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Old 07-05-2007, 11:54   #1
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Hurth / ZF M15A Transmission Failures

Just finished installing the THIRD ZF/Hurth M15A transmission in our Alberg 37 Yawl within about 910 hours (engine time). The PHASOR 37.5 Kubota based engine has worked flawlessly, but for some reason that I cannot figure out is why the transmissions fail. The original trans failed at about 1 year (at about 300 eng hours the trans would not always go into gear without reving up the engine) and Hurth/ZF provided a new replacement under warrantee. On April 29, as we were about 13 miles south of Thunderbolt SC on the ICW the second transmission (with about 600 engine hours on the second transmission - 900 total engine hours) failed again (appears that the forward clutches are worn - reverse worked OK. Arranged to have a NEW trans drop-shipped (for about 2.0 'boat units') and since I'm getting pretty good at swapping out transmissions, we were back in business within a bit over 24 hours. This is an 8 degree "down angle" transmission, which has been religiously(?) maintained, with regular transmission oil (Dexron III ATF) changes. Alignment is always good, and the shift lever throw is well within limits. Anyone out there have similar experiences with this transmission?? Comments appreciated....

Thanks, Tom (holed up Charleston, SC awaiting weather to continue home to the Chesapeake after spending the winter in the Abacos)
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Old 07-05-2007, 13:33   #2
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Did you ask Hurth what they thought was a cause?? It is possible that it was a faulty part, it can happen, but if it is due to wear, then surely they must have a comment on the install or the way it is used.
What happens when you sail?? Is it allowed to free spin???
Although Dex III is a good oil, maybe you should look for a full synthetic. I am having very good personal results with Amzoil,( although I have still yet to use it in my own gear box...will be later this year)
The other major point will be trans cooling. Do you have an oil cooler. Every 10deg cooler you can run a transmission adds approx 1000hrs to it life.
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Old 07-05-2007, 13:52   #3
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All I get from Hurth/ZF (Annapolis office) is they don't have any history of failures.....(surprise/surprise)...

Am considering adding the (optional) oil cooler which simply bolts on the side of the trans using heat transfer of the aluminum case. In the meantime, I am using the engine compartment blower to evacuate hot air from the area of the trans. May consider using synthetic oil - thanks for the response.
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Old 07-05-2007, 15:51   #4
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Do you HAVE to use a Hurth trannie?...with that failure record and particularly that response from the manufacturer I'd be looking for something else......
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Old 08-05-2007, 00:33   #5
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Just about to put one in brand new (the straight output version) would also love to hear of any more comments on the ZF 15 good or bad.....
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Old 08-05-2007, 01:01   #6
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Hurth is a good box. Well known and realiable. I would suspect heat as being your killer. If you have to use the blower to cool the compartment, there must be some substantial heat build up in there. Definitly get the cooler fitted. Synthetic can greatly aid heat reduction. DO NOT use an oil additive.
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Old 08-05-2007, 03:08   #7
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Duty Rating...

I had a chat with a ZF technical rep. a few months ago.

The main point that came from our discussion was that each transmission is rated for various duty cycles.

There is a formula used to calculate the suitability of each transmission for each engine and task that involves multiplying the revs by the rating. (And other things that are beyond my imaginings.)

If you go to the ZF Marine website and look up your transmission specifications you should get some idea of what I mean.

On the web site you should find the following definitions:-
PLEASURE DUTY DEFINITION Highly intermittent operation with very large variations in engine speed and power

LIGHT DUTY DEFINITION Intermittent operation with large variations in engine speed and power

MEDIUM DUTY DEFINITION Intermittent operation with some variations in engine speed and power

CONTINUOUS DUTY DEFINITION Continuous operation with little or no variations in engine speed and power

While they do allow large numbers of hours for the operations of their transmissions your use may fall under the definition of continuous duty.

When I checked for my proposed engine a transmission rated for continuous duty was twice the weight, size and cost of a light duty one.
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Old 20-09-2007, 19:48   #8
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Hi, Does anyone have experience pulling off a rusty hurth transmission? What might be the pitfalls/ problems encountered? Can you swap out types of transmissions without too much fuss, or is it another can of worms? I have read about hurths packing it in on another website (barge boat site), and in that posting it was stated that they were not One post I read said that it is 4 bolts and not too much fuss, but what about if things are mighty corroded? Is this something that a reasonably handy fella could deal to? I have pulled parts off engines and replaced them before, but for some reason this seems more scary to deal with.

Thanks
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Old 20-09-2007, 23:06   #9
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Corrosion of bolts is always a possible big problem. And you will always find that the bolt that is the hardest to move, is the last one you try. And usually, the last one you try is the one in the most difficult to get to position.
If you have a very tight bolt, give it a good solid crack on the head with a hammer. This helps loosen the frozen ones. You can use WD40 or CRC or what have you. It can help if you can leave it on over night. If you need to use heat, try using break fluid as the lubricant. It does not run away from heat like the petroleum based products do.
Removing the box should be straight forward. Not much holding them in there. As for offerring up another make of box, it is fairly straight forward, providing you have the adaptor plate to mate the box to the engine. And ensure you have the correct ratio of course. The main issue is ensuring that the output shaft is the same height and angle on the new box as was the old. Otherwise you will have to realign the engine to suit the box and that maybe a major.
Do be a ware that the rust on the outside may not be what the inside is like. They can look worse than they really are.

So what's the project Brian?? I have seen a couple of boxes on Tradme. Worth looking to see if any would suit.
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Old 20-09-2007, 23:38   #10
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First thing to note is the colour of the oil, black , smelly oil = it has been run hot. I know of a mate who motorsailed using minimal revs and ended up with overheated plates and burnt / black oil. He did an oil change using synthetic and hasn't had a problem since, No more motorsailing though. Reccomend the oil cooler it will definitely help.
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Old 21-09-2007, 00:42   #11
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Quote:
Originally Posted by tomj View Post
This is an 8 degree "down angle" transmission, which has been religiously(?) maintained, with regular transmission oil (Dexron III ATF) changes. Alignment is always good, and the shift lever throw is well within limits. Comments appreciated....
What motor are you running??? What prop are you running? The prop could be too big/heavy or over pitched.

And what failed in the trany's, gears, clutches or bearings??? If the gears/clutches then it would be too big of prop. or over pitched. Too hot would take out the bearings and seals especially at higher RPM's.........._/)
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Old 21-09-2007, 00:45   #12
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I'll footstomp Alan's recommendation - penetrating oil needs time to penetrate. Soak it good - do whack the heads with a calibrated hammer - wait 4 hours - repeat - wait for hours repeat...

Us excellent quality sockets, get a straight purchase on the bolt heads, use an appropriate length extenstion bar. The right equipment really helps. No room for adjustables spanners and vice grips in these situations.
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Old 21-09-2007, 02:08   #13
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...most of the gearbox cases are alluminium....DONT over tighten, unless you want to start re tapping the threads...
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Old 21-09-2007, 03:13   #14
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It’s usually better to “break” (free) frozen or damaged fasteners with a 6-point hexagonal socket or box wrench, rather than the more convenient 12-point.
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Old 24-09-2007, 13:00   #15
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"If you need to use heat, try using break fluid as the lubricant."

Use extreme caution using Brake Fluid - it is highly flammable. Noone told me that - I found it out myself when I tried to braze a broken brake line shut..."OK" I yelled - "Hit the brakes and I'll watch to see if it still leaks".
Having the Oxy/Acetylene torch still lit probably wasn't the best thing I could have done, 'cause when the brake fluid shot out of the not quite sealed line, it looked like a flame thrower - about a 10' long flame!

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