Transmission cooler- do I really need it?

[DennisDW]

Hi Friends,
I was rebuilding the water pump">raw water pump and succumbed to project creep. By the time I was done I had all hoses off and the exchanger core serviced and etc... Well, you know.

The engine is a Perkins 4-154, 62hp. The gearbox is an Hurth HBW 250-2R. The hose run to plumb the raw water cooling is quite convoluted, needing to go back into the aft bilge and turn and come forward again. Can't really inspect it back there and if fact, it was separated and weeping. 'Glad I did get project creep.

So, do I really, really need to cool that tranny? The cooler is nothing more than a box that bolts on to the side of the case and probably has only 6 square inches of surface area. I believe the 250 series is the first gearbox in the series that has cooling as an option. Aren't there larger engines with non- cooled transmissions?

I'm gonna try it anyway as I can easily measure it's temp with and IR thermometer underway and allowable running temps are published. I'm just interested in the opinion and experience of the community. Lot's of good smarts in CF.

Cheers,
Dennis

[Lepke]

Oil rapidly looses it's lube ability above 200°F. That's why you can't sail with a hydraulic transmission in neutral and the engine off. With just the shaft turning, over time the main bearings heat the oil to a high enough temp to cause wear and maybe failure on internal parts because the oil isn't circulating.
The same trans w/o an oil cooler will heat much faster when providing propulsion and absorbing prop thrust.

[Snowpetrel]

Interested to hear what happens, I'm thinking about adding a cooler to my HBW, as from all i've read they can vastly increase the gearbox lifespan, overheating being the number one HBW gearbox killer.

For what its worth mine runs at about 60 deg outside temp at the moment with no cooler, so the oil is probably much hotter than that, hopefully nowhere near the 130 deg c maximum at the moment...

I get the whole simplicity thing, but at least these coolers are pretty simple.

[TeddyDiver]

If your runs are short enough to hold temp down why not, but how can you know if that's what you will allways do..

[svadagio]

Have a BW velvet drive. Lost the oil cooler in Roatan. Ordered one but needed to move before it arrived. Called BW and ask about running without the cooler. Man said if you can put your hand on the area of the trans where the rear bearing is and not have to pull it away it is ok. I ran the engine about 24 hours with no problem.
Bob

[deblen]

In good weather(no head seas,etc) & not cramming the throttle or towing something,you can get away with a lot of things-maybe.
Hose is relatively cheap compared to replacing a tranny.
Tranny oil coolers are relatively cheap also-even available used on ebay,etc-& are basically simple & trouble free.

There is a reason the tranny designer included provision for an oil cooler.

Sorta like Vegas IMHO

Cheers/ Len

[bauer965]

There's a very technical answer to this. Maybe. Hurth (ZF) will have the answer, and I recommend you give them a call in lieu of using subjective information.
Your gear box is rated for a certain HP input, and that rating in turn in influenced also by its reduction ratio and the input speed. I believe the ZF manuals and / or specs listed online will have those ratings. Now that's step one, and step two I do not believe is specifically called out in manuals, so you will need ZF's assistance. Based on the percentage of the gear HP rating you are providing it (use the HP you are actually producing at your engines max rpm as installed, not what the engine is rated for at some absurdly high rpm that doesn't apply to you (Perkins installations are often installed in derated configuration)) a determination is made as to whether you need a cooler. You should let ZF make that determination.

I have a ZF 25M, which is very similar to yours, and in its installed configuration, is an overkill for the 38hp that I provide it. In fact, I'm giving it about 50% of its rating (in fact it make be much less, I looked this up last year and don't remember exactly).
Because that percentage is so low, I don't need a cooler. And I believe adding a cooler will not provide any performance or maintenance benefit.
At some point, as you start to getting closer to the gear's rating, a cooler will be recommended.
So it truly is simply a matter of how hard you are driving the gear, and that point where a cooler is recommended is a known entity.

As others have pointed out, if you plan to (though don't things boating related never go as planned?) baby the gear by not operating at near full power for long periods, then yes, you could probably go without a cooler if you are in that gray zone where a cooler is recommended but not mandatory. However, I'd stick with being conservative in the event you or the next owner ever have to.

