[hypoxia]

The original pressure switch in 1482 is a Noshok 300-1-2-15/225-8 and the replacement is a Noshok 300L-3-2-15/230-8,  I ordered one from Zoro.  I assume there is no way to test the switch since the system is pressurized.  I attached a document showing the RAS system.

Noshok 300L-3-2-15230-8 Pressure Switch in RAS Manifold.pdf Size: 164.87 KB  Downloads: 11

ZF RAS .pdf Size: 2.79 MB  Downloads: 10

[Richard]

Would you guys correct me if I am wrong. I am imagining the accumulator to conceptually much like a pressure tank on a well pump system. A steel tank with an internal bladder. The bladder is precharged to an adjustable level. Let’s say the pre charge is 50 psi. When hooked up to a water pump, when the water pressure coming from the pump exceeds 50 psi, the bladder starts to compress and continues to compress until the cut off pressure of the water pump. So if the cut off pressure of the water pump is 100, then IF you measured the bladder pressure it too would be 100 psi.

My point being, that although the accumulator starts off at it’s precharge pressure, as soon as the hydraulic side is pressurized above it’s 10 bar (147 psi) then the pressure in the accumulator will be whatever the hydraulic pressure is.

If my thinking is correct, then the N2 pressure sensor will read whatever the hydraulic mule charges the system to. And if Newell is charging to 275, the sensor in the picture show 230 as the upper limit I think. Just wondering out loud if overpressuring the sensor is the cause of failure.

[jdkskyking]

Richard I agree, my exact thoughts. 
How many coaches are running around with faulty pressure switches?

I will install a ball valve between the pressure switch and the block it is mounted to. When the system is serviced, close the valve to protect the switch. 

I’d also like to add a bleed valve to the accumulator connection at the bottle. 
I’d like to add a bleed valve for servicing the system to the outermost hydraulic port on the rear centering cylinder (anchored side). 

I believe these would be beneficial modifications to the existing system. 

JK

[hypoxia]

I didn't check other switch manufacturers but Noshok 300 series does have one with 30psig/290psig.  I just ordered an accumulator test kit which will arrive tomorrow.  One complaint I have seen is the lack of leak protection through the cap.  Some folks in the hydraulic industry are switching to a metal cap with a brass washer.  The cap thread size is 28mmX1.5

[jdkskyking]

Let us know what kit you purchased and how it worked. 
I’ll bet servicing the nitrogen would be helpful on many rigs.

[hypoxia]

Would you guys correct me if I am wrong. I am imagining the accumulator to conceptually much like a pressure tank on a well pump system. A steel tank with an internal bladder. The bladder is precharged to an adjustable level. Let’s say the pre charge is 50 psi. When hooked up to a water pump, when the water pressure coming from the pump exceeds 50 psi, the bladder starts to compress and continues to compress until the cut off pressure of the water pump. So if the cut off pressure of the water pump is 100, then IF you measured the bladder pressure it too would be 100 psi.

My point being, that although the accumulator starts off at it’s precharge pressure, as soon as the hydraulic side is pressurized above it’s 10 bar (147 psi) then the pressure in the accumulator will be whatever the hydraulic pressure is.

If my thinking is correct, then the N2 pressure sensor will read whatever the hydraulic mule charges the system to. And if Newell is charging to 275, the sensor in the picture show 230 as the upper limit I think. Just wondering out loud if overpressuring the sensor is the cause of failure.

I think your theory is correct. This is a sealed system. There is no pump or compressor. I think the only purpose of the accumulator is to absorb any shocks and keep the system stable. I am puzzled as to increasing the  pressure and what that accomplishes?  MCI had a recall on their buses, and it was basically just an inspection and using the same original pressures.  I am certain they all know more than I do!  If I would have read your post earlier, I would have purchased the sensing switch that had the higher pressures.

[Richard]

looking at the circuit, does increasing the pressure in the accumulator increase the pressure on the rear cylinder “centering” ? It is my understanding the centering is needed, because of the unusual design of the master cylinder up front. The bypass section in the master which permits +/- 5 degrees of movement would also infer that the rear cylinder is free to move +/- 5 on its own if that float is permitted in the hydraulic loop coming from the master. Therefore some mechanism is required to keep the tag cylinder centered.

So losing pressure in the accumulator would result in less recentering force.

I don’t have any hands on experience with this, my comments are just from trying to digest the literature that Jeff has provided. And I could have a really flawed understanding.

[jdkskyking]

ZF RAS eye candy…


               

Redundancy… two bottles? 

Needs bleed ports at a system high point… HIGH POINT…

Would the engine bay get too hot for the accumulator to live? It would become a high point for more effective bleeding. Just saying… heat is a downer…




   
This page is from the ZF SERVOCOM Manual. Again… just saying… 

Just some food for thought. 
JK

[jdkskyking]

looking at the circuit, does increasing the pressure in the accumulator increase the pressure on the rear cylinder “centering” ? It is my understanding the centering is needed, because of the unusual design of the master cylinder up front. The bypass section in the master which permits +/- 5 degrees of movement would also infer that the rear cylinder is free to move +/- 5 on its own if that float is permitted in the hydraulic loop coming from the master. Therefore some mechanism is required to keep the tag cylinder centered.

So losing pressure in the accumulator would result in less recentering force.

I don’t have any hands on experience with this, my comments are just from trying to digest the literature that Jeff has provided. And I could have a really flawed understanding.

I enlarged an area of the ZF RAS system explanation. 

The question I have is what exactly “centers” the TAG Axle?

   

As with the front “Master Cylinder” and the “Rear Centering Cylinder”, each cylinder has separate chambers that complete different work. 

In the Rear Centering Cylinder, the chambers on the actuating rod side react to the movement of the front Master Cylinder “CENTER CHAMBERS. 

The “anchored” side, IMHO, simply holds the axle in position with system pressure. Equal pressure applied to each chamber. 

So what “centers” the tires? Master Cylinder gets it close but then there’s the 5°… or is it 10° total? Details the ZF info doesn’t provide.

[jdkskyking]

if we apply equal hydraulic pressure to the ports of this cylinder, what will the results be?