Cat 950F Hydraulic Brake noise

[Liviu Popescu]

Hi Nige, all,


Just trying to figure out a way of attacking this tomorrow. Where we work in the quarry there is no cell coverage so I only find out stuff once the operators return at the end of the shift. Hence, whenever I ask them to check something, I try to think of all the possibilities and get the most out of the day/shift (as it will take another 24h until I get a chance to try another thing).

My main concern is that the both the parking brake and low brake pressure alert lights are on, as in the picture which I shared earlier. Also, I’ve taken into account the fact that the machine braked itself. Looking at an extract from System Operations for the 5SK743-UP, I see the following statement: “Loss of brake oil pressure will cause both brake oil pressure alert indicator (3) and action light (2) to flash. The action alarm will also sound if the engine is running. The parking and secondary brake will automatically engage when oil pressure drops below 6035 ± 515 kPa (875 ± 75 psi).” Now, our action light has never worked but all the alerts do function. So taking into account that the low brake pressure light is on, parking brake light is on and the machine braked itself, I can only assume the oil pressure dropped below 875psi.


I will assume that the pump is not the cause of this, as this would probably be the most expensive part and worth keeping for last resort.


Following the hydraulic oil route:

1. From the pilot pump, oil flows into the accumulator charging valve where it acts against a relief valve which limits brake oil pressure at 2400 psi. Could it be that this relief valve is stuck open? I don’t see what else would be causing such a brake failure taking into account that the accumulators were empty before charging last week and everything did work, except for the strange noise. Maybe the noise I was hearing was this relief valve on its way out.

2. From there is flows into the valve that control the accumulator charging. When it reaches 2100 psi then oil flows into the tank and the accumulators are charged.

3. When accumulator pressure drops to 1700 psi then oil flows again into the accumulator and charges it to the pressure at point 2.

For points 2 & 3, if the parking brake engages by itself then we’re not even at these pressures so it would all go back to either the relief valve or the pilot pump.

We will put a pressure gauge on the brake system, presumably at the port on the accumulator charging valve, and if by some miracle the pressure is above 875 psi and around what Nige suggested 1700-2100 then my logic would tell me that the pressure sensor on the accumulator failed and at the same time the bell-crank spool on the parking brake valve. This is probably highly unlikely but could be.


Is my logic flawed or any points worth adding? I’ve dreaded getting involved in hydraulics and was lucky enough not to have any issues so far but it seems I’m forced to take a leap of faith.

I refuse to believe that by charging the accumulators we somehow broke something, however if the accumulators were over-charged would it cause any behaviour as such – but again how would they?


Lastly any suggestions on the panel with the action alert towards the right of our CMS? It never works as you can see in the picture.

I really hope this helps others as well which are new to this as me.
Thanks,
Liviu

 

[Liviu Popescu]

Also, just noticed now, there is a Pressure Reducing Valve which is placed in between the pump and the accumulator charging valve so I guess point 1 would have 2 possible fault routes - Pressure Reducing Valve and then the Accumulator Charging relief valve. However, I can't seem to find any info on this PRV for the brakes, any help would be appreciated. Thanks

 

[Nige]

PRESSURE REDUCING VALVE. On models with hydraulic-over-hydraulic brakes, part of the oil flows from the pump through an oil line to the brake system. The rest of the oil flows through PRV inlet (18) into valve (16) that is a cartidge-type reducing valve. It maintains pressure to the pilot and steering circuits at 2410 ± 140 kPa (350 ± 20 psi). Reduced pressure oil flows to the pilot circuits through outlet (19). Excess oil returns to the hydraulic tank through outlet (17)

 

[Liviu Popescu]

Hi all, I got an initial feedback of one of our drivers. There is around 1000psi at the accumulator valve port, not going any higher. Steering and everything else is working ok so I guess I would rule out the pump. Would it be the accumulator charging valve being blocked / failed?

They disconnected the parking brake rod from the actuator to get the machine moving. With it disconnected, the service brakes apparently work ok however the alarm for low pressure still sounds. I will get a more accurate feedback later, but is there anything else that comes to mind? We haven't measured the pressure individually on the front/rear service brakes, that's probably going to be one for tomorrow when I come up with a revised plan.
Do you think it's worth removing the accumulator charging valve or maybe just removing the relief valve from the accumulator charging valve to see if it is blocked and maybe clean it, although they don't appear as separate part numbers on Sis.
Thanks
Liviu

 

[Liviu Popescu]

Also thought now that the 1000psi is the exact pressure that the accumulators were nitrogen charged as per the spec last week.
Looking at what Nige uploaded, under accumulator charging valve Procedure 2, Point 4 (this is assuming there is 0 pressure in the brake system): "Start the engine. Look for a stop (pause) in the increase of oil pressure. Make a record of the oil pressure at this point. This is the approximate nitrogen charge (pre-charge) pressure in the accumulator."
Could it be that it can't load more than the accumulator nitrogen pressure, but can't figure out what sense that would make.

Any help would be really appreciated.

 

[Liviu Popescu]

Not much to add to what I've already said above, the parking brake bell lever has been disconnected from the drum and the loader brakes work, both front and rear. Both parking brake and low pressure brake alarms are on. we've tried adjusting the cut in/off pressures but we've seen no change on the pressure gauge. From 0psi, once you start the engine, the pressure rises very quickly to 1000psi and then it just stays there no-matter what you do. I am surprised that the 1000psi is not enough to disengage the parking brake as it will engage at 875 +/- 75 psi but maybe we're right on the limit.
Any suggestions would be really appreciated. My next thought is to try and remove the relief valve in the accumulator charging valve and see if we see anything obvious.

