Torque Converter and Turning Gear System in Gas Turbine


I would like to know how my watch works. Can you help me? And, you can't see my watch (this is, after all, the World Wide Web, and we're not standing together and talking).

I didn't tell you if the watch was analog or digital. If it was wind-up, or battery-powered. If it has hands or a digital display, or both.

How would you help me? By asking questions?

Have you used the 'Search' field at the far right corner of the Menu bar at the top of every webpage? Because, torque converters (as used on GE-design heavy duty gas turbines) have been discussed off and on for more than 14 years here on, as have many of the various "turning gear" methods (slow roll; hydraulic ratchet; turning gear; etc.).

What kind of turbine are you working on?

What kind of torque converter does it have? One with variable torque converter vanes, usually manufactured by Voith? Or one with a fixed torque transmission method, usually manufactured by TwinDisc? (Again, these are torque converters typically used on GE-design heavy duty gas turbines; you haven't told us what kind of turbine is in question.)

What kind of "turning gear" mechanism is in use? An actual turning gear which rotates the shaft slowly and continuously? Or a hydraulic ratchet which turns the shaft a fraction of a turn every few minutes? Or does the shaft maintain RPM after shutdown using Auxiliary Oil Pump pressure through the hydraulic torque converter?

So, you see, we can't see the unit you are asking about. We don't know anything about the application and the equipment in question because we aren't there alongside you seeing what you see. And, there are very many types of torque converters and "turning gear" mechanisms and methods in use--just on one gas turbine design, of which there can are very many different turbines (sizes).

Help us to help you! Try to imagine someone asking you the same question you posted here--and how would you answer it. Or, if you're new to the turbine business, how would you answer someone who's asked you to explain how their watch works--when you can't see the watch in question.

yasin rupani


We have frame 9e gas fired machine, with normal turning gear motor (rated speed 110rpm) barring, and a fixed vanes Voith made torque converter(0-107 deg guide angle). Please help me to understand the exact function of torque converter with respect to turbine roll on from barring to base load (around 116MW).

Hope for assistance from your side,

Thanks & regards,
Yasin rupani
Yasin Rupani,

"... normal turning gear motor ... " Hmmm. I've worked on a LOT of GE-design Frame 9Es and I've never seen one with a turning gear motor. Who was the packager of the Frame 9E at your site? GE? John Brown? BHEL? (It doesn't really matter; I'm just curious.)

How is the turning gear mechanism coupled to the turbine-generator shaft? Is there a SSS (Shifting, Self-Synchronizing) clutch used? Or is there a gear mechanism that is mechanically shifted into place to connect the output of the turning gear to some teeth on the turbine-generator shaft? (Again, not having seen this type of turning gear I'm very curious.)

And, I'm also confused by the statement: "... a fixed vanes Voith made torque converter(0-107 deg guide angle)...." Are you saying the torque converter has variable guide vanes that are locked into some particular angle? What determines the angle? Is it speed?

Does the unit at your site have to be on turning gear to start? Or can it be started from zero speed?

In general the purpose of the torque converter is to serve as a means of providing a method to transfer torque from the starting (cranking) motor to the turbine shaft. Most electric starting (cranking) motors run at a relatively fixed speed--approximately 1500 RPM or approximately 3000 RPM. Approximate because the motors are usually induction motors which require some "slip" to operate (meaning they won't operate at exactly 1500 RPM or exactly 3000 RPM, because a slight difference in speed is necessary for the motor to operate properly). But, they run at a relatively constant speed, and one can't couple an electric induction motor directly to a turbine-generator shaft and accelerate the turbine-generator shaft from zero speed to firing speed and then to self-sustaining speed (approximately 50-60% of rated turbine-generator shaft speed). The electric induction motor would draw HUGE amounts of current trying to break the turbine-generator shaft away from zero speed--and will either overheat VERY quickly, or the motor protection relay will trip the motor to protect it.

So, a torque converter is used to provide for transmitting torque from the electric induction motor running at a relatively constant speed to the turbine-generator shaft to accelerate it from zero speed, or a very low speed, through firing and acceleration to self-sustaining speed. It's kind of like a clutch in an automobile or lorry (truck) with a manual transmission. Actually, it's more like a crude automatic transmission in a car or truck (lorry). In actuality, it's really a hydraulic pump (driven by the starting (cranking) motor) driving a hydraulic motor (the output shaft of the torque converter which drive the turbine-generator shaft). The hydraulic pump part of the torque converter runs at the same speed as the starting (cranking) motor, while the turbine-generator shaft speed accelerates during starting.

Once the turbine-generator is capable of producing enough torque on its own to maintain speed and accelerate to FSNL (Full Speed-No Load), the starting (cranking) motor is shut down. Some torque converters are directly coupled to the turbine-generator shaft, and some have a jaw clutch or SSS clutch to disengage the torque converter from the turbine-generator shaft when the starting motor is shut down.

The function of the turning gear (other than possibly providing an assist for helping break the turbine-generator shaft away from zero speed during starting) is to protect the axial compressor rotor from developing a bow, or sagging, during shutdown. If the unit is hot when it is shut down or tripped and the turbine-generator shaft is allowed to reach zero speed and not be slowly rotated within approximately 20-30 minutes the axial compressor rotor will begin to sag, or bow, under its own weight. This is because the metal through-bolts of the axial compressor are warm/hot, and in what's called their "elastic" range of temperature. If allowed to sit at zero speed for long the bolts will begin to elongate which will cause the axial compressor rotor to sag, or bow, which will--if the temperature is hot enough--allow the rotating compressor blades to come into contact with the compressor casing. And, if the rotor is rotated under these conditions, either the rotating blade tips will get damaged or the blades will break.

So, turning gear is one of many methods employed on GE-design heavy duty gas turbines to slowly rotate the turbine-generator shaft--of which the axial compressor is a very large, heavy part of--while the axial compressor and the rotor through-bolts cool. This prevents the bolts from elongating and protects the compressor from possible damage.

However, even if the axial compressor rotor isn't rotated slowly for approximately 24 hours after a warm/hot shutdown/trip and the rotor through-bolts do stretch--they are in their elastic range and left long enough they will return to very nearly straight. Not exactly straight, but close enough that using the turning gear for several hours will help to return them to nearly perfectly straight.

Hope this helps! If you could provide some information as asked above, that would be most helpful.
Hello CSA,

Thanks for your response.

GE is the supplier of Frame 9e machine at our site

Turnings gear motor (barring motor-88TG-1) keeps turbine shaft running on 110 rpm. On the same shaft we have cranking motor 88CR-1 that keeps turbine shaft running till 1800 rpm (self sustaining speed of turbine). We have Torque converter (88TM-1) on the downstream of 88CR-1 that has variable guide vanes, which changes its angle to pass lube oil to mention shaft speed. Angles are (43 degree-turning gear-110rpm, 15 degree-firing speed-350 rpm, 50 deg-during crank, 68 deg- 1800 rpm).