Quote:
Originally Posted by vstech
wait a minute, before you just agree to disagree...
please expound on your thoughts of why it's different from engine oil filter?
do you mean to say that since the oil pan is in contact with the bottom of the transmission, that oil is not being filtered due to splash from the pan?
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No!
Quote:
Originally Posted by vstech
... I'd like to understand what you see different.
thanks.
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Right John – I'll try and explain what I think is happening.
(Boy am I pissed off with ATSG that they don't have a proper explanation of operation chapter in either the 722.1 or 722.3/4 manual. I don't have access to the paper FSM version that has a chapter on automatic transmissions so I can't check to see if there is a description there.)
The purpose of the front pump is to provide hydraulic pressure.
The output from the front pump is connected to the valve body.
Springs and pistons in the valve body react to the pressure produced by the front pump.
EDIT:- The governor that is attached to the output shaft of the transmission controls when shifting should occur
For example, at a certain input shaft speed – somewhere in the valve body - a spring and piston reaction force will be overcome by a force due to the pressure created by the front pump. So a spring and piston will move at a certain speed...
...this spring and piston will be connected “hydraulically” to an actuator that will move either a brake band or a clutch. Gripping hold of parts or releasing parts of the sun and planet gears in the transmission is what changes the gearing – in other words the relationship between the input shaft rotational speed and the output shaft rotational speed.
The motion – the distances travelled – by the pistons and springs in the valve body are small. (Have a look at the pictures I posted in this thread
722.118 Automatic transmission rebuild (Monster DIY)) The volume of fluid pushed out of the valve body (due to movement of the springs and pistons within) is correspondingly small. I think that this type of movement of fluid through the valve body or “flow” is the only way in which fluid leaves the pressurised side of the front pump.
This type of “flow” is not equivalent to that of the “cleansing” situation of an engine oil pump (be it a diesel engine or not). This type of flow / movement of fluid through the valve body only occurs when it is time to change gear.
As the transmission filter is only connected to the input of the front pump it therefore does not perform a task that is similar to the “cleansing” process in an engine. It is behaving like a condom – it is just protecting the parts that are upstream; where the valve body is arguably the most crucial part. It is not behaving like a liver where all of the fluid (in the transmission) gets the treatment.
Phew! Deep breath – tea break.
Now you could argue that pressure is tapped off from the front pump and used to make sure that the moving parts in the gear set are nicely lubricated. My problem with that line of thought is that in the 722.118 transmission that I rebuilt and in the 722.120 transmission (that's also in bits) in my garage I can't see any evidence that the front pump does this job. The valve body only connects to the actuators. It doesn't have an oil supply line that goes to the bearings for example.
Besides why do you need this type of system in a gear box? Manual gear boxes and differentials don't need a pressurised fluid system to keep them lubricated.
You could also argue that the flow through the filter and the front pump is much greater – that the filter filters all of the oil in the transmission much like the lubrication system in an engine. Again, I have a problem with this line of thought, as I don't see an output from the “front pump system of plumbing” that would enable that kind of flow. I would expect to see an orifice of similar size to the input (shown in the pictures posted previously in this thread). If the front pump was for filtering and filtering alone you'd want to pump through as much fluid as possible – the exit hole would be obvious – you wouldn't mess about with piddly little passageways like the ones in the valve body.
From my observations of the piddly little passageways in the valve body I have reached the conclusion that the front pump is only used for one purpose – to represent input shaft speed as a function of hydraulic pressure. I don't think it is there for anything else.
(In desperation I will repeat that) The valve body uses the pressure produced by the front pump to move actuators that clamp onto the sun and planet cog systems in the gear set so that the transmission can change gear. The motion of the valves within the valve body are small and displace a small amount of fluid – which is why I think the transmission filter is performing the job of condom rather than liver.
I'll try and make it as clear as I possibly can (before I just give up and agree to disagree):-
I think the front pump is there to produce hydraulic pressure – it is not there to pull fluid through the filter.
@everyone:- Whether you agree with or not is my text clear and understandable?