Hydraulic assist for 3U 4904 Cat 12 Road Grader

My friend replaced the seals and O-rings in the hydrauluc assist steering gear in his road grader.  He managed to get one of the seals in backwards and it was pumping oil out the vent on top of the steering gear.  He ask me for assistance and I found the seal that was backward.  The first picture shows the steering gear with the hydraulic control valve and pinion gear removed.   [attachment=69239]Resized_20221023_160126 (1).jpeg[/attachment].  The next picture is of the control valve from the pinion end.  The center (spool) of the valve slides back and forth directing fluid to the output ports which feeds the cylinder to assist steering.  Notice the gap which is created when the center is pushed toward the pinion gear.  The pinion gear (rack gear)  turning the pinion shaft which is attached to the steering arm and tie rods create the force which moves the spool back and forth in the housing to direct the flow of fluid to the cylinder for steering assistance. [attachment=69243]Resized_20221023_160102.jpeg[/attachment].  Thrust bearings and washers with dowel pins hold the spool of the control valve in position to keep it from turning with the steering shaft.  The dowels on the washers ore only about 1/4 inch long so the center of the valve cannot move too far without the dowels comming out of place.  The next picture shows the valve from the steering wheel end, [attachment=69244]Resized_20221023_160057.jpeg[/attachment].  The center of the spool is slid all the way back against the body of the valve and the large nut on the steering shaft will allow the spool to travel a little more.  How tight should this nut be?  If it is tightened all the way down the steeing shaft is difficult to turn.  When I tighten the nut all the way down, it can anly be loosened about a turn and a half or the dowel pins which fit into the spool will come out.  Should it be loosened only enough to allow it to turn free or is there a specific amount to back the nut off?  There is a lock and a second nut which locks the  nut in place once it is in the corrrect position.  He does not have a servicemans reference manual to explain how tight this nut should be.  Can someone reference this?  We put the steering shaft and valve together as you see it in the pictures and used compressed air to simulate fluid flow and moved the spool back and forth to check the flow through the valve.  Everything worked as it should.  Thanks Brian Boaz

This is from the 8T service manual, is it what you need?

Thanks Josh, That is exactly what I needed. Looks exactly like the same unit. A perfect example of what this organization is all about. Brian Boaz

Hi, bboaz.
I knew which steering booster you were talking about but I have never been inside one of them and nor have I ever seen that manual page for them. Thanks to Josh for coming up with the goods.

This was/is a pretty reliable system but Cat discontinued use of this system on later models 'coz it had a tendency to shear off the vertical downshaft to the steering arm under heavy load or due to shock loading, usually just above the steering arm. They moved the booster down to the steering arm by means of a hydraulic ram connected directly to the steering arm. This eliminated the shearing problem.

Just my 0.02.

Deas Plant, That is some intresting information. This system has a 4 inch cylinder that is conneced directy to the vertical downshaft, so I can see where it culd fail under a heavy load or shodking load. The shaft is consideable smaller where it goes into the splined steeering arm creating a weak point for possibl;e failure. The main purpose of this grader will be to maintain about a mile of driveway, so I don't think it will be used in such a manner to place undue stress on the downshaft. I will let the owner know what you have said about the possible failure so he will be aware of it. We will hope for the better. Thanks for taking the time to pass along this information. Brian Boaz

Hi, bboaz.
It has taken me something over 1/2 a century to gather a lot of 'stuff' like that. I cain't see any point in just taking it to the grave with me if it can help somebody/anybody who'll be staying on the top side of the daisies for a bit longer than I am likely to.

Just my 0.02.

Awesome thread - it embodies what tis club is all about.