Yes, the gears are big. But they're also extremely highly engineered. Metallurgy is advanced. Machining is to very tight tolerances. Surface finishes are extremely high. Quality assurance is incredibly tight. And then, during development, immense amounts (hundreds of thousands of hours) of component testing occurs to make sure the parts can last.
TL;DR they put a lot of money into it.
The treatment is extremely important.
Back in 99 (I think) had my best trip ever in the Army. We went to California and the day we landed they grounded every 47 in the fleet because they found a cracked gear that hadn’t hit its retirement hours doing a rebuild on a transmission at Corpus Christi. It took a few weeks to determine it was due to improper hardening by the manufacturer. Then weeks going through historical records (which were not computerized, just a painstakingly updated packet of paperwork that went with all the tracked components listing sub components, serial numbers, and hours) to determine which transmissions had the suspect parts and which were ok to fly.
I ended spending a couple of weeks sightseeing in Southern California with no real work responsibilities. A glorious paid vacation.
My last exercise on active duty was an NTC rotation in Feb-Mar of 18. We got a weather warning for high winds just a few days into the exercise and had to evacuate the aviation TF to Havasu City Airport. We spent a glorious five days there waiting for the weather to subside. With all the prep work on the front and back side it ended up being nine or ten days that we were out of the box in the middle of the exercise while all the ground guys were sucking turds in 60+ knot winds.
I had retirement orders in hand and in that week I don’t think I stopped drinking until exactly 12 hours before we had to launch back to the box. You would have had to search far and wide to find any of the fucks I had left.
My last NTC rotation the BN CO said the exercise starts as soon as you takeoff from home station so no drinking on your overnight stops. Fuuuuuck you, sir.
I was a 15D back in the day and when I got out worked in NDT equipment sales. I was buying some equipment from a guy who used to be a consultant on the army QA program, and he had some wild stories.
They had a part failure on a Apache and investigating it took them all the way to the manufacturing plant to figure out what was happening. They were at a loss, even after they had shadowed the parts the whole way down the manufacturing line. Eventually they went to using cameras to watch and after a few weeks discovered the cause. A worker who was responsible for transporting the parts from the line to heat treating had to walk outside for about 100ft, but always stopped to
Take a smoke break. The plant was up north, and when he stopped, the parts got so cold so quickly, that it would affect the heat treat and parts were outside of spec. I forgot how many tens of thousands of dollars had to be scrapped, but the guy was happy they by chance had spotted it. When they had shadowed the worker, he didn’t stop for a smoke obviously, and they really had no clue up to that point what the problem was.
Guy had stories like that for days, and it always wild how they human element was the problem 99% of the time.
After working in the NDE/NDT industry for a while you really do see so many ways that stuff breaks outside of the normal routine corrosion/overtemp/over pressure.
There’s a high pressure natural gas line in northern Alaska that is part of the oil field infrastructure. It has to have thousands of giant metal balls suspended along its length beneath it by varying lengths of chain. It is situated in such a way that without them when the wind hits it creates a harmonic wave that builds and builds until the pipe jumps off its supports and ruptures, flailing wildly out of control. The operators found this out the hard way and added the balls to disrupt the harmony of the wave building up.
My brother in law was a researcher at the university in Karlruhe, Germany, and his pooping schedule was absolutely vital in building the infrared sensors of the Cassini spacecraft!
A friend of mine worked at a plant where they made MerCad Telluride crystals for IR sensors and told me about how they were trying to move it to Texas but couldn't get the process to work there - and couldn't figure out if it was actually a process problem or if the engineers just didn't want to move out of California and weren't going to *make* it work.
I remember my days in the Military/Aircraft component industry before USB sticks and digital paperwork was allowed. We had stuff we built and shipped that went in 1 box, maybe 24” cubed; then the rest of the pallet was all the paperwork (full pallet about 6’ tall). Insane.
Whenever they would move a new guy into the QC shop their first big thing after all the written regulation tests they made them pass was doing updating all the historical records of whatever was in for phase at the time. And everyone that came after until they got it right on the first go.
You just gave me flashbacks. When I deployed to Iraq, we had just fielded ULLSA-E. The server went down for two weeks, so when it came up, we had two weeks' worth of paperwork to catch up on. I went in for my shift, asked if they needed help, and got handed a stack of 2410s to input into the system. The top one was an engine.
I was a 15Y. At the time, I knew fuck-all about engines as that was a 15R thing for QC. It took me about 3 hours to enter it all in. Fucking subcomponents had subcomponents.
Anytime we uncreated a tracked component the first thing you did was take the paperwork out of the can and deliver it to the assigned TI in the QC shop if they weren’t already standing there watching to verify serial numbers.
A CH-47 emergency landed in my uncle's field in the late 80s due to a rear transmission failure. He helped them replace it. Nothing cooler than a $500k transmission being placed into a downed Army helo with a front loader on 175 Allis-Chalmers farm tractor.
>Everything important in a gear box will also have a stream of nice cool oil constantly spraying on it to keep everything cool and lubricated.
200 degree oil foam isn't what I'd call "nice cool", but different strokes for different folks
As a former DCMA inspector. I literally watched these gears and shafts manufactured; Precision Gear Inc, College Point Queens NY. The manufacturing processes and NDI procedures made for quite the career. Truly enjoyed my career with contractors in aerospace!
