Hi, Just looking for some help, I had a brake surging problem with my cat m320f. I had the accumulator replaced by a dealer and within 30m of moving the machine I blew all seals from rear hub brakes all the way back to seal on the driveshaft/dropbox. Anyone know how this may have happened? Any and all suggestions/information would be greatly appreciated. Thanks.
I should have mentioned the serial no fb200704. Cat 2019 wheeled excavator. There are 2 accumulators for the brake system and 1 was undercharged. They replaced 1 accumulator. Before the replacement I never had any warnings of low brake pressure and or any on dash displays. Only symptom was a pulse every 10-15sec. Before the seals blew out, I moved the machine about 15m and noticed that the brakes were kind of sticking and with the next 15m moving it back out of the way the brakes released and I noticed oil spilling out of the dropbox. Thanks
So if you had no warning messages or alarms how come the brake accumulator was tested and found to be undercharged in the first place.? Was there any suspicion of a problem in the brake system.? If an accumulator was undercharged how much pressure was in it.? How much pressure was in the other one for that matter.? The service report should contain the information. What Part Number of brake accumulator was installed when the "undercharged" one was replaced.? After installation was the precharge pressure checked.? Again the info will be in the Service Report. What testing was done on the brake system after the accumulator was installed.?
The brakes were pulsing every 10-15sec so I got dealer out to carry out an inspection. This had been happening for some time. I wanted to get it fixed. The original accumulator was tested to be down around 260psi. New accumulator p/n 237-1925. The only thing that was tested on the machine was the cut in and cut out pressure of the brakes. The s1/s2 port I think. There wasn't any other test done on the machine.
That P/N is listed as an UNCHARGED accumulator which is something pretty standard out of the parts system because there are restrictions on transport of pressurized cylinders. Was it charged after installation.? That is low. The specification is around 650psi @ 20° C. The precharge pressure will be slightly lower if the ambient temperature is cooler but there is no pressure/temperature correction table as far as I can see. What was the precharge pressure in the other (good) brake accumulator.? I dunno about aynone else on here (I don't class myself as an excavator expert) but I would not have thought an accumulator that was low on charge would cause pulsing on the brakes. This would be especially the case seeing as the machine has two accumalators rather than one, and where only one was found low on precharge pressure. Those symptoms for me point to the accumulator charging valve cycling excessively.
It is was an uncharged unit, but they charged it in the warehouse before coming to site. I don't know what they did or didn't do. The dealer mentioned a service letter about the f series but didn't/won't tell me any details about it. I think it has something to do with replacing the accumulator or how the accumulator affects the brakes system itself but that is speculation on my end. I can't get access to this letter. I have done some research into the brake system and the front brakes and back brakes are ran/worked of the 2 accumulators independently. This being the case, if the replacement accumulator had been over charged, could this have caused my brake piston seals to blow?
This is typical. Even wheel loaders have separate circuits for the front & rear wheel brakes. $64k question is whether the accumulator that was replaced because of low pressure was the one in the circuit for the rear brakes or not.? I can't find any information that definitively shows which brake accumulator is which. You may have to trace the lines from the accumulators to figure out which one is which. Looking at the accumulator installation logic would dictate that the one closest to the front of the machine is for the front brakes and the other one for the rear, but that may not be correct. I wouldn't have thought so, but as I said before I'm not the expert on these Rubber Ducks.
In terms of testing after an accumulator replacement. What all tests could/should be done to prove a working system/accumulator? Should the service brake system have been bleed for air? Should a standard brakes test have been completed?
First thing that should have been done (IMHO) was a test of accumulator precharge pressure if it was charged elsewhere and then installed on site. I would do that even if I was the guy who charged it back at the depot then brought it out in the van with me. It's all part of the T&A procedure for the accumulator charging valve anyway. See reply below. Have a detailed read of the attachment UENR5282-07 that I posted above. For replacement of an accumulator.? I would not have thought so. It's not a majorly invasive procedure to the brake hydraulic system.
While doing the test/adjust procedure. Just part 11 of the test procedure " If the nitrogen pre-charge pressure is not within the specification, refer to Special Instruction, REHS5464 , "Accumulator Discharging and Charging Procedures" for the required tooling, and the testing and charging procedure. Note: Do not attempt to charge the accumulator until all pressure has been removed from the wheel brake accumulator." What is that note telling me. Don't charge(with nitrogen or hydraulic) until all pressure(nitrogen or hydraulic) pressure is gone? Is this while it's attached to the machine? Also should the accumulator be gased while attached to the machine? Or it doesn't really matter?
Basically yes. The accumulator nitrogen gas precharge pressure cannot be checked until all hydraulic pressure in the brake system has been released. Makes no difference TBH. Just that if the accumulator is off the machine you don't have to worry about releasing hydraulic pressure in the brake system. It doesn't matter so long as all brake system hydraulic pressure has been released. Releasing hydraulic pressure is simple. With the engine off, key ON, slowly press & release the brake pedal repeatedly until you get an audible/visual warning for low brake pressure. Continue pressing/releasing the pedal until basically it goes solid under your foot. (Think of it like your car with a servo on the brakes. If you press the pedal more than X times it goes hard. An accumulator on a hydraulic brake system works exactly the same way.) I just thought of another important question. After installation of the replacement accumulator was the charging valve actually adjusted or simply tested cut-in/cut-out pressures and then pronounced to be good.?