I think you'll be surprised at the answer you get from ZF; my very wild guess is that if you are at 50% or greater of the gear's rating, they'll recommend a cooler.
My gear was installed by a very reputable shop in Seattle by the previous owner, and am confident they did it right. Without the cooler, I have had no issues with the oil getting too hot. I also like that that makes the seawater plumbing a tiny bit simpler.


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[rwidman]

Originally Posted by DennisDW

Hi Friends,
I was rebuilding the raw water pump and succumbed to project creep. By the time I was done I had all hoses off and the exchanger core serviced and etc... Well, you know.

The engine is a Perkins 4-154, 62hp. The gearbox is an Hurth HBW 250-2R. The hose run to plumb the raw water cooling is quite convoluted, needing to go back into the aft bilge and turn and come forward again. Can't really inspect it back there and if fact, it was separated and weeping. 'Glad I did get project creep.

So, do I really, really need to cool that tranny? The cooler is nothing more than a box that bolts on to the side of the case and probably has only 6 square inches of surface area. I believe the 250 series is the first gearbox in the series that has cooling as an option. Aren't there larger engines with non- cooled transmissions?

I'm gonna try it anyway as I can easily measure it's temp with and IR thermometer underway and allowable running temps are published. I'm just interested in the opinion and experience of the community. Lot's of good smarts in CF.

Cheers,
Dennis

Why bother to ask if you're going to do it anyway?

My thought is, if the transmission cooler wasn't necessary, the builder wouldn't have installed one in the first place.

Keep it in place and replace any defective hoses.

[bauer965]

I checked the ZF Service Manual, and for your gear, the maximum continuous HP that it can accept without cooling is 54hp. Sounds like you are good to not use one, assuming you don't continuously operate your engine over 90% power.

Your gear is the same exact model as mine, which is rated between 98-107 hp depending on what rpms you are running.

[DennisDW]

Thanks for all the technical input. I'm sure I could run a long time without it, then again I could run longer with it. Longer both in terms of time underway at high load and in lifespan. If the hose run was straight forward I'm sure I'd just clean it up and put it back in. For now I'll go without it and monitor the temp. I'll also look at preformed hoses to see if one might fit better than the last installation.

Cheers friends,
Dennis

[Dennis.G]

As stated by others, depends on how the transmission was selected against the HP of engine. I have a ZF transmission and do not have the optional cooler, it is not required for the HP being run, and the transmission does not seem to get very hot. I do not plan to add a cooler either.

[Dockhead]

Originally Posted by Dennis.G

As stated by others, depends on how the transmission was selected against the HP of engine. I have a ZF transmission and do not have the optional cooler, it is not required for the HP being run, and the transmission does not seem to get very hot. I do not plan to add a cooler either.

Whether you need a cooler or not is an objective question. If the oil temp doesn't exceed specs without a cooler -- in your installation and usage -- then you don't need it.

You should measure it, and that will give you your answer. I would guess that very few sailboat engines are operated much at maximum power output of the engine, and that even fewer of those are also equipped with gearboxes which are at the limit of their capacity, even at max engine power.

So as a pure guess, I suppose in many cases the oil cooler is not necessary.

But don't guess -- MEASURE.

[Dennis.G]

The ZF uses transmission oil, which has a normal desired operating temperature of 175 F to 200 F. OK to exceed these temperatures for short periods, up to about 275 F.

Although you don't want temperature too high, also note that over cooling the oil below about 150 F running temperature is also not a good thing. It is possible to overcool oil with an oil cooler, depending on transmission load, water cooling flow rate and water temperature.

[Snowpetrel]

I guess the other issue is the amount of airflow over the gearbox. If it's in a big cool space it's going to get a good bit of cooling from the air, but if the engine room is tight and poorly vented, and gets very hot, then the gearbox isn't going to get much cooling at all without the water cooler.

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[Jim Cate]

ZF and Hurth gearboxes do not have a great reputation for longevity. There have been several threads here on CF, nd many more elsewhere relating to their failure rates. The ZF10 in our boat failed shortly after we purchased it, and we did a bunch of research, hoping to avoid the infamous "dollar an hour" depreciation rate often reported for these mechanisms.

Consensus was that one needed to use only Dexron III ATF and to change it every time you change engine oil (200 hrs on our Nanni/Kubota). With that regime, the replacement tranny has lasted nearly 4000 hours and appears to be going strong. My take on your situation is that considering your position on the allowable power curve, if you do the proper ATF changes you should be OK without the cooler.

Good luck,

Jim