CaptainSlog
Part of things
Posts: 510
Club RR Member Number: 180
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1951 Pontiac ChieftainCaptainSlog
@captainslog
Club Retro Rides Member 180
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Enjoying the gearbox rebuild and good to know you can source the parts, Ive never liked autos as they always seem mega complex but this doesn't actually look too bad! For me I think it is the complexity, when compared to a manual 'box (scary enough by themselves) and fear of the unknown that puts us mortals off autos |
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 Well, that's all burned up.  That doesn't look none too clever either. Phil |
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 Other drum came apart with creative use of mallet and wood. This has friction material left but the seals are junk and the steels have heat fractures. Phil |
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So, the behavior of this is fairly straightforward.
The band is easy to understand, it wraps around the outside of the drum (you can see where it's worn the drum above), and a hydraulic piston in the servo pushes it tight, it stops the drum like a brake.
The clutches are a little more complex but not by much. The clutch pack itself is made up from a sandwich of alternating steel plates and rings with friction material on. You can see the teeth on the inside edge int he picture above; those are only on the friction rings. Those teeth engage with the sun gear in the center. The steel plates are toothed on the outside edge and are locked in place and rotate as part of the outer drum. You can see the 3 pegs that hold them around the edge. It's all spring loaded by the springs around the outside edge of the discs.
The lid is in two pieces, one that fits inside the other with an oil feed the same as a main bearing gets on an engine. Hydraulic oil is fed into the center shaft of the drum (dull cast iron piece), which goes into the void inside. That then forces the piston (silver alloy piece) to expand out and press against the clutch stack, forcing it to engage and grab.
The friction material on these is an organic fiber material around the outer edge and cork around the inner. As such, they can only tolerate a certain amount of heat- they really aren't meant to slip. They're meant to engage quickly and not heat up. If everything's adjusted incorrectly then they'll slip and fight and have to work harder than they should. In this case they've become so hot they've burned to a crisp and all that crispy stuff ended up in the oil pan. Then because there was no more friction material left the slip was worse and heated up so much it softened the bearings and ruined them.
The funny thing is, even with all that is wrong with it, the gearbox still functioned enough to drive the car down the road. It wasn't good but it did work. I guess that's testament to the design.
However, to try and prevent that happening again, I'm going to be setting this up properly...
--Phil
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Last Edit: Aug 7, 2019 15:04:57 GMT by PhilA
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 My snap ring pliers arrived! Phil |
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So, the behavior of this is fairly straightforward. The band is easy to understand, it wraps around the outside of the drum (you can see where it's worn the drum above), and a hydraulic piston in the servo pushes it tight, it stops the drum like a brake. The clutches are a little more complex but not by much. The clutch pack itself is made up from a sandwich of alternating steel plates and rings with friction material on. You can see the teeth on the inside edge int he picture above; those are only on the friction rings. Those teeth engage with the sun gear in the center. The steel plates are toothed on the outside edge and are locked in place and rotate as part of the outer drum. You can see the 3 pegs that hold them around the edge. It's all spring loaded by the springs around the outside edge of the discs. The lid is in two pieces, one that fits inside the other with an oil feed the same as a main bearing gets on an engine. Hydraulic oil is fed into the center shaft of the drum (dull cast iron piece), which goes into the void inside. That then forces the piston (silver alloy piece) to expand out and press against the clutch stack, forcing it to engage and grab. The friction material on these is an organic fiber material around the outer edge and cork around the inner. As such, they can only tolerate a certain amount of heat- they really aren't meant to slip. They're meant to engage quickly and not heat up. If everything's adjusted incorrectly then they'll slip and fight and have to work harder than they should. In this case they've become so hot they've burned to a crisp and all that crispy stuff ended up in the oil pan. Then because there was no more friction material left the slip was worse and heated up so much it softened the bearings and ruined them. The funny thing is, even with all that is wrong with it, the gearbox still functioned enough to drive the car down the road. It wasn't good but it did work. I guess that's testament to the design. However, to try and prevent that happening again, I'm going to be setting this up properly... --Phil Pretty much a motorbike clutch then? Just actuated differently. Do the steel plates wear grooves into the 'basket' like they do on bikes? |
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Exactly like a multi-plate motorcycle clutch, yes. They do wear grooves but it's less because the connection to the engine is not so direct through the fluid coupling and clutch plate style springs in the housing, so a lot of what causes them to chatter in a motorcycle isn't present in this gearbox.
