http://www.aulro.com/afvb/isuzu-land...tml#post743719
thats one of the places Ive previously mentioned that torque impulse does damage to gearboxes....
assuming that thats not what you want explained you need to do a couple of things..
first go rent out or buy Hoodwinked. Tell the people its for your kids, If you dont have kids take your wife/girlfriend to the bedroom with some chocolate and flowers then explain to her that next year youd like her to be able to celebrate mothers day properly.... if that doesnt work or you dont have a wife/girlfriend try
www.rusianbrides.com.ru then after the little tacker is out, wait 5 years and then watch the movie.
The whole point of that exercise is so you can see the bit where the already hyperactive squirrel takes a shot of coffee to go uber fast... Thats the sort of speed you need to be thinking at for all of this to make sense.
(the other thing is that if you havent got a child by the time you go through this exercise you will probabley be cursing the lack of income to repair it so you'll sell the rover and hopefully I'll be in a financially more suitable position to buy it for its engine)
Ok now weve got the whole relative speeds thing under control. lets start at the root of the problem. The isuzu diesel 3.9L (turbo or not the effects the same) more accurately we want to look at the crank shaft and the number of pistons and the way that they work..
What we know is that its a 4 pot engine of near as damnit 4l capacity that makes each pot about equivelent to a 1l engine in its own right. We also know that its a 4 stroke diesel and diesels have high compression rates.
conveniently the layout of a 4 pot 4 stroke engine means that 2 pots are coming up as 2 are going down and at any one time (excluding the exclusive TDC and BDC parts where the cylinders are changing strokes) you will have
- one Cylinder on the intake stroke
- one cylinder on compression
- one cylinder on power
- and the last cylinder on exhaust.
lets look more closely at the cylinder on compression and on power. actually lets do that a little later for now lets look at the mechanical relation between the piston the con rod and the crank and what that does to the speed of the fly wheel. This is the bit where if you were to try to think of it as its happening in real time you need to be twitchy on caffine. Having read it all before I hit the send lets look at the relative postion of the crank of a single cylinder on the power stroke and after I tell you what its doing I'll describe what the piston on compression is doing.
Lets start looking at it with a crank arm at just after top dead center and the fuel air mix is just getting into doing its job now the throw of the crank never changes (but it would be cool if we could make it do that reliabley) but the effective leverage it offers does and it does so following a sine wave. But for the purpose of this explination we'll just assume it does it linierally
I all the pics that follow
- The black circle is the flywheel,
- The yellow circle is the center of the crank and represent the crank journal about which the whole shebang rotates (the vertical line is just for you to pull a reference off of)
- The red circle represents the big end which is the bit the piston pushes on and again the line is only there for you to draw a reference from
TDC.jpg
thats how the crank looks (for a single pot) just after TDC you can see that therse not a lot of mechanical advantage being offered to the piston to turn the crank but right now thats cool, we need to get the combustion process sizzling to get maximum pressure and power out of our fuel air mix. (note the short distance between the red and yellow lines). What you cant see on this pic is the next cylinder, the one thats on compression right about now its offering a little bit of resistance and mainly just from internal friction so its trying to slow the crank down but its not very effective right now.
power.jpg
Now were getting places the crank is starting to offer more leverage to the piston (bigger gap between the lines yeah) and the fuel air mix is giving it some oompgh so the crankshaft will accelerate on the way down and as it does so the crank moves further "out" from center and offers even more leverage. Now when your idling you dont get a lot of power so lets just say that the power is only put onto the crank from here to mid stroke. At the same time not illustrated is the same cylinder that wasnt ilustrated before its crank is moving in the same way but now its acting like a brake (think of a see saw) its not overly effective right now as its only pushed the piston up against the air a little bit so the pressure resisting the motion isnt very high
midstroke.jpg
and that would be midstroke if you had more power on then the crank would be offering maximum torque at this postion providing that the fuel air mix was still burning (biggest distance between the 2 lines) nowfrom here on it begins to get interesting. The crank is no longer being pushed by the piston as hard as it was as the gasses arent expanding as much and what little effort is being made is being made against a shortening lever but at the same time the piston thats coming up on compression is starting to get some serious resistance to it and is slowing the whole lot down.
end.jpg
Now is the worst time for the slow down. The piston is near the end of its travel the gasses have done all their expanding and everything is still running on inertia but the piston on compression is still resisting due to the now high pressure being compressed on top of the piston to make it worse not far from here is when the exhaust valve opens and any residual pressure that was helping the piston will naff off out of the exhaust. On top of all of this at the same time you have a piston thats being used to push a lot of exhaust gas out of a small valve and another one thats being used to create a low pressure area to draw air in (unless its turbocharged and on boost but that doesnt happen at idle) so theres even more resitance offered to the one pot thats trying to provide power and the same rules apply those pots but not to the same extent.
and then the piston thats on compression gets its fuel injected into the hot air on top of the piston, the crank carries on on intertia and the whole lot begins again... at 700 RPM or just over 11 times a second.
so in a nutshell every half turn of the crank it gets faster and slower... hold onto that thought
now using my Extra magic gas axe IVe sectionalised a contact section of the teeth of a helical cut gear set, lets call it the input shaft to the layshaft gear set of your gearbox
- the black line represents the axis of rotation
- the red section is the input shaft gear thats driven by the engine (and the arrow shows which way it wants to apply force
- the green section shows the lay shaft main gear and the arrow shows which way it wants to apply force
slip.jpg
Now you can imagine that if force was applied to my over angled tooth section it would force the gears away from each other and into whatever else happend to be in the direction that they want to move in and that just happens to be the bearing.
so picture if you will a shaft thats turning at say 700rpm and jumping back and forth into the bearing 11 times a second. Yes I know what your thinking relatively speaking the shaft isnt that heavy but its persistant and it gets better at doing it if you labour the engine at low RPM in a tall gear (or even low first if your trying to walk it over rocksnstuff).
Its about the equivelent of using a bucket to empty an olympic swimming pool, It takes an age but it can be done and as the bearing goes out of spec from hammering it takes longer for the damage level to be increased as the extra distance that the shaft has to travel to cause an impact means that by the time it gets there then the force driving it has reduced much like once the water level has gone down you have to climb down the ladder to get to the water to get the next bucketfull so it takes you longer to get each subsequent bucketfull out.