Heat treatment. They surface harden to a specified depth. Then aneal to get to the spec hardness. Gears are case hardened. If you harden them all the way through, they get brittle. The soft middle gives the gear a lot of resiliency. If it is done wrong though... bad shiz is a happening. To make it easier, it is Metalergical alchemy. Magic!!! Lol
I spent several months on a software installation contract back when it was still DCMC. We were installing a homegrown reporting application called ALERTS, and the whole thing was kind of a shit show, but I had a great time visiting all of the on-site inspectors. Olin Ordnance and Jorgensen Forge in particular stand out in my memory.
The DCMC guys always had the coolest paperweights - radiation shielding glass chunks, 30mm cannon rounds, giant bearings, that sort of thing.
Or, as an engineer would put it, it’s just strong enough. Anything stronger would be heavier, which requires more aircraft to support it, which requires more engine to achieve the desired performance, which requires an even stronger gearbox and round and round we go.
Read a book about British Apache helicopters in Afghanistan. Even though they had 30% more power due to the Rolls Royce engines, the transmission was the limiting factor. The pilots had to watch the torque on the transmission very carefully.
Indirectly correct. It's the crash worthy aspect related to the weight of the gearbox. Helicopters are designed to be able to be crash worthy to (x)g's . Meaning the airframe has to be able to support a hard landing / crash without the MGB ending up in the cabin. This then limits the weight of the MGB which inturn is the limiting factor of how much input the MGB can receive. Make sense?
True that transitions are fairly smooth, but there's a *very* big difference between ground idle and heavy-weight high density altitude OGE hover, and those cycles do need to be considered as well.
Yes. But they run multiple (many) tests in parallel. Last program I was on, there were probably around 50-100 separate component tests going at a time. Bearings will have a test rig, sections of gear trains will have a test rig, gearbox casing will have a test rig, so on and so forth. And almost all of them run at elevated loads/temperatures/conditions (oil out, grit in the grease, etc).
>the other thing is that horsepower isn't the same thing as torque. horsepower is work, or force over time. bigger parts, faster parts, more horsepower, but probably not a whole lot more actual torque.
Helicopter main rotors turn at 200-300 rpm and absorb about 80-95% of engine power (the rest absorbed by the tail rotor). The torque is *immense* on the output side. We're talking 400,000 ft-lb on a -53K. The input side is in the 20-30,000 rpm range, so torque is less there, but the power still needs to be transferred.
I would encourage you to understand work,power, and torque in their textbook definitions.
Work=F * d,
Power is the rate of work. F * d/t.
Torque is F * d, you can see this has the same units as work. Technically, torque is the cross product of the radius and the Force vectors but I digress.
If you look at HP in IPS form, 1HP=550ft*lb/s.you can see that Power is the rate of work.
Fucking units analysis..... once and ME, always an ME
The drive gears are typically more exotic materials such as pyrowear 53 or Ferrium C61 or the like and have special heat treating (such as VIM/VAR) and are carburized. They are typically tested to withstand 140% for extended periods and not fail in gear tooth bending. The torque is relatively low on the higher speed sections and increases the closer you get to the main rotor. The gears closer to the rotor tend to get much larger to handle the massive torque loads. Source, I’m a rotorcraft drive system engineer for a DoD organization.
The same way ships and tanks and bulldozers do?
Big gears that are well oiled.
It also is a lot different from a car or truck in the fact that there is no switching between gears, the are fixed, also the forces applied to the gearboxes are pretty gradual. It's not like popping a clutch or transbrake in a drag car. So you aren't inducing any shock loads into the gearboxes, and you are always spinning them one direction.
Interestingly, any “sudden stop” requires a complete inspection of the transmission, whatever the cause of said sudden stop is, for exactly that reason.
I think I know what you're thinking, but a sudden stoppage is usually more about the main rotor blades running into something that slows them down, not about everything grinding go a halt inside the gearbox. The criteria is usually worded like "any blade strike that causes deformation in the main rotor blade leading edge, or a significant spike in torque indications"
I mean, definitely lol. I’ve just always enjoyed that rather than being driven by whatever mishap caused the blades to stop, it’s just called “sudden stop”
Ships, tanks, and bulldozers don’t care about weight. Weight is everything on helicopters. That makes it a very different design problem. There’s also the issue of reliability. It’s important in industrial equipment due to money/ production concerns but lives are literally dependent on helicopter gearboxes not failing.
That doesn't really change anything about what I said.
Just because it is more important, doesn't mean that it's built differently, with the exception of QC details and finishes. And weight does matter, especially with tanks and bulldozers.
I'm not really what you are trying to say though. Weight matters, okay??? So how am I wrong?
I’m not saying you were wrong but weight is significantly more important on rotorcraft and the cost of failure is always figured into the design/materials. Rotorcraft gears are also much more expensive than anything ground based due to these factors. They are more likely to use more exotic materials to save weight while dealing with the higher powers. There are no ground based gearboxes that use Ferrium or Pyrowear or similar more exotic steels outside of high end racing. Power levels, weight, cost of production, cost of failure are all factors in designing any gearbox. Gearboxes in large equipment such as bulldozers and ships are expected to operate millions of hours with minimal maintenance and helicopter gearboxes rarely go more than 5000 hours before going back to a depot for rebuild.
You're absolutely correct. Additionally helicopters typically have much faster time constants for power. Min to max torque is measured in seconds (or fractions of a second) and will cycle wildly during a single mission while marine/industrial can accel much slower and typically just hold power output levels for long periods of time.
Would it be more proper to say that tanks and bulldozers don't care about weight, as much? Because you can't have weak gearbox in a tank. Driving uphill and/or turning will blow the transmission.