However, if they could they would
--Phil
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Well. I've enjoyed reading that, probably more than you've enjoyed finding the problems... I do love the approach you're taking to repair each part in turn. Proper job in getting the old gal up and running for another 50 years - I've no idea what's going on in that gearbox, (not helped by a couple of glasses of Shiraz  ), but it's all looking like you know what's happening and that's the impressive part! Many thanks again for taking the time to document this - definately my favourite thread on here! |
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1968 Mini MkII, 1968 VW T1, 1967 VW T1, 1974 VW T1, 1974 VW T1 1303, 1975 Mini 1000 auto, 1979 Chevette, 1981 Cortina, 1978 Mini 1000 1981 Mini City, 1981 Mini van, 1974 Mini Clubman, 1982 Metro City, 1987 Escort, 1989 Lancia Y10, 1989 Cavalier, 1990 Sierra, 1990 Renault 19, 1993 Nova, 1990 Citroen BX, 1994 Ford Scorpio, 1990 Renault Clio, 2004 Citroen C3, 2006 Citroen C2, 2004 Citroen C4, 2013 Citroen DS5. 2017 DS3 130 Plenty of other scrappers!
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Some of this is common knowledge; general engineering principles that I have been taught or learned.
The rest I'm tending to follow the manual which has given good methodical approach to repairing these things
The Internet is kinda helpful too. I like to understand what I'm working on. I might not do it properly but I do try to understand.
--Phil
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Some of this is common knowledge; general engineering principles that I have been taught or learned. The rest I'm tending to follow the manual which has given good methodical approach to repairing these things The Internet is kinda helpful too. I like to understand what I'm working on. I might not do it properly but I do try to understand. --Phil clearly not a haynes manual then!!! |
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'80 s1 924 turbo..hibernating
'80 golf gli cabriolet...doing impression of a skip
'97 pug 106 commuter...continuing cheapness making me smile!
firm believer in the k.i.s.s and f.i.s.h principles.
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Whilst I'm very patient with my restorations & paintwork - taking a auto box to bits is well out of my league - and would see me having it down the road to a specialist - but what ever makes you happy - its not often that we see a retro auto box rebuilt on here - hope that it all goes to plan
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Phil, you are a very skilled, and brave man. My cap is well and truly doffed. I've just read about 7 pages and understood less than the last lot of stuff I didn't understand. Keep up the good work. |
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 Got busy measuring stuff last night. The thrust washers either side of the primary drum are worn. Both are about 0.087"-0.088", one has gone through the copper. The place I'm getting the spares from doesn't have any left, only 20 thou' oversize which is no good. I had measured the endfloat of the main shaft, that is compound of all the slack from the central bearing to the end of the shaft, 4 thrust washers. It totaled 0.019" which is just out of limits. If I were to add a 0.109" washer it would tighten it up solid, which is no good. The minimum clearance is 0.004" A guy has them listed so 0.090 x2 takes up about 0.004" of slack which brings the tolerance back into spec on the main endfloat. Let's hope he's still in business... Gonna call him up this morning. Phil |
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Last Edit: Aug 8, 2019 11:19:10 GMT by PhilA
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Well, in good news I have just ordered the first parts to repair the gearbox. $28 of standard thickness thrust washers (the one I had the micrometer on in the photo) that sit either side of the rear drum hub. They'll be winging their way from Massachusetts today. Thanks to Dave at autotran.us/--Phil |
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Was looking at slightly less modern tachometers than the little one I have- as such there's little point in having one that reads up to 8000 RPM because the engine won't go anywhere near that.
Figured a 0-4000 being as the "red" is 3750, the "orange" starting at 3400.
I spoke to some people and they said 55mph is 2000 RPM which the engine is really quite happy at. That's good.
