. My fear is that the motor wont be able to take the thrust load, hence why I was going to go with the tapered rollers. As for the oil bath, I've been contemplating doing just that. I have seen some designs hold up well when the shaft is filled with ATF. Since it will only be used during duck season (ie, cold water), I don't think cooling will be an issue.

Thanks again for the info and keep it coming. This is going to be a project over the winter so I will definitely proide pictures and updates as I move along. Thanks again.

Jon

 

[mwb]

Cool, someone else here wants to build one of these! I have been working in AutoCad ironing out these details also. I have two designs in process, a long-tail like yours and a short-tail like a Mud Buddy and others.

I am concerned about the rpm spec for "tractor" u-joints; these are typically rated at 540, not 3200 - 3600. I bet this is due to balancing.

One option you might consider is to put your thust bearing at the top of the shaft and use a bronze or teflon type radial bushing at the base. This would eliminate the need for good seals and eventual bearing failure at the prop end. It would be easier to keep sealed at the top (not submersed).

Here is quick sketch of one of my bearing mounts (not complete) for a short-tail. I plan to use two outboard seals and one inboard of the bearing. The bearing would have a grease zert on top. The seals ride on a raised bushing that has to be pinned or somehow attached to the drive shaft to transfer the thrust load to the bearing - and be removable to service the bearing. This would have to be water tight of course. At the other end the drive shaft is supported by radial ball bearings (belt drive for short shaft).


I have found lots of info on the web and some good pics of homemade longtails; here is one thread that has lots of info Thai Longtail/mudmotor With 6hp Lifan/honda - Boat Design Forums

You building the boat too?

 

[DuckHunterJon]

MWB - I see one issue with your design - I had the same issue and it's been my biggest stumbling block. The tapered roller bearings have to be preloaded somehow to a specific amount - the size I'm looking at (.75 ID, 1.781 OD) needs a preload which will equate to approx 140 deg F free running temperature. The only way I've been able to come up with is to use tapered bearings at both ends and preload them with jam nuts towards each other. Otherwise, you have nothing to push/pull against. I'll try to add a diagram of what I'm planning (it isn't in CAD, but hopefully will convey the driveshaft arangement). Take a look and let me know what your thoughts are.

Also, I have to ask, as I have been going back and forth - have you decided on long vs short tail? I have weighed the pro's and con's of each, and figured they were dead even. I'm going with the long tail as it seems easier to build. But my mind isn't set in stone if you have differing thoughts.

Thanks.

Jon