Planetary gears

[Arthur Marshall]

Being a bloke of no engineering knowledge, can someone explain this in idiot-proof terms these for me? According to the engineer, something seized up in the planetary gears in my Lister gearbox which then broke the reversing mechanism. I still had forwards and neutral OK.

I realise I probably won't completely understand it, but I like as far as possible, not to appear a complete ignoramus... I've got a parts manual somewhere so can try to have a look at whatever you can tell me.

[nicknorman]

I don't know about this specific gearbox but planetary gears are part of an epicyclic gear train. You could look it up on Wikipedia etc. There is a central (sun) gear, and outside that a much larger ring gear (teeth on the inside of the ring). Between these two gears are the planet gears in a carrier, that revolve around the sun gear and the mesh with both the sun gear and the outer ring gear. If the outer ring gear is just free to rotate, nothing happens as it just spins. If the outer ring gear is held stationary, the drive is transferred between the sun gear and the planet carrier. Typically a brake band is used to stop the ring gear when the gear selector is moved out of neutral. Epicyclic gear trains are good for slow rotation speeds with high torques.

 

If there is a problem with a planet gear, eg cracks resulting in bits like teeth breaking off or even the whole gear cracking open, the debris tends to get jammed in all the meshing of the gears.

Edited November 21, 2016 by nicknorman

[Arthur Marshall]

Thanks... I've had a look at some videos and sort of understand! Really wish I'd done some work on this thirty years ago.

[Bee]

Yeah, I kind of understand. Now differentials, they are weird.

[ditchcrawler]

Yeah, I kind of understand. Now differentials, they are weird.

I didn't get my head round them until I built a Maccarno one

[MoominPapa]

I don't know about this specific gearbox but planetary gears are part of an epicyclic gear train. You could look it up on Wikipedia etc. There is a central (sun) gear, and outside that a much larger ring gear (teeth on the inside of the ring). Between these two gears are the planet gears in a carrier, that revolve around the sun gear and the mesh with both the sun gear and the outer ring gear. If the outer ring gear is just free to rotate, nothing happens as it just spins. If the outer ring gear is held stationary, the drive is transferred between the sun gear and the planet carrier. Typically a brake band is used to stop the ring gear when the gear selector is moved out of neutral. Epicyclic gear trains are good for slow rotation speeds with high torques.

 

If there is a problem with a planet gear, eg cracks resulting in bits like teeth breaking off or even the whole gear cracking open, the debris tends to get jammed in all the meshing of the gears.

 

Lister gearboxes are almost but not exactly like this. Describing the exact arrangement is difficult, but at least be aware that taking Nick's excellent description too literally may lead you astray.

 

MP.

[MoominPapa]

In a Lister gearbox, there's an output shaft, a carrier with small planetary gears, and an input shaft. The planetary gears mesh with both the input and output shaft. The forward clutch locks the output shaft to the planetary gear carrier. Since the PGs are in mesh with the output shaft, they're stopped from rotating relative to the carrier, and because they're in mesh with the input shaft, the whole mechanism; carrier and output shaft are driven round at input shaft speed.

 

In reverse, the break band stops the gear carrier from turning, the input shaft rotates the planetary gears, and they rotate the output shaft backwards.

 

In neutral, the carrier is free and it is driven round by the input shaft in such a way that the output shaft doesn't rotate. This, paradoxically, is the most difficult mode to get your head around.

 

It's slightly difficult to see what failure mode would jam the gearbox in reverse, but leave forward and neutral working. A jamb between the gear carrier and output shaft would cause the whole thing to seize solid when reverse was selected, but that would also give no neutral.

 

MP.

Edited November 21, 2016 by MoominPapa

[Arthur Marshall]

What seems to have happened (as I understand it, which I may well not) is that something happened to the planetary gears and it broke the brake band mechanism. There's an adjuster for it on top of a sort of spindle, and it seems it's that that's bust. At least, he can just lift it out and it's not attached to anything any more, which I suspect it should be. The band itself may also have disintegrated, though I'm not sure about that.

PS I really appreciate your explanations!

It wasn't jammed in reverse, it just stayed in neutral and raced the engine. Going into reverse simply had no effect.

[Tony Brooks]

Agree with MP - I would say a suspect diagnosis. More likely some ham fisted attempt to adjust/remove the brake band snapped the adjuster or its simple metal fatigue.