Phil
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Duggy
Part of things
Posts: 129 
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George, I had typed this up- it may help a little in describing how this does what it does. The gearbox is specified in a strange way compared to automatic gearboxes I am used to. Most modern automatics have the engine drive the torque converter housing, which turns the pump, and the inner vanes of the torque converter drive the mainshaft. The output of that is then passed through the reduction gears. Pontiac specified that the vehicle should not creep at idle, in Dr (drive) position. Even with the engine idle specced lower than the manual gearbox version (375 versus 550 RPM) the fluid coupling imparts enough force at 375 RPM to move the car forwards. At a guess the rationale for this was to not alienate their customers who had never experienced an automatic gearbox before. Get in, sit down, start up and put it into gear, look there, the car does not move.. gently squeeze on the gas pedal sir.. there, the car begins to move- press a little harder, and just take your foot off the gas and onto the brake to stop. Yes, just like that sir, isn’t that a fantastic marvel of modern technology? To stop the car from creeping yet retain an acceptable idle (the engine gets a bit unhappy as it approaches 300 RPM or lower) the engine drives the outer coupling housing directly (which is not connected to either inner torus) and that is taken inside the gearbox and through a 1.45:1 epicyclic reduction gear. The output of that is then sent back to the driving torus at 7/10 the speed of the input. As a result the driving torus is turning at about 260RPM at idle. At this speed it does not impart enough torque into the driven torus to move the car. The output of the driven torus goes back deep inside the gearbox and drives a second epicyclic reduction gear of 2.63:1 ratio. The two gears driving each other in reduction results in an overall gearbox ratio of 3.82:1 in first gear. The epicyclic reduction gears can be locked by the internal clutches to either reduce the output speed or be locked solid at 1:1 ratio. Combining the 2 clutches in a binary style fashion (00 01 10 11) provides 4 reduction options- 3.82:1, 2.63:1, 1.45:1 and 1.00:1 (direct). The final drive passes through a third epicyclic gear, which, by holding the planet set still with a clutch will reverse the motion of the output shaft and provide a reverse gear. It is also locked out internally to first gear full reduction only- without that there would be 4 speeds in reverse with a top speed of about 60mph. Not ideal! The gearbox also has a few interesting design features- two oil pumps, one on the engine driven side, the other on the output shaft. This is to allow oil pressure to be built up as the vehicle is towed, for two reasons. First, it provides lubrication without the engine running, second it provides oil pressure so that if the transmission is dropped from N to Dr it will engage a gear and be capable of bump starting the engine. The oil pumps are designed to provide high pressure at low speed. This pressure is kept constant by a spring relief valve. The gears are selected by valves with varying strength springs. A governor valve (centrifugal weight that spins with road speed) increases pressure on the valves as the road speed changes, initiating gear changes. The throttle pedal is attached to the gearbox and serves two functions. First, it opens a valve that applies main pressure to the opposite side of the gear shift valves, meaning greater governor pressure needs to be built up before the valves will move and gear changes occur. Second, as the gears are changed it adds boost pressure to the servos so the bands clamp more firmly and quickly. This makes low speed, light throttle gear changes smooth to engage at low speed, and hard acceleration changes to snap tight quickly at higher road speeds. It all sounds good but there are a lot of moving parts that all need to move in synchrony. If something jams, it’ll either disengage drive fully or attempt to engage two gears at once which causes undesirable operation, neck-snapping jerks in the drive or a redline-flare of the engine. In this case, it would change gear at a much higher RPM/roadspeed than it should have, and only when the throttle was snapped shut. Hopefully that can be remedied. Fascinating engineering from a post-war era where a 'can-do' attitude shone through. Good luck getting it fixed, I bet with the new parts it'll work like a dream. |
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Last Edit: Aug 9, 2019 12:39:07 GMT by Duggy
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The other interesting feature it has is that above about 5 mph a piston moves out and engages a locking mechanism. You cannot engage reverse once you are moving forwards fast enough to break it; however they also say due to the design you do not need to be at a full stop before you can engage reverse or drive- specifically so you can "rock" the car back and forth to try get unstuck from a muddy or snowed-in rut, for instance.
--Phil
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Excellent work, this thing is going to be smooth a silk once its back together with its straight 8!
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I hope so.
Maybe not silk, but certainly cotton, perhaps linen.
They were never silky-smooth, even new with that gearbox.
--Phil
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), but it's all looking like you know what's happening and that's the impressive part! Many thanks again for taking the time to document this - definately my favourite thread on here! 
