How does it work?

[bizzard]

Dear old Tim is probably still recovering from a bit of set to with me yesterday over machining a belt groove in a flywheel. bizzard.

 

So during this interlude i shall explain in simple terms how to convert an Aston Martin ZF 5 speed,all synchromesh gearbox for marine use with chains and sprockets,or perhaps another time. bizzard.

 

Interlude continues.

 

When i was an apprentice mechanic in the motor trade.I was assigned by a foreman the job of taking out and overhauling and fitting a new crownwheel and pinion to the differential of an old Morris 10.

All went smoothly. I refitted the unit back into the axles banjo case,connect up its propshaft,halfshafts ect.

Got in started the car up to give it a little test run around the yard.

Low and behold i put into 1st gear and the car went backwards,then into reverse and the car went forwards. UM,

What i'd done was put the diff unit back in the axle casing upside down,which reverses the whole motion. Those were one of the very few vehicles that this mistake could be made.

[Chertsey]

right,on the right of your picture you can see a bronze bit that is a rowlock ,but upside down.it is commonly called a YOKE/

 

ahead is engaged when the shiny silver tapered bit is forced forward by the yoke and the three TOGGLES are splayed out as they rise up the ramp.

 

when the TOGGLES are splayed out the lock up the CLUTCH PACK inside the big rotating lump in the middle of the gearbox housing.

 

 

when astern is required,another lever,which cannot be seen in your picture,as it's working part are suberged in oil at the bottom of the gearbox,causes a BRAKE BAND to lock the the big rotating lump and hold it still.

 

 

within the big lump there is a set of gears which do function in a similar manner to a vehicle differential.

 

if you put a vehicle in gear with the driving wheels off the ground and prevent one of them from rotating,the other wheel would rotate in the OPPOSITE direction.

 

so,when you lock up the big lump with the BRAKE BAND,the inner geartrain causes the OUTPUT SHAFT to rotate in the opposite direction,thereby giving you reverse THRUST.

 

 

this is not easy to explain without a good deal of engineering terminology.

 

so ,are you any the wiser?

Yes, thank you, I am groping my way towards it I think. What would be really good would be a picture or diagram of the bit I can't see.

[Guest]

Interlude continues.

 

When i was an apprentice mechanic in the motor trade.I was assigned by a foreman the job of taking out and overhauling and fitting a new crownwheel and pinion to the differential of an old Morris 10.

All went smoothly. I refitted the unit back into the axles banjo case,connect up its propshaft,halfshafts ect.

Got in started the car up to give it a little test run around the yard.

Low and behold i put into 1st gear and the car went backwards,then into reverse and the car went forwards. UM,

What i'd done was put the diff unit back in the axle casing upside down,which reverses the whole motion. Those were one of the very few vehicles that this mistake could be made.

 

Interlude part 2

 

Wow like an Italian tank then - one forward and four reverse gears...

[Chertsey]

Another Parsons-with guts exposed

 

 

Edit....you can just see the crownwheel and pinions.

Ah, that looks useful.

 

They were so right: 'Zeus has now been sold. I leave this page here in case there is anyone interested in looking inside a Parsons Marine gearbox. You never know....'

 

Before I get too into it, is this the same as wot I've got?

Edited October 10, 2011 by Chertsey

[cereal tiller]

Yes, thank you, I am groping my way towards it I think. What would be really good would be a picture or diagram of the bit I can't see.

look up epicyclic gear on wikipedia,quite good with a clear diagram.

[Chris Pink]

Unfortunately you can't embed swf on this forum;

 

 

http://www.mekanizmalar.com/transmission.swf

[Timleech]

Here is a picture:

 

Alan, you've frightened me now... I was told they were bombproof!

 

As has been explained, the cone on the right of the pic forces out the toggles which then press the clutch plates together to engage ahead. The bronze castellated ring inside the toggles is the ahead clutch adjuster, you can just see the locking screw & plate which stops this from turning on its own.

 

The big red thing with the copper rivets in the middle of the pic is the brake band, this is clamped around the clutch body to engage astern gear. The clutch plates are inside that bit.

 

There's a big bevel wheel permanently driven from an extension to the crankshaft. This is permanently in mesh with either 2 or 4, can't remember, small bevel pinions, the big castellated steel nut at the left of the pic holds the stub shaft & bearings for one of these pinions. These small pinions are in their turn permanently engaged with another big bevel gear, this one is mounted on a big splined shaft which carries the inner clutch plates and is also keyed to the output shaft.

If the clutch plates are compressed by the toggles, the whole central assembly, except for the brake band, revolves as one unit and the output shaft turns with the crankshaft. The bevel gears don't revolve against one another.

 

If astern is engaged, the clutch plates are slack and free to rotate against one another. The brake band holds the clutch casing, and with it the small bevel pinions, fixed and unable to rotate. This means that the small pinions turn on their stub shafts, and they in turn force the second large bevel gear - the one carrying the clutch plates - to rotate in the opposite direction (and ultimately the propellor is turned backwards).

 

Probably clear as mud

 

Tim

 

Edit to add - for neutral, the brake band and clutch plates are slack, the output shaft is free and shouldn't rotate either way. One thing these boxes do suffer from when they get to 'a certain age' is that the splines on the output bevel, which carry the inner clutch plates, develop notches from the clutch plates pressing against them. These will eventually reach the stage that they hold the clutch plates from sliding freely along the splines, and it becomes difficult or impossible to get a true neutral.

 

Another edit, right at the left partially covering the castellated nut is the oil thrower, this rotates with the input shaft, picks up oil from the bottom, and it's caught by a sort of gallery thing fixed to the underside of the gearbox lid from whence it drops into the appropriate spots within the box.

The fixing of this spun steel thrower is a bit delicate, it's not unknown for them to come loose.

