UPDATE: Twin Disc Trans Low Oil Pressure

[Bill Root]

The diesel mechanic (Ken Ruse) has been at the boat to determine cause of starboard trans low/erratic oil pressure and slipping in/out of gear at low RPM. Transmissions are TD 506's attached to CAT 3126/300's installed as OEM distributor re-builds four years ago. Engine hours are less than 200.

Ken checked the oil, regulator valve, filters, etc and all were OK. What he found is that the oil pump drive gear (driven off the reverse gear) is rotating freely on the shaft. The pump drive gear is pressed onto the shaft. There is no mechanical fastening system. It can not be repaired in the boat.

We have pulled the trans out of the boat (awful job) and sent it to his shop.
I appear to have three options to repair the unit (in additon, of course,to labor charge for removing and re-installing):

1. Provided the shaft and pump drive gear are OK, re-press the gear onto the shaft, re-mount in the transmission and test for alignment, then tack weld the gear in a couple of spots to preclude recurrance. Cost is about $200.

2. Buy a new gear assembly from Twin Disc. Cost is about $1200. Yes, that's twelve hundred dollars!, with no assurance it wouldn't do the same thing.

3. Buy a used gear from twin disc. They claim it would be thoroughly tested before they sold it to me. Cost is $700.

The mechanic is recommending option 1, claims to have done it successfully several times. Does anyone out there have experience with this? I will be cruising the boat extensively, so want to do it right, but based on this experience there's no guarantee a new gear would be the best choice.

Bill Root
43' DCMY

 

[yachtsmanbill]

A good welder should be able to accomplish this pretty easily. First off, you need to run the gear and shaft in V blocks to make sure its straight. If its good, pre-heat to around 400F and weld. POST heat for a gradual cool down and check for runout again when cool.
When pre-heating it DO NOT LET IT TURN BLUE. A lot of this stuff is nitrided / case hardened and that will burn it off. I'm really surprised TD let a critical part be just pressed on. ws

 

[Brian Degulis]

If I understand you corectly I would not weld on it there is no way to insure the heat of welding will not soften and distort. Why not pin it is there a shoulder to work with? If not could a small hardened pin be put in between the teeth driven in to a tight fit then peened or punched on each end to insure it stays in? Maybe instead of leaving up to the mechanic could you bring the part to a competant machine shop?

Brian

 

[yachtsmanbill]

I agree Brian... Theres several ways to do it proppa. The shaft could be knurled and the gear pressed back on. It should really be pressed and keyed.
To pin it, it really needs to be drilled on a vertical mill with a cutter, not a drill bit, which would be tough to keep from walking, especially on a hardened shaft.
I rebuilt a T10 4 speed that had a loose bearing race in the case and merely center puched the case in about 20 spots and tapped the race back in, with some bearing mount locktite of course.
I would really only weld it if it broke in Papua N.G. as a get home fix. ws

 

[Boatsb]

Charles Scakett machine chop in Orlando could probably do what ever is needed to fit the pieces together right. They do work on radial aircraft engined for air boats and custom machining and fabrication. Great shop and fair prices.

 

[LENZ]

Can you have a new shaft made and have the diameter where the gear is pressed onto a couple of tenths bigger then it is now? or does it have gears machined on to it? what about loc-tight (red) If it's just a shaft with a couple of brg races draw it up and send it to me and I'll whip one up for you in my machine shop for you. I hate to see anybody pay a premium for a part to a boat just because it goes on a boat especially if they know it's for a HATTERAS!!

Thanks Lenz

PM me if you need a shaft made, It would take a day or so.

 

[Trojan]

How you repair it depends on the rpm and the amount of torque that is on the shaft. It can't be to much in as much it was just a press fit. I think I would Lock Tite it and drill and ream a small hole on the mating line. Then press fit a harden pin in the hole. Lock Tite makes a special Lock Tite just for loose gear fits to shaft. I don't think I would weld it. The gear is most likely case hardened and you could mess it up structurally. Any machine shop could do this.
BILL

 

[krush]

Sounds like somebody was off on the tolerances of the interference-fit!

Pumps take a bit of HP to drive and the torque on the shaft may be greater than you think it is.

If the part could be accessed and replaced easily, I'd say weld it up and keep a spare.

But since you have to pull the tranny to swap out the part, I would consider hitting up a machine shop to either A) make a new shaft that is larger so press fit is better or B) do some type of splining.

You could really hi-tech and friction weld it LOL!

The concern with welding is like bill said, distortion. Additionally, you may create some yucky stress concentrations. If you weld, you may need to do post process heat treating too.

 

[Bill Root]

Guys:

This is all great feedback. Thanks very much.

Actually the machanic wasn't going to do the work himself. He has a marine transmission shop in Deland that he works with. They would do the work, although I may take Lenz up on his offer. My biggest concern is that if TD pressed the gear on, knowing it could come loose, they must have had a reason. Was it intentionally designed to spin on shaft if the load was too great? I would hate to create a problem by welding or keying that could grind up the entire transmission.

I'll let you know what we decide to do and how it works.

Bill

 

[LENZ]

Guys:

This is all great feedback. Thanks very much.

Actually the machanic wasn't going to do the work himself. He has a marine transmission shop in Deland that he works with. They would do the work, although I may take Lenz up on his offer. My biggest concern is that if TD pressed the gear on, knowing it could come loose, they must have had a reason. Was it intentionally designed to spin on shaft if the load was too great? I would hate to create a problem by welding or keying that could grind up the entire transmission.

I'll let you know what we decide to do and how it works.

Bill

Keep my posted Bill, Also, you might be able to have the area where the gear rides chromed and then re-ground back to the proper dia. Industrial chroming in pretty reasonable.

Lenz

 

[Bill Root]

After considering all the pros and cons of the various recommendations for repairing the reverse drive gear that drives the oil pump, the mechanic and I decided that we were not comfortable with welding or keying the gear due to the inherent risks to the integrity of the metal associated with heating or machining. So, after checking the gear shaft for run-out, we re-pressed the gear onto the shaft and sealed it with commercial grade Loctite. We re-installed the transmission with repaired reverse gear yesterday. The oil pressure quickly came up to the spec'd 300lbs. and stayed there. I'll do a sea trial this weekend.

By the way, the marine transmission shop in Ft lauderdale told me this is not an uncommon problem with the Twin Disc transmission; he had two others in for the same repair. Thanks to all who provided advise.

Bill

 

[krush]

By the way, the marine transmission shop in Ft lauderdale told me this is not an uncommon problem with the Twin Disc transmission; he had two others in for the same repair. Thanks to all who provided advise.

Bill

Did they ever have comebacks? Loctite isn't going to take any type of load on it, but hopefully you won't have the problem again.

 

[Bill Root]

They claim this repair method has never failed. We'll see. I'm certainly no transmission expert, but as I understand it, most of the load is on the clutch, not the gear. Probably why TD thought pressing it on was sufficient.