reverse piston replacement

[James.uk]

Nice going Reece.

Erm what piston? I can't see any in the pics? or if it is i can't recognise it as such.

And I assume the solenoids are housed elswhere?
...

[crofts]

Originally Posted by James.uk

Nice going Reece.

Erm what piston? I can't see any in the pics? or if it is i can't recognise it as such.
And I assume the solenoids are housed elswhere?
...

Well you see a piston aint a piston innit. It's one of those circular bits in the photo.One wonders how it gets the name piston !
The cracks I have seen are hairline so not immediately obvious
Solenoids are in another location.
Bet I've got something wrong............usually do ! !

[James.uk]

>>>Well you see a piston ain't a piston innit.<<<

Ah well, that makes everything as clear as mud innit, so I'm off to dangle me feet in the jelly till Reece gets back.

Maybe what appears to be a ball race in the pics, is actually a house brick used in building upside down light houses for submarines innit..
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[rrobson]

just for you james, heres what the upside down house brick looks like

heres the assembly



outer removed



this is the piston on its own







this is the crack, it is right on the edge but its difficult to get a pic off. i will have to rub some muck into to make it more visible



mr crofts is quite right, but i have no idea why its called a piston. it probably moves in a similar way to an engine piston, if not quite as much.

[crofts]

Originally Posted by James.uk

>>>Well you see a piston ain't a piston innit.<<<

Ah well, that makes everything as clear as mud innit, so I'm off to dangle me feet in the jelly till Reece gets back.

Maybe what appears to be a ball race in the pics, is actually a house brick used in building upside down light houses for submarines innit..
...

Oh James you are wasted. With your immediate grasp of the complexities and workings of an auto box you really should be putting your talents to designing and producing something better than upside down light houses for submarines. What about a square piston, now that would be truly revolutionary, with so many uses. (Can't think of any offhand but will let you know when I do ! ! ! )

[James.uk]

Hmmm ... Epicyclic gearing or planetary gearing, but then it's not is it? Looks more like a ring gear to me Reece, but no star wheels to go inside it. I assume it just slides up and down on it's internal splines, engaging and disengaging on the outer teeth...?

Nice piece of engineering innit.. But I can't see why the casting should have failed, must be under a huge amount of pressure-stress innit...

When you have finished this one Reece, I reckon you will have a queue of orders waiting...

OFF TOPIC... (Croft) Dave... .. When I was at college.... For my final test piece:-- I made a switch box with the switch arm on the outside. when you pressed the switch down, a lil brass hook appeared through a hole above it, operated by a small clockwork rack and pinion. This then dropped over the switch and was retrieved by a spring, switching itself back off as it retracted...

I got no points at all for it? teacher couldn't get why I made something that just switched itself off, so I got my pass based on my last years work innit... heh heh....
...

[beinet1]

The so called "reverse piston" is a piece with a lot of functions. Look at the first picture in the post above, you can see the piston installed inside its "cylinder". The spline`s task is to transfer torque between the cylinder and the piston. When pressure is applied to the outside of the reverse piston, it will compress the reverse clutch so it engages.
The reverse piston is also the "high gear cylinder". The first picture in the post above, also shows the "4th and 5th. gear piston installed inside the reverse pistion. When pressure is applied to this, it compress the high gear clutch and engages 4th or 5th gear. When the reverse piston is cracked, it will leak pressure out of the "reverse cylinder" and/or "high gear cylinder". Depending on how much leak there is, it will decide if the clutches will only slip or have a total loss of torque transfer.

The hydraulic pressure in these boxes are typical around 15-30 bars

Did this make`s thing more clear??

P.S.
Here is another picture of a cracked piston:

[rrobson]

Originally Posted by James.uk

When you have finished this one Reece, I reckon you will have a queue of orders waiting...

well, my services are avaliable to those who require it, doesnt take long to do it

Originally Posted by beinet1

The so called "reverse piston" is a piece with a lot of functions. Look at the first picture in the post above, you can see the piston installed inside its "cylinder". The spline`s task is to transfer torque between the cylinder and the piston. When pressure is applied to the outside of the reverse piston, it will compress the reverse clutch so it engages.
The reverse piston is also the "high gear cylinder". The first picture in the post above, also shows the "4th and 5th. gear piston installed inside the reverse pistion. When pressure is applied to this, it compress the high gear clutch and engages 4th or 5th gear. When the reverse piston is cracked, it will leak pressure out of the "reverse cylinder" and/or "high gear cylinder". Depending on how much leak there is, it will decide if the clutches will only slip or have a total loss of torque transfer.

The hydraulic pressure in these boxes are typical around 15-30 bars

Did this make`s thing more clear??

thanks for that, it makes its operation much clearer

suprising how a hairline crack can such a big impact on the gearbox. 30 bar isnt exactly a lot of pressure

[rrobson]

Quick update, I've spoken to the very helpful people at automatic choice today who have sorted me out with the parts I need to repair the box.

First thing they said was to check the clutch plates, especially the two bigger ones and I found them to be worn! So top marks there. They said these parts usually wear when the piston goes and they tell everyone to check them because there's no point doing a job twice an sending two packages.

So for a reverse piston (uprated), o ring kit, circlips, two clutch plates and next day delivery it cost me 160 quid, so that plus oil and its set me back a little over £200. So not a bad result if it all works as it should so fingers crossed!

Will be doing the repair either fri or sat so I will keep you posted

[beinet1]

Originally Posted by rrobson

well, my services are avaliable to those who require it, doesnt take long to do it



thanks for that, it makes its operation much clearer

suprising how a hairline crack can such a big impact on the gearbox. 30 bar isnt exactly a lot of pressure

30 bars is not that much, but it will generate much force when applied to a greater piston area. I measured my old piston and calculated the piston force. At 30 bars, the high gear clutch will be compressed with a force of 17kN/ approx 1,7 tons and the reverse clutch will be compressed with a force of 41,8kN/ approx 4,18 tons.
This gives an idea of the loads put on this piston I guess that the hairline crack will open up a bit when the pressure is applied

In my case, the crack caused the reverse clutch to slip. Seems like the previous owner continued to drive which resulted in a completely worn reverse clutch which again resultet in excesive travel for the reverse piston which made its seals to travel of it seal surface and shred it seals. The result was no drive in the high gears. Have you had a closer look at the seals inside the reverse piston and high gear piston??

This is how mine looked:



Good luck with the reassembly. Keep us updated

Tip of the day: "Housekeeping" is one of the keys to success during re-assembly. Don`t let any dirt or debris left in your box