If I find that the accumulator changed is infact linked to my rear brakes what would my next course of action be? Would this be a viable link to why my seals have gone?
O I did forget to mention that after the accumulator was installed, the brakes were kind of dragging, could this have been a direct link to a new accumulator, what could have caused this drag?
I would say in that case you could quite reasonably approach the company who did the accumulator replacement with the "this has to be a bit more than a coincidence surely.?" It might be related, but at this moment in time I am struggling to understand how. At the risk of repeating myself the big question is whether or not the cut-in/cut-out pressures of the accumulator charging valve were adjusted after the accumulator replacement or simply checked.
I'm honestly not sure how the accumulator is linked in the system. But reading all the service material I believe it is directly linked to the brakes as well as being a backup for if I lose hydraulic pressure. This is taken from service brake system operation. When service brake pedal (6) is depressed, stored oil in brake accumulators (12) is directed to the pistons of service brakes (1) and (4) in order to engage the brakes. My assumption here is that the accumulator is directly linked to the piston when I press the brake, so if the accumulator was overcharged that additional pressure would have been transferred to my piston seal when I applied brakes, In terms of the brakes dragging, I know that they rely on springs to release the brakes, and that additional pressure could overcome the springs, I just don't know why this would have been the case during normal operation.
The accumulator is a way of ensuring that there is always pressure in the system as far as the brake control valve even if the engine is not running. So the idea is that you will always have brakes provided that the accumulators are charged up correctly. The accumulators also function as a secondary source of pressurised oil if the demand from the brakes is greater than the capability of the hydraulic pump to supply it. e.g. when the engine is idling and the hydraulic pump output is relatively low.
Let's look at this a different way. Regardless of nitrogen precharge pressure, an accumulator only stores pressure (or smoothes out pressure pulses). As far as nitrogen precharge pressure, if the precharge is low, the dampening of hydraulic pulsing increases, but available reserve hydraulics is low. If the nitrogen precharge pressure is too high, the dampening of hydraulic pulsing decreases or becomes non-existent, and available reserve hydraulics is low. At no point can a nitrogen accumulator increase hydraulic pressures in a braking or pilot circuit. So, if components damaged in the braking circuit were overpressurized, then the pump regulator or pressure reduction valve are suspect. Just grabbing a wet brake example, my Terex main pump runs at 3600psi, yet, the brake circuit is reduced to 1750psi. Whether my brake accumulator(s) are charged, overcharged, or depleted, the brakes only receive 1750psi.
Any idea on why brakes were dragging after the installation/replacement? If it wasn't the accumulator itself, what could have caused it instead?
If the brakes drag then there are two likely causes - either somehow pressure from the wheel brake pistons is not releasing back through the brake control valve to tank, or the parking brake located in the transmission is not fully released. The problem is that from the operator seat it must be impossible to tell which of the two possibilities is causing it. Question: When you moved the machine that 30m any you felt the brakes dragging before oil started leaking on to the ground did you see/hear any low brake pressure alarms.?
No alarms or alerts on the dash. Nothing at all. The brakes were dragging from when I turned the machine on until the seals blew(within the 30m) then I felt the relief on the machine(when seals blew)
That would appear to indicate that there was pressure in the service brake system. Again it goes back to the (so far unanswered) question whether or not the cut-out/cut-in prressures at the accumulator charging valve were simply tested and pronounced OK or were tested and then adjusted to get them within spec. There is a big difference. The Service Report should contain the information.
See attached service report
Unfortunately that's a sadly deficient Service Report. "Tested machine all OK." covers a multitude of sins. I don't suppose you were hanging around while he was doing all the work were you.?
Only bits and pieces, what I had seen was he depressurized the system (press brake pedal) (no warning alarms etc) then linked a pressure gauge to s1,s2 port. That was as far as I had seen. When I came back he was finished and gone, pulse had stopped, brakes were dragging, then blew all seals.
The attached infromation may be of some interest. The new brake pistons referred in the article were fitted to all FB2-prefix M320F machines from Serial Number 813-Up (July 2020-ish), which is after your machine was built. Actually it is not the latest available information because on Page 5 it states - "Follow the below replacement procedure to replace the former 328-8803 [Front Axle brake] Piston with a new 614-9638 Piston and to replace the 328-8804 [Rear Axle brake] Piston with the same Part Number listed in Table 1." The information regarding the rear axle brake pistons has changed. The 328-8804 Piston part number mentioned in the article has now been replaced by an updated Part Number of 614-9640. Note that the Parts List contained in the procedure is the quantity required for one Wheel End only. If both wheels on an axle (front or rear) were to be modified then double the listed quantities of parts would be required.