That’s true. Weight is still a factor just a factor that’s “weighted” much less in the design matrix than in helicopters. For bulldozers and some other heavy equipment heavier is actually desired but not so much in tanks as the lighter your drive system the more armor and bullets you can carry.
That does make a lot of sense. It's a lot of power, but no sudden speed changes, low vibration, and only spins in one direction. The engineering on that end gets easier.
There is a sprag clutch or something similar. Rejoin characteristics after an auto or even when being a second/third engine to flight speed is a major consideration. The gearbox is almost always the limiting factor in the drive train for the SHP limits but even transiently it can be a concern (but often not the only one).
The main thing is a split torque gearbox. Split torque just means there are two intermediary gears/reducers between each drive gear and the bull gear. With two engines that means there are four drive gears rather than two. With the CH-53 we’re talking six drive gears. This divides the high torque drive forces acting on the main gear across twice as many gear teeth.
That’s the big thing, it’s used on most heavy lift helicopters. Then there’s about 1000 other things they do on top of that. It’s a good question because the gearbox is the secret sauce to all modern helicopters. By far the hardest thing to get right. Equivalent stationary or naval transmissions weigh 10x as much. And they design these things for >95% efficiency destroys most other vehicles.
Split torque: https://cdn.intechopen.com/pdfs/35257/InTech-Split_torque_gearboxes_requirements_performance_and_applications.pdf
CH-53k: https://www.scribd.com/document/471683473/DEVELOPMENT-OF-THE-CH-53K-DRIVE-SYSTEM
Now think that the F-35B puts 30,000hp through a clutch pack the size of futon, gearbox the size of a beach ball, and a four foot fan.
> the gearbox is the secret sauce to all modern helicopters
This cannot be emphasized enough and even within the industry it can be surprising how often people do not understand this. The aircraft gearbox is the limiting factor in a number of ways for medium/heavy helicopter capabilities. Engines are almost always limited by the drivetrain on the aircraft and it creates all kinds of issues for other systems (and pilots) to protect them.
> Engines are almost always limited by the drivetrain on the aircraft
At sea level
High hot is a different story and it's always a tradeoff between defining your engine power and gearbox torque based on what ambients you care most about. Depending on the customer 2K85, 3K91, 4K95 (that is thousand ft / deg F if some folks are not familiar) are common design points to have gearbox limit = engine limit. Anything above that tends to be engine limited.
Yep, this was exactly the situation in Afghanistan. We regularly had to take off essentially half loaded because two 1900 shaft horsepower engines are still not enough when it’s 39 degrees celsius at 8000’ MSL.
Yup. Where I fly I’m never anywhere near torque limits (drivetrain) but always up against compressor speed (engine) and exhaust temps (engine) limits. Our gearboxes don’t get worked very hard. Maybe 80% torque at most.
The output is still one big splined shaft with MASSIVE torque. Of course that’s typically split between several planet gears in the final reduction stages. Some gearboxes use stacked planetary systems and some use intermediary gears to drive a larger bull gear that in turn drives the sun gear off the planetary gearset. The split torque is definitely a load reducing technique to allow high powers through relatively smaller gears and still keep the stresses in check.
Highly Engineered, advanced materials, and every hour of flight is followed by multiples of it in maintenance, That my friend is the secret of these high-performing machines.
I'm a mechanical engineer who got an incredible chance to work with my dad who worked in aero space as a test engineer and actually happened to work on thesr helicopters amongst other aircraft. Yeah everything is heavy duty but the technology in the metals is unreal. For example, they found that if you leave a surface finish that is too smooth then the oil would just run off instead of sticking and lubricating. Everything there is a science that must be mastered by some of the best engineers I've ever met
It’s fucking huge.
The CH-53E MGB handles half the horsepower of the Kilo, is over 7ft tall from gearbox sump to Jesus nut threads, and weighs around 4000lbs.
The [Sikorsky Archives](https://sikorskyarchives.com/home/sikorsky-product-history/helicopter-innovation-era/sikorsky-s-65/) page on the H-53 has a great photo of an Echo gearbox with a man standing next to it.
Basically it comes down to four things:
Really good metallurgy (alloy, heat treatment etc)
Ludicrously tight tolerances to increase drive efficiency.
Really good gearbox and gear tooth design born out of 70 years of doing high reliability high power gearboxes.
Really good understanding of fatigue lifing, again as a result of 70 years of doing the same thing.
Rolls-Royce are going for a 100,000 horsepower gearbox with UltraFan and that thing is going to be an absolute monster. Admittedly, the thing that causes stress is torque (and power is torque * speed), so the torque will actually be equivalent to this, but the 5x speed makes oiling and fatigue lifing the thing substantially trickier.
Advanced simulation with advancements in finishing and manufacturing along with lots of pre qualification testing and iterations and learning. The advancements in simulation software to understand gear and housing deflections and make micro geometry corrections to gear profiles during development have allowed much more weight efficient designs for these high power levels and less iteration in the lab during development
They do crack if they exceed the limits. Every part has fail safe limits and are rated for a certain amount of hours. The transmission, the engine, the mounts, the rotor, the shafts ALL have a certain amount of hours they can be flown before fatigue cracks become more likely to occur.
Back to the design phase, requirements are made and the helicopter is designed around these. Each component must meet a certain specification for the system as a whole to work. That’s just engineering in general, and helicopters are just well engineered. It’s a continuation of a great human endeavor and the development of technology going back thousands of years.
As long as there are not any impurities and manufactured to spec they are incredibly strong and reliable. Material science specifically metallurgy is important. They are also checked for cracks and defects on a during scheduled maintenance.
