When dry sump oiling is preferred over wet sump systems? 2

[Onemorechance]

I have a splitter bevel gearbox with shaft rotation 6000 rpm and gear pitch circle rotation speeds above 30 m/s, transmitting a power of 400 HP. According to me it would be necessary to use a dry sump oiling system with jet spray lubrication to both the gears and the bearings. With such high rotation speeds the oil in the wet sump would only be churned by the gears and not lubricate and cool the gear teeth adequately, resulting in loss of power and reduction of gear life. Also the oiling of the bearings is not good at high speeds and would result in cage slipping which leads to increased heat and reduced bearing life.
Is all of this right? Are there arguments against this? In this case, if a wet sump oiling system would be used, would this dramatically reduce power and the life of the gearbox? I ask this because some people are telling me that it is cheaper and less complicated to use a wet system over a dry system and that the loss in power and life is insignificant. I don’t have enough experience to judge this. Can some people with more experience give me some arguments for or against so that I can learn more about this and make a better decision?

OneMoreChance.

 

[Tmoose]

The gear efficiency ratings are terribly deceiving especially with splash or re-circulating lube when the revs go up, and with many gear meshes, like in a planetary or epicyclic gearbox. (4) 97% efficient gear meshes in series make an 89% efficient machine.

A few years ago we had separate bouts with 2 large name brand industrial gearboxes that ran at least 5O DEG F hotter than we were "promised." That darned 97% - 98% efficiency figure kept being tossed our way.
By actual measure a 50 HP rated gearbox with 1/4 HP coming out the output shaft used up about 6 HP to tear up its oil. That is NOT 98% efficient. Its more like 5% efficient. All our spindle testing was at operating speed, but not cutting metal. I'm guessing even at full working load it would still lose something like 6 HP to oil pumping/shear, and the efficiency would be more like 85 to 90%.

And, then, when they finally sent THEIR test data, OUR high temps were declared normal. Then we received factory permission to run 4X the recommended oil flow in in a desparate effort to help cool the danged thing down. Both manufacturers were researching "minimal" spray lubrication as a means to quickly chop the temp back down.
That was too late for us.

 

[Fabrico]

It looks like you are using the term “wet sump” to describe splash lubrication. Wet sump typically means a sump with an oil pump drawing from that sump to lubricate things, as in a typical automotive engine. Splash lubrication typically means a sump of oil that all or some of the gears sit in and simply pick up as they rotate, as in a typical stick shift transmission or rear end.

The biggest single advantage of a dry sump system is its ability to avoid oil starvation. The next is keeping oil away from fast moving parts and virtually out of the engine. As far as lubrication, it does little more than a pump-in-the-sump system can do.

Splash lubrication is used in trucks of all sizes, many race cars, motorcycles, etc. with great success. 400 HP is a lot of power, what is the duty cycle? What is the gear case made of? How much air flow is there? What type of bevel gears are they? Does the machine significantly bounce or accelerate? Splash lubrication can be sufficient if other things are up to the task.

 

[Onemorechance]

I don' t have all of the details here, but it is a bevel gearbox with 2 carbon steel straight bevel gears positioned at 90 ° with pitch diameter around 100 mm. 6000 RPM. It is fed by a gas turbine engine (no big vibrations, so its running smoothly without big accelerations) A typical operational mission (duty cycle) is say 3 minutes at 420 HP, then 2 hours at maximum continuous which would be 336 HP and finally again 3 minutes at 420 HP. The life of the box would have to be 3000 hours. The gear case would have to be in aluminum and there is no free air flowing around the box !! It's for an aviation application.

By the way, just use the figures as an indication , I don't want anybody to solve my problem, just some advice from a more experienced person would help.

 

[sreid]

I think that your words "No free air" will be the heart of the problem. If you are running 336 HP with 97% efficiency, there will be a power loss of

336 HP x 0.03 = 10 HP x 746 W/HP = 7,460 Watts

With 100 mm pitch line gears, assume a cubic transmission case 1 ft x 1 ft x 1 ft. Further assume you'd allow a temperature rise of 200 degrees F (say, 270 degrees F).

Then assume free air convection over the bottom, sides and top of the case. A quick calculation shows that only 350 Watts will cause a 200 degree F temperature rise. Case Fins and forced air cooling won't get you to 7,460 Watts. So you'll need a pump and an oil cooler in the aero air stream. Once you have the pump you might as well use jet stream lubrication to increase the gearbox efficiency.

 

[tbuelna]

onemorechance,

most of the high performance aircraft gearboxes I have designed/worked with were dry sumped. That is, they had separate lube scavenge pump(s) and pressure pumps. Keeping the gearcase of high-speed gear train gearbox free of as much oil as possible is very critical for efficiency, because oil churning and windage can produce significant "hydraulic" losses. In fact, windage losses can be several times as high as friction losses. And not only do those losses reduce mechanical efficiency, your lube circuit's heat exchanger capacity must be increased to reject the additional heat load. For aircraft, that also can mean more drag losses.

On modern aircraft, gearbox lube systems can be quite complex. They can include pressure and (multiple) scavenge pumps, filters, bearing/gear mesh lube jets, oil de-aerators (swirl pots), high speed centrifugal air/oil separators, chip detectors, and oil tanks with remote replenishment capability. If the gearbox must also function under various attitudes and sustained +/- g-loads, the oil scavenge system can become extremely complex.

As for your 400 hp gearbox, with a single straight bevel mesh, I would agree with sreid. You can expect a mechanical efficiency of about 97% to 98% per mesh, depending upon the actual gear ratio. For 400 hp input, that means you need to provide about 8 to 12 hp (339 to 509 Btu/min) cooling from your lube system. Your bearings will also require a small amount of cooling.

Finally, be careful where you locate your cooling jets. Cooling jets for your gear mesh should be directed at the exit side of the mesh. Your bearings should be provided plenty of drainage, because any oil trapped within/behind the rolling bearing elements will be churned and will cause rapid overheating of the bearing. The temperature limit of most common gear and bearing materials is about 350degF.

You mention a pitch line velocity of 30 m/sec. That is fairly high, so I would recommend that you specify a high quality level for your gear manufacture/inspection (AGMA 10 or better). And be sure to do a torsional vibration survey of your complete drivetrain. It's a lot of work, but it will likely save you from grief and embarrassment later on.

Good Luck.
Regards,
Terry

 

[Onemorechance]

Thanks very much for your replies, I appreciate it.


OneMoreChance.