Cat hydrostatic transmission overhaul.

[Cmark]

The next component of interest is the main control valve. (Front view)




(Back view).
There's several things going on in here including neutral start function, braking and and couple of pressure controls. What interests us though is.......




This component with the two O rings. It's the venturi. It takes the flow from the charge pump and (as we all know from high school physics class, duuh) the pressure at a venturi is lower than the pressure upstream of the venturi and the faster the flow, the lower the pressure at the venturi.

This provides us with two pressures. The difference between the pressures (delta pressure) is directly proportional to the flow and, because the flow is provided by the charge pump, the delta pressure is directly proportional to the engine speed.





BTW, apologies to any physicists reading. I'm only a diesel fitter and if my take on any of the fundamentals is way off, please let me know.

 

[Scrub Puller]

Yair . . . Cmark. Thanks for reply to my query re hours upthread . . . and I reckon you oughta drop the "only" tag from your job description. (big grin)

Cheers.

 

[Cmark]

Back view of the replenishing valves.




And the replenishing valves in place.




Main control valve manifold.




And the main control valve in place.

 

[Cmark]

Next we install the servo valve housings and connect the servo cylinder lines.



At this stage, it's vitaly important for your sanity that you don't drop anything down the sides

 

[Cmark]

This is the next component in the load sensing system. It's callled the underspeed valve. It takes the two different pressures created by the venturi and uses them to vary the ratio of input from the FNR lever to the amount of movement at the servo control valves.

Or let's put it this way. See the shaft sticking out on the right? That's directly connected to the FNR lever. The silvery thing in the centre is actualy the back of a ball joint which is connected to the servo control valves. If the engine is idling, we have a low delta pressure and if we move the FNR lever 100% FWD, none of the movement gets sent to the servo valves.

However, if the engine is at high idle and we move the FNR lever 100% FWD , the underspeed valve sends all that movement to the servo valves and both pumps go to full stroke FWD.

When the machine comes under load, the drive pressures increase and the engine slows down. This causes a drop in the delta pressure and the underspeed valve reduces the command to the servo valves, the pumps destroke a little and the load on the engine decreases.

This is what allows the operator to drive into a load and not ease back on the FNR. If an operator finds he has to modulate the FNR to reduce the load on the engine, there's a problem. This is also why it's important to run the engine at full RPM.

Also note the load on the engine doesn't have to come from the transmission. If the implement hydraulics slow the engine down, that will also cause the transmission to destroke. The whole set up is designed to keep the engine RPM from dropping below about 2000 RPM (depending on model).




But what if we want to move the machine slowly, around the yard, or loading it onto a truck? We don't want to run the engine at full throttle, do we? We need to override the underspeed system somehow.

OK, so let's put in an underspeed-override valve, just like this one.

This partially dumps one side of the delta pressure to artificially raise it when the governor lever is not set wide open. It is mechanically connected to the governor lever and is the reason why a faulty machine will sometimes increase ground speed when the governor lever is moved back towards idle; the underspeed override valve has opened and raised the delta pressure.




Here's the underspeed valve in its position.

 

[Cmark]

These are the servo valves and the associated linkages that keep the two sides synchronised and allow each side to be independently slowed down for steering.

The only common wear points here are the two bronze bushes in the plate at the bottom of the picture. These are the bearings for the steering linkages. If all the external linkages are correctly adjusted and the machine won't counter rotate, wear in these bearings is a possibility.




The servo valves and linkages all built up.




And installed in the case.

Note FNR input top left, underspeed valve left, underspeed override valve bottom left, top of the two servo valves middle of picture and steering input levers and linkages top right.

 

[Cmark]

And the back side built up. You can see the charge pump in the centre and the filter housing top right. Early models had a different filter housing that stood vertically. Both do the same thing but the later ones like this can be serviced without having to lift the cab. You can just make out a brass valve below the filter. This is to drain the housing before servicing. It's important to do this because if you don't, oil from the dirty side can enter the clean side when the filter is removed.

Anyway, that's about it. I hope someone found it to be useful or of interest. :thumbsup

 

[AllDodge]

This was great Cmark very well done. Many thanks

 

[Bob/Ont]

This was great Cmark very well done. Many thanks

Yes very informative and pic's of parts rarely seen by most people. Thanks Cmark.
Later Bob

 

[HATCHEQUIP]

Great thread

 

[OldandWorn]

Anyway, that's about it. I hope someone found it to be useful or of interest. :thumbsup

Are you kidding? Now, if only I can get my jaw back to where it belongs. :notworthy

 

[still learn'n]

My question is what all is different on the new ones and u will pry say they are all electrically controlled with solenoid valves and computers so they wont have all the linkages is that right? Thanks for posting all this it is very interesting and mind boggling to me if you don't do them very often how I would remember all that! Was gonna say that we have 850J deere dozer so that why I was wondering what difference was if 850 looked at all like that on the inside?