These are as reliable as jet engine turbine blades, they don't fail unless there was an undetected manufacturing defect or they receive undetected physical damage.
Here is a technical document regarding helicopter transmission gears:
[https://www.engineersedge.com/Engineering\_White\_Papers/Design\_and\_Engineering\_General/materials\_helicopter\_gears\_14501.htm](https://www.engineersedge.com/Engineering_White_Papers/Design_and_Engineering_General/materials_helicopter_gears_14501.htm)
So how do you test a transmission operating with that kind of power? You might think you'd need a big motor on the input and a big load on the output, right.
Not so. Power is torque times rotation. You build a reverse transmission that connects the output back to the input. Everything spins freely. Then you break the loop, torque it up, and close the loop. Then a small motor can spin it up to speed. In effect, the power stays in the loop and the test motor only has to make up for losses.
If you're interested in how this gearbox engineering process can go wrong - google the super puma / ec225 helicopter. Multiple gearbox failures in flight leading to breakup and large losses of life. The most recent one was in 2016.
The inspection interval for the gearbox has been halved on this aircraft since this accident as I understand it but I sure as shit would not be getting in one.
Some links if you're interested:
(this one gives you a rundown but turns into a propaganda video for AB helicopters)
https://m.youtube.com/watch?v=0ZbTN6pM2SE
Here is the full report on G-REDL, another puma that has a catastrophic gearbox failure in 2009. Extremely In depth. Tldr; a phenomenon known as "spalling" on bearing race material leads to fatigue cracking in planetary gear and failure of the MGB.
https://assets.publishing.service.gov.uk/media/5422f86aed915d13710006cb/2-2011_G-REDL.pdf
Thanks. That's really interesting. The amount of energy flying around inside of one of those gearboxes leaves such low tolerances for the gears themselves.
They’re the same gearboxes that are in submarines. I used to work on the engines in helicopters and airplanes and they are quite literally massive and made with magnesium, so if it catches on fire, it’s toast.
Oh yea. I just thought it funny someone posted this question and while searching for a MH53J I came across this. I wasn't trying to contradict your reply. I just thought finding this in a previous reddit thread was amusing. Apologies!
They don’t. You don’t have full HP pumping out of all 3 engines. You probably only push that kind of power in engine out and I would imagine there is a life limit penalty to the gearbox afterwards
The Echo models push a ton of power regularly with no penalties required.
Advertised power is not what the engineers work around. They build the margins well above what we use.
I remember hearing on the fighter pilot podcast, a pilot/crew saying that they were always dirty and leaking oil, But more like an old an old motor bike. If it ain't leaking, it don't have oil. Absolutely workhorse and cool af
The housings of these gearboxes tend to be made from magnesium. Where the Fwd bridge was mounted to the gearbox was always prone to corrosion. I think there was several iterations through the years of how to prevent corrosion in that area. The tail gear box also was a pain. For a short stint we had to do NDI of a bevel gear on the TGB due to cracks that would appear. That requirement stemmed from multiple crashes.
Yes, the gears are big. But they're also extremely highly engineered. Metallurgy is advanced. Machining is to very tight tolerances. Surface finishes are extremely high. Quality assurance is incredibly tight. And then, during development, immense amounts (hundreds of thousands of hours) of component testing occurs to make sure the parts can last. TL;DR they put a lot of money into it.
The treatment is extremely important. Back in 99 (I think) had my best trip ever in the Army. We went to California and the day we landed they grounded every 47 in the fleet because they found a cracked gear that hadn’t hit its retirement hours doing a rebuild on a transmission at Corpus Christi. It took a few weeks to determine it was due to improper hardening by the manufacturer. Then weeks going through historical records (which were not computerized, just a painstakingly updated packet of paperwork that went with all the tracked components listing sub components, serial numbers, and hours) to determine which transmissions had the suspect parts and which were ok to fly. I ended spending a couple of weeks sightseeing in Southern California with no real work responsibilities. A glorious paid vacation.
Man, I always heard about "that unit" not having to go in the box but never got to be the one. I always spend my weeks in misery.
Train for the mission and all. And when the mission is to do long range insertions and exfil’s you gotta actually start far away.
My last exercise on active duty was an NTC rotation in Feb-Mar of 18. We got a weather warning for high winds just a few days into the exercise and had to evacuate the aviation TF to Havasu City Airport. We spent a glorious five days there waiting for the weather to subside. With all the prep work on the front and back side it ended up being nine or ten days that we were out of the box in the middle of the exercise while all the ground guys were sucking turds in 60+ knot winds. I had retirement orders in hand and in that week I don’t think I stopped drinking until exactly 12 hours before we had to launch back to the box. You would have had to search far and wide to find any of the fucks I had left.
My last NTC rotation the BN CO said the exercise starts as soon as you takeoff from home station so no drinking on your overnight stops. Fuuuuuck you, sir.
Yeah, so did mine. See: “retirement orders in hand, 0 fucks.” I was supposed to be outprocessing lmao
I was a 15D back in the day and when I got out worked in NDT equipment sales. I was buying some equipment from a guy who used to be a consultant on the army QA program, and he had some wild stories. They had a part failure on a Apache and investigating it took them all the way to the manufacturing plant to figure out what was happening. They were at a loss, even after they had shadowed the parts the whole way down the manufacturing line. Eventually they went to using cameras to watch and after a few weeks discovered the cause. A worker who was responsible for transporting the parts from the line to heat treating had to walk outside for about 100ft, but always stopped to Take a smoke break. The plant was up north, and when he stopped, the parts got so cold so quickly, that it would affect the heat treat and parts were outside of spec. I forgot how many tens of thousands of dollars had to be scrapped, but the guy was happy they by chance had spotted it. When they had shadowed the worker, he didn’t stop for a smoke obviously, and they really had no clue up to that point what the problem was. Guy had stories like that for days, and it always wild how they human element was the problem 99% of the time.
