Mr Spaceman From Canada, joined Mar 2001, 2780 posts, RR: 15 Posted (8 years 5 months 1 week 2 days 14 hours ago) and read 5620 times:
I have a few questions about jet engines and their starters, which I couldn't find the answer to after I did a search.
The search did remind me of certain facts such as .................
Bleed air to the starter is supplied from either the APU, a ground cart, or another engine. The starter will disengage at different engine N2% speeds (such as 48%N2) for different engine types. Starters are generally mounted on the accesory gearbox (AGB), etc, etc.
My questions are .......
Does a jet engine have a large "Flywheel" disc/ring with teeth all around it located somewhere among the N2 compressor blades that a pinion type gear from the starter meshes with (like in a car's engine) to start turning the engine during startup?
I understand that jet engine starters are caused to spin inside due to bleed air, but what I don't understand is what happens next ....... to start turning the engine's compressor blades.
The closest I've come to learning how this works was from this explanation from my search ......
"The starter turns the accesory gear box (AGB), which turns the quill shaft, which turns the main shaft."
That's fine info, but, I'd like to learn about it in finer detail.
I also learned these 4 terms from my search ...... Quill Shaft, Spline Shaft (about the starter), and N2 Spool, Rotor Spool (about the engine's main shaft).
So, armed with the above quoted statement, and these 4 terms, is it safe to say that when the starter is engaged, a spinning Quill Shaft extends from the AGB and meshes with a Rotor Spool which is a large flywheel?
Or, perhaps the Quill Shaft extends farther and meshes with a smaller gear (Rotor Spool) on the main engine shaft deeper inside the engine?
Sorry for the long post guys. I'm just trying to put these pieces together.
Air2gxs From , joined Dec 1969, posts, RR: Reply 1, posted (8 years 5 months 1 week 2 days 13 hours ago) and read 5554 times:
The starter turns the gearbox. The gearbox is connected to a tower shaft through a bevel gear. The tower shaft is connected to the N2 (N3 on RR) rotor through a becel gear. When the starter turns, the N2 rotor turns, pulling compressed air through the engine. After the engine is running, the process reverses. The compressor eventually turns the gearbox through the same system.
I don't like to use the term quill shaft ith the starter. Quill shafts (in my experience) belong on fuel controls. They turn in respect to engine speed and tell the fuel control how fast the engine is turning. I understand why some would call the starter shaft a quill shaft, but I feel its incorrect.
The spline shaft just refers to the output shaft of the starter. It has spines which engage the gear train in the gearbox.
Rotor, spool and N2 are pretty much synonymous when talking about jet engines. The N2 compressor can be called the N2 rotor or N2 spool.
I just realized that I really don't know how the starter disconnects. It doesn't work like an electric starter where the shaft engages or disengages. I'm thinking something like a clutch coupling in the gear box. I'd have to check. The internals of the gearbox are not my strong suit.
Mr Spaceman From Canada, joined Mar 2001, 2780 posts, RR: 15 Reply 2, posted (8 years 5 months 1 week 2 days 13 hours ago) and read 5513 times:
Thanks for your reply.
OK, so there's no huge flywheel with hundreds of teeth on it. I can finally get that idea out of my mind.
How wide in diameter would the bevel gear on the N2 rotor be? Is it roughly the same as the diameter of the main engine shaft?
I'm just trying to imagine how long a tower shaft must be to cover the distance between the gearbox on the outside of the engine casing and the internal N2 rotor. Is it several feet long?
I believe I understand correctly now that these gears and shaft are permanently in place, nothing extends or retracts during the start process ..... especially if the process reverses after the engine is running.
Air2gxs From , joined Dec 1969, posts, RR: Reply 3, posted (8 years 5 months 1 week 2 days 11 hours ago) and read 5449 times:
NOt sure how big the bevel gear itself is (having never seen one), but the tower shaft isn't that long. It just has to travel from the spool to a gearbox. I simplified my earlier post. Some engines use what's called an angle gearbox, which is mounted closer to the core engine, thus making the tower shaft short(er). From this angle gearbox extends a drive shaft which drives the main gearbox. Of course this varies from manufacturer to manufacturer and engine to engine.
SlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 71 Reply 4, posted (8 years 5 months 1 week 2 days 10 hours ago) and read 5433 times:
In most jet engines the bevel gear on the N2 shaft will be slightly smaller than the diameter of the hub of the adjacent compressor stage. This would be no more than a few inches. The gears in the accessory section smaller still.
