Of particular interest was an excellent page on the inner workings of the hydraulic Constant Speed Drive (CSD) mechanism used in the Integrated Drive Generator (IDG) as seen on many commercial types.
The actual mechanism is far more complex and clever than I first imagined. I was always under the impression that the variable / fixed hydraulic pump combination was the only component responsible for achieving the constant output RPM. However, it appears that the variable / fixed hydraulic pump combination acts on a planetary gear-set and only pumps oil when there is a speed difference between the input and output sides.
If the input and output speeds are similar, there is a direct drive through the planetary gear-set. This approach makes a lot of sense, for if there was only a variable / fixed hydraulic pump combination as I first thought, such a mechanism would still need to pump an appreciable amount of oil for the direct drive (input speed equal to output speed) situation, which would of course entail the IDG consuming much more power.
Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair.
tb727 From United States of America, joined Jun 2005, 1140 posts, RR: 4 Reply 1, posted (4 months 1 week 3 days 14 hours ago) and read 2267 times:
The CSD is a really cool design. You've gotta check those "In" and "Rise" temps! What is the schematic off of? 737-200? Looks Boeing.
The CSD Disconnect is an interesting feature too, it "screws itself out of a job" and can only be reset on the ground so if you're gonna do it, make sure you really want to do it!
tdscanuck From Canada, joined Jan 2006, 11032 posts, RR: 72 Reply 2, posted (4 months 1 week 3 days 13 hours ago) and read 2253 times:
Quoting tb727 (Reply 1): What is the schematic off of? 737-200? Looks Boeing.
Not sure what aircraft, but that's straight from a Boeing System Schematic Manual...that layout, formatting, and fonts are pretty unmistakable.
Quoting tb727 (Reply 1): The CSD Disconnect is an interesting feature too, it "screws itself out of a job" and can only be reset on the ground so if you're gonna do it, make sure you really want to do it!
This is fairly common for generator disconnects...Boeing IDG's and VFSG's work the same way.
B747FE From Hong Kong, joined Jun 2004, 225 posts, RR: 4 Reply 3, posted (4 months 1 week 2 days 15 hours ago) and read 2116 times:
Quoting jetmech (Thread starter): However, it appears that the variable / fixed hydraulic pump combination acts on a planetary gear-set and only pumps oil when there is a speed difference between the input and output sides.
Correct!
Underdrive mode = input speed higher than required = hydraulic unit works a a motor.
Overdrive mode = input speed lower than required = hydraulic unit works as a pump.
When in straight through mode, the generator is then driven directly through the differential. (Wobbler perpendicular to the pumping pistons)
On a JT9, at around 65% N2, sometimes you can actually recognize when the CSD is in straight through because there is a slight load oscillation between generators.
Quoting tb727 (Reply 1): What is the schematic off of? 737-200?
Regards,
B747FE.
"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"
tb727 From United States of America, joined Jun 2005, 1140 posts, RR: 4 Reply 4, posted (4 months 1 week 2 days 13 hours ago) and read 2091 times:
Quoting B747FE (Reply 3): On a JT9, at around 65% N2, sometimes you can actually recognize when the CSD is in straight through because there is a slight load oscillation between generators.
737tdi From United States of America, joined Sep 2007, 317 posts, RR: 0 Reply 5, posted (4 months 1 week 2 days 13 hours ago) and read 2086 times:
Not to be overly picky but the use of IDG in this case is not really correct. In this case the CSD and Generator are separate units where as an IDG is the combination of CSD/Generator in one unit. Good stuff either way. I sure know I prefer IDG's when it comes to replacement.
jetmech From Australia, joined Mar 2006, 2446 posts, RR: 54 Reply 7, posted (4 months 5 days 15 hours ago) and read 1722 times:
Quoting B747FE (Reply 3): On a JT9, at around 65% N2, sometimes you can actually recognize when the CSD is in straight through because there is a slight load oscillation between generators.
Was this N2 typical of cruise conditions? Seems like it may be associated more with idle or lower power settings. I would have imagined that you would want the straight through (direct drive) condition to occur at an N2 close to that associated with typical cruise power settings .
Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair.
B747FE From Hong Kong, joined Jun 2004, 225 posts, RR: 4 Reply 8, posted (4 months 5 days 10 hours ago) and read 1706 times:
Quoting jetmech (Reply 7): Was this N2 typical of cruise conditions?
No, just above idle thrust. (N2 in cruise is around 20 to 25% higher)
On the P&W engines, I used to see this during the taxi out.
On aeroplanes with RR (211-524 with IDG) or GE CF6 it was during descent, but I don't remember the exact speed. (Should be at faster speed than the JT9D)
Quoting jetmech (Reply 7): I would have imagined that you would want the straight through (direct drive) condition to occur at an N2 close to that associated with typical cruise power settings
Sure, but either way even if it was at cruise thrust setting it was a very narrow speed range in which the oscillations happened (+/- 2 or 3% max) and it didn't necessarily happened on all engines at the same time.
Needless to say the CSD's didn't really spend a lot of time in straight through mode.
Regards,
B747FE.
"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"
aviopic From Netherlands, joined Mar 2004, 2676 posts, RR: 51 Reply 10, posted (4 months 4 days 20 hours ago) and read 1609 times:
Quoting 737tdi (Reply 5): In this case the CSD and Generator are separate units where as an IDG is the combination of CSD/Generator in one unit.
To Illustrate the matter I' ve uploaded a few photos that I made at work for PR purpose into an album.
It contains a typical B73 setup, the shiny bit is the Generator and the black bit the CSD together forming a huge lumb
An A320 IDG.
And an internal view of a F70/100 IDG.
The truth lives in one’s mind, it doesn’t really exist
jetmech From Australia, joined Mar 2006, 2446 posts, RR: 54 Reply 11, posted (4 months 4 days 13 hours ago) and read 1562 times:
Quoting B747FE (Reply 8): Needless to say the CSD's didn't really spend a lot of time in straight through mode.
Certainly. No doubt N2 would also vary somewhat for different flights and during a flight, so any benefits of operating in direct drive would only manifest for a small portion of the total flight time.
titanmiller From United States of America, joined exactly 6 years ago today! , 71 posts, RR: 0 Reply 14, posted (4 months 3 days 23 hours ago) and read 1486 times:
Absolutely fascinating! Thanks for sharing your discovery.
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