 

[Cmark]

My question is what all is different on the new ones and u will pry say they are all electrically controlled with solenoid valves and computers so they wont have all the linkages is that right? Thanks for posting all this it is very interesting and mind boggling to me if you don't do them very often how I would remember all that! Was gonna say that we have 850J deere dozer so that why I was wondering what difference was if 850 looked at all like that on the inside?

The Cat C series use two "off the shelf" hydraulic pumps, driven by a simple splitter box and controlled by solenoids and an ECM.

I haven't got a clue about Deere dozers I'm afraid, but I would guess they work on a similar principle to the C series.

I haven't seen anything else ever in the world in space that looks anything like one of the Cat HPCUs.

 

[AndrewC]

My question is what all is different on the new ones and u will pry say they are all electrically controlled with solenoid valves and computers so they wont have all the linkages is that right? Thanks for posting all this it is very interesting and mind boggling to me if you don't do them very often how I would remember all that! Was gonna say that we have 850J deere dozer so that why I was wondering what difference was if 850 looked at all like that on the inside?

still learn'n. The deere isn't even close although the principles are the same. Its very close to this http://powersolutions.danfoss.com/s...ons/documents/product_literature/520l0603.pdf It is two pumps for the tracks one behind the other, one for each side and then an another series 45 for the hydraulics.
Cmark. Really liked reading this, its nice to see another mousetrap.

 

[kshansen]

Cmark, Looks like lots of fun!

Do you ever have someone ask, "Why are you so fussy about getting every thing so clean?"

Had a guy ask us that once when we were rebuilding an Allison 6 speed CLBT 5860 transmission

That guy is now a superintendent at one of our smaller plants.

Could never understand why just because this equipment works in the dirt they have to be covered with dirt all the time.

 

[Scrub Puller]

Yair . . . Excellent comprehensible thread and pictures Cmark.

A beautiful bit of gear but you would really have to wonder about the designer who first got his head around the concept and put it down on paper . . . it makes my head ache just thinking about it.

Cheers.

 

[Bob/Ont]

Yair . . . Excellent comprehensible thread and pictures Cmark.

A beautiful bit of gear but you would really have to wonder about the designer who first got his head around the concept and put it down on paper . . . it makes my head ache just thinking about it.

Cheers.

I had the good fortune to meet one of the engineer's (Hugh Piper) involved in the later stages of their development.
According to him the concept was from the 1960's worked on then put on the shelf. Hugh got involved when the got out the drawings in the late 70's. They took the linkage drawings and built large scale cardboard copy's of the parts, assembled them together and worked them back and forth to perfect things. Then they built parts from steel. A large fleet of the bigger ones worked near here and provided much research and development information.
Later Bob

 

[kshansen]

They took the linkage drawings and built large scale cardboard copy's of the parts, assembled them together and worked them back and forth to perfect things. Then they built parts from steel

And I thought that was just the way guys like me figured things out, cardboard or scraps of plywood to design brackets and such?

 

[Cmark]

I had the good fortune to meet one of the engineer's (Hugh Piper) involved in the later stages of their development.
According to him the concept was from the 1960's worked on then put on the shelf. Hugh got involved when the got out the drawings in the late 70's. They took the linkage drawings and built large scale cardboard copy's of the parts, assembled them together and worked them back and forth to perfect things. Then they built parts from steel. A large fleet of the bigger ones worked near here and provided much research and development information.
Later Bob

That's interesting. I'd love to chat with the designers and find out how this thing came to being.

What doesn't come across in the pictures is the weight of the components. Every item is solid and heavy as billyo. It must have cost a fortune to tool up to produce the things.

Seeing as how, to the best of my knowledge, they were only fitted in the track loaders, I wonder if Cat ever made their money back?

 

[Dickjr.]

I have noticed the D6K finals are very similar in appearance to a 953. I wonder if the 2 machine share the same hydrostat , engine setup. I never understood why they didn't use the 53 drive on a dozer. Not to sidetrack this thread but if this happened earlier on there would have been a hydro dozer in the late 80s. I ran a Case 1550 which operates almost identical as a 953. I was always told they had issues with cross contamination and failures in the drive system. But this machine could have been the Achilles heel if it had proven out. Cmark I guarantee CAT made there money back. My neighbor is an engineer , has his own dozer , we talk about equipment a lot , he has told me its so much cheaper to do pilots over conventional linkage these days. I think most of the switches are made in China or over seas in cheap labor areas.