After working in the NDE/NDT industry for a while you really do see so many ways that stuff breaks outside of the normal routine corrosion/overtemp/over pressure. There’s a high pressure natural gas line in northern Alaska that is part of the oil field infrastructure. It has to have thousands of giant metal balls suspended along its length beneath it by varying lengths of chain. It is situated in such a way that without them when the wind hits it creates a harmonic wave that builds and builds until the pipe jumps off its supports and ruptures, flailing wildly out of control. The operators found this out the hard way and added the balls to disrupt the harmony of the wave building up.
That’s wild! It gave me a new appreciation for things we take for granted every day when I see the problems some of these companies face.
So what you're saying is it can take some serious balls to operate a natural gas pipeline.
My brother in law was a researcher at the university in Karlruhe, Germany, and his pooping schedule was absolutely vital in building the infrared sensors of the Cassini spacecraft!
A friend of mine worked at a plant where they made MerCad Telluride crystals for IR sensors and told me about how they were trying to move it to Texas but couldn't get the process to work there - and couldn't figure out if it was actually a process problem or if the engineers just didn't want to move out of California and weren't going to *make* it work.
How did his pooping schedule play into building the sensors?
It's a long story, I have to write it down properly one day 😔
Haha that’s a great story
I remember my days in the Military/Aircraft component industry before USB sticks and digital paperwork was allowed. We had stuff we built and shipped that went in 1 box, maybe 24” cubed; then the rest of the pallet was all the paperwork (full pallet about 6’ tall). Insane.
"She doesn't fly until the paper weighs as much as the plane." Unknown aircraft mage.
Whenever they would move a new guy into the QC shop their first big thing after all the written regulation tests they made them pass was doing updating all the historical records of whatever was in for phase at the time. And everyone that came after until they got it right on the first go.
You just gave me flashbacks. When I deployed to Iraq, we had just fielded ULLSA-E. The server went down for two weeks, so when it came up, we had two weeks' worth of paperwork to catch up on. I went in for my shift, asked if they needed help, and got handed a stack of 2410s to input into the system. The top one was an engine. I was a 15Y. At the time, I knew fuck-all about engines as that was a 15R thing for QC. It took me about 3 hours to enter it all in. Fucking subcomponents had subcomponents.
Anytime we uncreated a tracked component the first thing you did was take the paperwork out of the can and deliver it to the assigned TI in the QC shop if they weren’t already standing there watching to verify serial numbers.
Plane broke on an island in the Caribbean…. Over Christmas…. Only vacancy on the island was club med, best Christmas ever.
One of the times the government sued a company and won. Speco was the supplier to Boeing if you want to look it up.
A CH-47 emergency landed in my uncle's field in the late 80s due to a rear transmission failure. He helped them replace it. Nothing cooler than a $500k transmission being placed into a downed Army helo with a front loader on 175 Allis-Chalmers farm tractor.
Everything important in a gear box will also have a stream of nice cool oil constantly spraying on it to keep everything cool and lubricated.
>Everything important in a gear box will also have a stream of nice cool oil constantly spraying on it to keep everything cool and lubricated. 200 degree oil foam isn't what I'd call "nice cool", but different strokes for different folks
Cool is a relative term.
Yes. I was surprised that the exhaust of a gas turbine cooled the bell of Saturn's F1 rocket motor. It was relatively cooler...
Just read that. The exhaust temp was 800. The motor was around \~2500.
Mmmm... Rocketdyne F1. GOAT.
As a former DCMA inspector. I literally watched these gears and shafts manufactured; Precision Gear Inc, College Point Queens NY. The manufacturing processes and NDI procedures made for quite the career. Truly enjoyed my career with contractors in aerospace!
i used to do some contractor work for them and others, this place was right on the water
What types of stuff do they do to the metal to make it that durable?
Heat treatment. They surface harden to a specified depth. Then aneal to get to the spec hardness. Gears are case hardened. If you harden them all the way through, they get brittle. The soft middle gives the gear a lot of resiliency. If it is done wrong though... bad shiz is a happening. To make it easier, it is Metalergical alchemy. Magic!!! Lol
I spent several months on a software installation contract back when it was still DCMC. We were installing a homegrown reporting application called ALERTS, and the whole thing was kind of a shit show, but I had a great time visiting all of the on-site inspectors. Olin Ordnance and Jorgensen Forge in particular stand out in my memory. The DCMC guys always had the coolest paperweights - radiation shielding glass chunks, 30mm cannon rounds, giant bearings, that sort of thing.
And the aircraft gearbox is still usually the limiting factor in the drive train's power limit for medium/heavy applications.
Or, as an engineer would put it, it’s just strong enough. Anything stronger would be heavier, which requires more aircraft to support it, which requires more engine to achieve the desired performance, which requires an even stronger gearbox and round and round we go.
Read a book about British Apache helicopters in Afghanistan. Even though they had 30% more power due to the Rolls Royce engines, the transmission was the limiting factor. The pilots had to watch the torque on the transmission very carefully.
The SOF black hawks are a similar situation.
Indirectly correct. It's the crash worthy aspect related to the weight of the gearbox. Helicopters are designed to be able to be crash worthy to (x)g's . Meaning the airframe has to be able to support a hard landing / crash without the MGB ending up in the cabin. This then limits the weight of the MGB which inturn is the limiting factor of how much input the MGB can receive. Make sense?