These air-driven starter motors have tremendous torque.
Also want to expand on something Air2gxs said: "When the starter turns, the N2 rotor turns, pulling compressed air through the engine." This sort of sounds like pneumatic compressed air is being pulled through the engine core. Not so, it is outside air, through the intake being drawn deeper and deeper into the compressor section, being compressed along the way. The pneumatic compressed air that drives the starter simply exhausts overboard.
Happiness is not seeing another trite Ste. Maarten photo all week long.
Broke From United States of America, joined Apr 2002, 1322 posts, RR: 4 Reply 5, posted (8 years 5 months 1 week 2 days 10 hours ago) and read 5413 times:
Starters have one of 2 methods of disengaging when the engine begins to accelerate to idle speed during the start sequence.
One is called a sprague clutch. This looks like a roller bearing with a sloppy fit to the bearing cage. As the engine accelerates, the centrifugal force of the spinning starter shaft pulls the rollers away from the inner portion of the "cage" disengaging the starter from its shaft.
The other is a pawl type clutch where the spring loaded pawls on the starter shaft engage a notched ring. As the engine accelerates, the centrifugal force overrides the springs and the pawls disengage the ring.
In both cases, the starter shaft spins with the engine gear box while the rest of the starter is not spinning; unless, of course, the start valve is still open.
HAWK21M From India, joined Jan 2001, 31201 posts, RR: 58 Reply 6, posted (8 years 5 months 1 week 2 days 2 hours ago) and read 5008 times:
In a typical case,it may vary with other Engines.
The Starter recieves bleed Air that strikes the Turbines in the Starter Assy causing the torque produced from High speed low torque to Low speed high torque with the help of the Gear mechanism in the Starter Assy.
From there the starter transmits motion thru a spline shaft that moves fwd with rotation engaging a quillshaft in the Gearbox assy that is interconnected to a tower shaft which drives the N2 compressor.
After 40% N2 is reached a Centrifugal switch mounted in the Starter assy is contacted by Flyweight operated sw to send an Electrical signal to disconnect the Start Sw in the cockpit.causing the starter drive shaft to slide back away from the Gearbox quill shaft.
TimT From United States of America, joined Jun 2001, 168 posts, RR: 0 Reply 8, posted (8 years 5 months 4 days 6 hours ago) and read 4681 times:
I counted the teeth and figured out the ratio on a JT-9 many years ago but don't remember it now. Old age creeping in!. The speeds I remember are something like - N2 7000 rpm/ tower shaft was 3 or 4 times that. I do remember the diameter of the drive gear on the N2- inside diameter is around 10 inches and the tower shaft gear is about 4 inches.
MD11Engineer From Germany, joined Oct 2003, 13337 posts, RR: 64 Reply 9, posted (8 years 5 months 4 days 5 hours ago) and read 4677 times:
The starter motors contain a small air turbine wheel of about 4 inches diametre. It spins at about 100.000 rpm, but the speed gets reduced through a planetary gear. The problem comes if the starter clutch doesn´t disengage or the starter valve doesn´t close at starter cut-out, because then the engine will drive the starter through the now reversed gear and spin it to excess rpms (250.000- 500.000 rpm). As a result the starter will fly apart from the centrifugal forces.
I´ve heard of accidents happening especiallly during manual starts (when the starter valve can´t be controlled electrically and has to be manually overriden by a mechanic sitting right under the engine with a ratchet and an extension, he just opens the butterfly valve manually). The mechanic absolutely depends on the pilot to monitor the engine N2 rpm and to tell him when to close the vaklve (when N2 approaches, depending on the engine, 45-50% N2).
If not, the starter will go off like a handgrenade. The other thing is the idea of sitting right below a running engine...
CCA From Hong Kong, joined Oct 2002, 708 posts, RR: 14 Reply 10, posted (8 years 5 months 4 days 4 hours ago) and read 4679 times:
This is from a RR RB211-524 starter operation Manual.
The single stage Axial Flow Turbine connects through the reduction gear and gear hub to the Output Drive Assembly. The Output Drive Assembly consists of an output drive shaft, and an override clutch mechanism which disengages the Starter drive on the Engine. The Splined Output Drive Shaft mates with the High Speed External Gearbox input shaft, and drives the N3 Rotor Assembly.