 

[Nige]

Pull the accumulator charging valve apart and inspect it would be the next logical step I think.

 

[Nige]

The accumulator charging valve 8R-5603 that was installed on your machine when it was originally built was upgraded to 138-6875.
There are no serviceable parts in either one.

 

[Bluox]

Hi all,

Update: both accumulators are now charged. We were lucky that our guys who do the drilling for the blasting offered to refill our accumulators.

The downside is that, although I told everybody to check for oil as Nige suggested, everybody forgot. I have understood though that they were not completey empty which might mean the seals are ok, but I guess we will see soon.

We’ve stopped for the blast so haven’t worked with the loader but initial feedback seems that the noise is gone. We will see next week when we start again. Nige was spot on with this advice - after the accumulators were charged it now takes 10 pedals for theoressure light to come on.

I’ll keep everybody up to date and thanks again for your help Nige!

Nige is not quite spot on ,Cat accumulators are assembled with a couple of quarts of hyd oil on top of the gas side of the piston.
This helps git rid of air in the cylinder.
On the first charge the gas side is bleed of air and excessive oil thru the gas charging port and the nitrogen charge is applied.
Any complete drain of the nitrogen charge will likely show oil.
Bob

 

[John C.]

Thee are different types of accumulators. The piston type requires some oil on the gas side to lube the seals on the piston. That is also a reason you usually find the piston type accumulators mounted vertically. The diaphragm type does not require any oil on the gas side but you will usually find some in there. Either type being full of oil is when you have a problem.

 

[Liviu Popescu]

Thank you both for your clarifications. They weren’t full of oil as far as I’ve been told. I’m stuck with this now as the dealer has quoted over 2000 Euros for the accumulator charging valve and can’t find it anywhere else other than CAT.
The machine works at the moment (no parking brake) so I’m holding back at dismantling the accumulator charging valve as it might break entirely.

Does anybody know of a more universal part that might replace that charging valve? The CAT one seems to be a bespoke design by WABCO so didn’t have much luck until now. It’s the 8R5603:

https://www.wabco-customercentre.com/catalog/docs/81463013charging_valve-caterpillar.pdf

 

[Liviu Popescu]

View attachment 257059
PRESSURE REDUCING VALVE. On models with hydraulic-over-hydraulic brakes, part of the oil flows from the pump through an oil line to the brake system. The rest of the oil flows through PRV inlet (18) into valve (16) that is a cartidge-type reducing valve. It maintains pressure to the pilot and steering circuits at 2410 ± 140 kPa (350 ± 20 psi). Reduced pressure oil flows to the pilot circuits through outlet (19). Excess oil returns to the hydraulic tank through outlet (17)

Hi Nige, I am going to ask what is surely a stupid question. From what I can tell, the pilot hydraulic oil comes into this Tee and then either goes upward towards the steering system or downwards towards the accumulator charging valve. If I have 1000 psi at the charging valve measuring port would there be a possibility that the PRV valve could be stuck open and would be lowering the overall circuit pressure to 1000 psi. The way I see it is that the oil would take the path of least resistance.

I struggle so hard to understand how these hydraulic circuits work - I probably always try to look at them as an electrical system. but anyway, back to the topic, the steering system works... I've noticed that both the relieve valves in the PRV and the one in the charging valve are set at 2400 psi. But back to my question if I have 1000psi at the charging valve how does the steering system work.

Also what is the difference between the high pressure system and the pilot system (is the pilot like a control system for the high pressure one - like you would have in a relay for example?)

I've attached a picture of what I'm referring to. I'm not trying to avoid buying a charging valve necessarily but want to fully understand the logic to how I got there in case it really is the charging valve.

Thanks
Liviu

 

[Nige]

I've noticed that both the relieve valves in the PRV and the one in the charging valve are set at 2400 psi.

Are you sure you are not confusing psi and kPa.? The PRV relief is set to 2400 kPa which is 350 psi.
The ACV relief valve is set to 16,550 kPa which is 2,400 psi.

The function of the PRV is to supply oil at a reduced pressure of 350 ± 20 psi to the implement pilot & steering circuits.
It is a simple way of using one pump to supply more than one circuit but the circuits in question have different pressure requirements. In this case the implement pilot and steering systems operate at around 350psi, the brake circuit operates at 2000+ psi.

I marked oil coming from the pump as Hose "A" on the illustration below. Hose "A" connects to a tee where the flow can go one of two ways. Up into the PRV or down into the ACV.
The outlet from the PRV to the pilot/steering circuits is the fitting/hose in the red cricle directly above Hose "A". It's not shown very well on this illustration.
You can easily check if the PRV is working correctly by testing the implement pilot pressure at Test Point #1. Is it within specification.?



Full pressure from the pilot pump flows through the tube #56 and enters the ACV at Port #3.
Oil to charge the accumulators, and to supply the brake system, outlets from Ports #1 & #4 of the ACV to the front and rear brake accumulators via tubes #57 & #58.
When the ACV reaches cutoff pressure (2100psi) oil flows from the ACV return port #5 through tube #52. It meets with the excess oil from the PRV at tee #63 and the combined flow then returns back to the tank via tube #71, manifold #62, and hose #61.