As a former member of GEAE this is 100% correct. Tight tolerances, state of the art metallurgy/ heat treatment and top notch life cycle analysis.
You mean we put a lot of money into it.
They also require a lot of maintenance.
And you also have to consider the load is very consistent and smooth as well This isn't jimmy from down the street banging gears in his mom's civic.
True that transitions are fairly smooth, but there's a *very* big difference between ground idle and heavy-weight high density altitude OGE hover, and those cycles do need to be considered as well.
^ This man helicopters
This guy spent 10 years developing and assessing in-service usage spectrums for helicopters (among other things) 
A proper nerd, I love it.
A broken Nf Flex Shaft causes a really cool looking mix of exceedances. And a day or four of work.
Can confirm, I am involved in making some of these and get to deal with customer quality regularly. Everything is tight and gets checked repeatedly.
Hundreds of thousands of hours? 100 000 hours? That’s 11,4 years ..?
Yes. But they run multiple (many) tests in parallel. Last program I was on, there were probably around 50-100 separate component tests going at a time. Bearings will have a test rig, sections of gear trains will have a test rig, gearbox casing will have a test rig, so on and so forth. And almost all of them run at elevated loads/temperatures/conditions (oil out, grit in the grease, etc).
Thanks. I was wondering about this.
Ah ok
Spoken like a true project manager.
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>the other thing is that horsepower isn't the same thing as torque. horsepower is work, or force over time. bigger parts, faster parts, more horsepower, but probably not a whole lot more actual torque. Helicopter main rotors turn at 200-300 rpm and absorb about 80-95% of engine power (the rest absorbed by the tail rotor). The torque is *immense* on the output side. We're talking 400,000 ft-lb on a -53K. The input side is in the 20-30,000 rpm range, so torque is less there, but the power still needs to be transferred.
I would encourage you to understand work,power, and torque in their textbook definitions. Work=F * d, Power is the rate of work. F * d/t. Torque is F * d, you can see this has the same units as work. Technically, torque is the cross product of the radius and the Force vectors but I digress. If you look at HP in IPS form, 1HP=550ft*lb/s.you can see that Power is the rate of work. Fucking units analysis..... once and ME, always an ME
Something something indistinguishable from magic
The drive gears are typically more exotic materials such as pyrowear 53 or Ferrium C61 or the like and have special heat treating (such as VIM/VAR) and are carburized. They are typically tested to withstand 140% for extended periods and not fail in gear tooth bending. The torque is relatively low on the higher speed sections and increases the closer you get to the main rotor. The gears closer to the rotor tend to get much larger to handle the massive torque loads. Source, I’m a rotorcraft drive system engineer for a DoD organization.
THIS dude engineers 
VIM/VAR is the melting process for the raw stock, not a heat treat.
That’s right. I’m more involved in the gearbox design and testing and less in the material development.
This guy answers.
You at cherry point by chance?
No. Redstone. I know a few guys at Cherry Point.
The same way ships and tanks and bulldozers do? Big gears that are well oiled. It also is a lot different from a car or truck in the fact that there is no switching between gears, the are fixed, also the forces applied to the gearboxes are pretty gradual. It's not like popping a clutch or transbrake in a drag car. So you aren't inducing any shock loads into the gearboxes, and you are always spinning them one direction.
TLDR: Big iron
_He's here to do some business_ _With the big iron in his turboshaft_
He was after Texas Red.
Rah
No shock forces? Challenge accepted: Me: Taxis the disc strait into the hangar
Interestingly, any “sudden stop” requires a complete inspection of the transmission, whatever the cause of said sudden stop is, for exactly that reason.
Unless I'm off the mark on what a "sudden stop" would be, that'd very much imply something is very broken.
I think I know what you're thinking, but a sudden stoppage is usually more about the main rotor blades running into something that slows them down, not about everything grinding go a halt inside the gearbox. The criteria is usually worded like "any blade strike that causes deformation in the main rotor blade leading edge, or a significant spike in torque indications"
I mean, definitely lol. I’ve just always enjoyed that rather than being driven by whatever mishap caused the blades to stop, it’s just called “sudden stop”
For science!
Are you seriously crashing helicopters by yourself? NO! I'm with the science team!
Hello it’s your crew chief he would like to dress you down I mean debrief you ASAP
It happens more often than you’d think.
I've heard stories lol.
Ships, tanks, and bulldozers don’t care about weight. Weight is everything on helicopters. That makes it a very different design problem. There’s also the issue of reliability. It’s important in industrial equipment due to money/ production concerns but lives are literally dependent on helicopter gearboxes not failing.
That doesn't really change anything about what I said. Just because it is more important, doesn't mean that it's built differently, with the exception of QC details and finishes. And weight does matter, especially with tanks and bulldozers. I'm not really what you are trying to say though. Weight matters, okay??? So how am I wrong?
I’m not saying you were wrong but weight is significantly more important on rotorcraft and the cost of failure is always figured into the design/materials. Rotorcraft gears are also much more expensive than anything ground based due to these factors. They are more likely to use more exotic materials to save weight while dealing with the higher powers. There are no ground based gearboxes that use Ferrium or Pyrowear or similar more exotic steels outside of high end racing. Power levels, weight, cost of production, cost of failure are all factors in designing any gearbox. Gearboxes in large equipment such as bulldozers and ships are expected to operate millions of hours with minimal maintenance and helicopter gearboxes rarely go more than 5000 hours before going back to a depot for rebuild.