The Turbine Rotor Wheel has a reduced cross section (fuse) below the Turbine Blade Rim to effect separation of the outer rim and blades by two cutters, that cut into the fused area in the event of a rotor wobble caused by bearing malfunction.
In the normal zero speed position, springs cause the Clutch Pawls on the Output Drive Shaft engage with the ratchet ring on the rear face of the gear hub; leaf springs acting on the pawls form a centrifugal clutch, to allow the Starter to accelerate the Engine.
Air flows to the Starter Turbine Wheel Blades through the Turbine Nozzles. This Drives the Gear Train, Clutch and Output Shaft, which in turn causes the N3 Rotor to rotate.
At Starter cutout, the Air Supply to the Starter is shut off and the Starter Turbine decelerates to a stop. In creasing centrifugal force caused by the Engine rotation, causes the Clutch Pawls to release and ride free above the Ratchet Teeth, while the Engine Continues to accelerate.
If the Start Valve fails to close, at approximately 55-60% N3 the Turbine Nozzle will choke due to the design of the Inlet and Turbine. This removes Starter torque, releasing the Ratchet/Clutch.
With the Clutch released, the disengaged Turbine will accelerate up to the choked designed speed, (which is less than the burst speed or 25% above MAX design speed - 95000 RPM) until the Bearings fail due to the over heated oil losing it's lubricating properties. When the Bearings fail the Shaft/Turbine vibrate causing contact with the tungsten cutter to separate the Turbine from the Disc.
If the Clutch fails to disengage the Starter will be driven by the Engine to the Rotor Burst Speed of approximately 95,000 RPM. The Turbine Disc separates below this RPM in order to reduce overall damage.
MD11Engineer From Germany, joined Oct 2003, 13337 posts, RR: 64 Reply 11, posted (8 years 5 months 4 days ago) and read 4661 times:
I´ve got the remains of a selfdestructed B737-300 starter somewhere in my basement. In this case it wasn´t the turbine disk, but the cage and the outer wheel of the planetary gear which failed. The results was the same, the starter flew apart.
Acc. to one of our pilots, our airline had a starter flying apart a few months ago during a manual start at another station due to the pilot not telling the mechanic when to close the start valve.
Air2gxs From , joined Dec 1969, posts, RR: Reply 14, posted (8 years 5 months 1 day 13 hours ago) and read 4492 times:
I've done hundreds (no exageration) of manual starts on aircraft (mostly the CFM 56 on DC8s) and I would still need a que from the headset man to release the T handle.
My experience tells me about how long the start sequence should take, but I've started enough of them in the cockpit to know that the timing is dependent on several variables.
Listen for the the right point? I can't tell the difference between an engine at 40%, 50% or 60%. I can hear the acceleration and only guess at the speed.
And maybe the mechanic just hadn't developed the instincts yet. Not too experienced or hasn't ever done it before. All I know is that I rely on the headset man (who is also a mechanic) to remind the flight at the initiation of the start sequence that starter cut-out must be relayed to him, and then to me.
MD11Engineer From Germany, joined Oct 2003, 13337 posts, RR: 64 Reply 16, posted (8 years 5 months 6 hours ago) and read 4394 times:
The problem is, if you close the start vslve to early you´ll get the mother of all hot starts. There is no way you can hear from experience at which speed an engine is turning.
I´ve done leak check s on engines running on idle, the last was on a Tay powered 727. Almost all engines I´m workung on now have the accessories and gearbox under the reverser cowl, which as a load sharing cowl can´t be opened while the engine is running. You´ll clean everything, close the cowling, run the engine and then open the cowling again and look for leaks.
Having done leak checks and trimmed running Tays and JT8Ds doesn´t mean that I liketo do the job or that I´m comfortable around a running engines. A former Irish colleague of mine got sucked into a Ryanair 737-200 and luckily escaped with his life. He "only" lost his right arm. Jet engines are dangerous.
HAWK21M From India, joined Jan 2001, 31201 posts, RR: 58 Reply 17, posted (8 years 4 months 4 weeks 1 day 22 hours ago) and read 4378 times:
Guys....I agree its the right procedure,but when there is a chance of a starter Disentegrating,the Injuries can be serious.
After lightup,one can judge approx the time to release the start lever by the sound.[Not exactly but quite close].
I still feel if the Signal does not arrive from the Man on the headset via the Cockpit,Use ones experience rather that risk flying metal.
A starter disentegrating & penetrating the Cowl can be serious.