Shear Section In The Drive Shaft Gearbox Component???
 

Hi,

-Are the drive shaft of ALL gearbox components (like oil pump, fuel pump, EDP, IDG,...etc) fitted with a shear section?
If not what are the components that are not fitted with a shear section? I am wondering because I read that certain oil pumps are not fitted with a shear section.

-How often the gearbox or the tower shaft fails?

Is it possible that the splined drive (the splined item in the gearbox that drives the drive shaft of a gearbox component, I am not sure about the exact name of this technical word) fails and cannot drive an operative drive shaft of a gearbox component (like EDP, IDG,...). How often this happens?


Feedback appreciated. Thank you.
Best regards

From my 20 odd years experience...
I would say at least every second flight.....:yuk:

I suppose it depends on the individual Aircraft type.

Fuel pumps can be electrically powered, IDG will have quill drive. Whichever aircraft are you referring to will give you differing answers dependant upon the cleared design spec.

Incidence of fail to safety shear has a thousand and one definitions. Sorry can't be more specific.

TW

Hi,

Thank you for your posts.
I am not refering to a specific engine, I meant a shaft shear section of a gearbox component in a modern turbofan engine (in general) :CFM56, V2500, GE90,...etc.
I was wondering about the shear section after reading an article "A380 cold soak" where the oil pump was replaced because the input shaft was broken.

Feedback appreciated. Thank you.
Best regards

Engines have a min temp at which the engine my be cranked which is related to oil type brand etc eg Exxon 2380 has a lower pour point than Mobil jet 2.

If the engine is cranked at a temp below the Min temp there is a very good chance that one of the following things will happen

1. The oil pump input shaft will shear due to over load and the shaft bearings will receive insufficient lube as the engine comes up to speed.
2. The oil will be too thick and the bearings will receive insufficient lube as the engine comes up to speed.


APUs have de-oil solenoids that allow a cold soaked engine to start....... Large turbo fans do not

Hi,

Thank you for your post.

Originally posted by Bolty McBolt
What's the role of de-oil solenoid? (scavenge the oil from bearings or gearbox and heat up this oil...?) may be it's needed for ETOPS aircraft?

Any comment regarding the failure of the drive shaft, splined drive (if it is little bit common, but it seems that the failure of the gearbox or the tower shaft is very rare?)

Feedback appreciated. Thank you
Best regards.

De-oil soleniod

NO its has nothing to do with ETOPS.
A De-oil solenoid is even on a Garret 660 its purpose is to allow a cold soaked APU to start.

Your drive shaft Question...Spare us all.

I encountered one case of the tower shaft failing (shearing) - the test cell operator was making the first start of a preserved engine. The preservative oil started to burn in the LPT or tailpipe - he shutdown but the residual oil was still burning (smoky too) so he hit the starter again before the core had stopped. The impact load sheared the shaft.

At that point we had very strange symptoms - can you guess what they were?

:cool:

Clue: the N2 tach is driven by the gearbox.

Aerotech,
"Shear sections" on drive shafts are put there to protect the component being driven, from being damaged when normal operating conditions are not encountered. EG: cold oil being too viscious and rotating the pump will cause unecessary damage to the pump.
The shear section also protects the component when you loose oil/fuel for lubrication. Thats good coz you fix the leak, replace the broken drive shaft and carry on. Thus saving an expensive bill to replace the damaged component.
As for how often this happens, depends on how old the part is, how much money you spend on overhaul(cheap overhaul means cheap job done), and the culture of standard practices( she'll be right mate type attitude).
As for "de-oil solenoid", now thats the guys who made the engine, found out that the starter was not "grunty" enough to ratate the engine at a sufficent speed to get it started without other problems. Having air in the oil reduces the friction on rotating the engine.
So bascially "shear sections" mean what it says. they are a design feature.

Hi,

Thank you for posts.

Originally posted by Bolty McBolt
I am not postulating, here what I know According to some forums:
In long ETOPS flights (207 or 180 minutes), the APU is cold soaked after long flight and may not start at high altitudes, therefore certain APUs are fitted with such devices to allow cold soaked APU to start. (it will be nice if someone can enlight me regarding such devices in ETOPS APUs in 777, A 330, 767,...)

-May be these de-oil solenoids are also fitted in APUs that usually operate in cold airports?

Barit1, may be no indication of N2 & oil pressure ?

aintsaying, what are gearbox components (IDG, IDG,...) that are fitted with shaft shear section?

Feedback appreciated. Thank you.
Best regards.

Nope - the other way around. We had N2 INDICATION(!) & OP (gearbox driven by starter), we could initiate FF, but no lightoff (no airflow - the core wasn't turning - duh!) The starter would spin up to cutout speed since it had minimal load, then everything coasted to a stop.

When we finally realized there was no turbomachinery rotation (despite the positive N2 indication), the first thing we checked was the tower shaft. :8

I had expected a brittle fracture in the shear section - but instead it was a very smooth plastic failure. The surfaces looked like they were machined & polished. :O

AeroTech

You have missed the point. The APU compartment on is non pressurised or heated area...therefore the the APU is subject the atmospheric conditions the aircraft is in.
Most large aircraft will cruise at 30,000 to 42,000 feet altitude.
The airtemp at this height is somewhere around -40 to -56 deg C
You could land in the middle east on a 42 deg C day and still reqire a de-oil device during start due to the fact that the APU has been cold soaked from the previous flight.
doh....

NB i seem to remember the 331 fitted to the A330/A340 has a small heating element that kicks in at sub zero oil temps to prevent the oil from freezing

777 APU oil is heated electrically.