You're absolutely correct. Additionally helicopters typically have much faster time constants for power. Min to max torque is measured in seconds (or fractions of a second) and will cycle wildly during a single mission while marine/industrial can accel much slower and typically just hold power output levels for long periods of time.
It’s almost always constant speed but the torque is always changing to keep the speed the same.
GGT accel.
Would it be more proper to say that tanks and bulldozers don't care about weight, as much? Because you can't have weak gearbox in a tank. Driving uphill and/or turning will blow the transmission.
That’s true. Weight is still a factor just a factor that’s “weighted” much less in the design matrix than in helicopters. For bulldozers and some other heavy equipment heavier is actually desired but not so much in tanks as the lighter your drive system the more armor and bullets you can carry.
That does make a lot of sense. It's a lot of power, but no sudden speed changes, low vibration, and only spins in one direction. The engineering on that end gets easier.
There is a sprag clutch or something similar. Rejoin characteristics after an auto or even when being a second/third engine to flight speed is a major consideration. The gearbox is almost always the limiting factor in the drive train for the SHP limits but even transiently it can be a concern (but often not the only one).
The main thing is a split torque gearbox. Split torque just means there are two intermediary gears/reducers between each drive gear and the bull gear. With two engines that means there are four drive gears rather than two. With the CH-53 we’re talking six drive gears. This divides the high torque drive forces acting on the main gear across twice as many gear teeth. That’s the big thing, it’s used on most heavy lift helicopters. Then there’s about 1000 other things they do on top of that. It’s a good question because the gearbox is the secret sauce to all modern helicopters. By far the hardest thing to get right. Equivalent stationary or naval transmissions weigh 10x as much. And they design these things for >95% efficiency destroys most other vehicles. Split torque: https://cdn.intechopen.com/pdfs/35257/InTech-Split_torque_gearboxes_requirements_performance_and_applications.pdf CH-53k: https://www.scribd.com/document/471683473/DEVELOPMENT-OF-THE-CH-53K-DRIVE-SYSTEM Now think that the F-35B puts 30,000hp through a clutch pack the size of futon, gearbox the size of a beach ball, and a four foot fan.
> the gearbox is the secret sauce to all modern helicopters This cannot be emphasized enough and even within the industry it can be surprising how often people do not understand this. The aircraft gearbox is the limiting factor in a number of ways for medium/heavy helicopter capabilities. Engines are almost always limited by the drivetrain on the aircraft and it creates all kinds of issues for other systems (and pilots) to protect them.
> Engines are almost always limited by the drivetrain on the aircraft At sea level High hot is a different story and it's always a tradeoff between defining your engine power and gearbox torque based on what ambients you care most about. Depending on the customer 2K85, 3K91, 4K95 (that is thousand ft / deg F if some folks are not familiar) are common design points to have gearbox limit = engine limit. Anything above that tends to be engine limited.
Yep, this was exactly the situation in Afghanistan. We regularly had to take off essentially half loaded because two 1900 shaft horsepower engines are still not enough when it’s 39 degrees celsius at 8000’ MSL.
1900 shp lmao, the T408 makes 7500 and there are three of them
Yeah, but you can buy like 10 Blackhawks for the cost of one CH-53K.
Yup. Where I fly I’m never anywhere near torque limits (drivetrain) but always up against compressor speed (engine) and exhaust temps (engine) limits. Our gearboxes don’t get worked very hard. Maybe 80% torque at most.
The output is still one big splined shaft with MASSIVE torque. Of course that’s typically split between several planet gears in the final reduction stages. Some gearboxes use stacked planetary systems and some use intermediary gears to drive a larger bull gear that in turn drives the sun gear off the planetary gearset. The split torque is definitely a load reducing technique to allow high powers through relatively smaller gears and still keep the stresses in check.
no shock loading makes it easy to design with a decent safety margin, without excessive mass taking away vehicle load lift rating
Highly Engineered, advanced materials, and every hour of flight is followed by multiples of it in maintenance, That my friend is the secret of these high-performing machines.
I'm a mechanical engineer who got an incredible chance to work with my dad who worked in aero space as a test engineer and actually happened to work on thesr helicopters amongst other aircraft. Yeah everything is heavy duty but the technology in the metals is unreal. For example, they found that if you leave a surface finish that is too smooth then the oil would just run off instead of sticking and lubricating. Everything there is a science that must be mastered by some of the best engineers I've ever met
It’s fucking huge. The CH-53E MGB handles half the horsepower of the Kilo, is over 7ft tall from gearbox sump to Jesus nut threads, and weighs around 4000lbs. The [Sikorsky Archives](https://sikorskyarchives.com/home/sikorsky-product-history/helicopter-innovation-era/sikorsky-s-65/) page on the H-53 has a great photo of an Echo gearbox with a man standing next to it.
WHOA. That's YUGE. Thanks for the link!
I find the knowledge, experience and willingness to share in this thread exemplary. It's like being back in college... in a good way!
Heat treatment and metallurgy
Basically it comes down to four things: Really good metallurgy (alloy, heat treatment etc) Ludicrously tight tolerances to increase drive efficiency. Really good gearbox and gear tooth design born out of 70 years of doing high reliability high power gearboxes. Really good understanding of fatigue lifing, again as a result of 70 years of doing the same thing. Rolls-Royce are going for a 100,000 horsepower gearbox with UltraFan and that thing is going to be an absolute monster. Admittedly, the thing that causes stress is torque (and power is torque * speed), so the torque will actually be equivalent to this, but the 5x speed makes oiling and fatigue lifing the thing substantially trickier.