[MoominPapa]

What seems to have happened (as I understand it, which I may well not) is that something happened to the planetary gears and it broke the brake band mechanism. There's an adjuster for it on top of a sort of spindle, and it seems it's that that's bust. At least, he can just lift it out and it's not attached to anything any more, which I suspect it should be. The band itself may also have disintegrated, though I'm not sure about that.

PS I really appreciate your explanations!

It wasn't jammed in reverse, it just stayed in neutral and raced the engine. Going into reverse simply had no effect.

By "jammed in reverse" I meant "unable to rotate in reverse" not "stuck in reverse". Sorry for the confusion. Something which forced the gear carrier to rotate despite the brake brand and therefore broke it.

 

MP.

[Arthur Marshall]

Agree with MP - I would say a suspect diagnosis. More likely some ham fisted attempt to adjust/remove the brake band snapped the adjuster or its simple metal fatigue.

No one's touched the adjuster for a few years, so I suspect metal fatigue too. It's been doing its stuff for over fifty years after all.

[MoominPapa]

Is this a manual or hydraulic box? If it's manual, there's a simple test, but it won;t work on the hydraulic 'box because that needs oil pressure to select gears,

 

With the top off the gearbox and the selector in neutral, rotate the prop shaft. You should see most of the internal parts of the gearbox rotating, and specifically the drum that the brake-band works on. If that's happening without making awful noises then the planetary gears are probably OK. Manually stop the brake drum from turning. It should now not be possible to rotate the prop shaft. That confirms that the prop shaft and engine are engaged via the planetary gears.

 

MP.

[Tony Brooks]

By "jammed in reverse" I meant "unable to rotate in reverse" not "stuck in reverse". Sorry for the confusion. Something which forced the gear carrier to rotate despite the brake brand and therefore broke it.

 

MP.

 

The gear carrier or drum is locked to the output shaft so it rotates under engine power all the time the box is in ahead and for a few seconds upon every start up while oil pressure builds.

 

I would expect anything jambing the drum would stall the engine before anything snapped but there is always a first time.

 

If the brake band lining snapped and became detached it could or maybe would jamb the drum but I doubt the engine could be started and if it did it while running I would expect the engine to overload (black smoke/slow down) and stall.

[Giant]

In a Lister gearbox, there's an output shaft, a carrier with small planetary gears, and an input shaft. The planetary gears mesh with both the input and output shaft. The forward clutch locks the output shaft to the planetary gear carrier. Since the PGs are in mesh with the output shaft, they're stopped from rotating relative to the carrier, and because they're in mesh with the input shaft, the whole mechanism; carrier and output shaft are driven round at input shaft speed.

 

In reverse, the break band stops the gear carrier from turning, the input shaft rotates the planetary gears, and they rotate the output shaft backwards.

 

In neutral, the carrier is free and it is driven round by the input shaft in such a way that the output shaft doesn't rotate. This, paradoxically, is the most difficult mode to get your head around.

 

Our box turns the output shaft gently forwards when in neutral - I've not got into working out why yet, but I guess from this description it could be the brake band not quite fully releasing the carrier?

[Tony Brooks]

I would suggest oil drag, possibly between the ahead clone clutch parts. If the brake band was dragging the shaft would rotate in astern direction.

 

Remember these boxes lock themselves into ahead when the engine stops so always turn in the ahead direction as you start the engine until the internal oil pressure has built up.

 

If you can stop the shaft rotating by standing on it in neutral I doubt there is anything wrong with it.

Edited November 23, 2016 by Tony Brooks

[bizzard]

A severe whack and sudden stoppage of the propeller whilst in astern gear can bust the reverse brake band anchorages. A drunken bloke I know whilst driving his Dutch barge did it. I can't remember what the gearbox was now but it wouldn't matter whether it was hydraulically or mechanically operated.

[Tony Brooks]

That makes sense Biz.

[Giant]

I would suggest oil drag, possibly between the ahead clone clutch parts. If the brake band was dragging the shaft would rotate in astern direction.

 

Ah OK - I figured it was either the reverse band or the forward clutch being not quite free, but couldn't quite work out from MP's description which was indicated by a forward rotation in neutral. Oil pressure is OK so I don't think it's a lack of that.

 

If you can stop the shaft rotating by standing on it in neutral I doubt there is anything wrong with it.

 

I'm not sure anything could convince me to touch a rotating propshaft. It's turning a 29" prop so must have a decent bit of torque behind it, even if only turning slowly.

 

There probably is something wrong with it, but there are plenty of other things wrong too, so it will all be getting done at some point.