 

Another edit - the gasket in the pic looks as though it might be encroaching into the space needed for proper lubrication, it should just be the width of the lands at the sides all the way round.

Edited October 10, 2011 by Timleech

[by'eck]

Don't know if this helps - clutch & epicyclic reverse gear from Blackstone box on JPM.

 

 

I think (waits to be shot down in flames ) the brake bands (74) acting on clutch body (39) stop it turning allowing planetary gears to come into play & reversing output shaft direction .

[RLWP]

It isn't exactly an epicyclic gearbox. It's got a set of bevel gears instead of the sun and planet ones

 

Richard

[by'eck]

It isn't exactly an epicyclic gearbox. It's got a set of bevel gears instead of the sun and planet ones

 

Richard

 

Maybe not but Lister describe it as having Built-in multiplate clutch and epicyclic reverse gear of robust design and construction

[Timleech]

Maybe not but Lister describe it as having Built-in multiplate clutch and epicyclic reverse gear of robust design and construction

 

I think Richard meant the Parsons box, the subject of the thread, isn't epicyclic

 

Tim

[bizzard]

It isn't exactly an epicyclic gearbox. It's got a set of bevel gears instead of the sun and planet ones

 

Richard

And when all these planetry bevel gears plus come into play when astern is selected,the gearbox is usually much noisier,''a wurring noise''.And its not recommended to run in astern for too long without frequent breaks for fear of overheating and so damaging it. This applies to most marine gearboxes without extra oil cooling facilities. bizzard.

[David Mack]

And when all these planetry bevel gears plus come into play when astern is selected,the gearbox is usually much noisier,''a wurring noise''.And its not recommended to run in astern for too long without frequent breaks for fear of overheating and so damaging it. This applies to most marine gearboxes without extra oil cooling facilities. bizzard.

 

Indeed, but when we took Fulbourne to the St Ives National in 2007 we had to go through Stanground Sluice backwards (because the lock chamber is only deep enough for our 3 ft stern draft at the downhill end), and reverse for a mile before we could wind. I was a little concerned about how the Brunton gearbox would cope with reversing for this long, but all was well.

 

David

[Chertsey]

Don't know if this helps - clutch & epicyclic reverse gear from Blackstone box on JPM.

 

 

I think (waits to be shot down in flames ) the brake bands (74) acting on clutch body (39) stop it turning allowing planetary gears to come into play & reversing output shaft direction .

Somebody actually drew that. Don't you think that's wonderful?

 

Thank you Tim for your post, that is really helpful. I think I can picture it and I can certainly understand the principle now.

Edited October 10, 2011 by Chertsey

[RLWP]

I think Richard meant the Parsons box, the subject of the thread, isn't epicyclic

 

Tim

 

{psst, it is actually - it's just a different arrangement using bevels. But let's keep that quiet as it could be confusing}

 

Richard

[PaddingtonBear]

Ah, epicyclic gearboxes, brings back happy memories of my Austin Champ radio truck which I owned in 1967/8. It had an epicyclic gearbox fitted to the generator to power the radio transmitter and a battery bank. I only had 2x12v batteries in series under the passenger seat and due possibly to serious overcharging the batteries exploded whilst driving along near our village, blowing the passenger onto the footpath. "Wow man!!" (for that is how people spoke then), he, she or it exclaimed and for some reason my offers of lifts were often declined. Good job too at 12 mpg

Edited October 10, 2011 by PaddingtonBear

[Chertsey]

And when all these planetry bevel gears plus come into play when astern is selected,the gearbox is usually much noisier,''a wurring noise''.And its not recommended to run in astern for too long without frequent breaks for fear of overheating and so damaging it. This applies to most marine gearboxes without extra oil cooling facilities. bizzard.

It does have a distinct whine in reverse. I have assumed this to be normal?

[bizzard]

Somebody actually drew that. Don't you think that's wonderful?

 

Thank you Tim for your post, that is really helpful. I think I can picture it and I can certainly understand the principle now.

Simple init Chertsey. That exploded view is what happens if you run too long in astern.

 

It does have a distinct whine in reverse. I have assumed this to be normal?

Although i'm not paticulally conversant with your gearbox i'd say thats okay.

 

PS. As long as its not a high pitched shriek which would mean lack of oil somewhere.

Edited October 10, 2011 by bizzard

[Timleech]

It does have a distinct whine in reverse. I have assumed this to be normal?

 

Yes.

 

Tim

[Timleech]

{psst, it is actually - it's just a different arrangement using bevels. But let's keep that quiet as it could be confusing}

 

Richard

 

Surely nothing in the Parsons box describes an epicycloid curve? Yes I know it's doing the same job

 

Tim

[mykaskin]

I have a Blackstone reverser and reduction box. I thought the whine was from the reduction box running backwards. Mine also whines in forwards at cirtain revs.

 

Mike

[cereal tiller]

I have a Blackstone reverser and reduction box. I thought the whine was from the reduction box running backwards. Mine also whines in forwards at cirtain revs.

 

Mike

make sure that the brake band is adjusted correctly,i could be slightly binding at certain speed/vibration levels.

[RLWP]

Surely nothing in the Parsons box describes an epicycloid curve? Yes I know it's doing the same job

 

Tim

 

Heading further

 

True, but it has a sun gear, a planet gear, a ring gear and two brakes - but they're all bevels.

 

Which bit describes an epicycloid curve in an epicyclic box then?

 

Richard

[MoominPapa]

Which bit describes an epicycloid curve in an epicyclic box then?

The teeth of the small reverse gears, but only when the gearbox is in neutral.

 

(Lister gearbox, that's the only one I know about.)

 

MP.

[Evo]

Wheres me bladdy Spirograph (even more complex than epicyclic).

Edited October 10, 2011 by Evo