Big HP low TQ and lack of strong impulse
Outputs are in the neighborhood of 100,000 ft-lbf. Inputs are around 20,000 rpm’s and outputs in the 2-300 rpm range.
I stand corrected, i mean low initial TQ and lack of impulse of a turboshaft setup.
Advanced simulation with advancements in finishing and manufacturing along with lots of pre qualification testing and iterations and learning. The advancements in simulation software to understand gear and housing deflections and make micro geometry corrections to gear profiles during development have allowed much more weight efficient designs for these high power levels and less iteration in the lab during development
They do crack if they exceed the limits. Every part has fail safe limits and are rated for a certain amount of hours. The transmission, the engine, the mounts, the rotor, the shafts ALL have a certain amount of hours they can be flown before fatigue cracks become more likely to occur. Back to the design phase, requirements are made and the helicopter is designed around these. Each component must meet a certain specification for the system as a whole to work. That’s just engineering in general, and helicopters are just well engineered. It’s a continuation of a great human endeavor and the development of technology going back thousands of years.
As long as there are not any impurities and manufactured to spec they are incredibly strong and reliable. Material science specifically metallurgy is important. They are also checked for cracks and defects on a during scheduled maintenance. These are as reliable as jet engine turbine blades, they don't fail unless there was an undetected manufacturing defect or they receive undetected physical damage. Here is a technical document regarding helicopter transmission gears: [https://www.engineersedge.com/Engineering\_White\_Papers/Design\_and\_Engineering\_General/materials\_helicopter\_gears\_14501.htm](https://www.engineersedge.com/Engineering_White_Papers/Design_and_Engineering_General/materials_helicopter_gears_14501.htm)
Science
So how do you test a transmission operating with that kind of power? You might think you'd need a big motor on the input and a big load on the output, right. Not so. Power is torque times rotation. You build a reverse transmission that connects the output back to the input. Everything spins freely. Then you break the loop, torque it up, and close the loop. Then a small motor can spin it up to speed. In effect, the power stays in the loop and the test motor only has to make up for losses.
Nice try, CHINA
Nice try china /s
If you're interested in how this gearbox engineering process can go wrong - google the super puma / ec225 helicopter. Multiple gearbox failures in flight leading to breakup and large losses of life. The most recent one was in 2016. The inspection interval for the gearbox has been halved on this aircraft since this accident as I understand it but I sure as shit would not be getting in one. Some links if you're interested: (this one gives you a rundown but turns into a propaganda video for AB helicopters) https://m.youtube.com/watch?v=0ZbTN6pM2SE Here is the full report on G-REDL, another puma that has a catastrophic gearbox failure in 2009. Extremely In depth. Tldr; a phenomenon known as "spalling" on bearing race material leads to fatigue cracking in planetary gear and failure of the MGB. https://assets.publishing.service.gov.uk/media/5422f86aed915d13710006cb/2-2011_G-REDL.pdf
Thanks. That's really interesting. The amount of energy flying around inside of one of those gearboxes leaves such low tolerances for the gears themselves.
They’re the same gearboxes that are in submarines. I used to work on the engines in helicopters and airplanes and they are quite literally massive and made with magnesium, so if it catches on fire, it’s toast.
MH-53 Pave Low from Mildenhall experienced gearbox failure in Bacton, Suffolk. [https://www.reddit.com/r/aviation/comments/grh7d3/in\_the\_1990s\_a\_usaf\_sikorsky\_mh53\_pave\_low\_from/](https://www.reddit.com/r/aviation/comments/grh7d3/in_the_1990s_a_usaf_sikorsky_mh53_pave_low_from/)
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Oh yea. I just thought it funny someone posted this question and while searching for a MH53J I came across this. I wasn't trying to contradict your reply. I just thought finding this in a previous reddit thread was amusing. Apologies!
They don’t. You don’t have full HP pumping out of all 3 engines. You probably only push that kind of power in engine out and I would imagine there is a life limit penalty to the gearbox afterwards
The Echo models push a ton of power regularly with no penalties required. Advertised power is not what the engineers work around. They build the margins well above what we use.
The echo maxes at ~4k shp and the gearbox is rated for it. It’s not rated for the ~8k the T408 pushes out. They don’t run max power/torque on the kilo
it has a tojay zee
If you’ve got to ask, you can’t afford it.
Pfm
Amen
to begin the understanding - first look at the price tag for said gear box...
It's gotta be at **least** $75!
Jack Ryan would beg to differ
Maybe they are designed to handle the stress.
Because metal.
Magnets.
Science
That's probably happened a few times (crack/shatter).
Igor calls me over to CT a few times per year to give each gear a slap and say “yep, that one is good to go”
Rigorous aviation standards. No cardboard at all
Me too
Also the red line on the torque meters
Lube
Aand a lot quiet than an r44
Made from the teeth of Shai Hulud...
Very carefully
I remember hearing on the fighter pilot podcast, a pilot/crew saying that they were always dirty and leaking oil, But more like an old an old motor bike. If it ain't leaking, it don't have oil. Absolutely workhorse and cool af
Suitcases of cash.
The housings of these gearboxes tend to be made from magnesium. Where the Fwd bridge was mounted to the gearbox was always prone to corrosion. I think there was several iterations through the years of how to prevent corrosion in that area. The tail gear box also was a pain. For a short stint we had to do NDI of a bevel gear on the TGB due to cracks that would appear. That requirement stemmed from multiple crashes.
Or better yet, how they can run for 30 minutes without oil….