one way clutches

[vorticey]

i metioned this in a thread wich i cant seem to find so, on a robbie,if a main or tail rotor gearbox seizure happends the rotors stop aswell. true? because the one way clutch is only isolating the engine.. .is this the same as most helicopters?? <img src="eek.gif" border="0">

[rotormatic]

You are correct..the freewheel only isolates the rotor drive system from the engine. Depends on the design of the freewheel how much of the main transmission will be isolated from the problem.

Some helicopters, like the Robinson, have the freewheel on the input to the main transmission...Other designs have the freewheel inside the main transmission, allowing some gearbox failures to be isolated by the freewheel unit.

If the main gearbox fails above the freewheel, everything stops...as for the intermediate and tail rotor gear boxes, if they fail, the main rotor usually has enough energy to continue rotation after the little gearboxes fail...

[Hone22]

ON the H300 (& possibly other makes) there are shear pins/bolts afixing the mast to the tranny output. If the tranny sizes, pins/bolts shear and there is still (hopefully) a certain amount of rotation by the main blades.

Thinking about it though, the tail rotor would stop with the tranny. So there's a failed TR situation to contend with at the bottom of the auto.

Cheers & fly Safe. .Hone

Ps: The shear pins/bolts were explained by a nice LAME after the same idea struck me (hope he wasn't doing the lame, lets have this pilot on thing)

[Lu Zuckerman]

To: Hone 22

I may be totally wrong but to the best of my recollection the H-269 / 369 models have static masts. The rotorhead is mounted on the static mast and is isolated by thrust bearings and held in place with a Jesus nut that was designed by Sikorsky and used under license by Hughes. A drive shaft is installed and it is splined to the output gear and is attached to the rotorhead by a series of bolts. I believe that these bolts are of sufficient strength that the seizure of the transmission will not cause the bolts to shear. The energy required to shear these bolts would have to come from the moment of inertia of the rotating rotorhead. The energy required to fracture these bolts would be so great as to totally fail the rotorhead. At least I believe this to be true IMHO.

The EH-101 was designed to have a shear point that would fracture if the transmission seized. The transmission is designed so that if this fracture were to occur the tail rotor would continue to rotate and one hydraulic pump would continue to function. As indicated above the energy to cause the fracture is the kinetic energy of the rotorhead. This kinetic energy is transmitted from the blade to the damper and then to the rotorhead. There is one small problem. The damper was designed to have a maximum tensile and compressive load of 1800 pounds with a safety factor of 1 1/2 or 2700 pounds. The load that is required to initiate the fracture is considerably higher than 2700 pounds so that if a fracture would occur the dampers would be destroyed. The destruction of the dampers would result in loads being inputted into the elastomeric bearings most likely causing them to fail as well.

[Rob_L]

I believe that 300 and 500 series have a weak point at the bottom of the main rotor drive shaft.. .If the box seized this would shear and the head would be retained by the main thrust bearings mounted at the top of the mast. It would continue to rotate.

Both have aluminium tail rotor drive shafts which would shear if the TRGB seized.

[Lu Zuckerman]

To: Rob_L

While working at Hughes I was made aware of a minister that used his 300-C for cross border church purposes. On a return flight to Arizona from Mexico he mistook a river for the active runway. He landed and the tail rotor struck the water, which immediately effected the synchronization between the main rotor gearbox and the tail rotor gearbox. An Arizona National Guard UH-1 lifted him out of the water. The only obvious damage was to the tail rotor but when they pulled the gearbox off of the boom they inspected the tail rotor drive shaft. It did not shear instead it twisted which made it shorter and it pulled on the diaphragm couplings expanding them like an accordion. This placed them under tension and they pulled off of the tail rotor gearbox and the main transmission. Nothing sheared, the whole assembly just got seriously distorted.

I was not aware of the shear point on the transmission drive shaft but then again I was being paid to work on the Apache.

One strange thing, the minister sued Hughes for the damage to his helicopter, as it was his feeling that nothing should have broken. He lost.

[Rob_L]

Hi Lu

I should probably have said "could" shear.

Might there be a difference between a seizure in flight and a sudden stoppage? I must admit that all the 300 incidents I have seen are in the latter category with substantial damage all round and as you say the TRDS has twisted. With the 500 I have seen a sheared TRDS but that was the least of that aircrafts problems.!!!!!!!!