Update on all 7520s

[SPYDERLK]

I know this was mentioned before, but this really IS the answer.....

I know you, MtnViewRanch, and Dougster are saying this in regard to testing the hydraulic output using a flow meter. The problem I see is that regardless of the method used, getting a lower than nominal flow does not tell you the reason for it. -It could either be small pump displacement, incorrect pump gearing/rpm, or corrupted flow to, -or restricted or diverted flow from- the pump.

The reported likelihood of entrained air due a suction leak throws all measurements of pump capacity in doubt. What we measure is pump delivery. A mass flow sensor meter would give an accurate measure of fluid delivery regardless of air entrainment and pressure. The source of error in such a sensor would only be in its measurement and inclusion of the mass of the entrained air in the mass flow. This airmass would be small since air in light. The small error would be inconsequential. By contrast, measurement using a more common and much cheaper rotor type meter would give unpredictable errors when air is present in the fluid. Correction of this error in measured delivery would require a complex detection scheme to determine exact air quantity and its pressure as it passes thru the meter. - - So now the meters agree with the correct reading initially given by the mass flow meter. How does this help us in any way different from timing the displacement of cylinders having known volume? Well, we eliminate timing error - - but in reality, repeated measurements by a discerning self critical user does a close enuf approximation to say that 6 to 7s timed is never 5s actual. This says the pump is delivering 14gpm not 17gpm. It does not say what the capacity of the pump is. The pump specs and its gearing wrt the crank would tell us whether to look for a malfunction. I see this as the most direct avenue to a proper corrective action , be it the pump itself or corrupted flow. In a lab it would be easy to verify pump capacity experimentally. Not so easy on a tractor. I dont relish looking for 17gpm from a pump that the specd 6.2s loader time indicates that it only delivers 13gpm. Could be chasing a wild goose. Until I saw a 5.4s loader time reported i was even more worried. Now I think the performance is actually there and will be realized thru a meticulous overhaul of the suction line. MtnView, I know you are satisfied by your loader speed - Mine has improved from 8 to 7s and it is satisfactory. If it were 60% faster it would be more satisfactory tho since I can always slow the engine to reduce flow without decreasing lift capacity- according to the pressure gauge I have on the pump output. I am not satisfied in getting less than I bought however. It is the loaders indication of an under spec delivery rate that dissatisfies me. !7gpm is borderline sufficient for many hydraulic motor PTO applications. !3gpm is below the line in many cases.

All; Point of information - pump gearing is 1.09 times engine speed. The crank gear is 36t driving the 72t cam gear that then drives the 33t pump gear. I learned this when watching mine being repaired. The idler function performed by the cam gear is quite strenuous and resulted in mine breaking. The helical tooth profile results in thrust loadings of the gearface - the gearface is pushed forward by the angular component of the crank gears torque force, and pulled backward on the opposite side as it imparts torque to the pump gear. This continuously occurring several hundred pound cyclic thrust reversal caused the web of mine to break. Mahindra is on the second upgrade addressing this problem. The latest upgrade is still too spindly to inspire my confidence in long term high rated pressure use of the pump. Probably last forever on 3pt implement use, which almost never requires over 1000psi. Use of the loader or running a hyd PTO however frequently requires over 2000psi and up to the 2500 relief pressure. I think that those using their hydraulic pumps at full rated capacity will add to the failure statistic.
larry

 

[Dougster]

I know you, MtnViewRanch, and Dougster are saying this in regard to testing the hydraulic output using a flow meter.

I was really planning to stay clear of any further input into this thread once the 7520/ML275 dealer test results were in and proved rather convincingly that the "new & clean" implied contractual guarantees for the 7520/ML275 combo were being met or bettered... but now I see that I have been misquoted.

Please do a search on this thread. I never recommended the use of a flow meter. My approach would have been less costly and far more convincing in a practical sense involving the use of a stopwatch, precision tachometer, calibrated thermometers, some extra hydraulic oil, some hose and two calibrated reservoirs.

That fact notwithstanding, a properly selected and calibrated flow meter could work just fine (within its manufacturer's specified tolerance) provided you followed one of the key fundamental prerequisites of contractual performance testing: Inspect and properly prepare the system for testing prior to starting. In other words, one must assure that there are no air leaks (or other issues potentially affecting performance) prior to the start of any such performance testing.

Fluid system performance testing will almost never prove the existance of a particular component or installation flaw or problem. It can only prove that a complex fluid system will or will not meet a specified or guaranteed level of performance. Ultimately, only individual equipment or component testing can prove actual equipment or component flaws or faults.

I still do sympathize with those folks who do not feel they got what they paid for. I can understand the frustration of those whose calculations show that better-than-specified performance should be achievable. I feel your pain and it's my (engineer's) nature to want to help. Just please don't misquote me. Thanks!

Dougster

 

[bindian]

I was really planning to stay clear of any further input into this thread once the 7520/ML275 dealer test results were in and proved rather convincingly that the "new & clean" implied contractual guarantees for the 7520/ML275 combo were being met or bettered... but now I see that I have been misquoted.

Please do a search on this thread. I never recommended the use of a flow meter. My approach would have been less costly and far more convincing in a practical sense involving the use of a stopwatch, precision tachometer, calibrated thermometers, some extra hydraulic oil, some hose and two calibrated reservoirs.

That fact notwithstanding, a properly selected and calibrated flow meter could work just fine (within its manufacturer's specified tolerance) provided you followed one of the key fundamental prerequisites of contractual performance testing: Inspect and properly prepare the system for testing prior to starting. In other words, one must assure that there are no air leaks (or other issues potentially affecting performance) prior to the start of any such performance testing.

Fluid system performance testing will almost never prove the existance of a particular component or installation flaw or problem. It can only prove that a complex fluid system will or will not meet a specified or guaranteed level of performance. Ultimately, only individual equipment or component testing can prove actual equipment or component flaws or faults.

I still do sympathize with those folks who do not feel they got what they paid for. I can understand the frustration of those whose calculations show that better-than-specified performance should be achievable. I feel your pain and it's my (engineer's) nature to want to help. Just please don't misquote me. Thanks!

Dougster

I don't have access to the hydraulic schematics of the ML275, so I am in the dark here as to where check valve(s) location(s) in the system. But if I was a Mahindra dealer mechanic with a not too happy customer with a slower than advertised loader raise time, I would pressuize the return lines with nitrogen to just above system pressure and do an old fashion leak check with soapy water. If I was a well equipped dealer mechanic, I would just operate the system and check ALL connections with a hand held ultrasonic leak detector. The ML275 Operators manual states all air must be purged from the system before operation. That means no suction leaks. If no leaks were found, I would replace the pump and time the loader again. All this, of course, after I exhausted all the troubleshooting procedures in the Ops manual. That said, I am now taking 4 Advil after reading the last two posts.
hugs, Brandi

 

[SPYDERLK]

Quote from Dougster: I was really planning to stay clear of any further input into this thread once the 7520/ML275 dealer test results were in and proved rather convincingly that the "new & clean" implied contractual guarantees for the 7520/ML275 combo were being met or bettered... but now I see that I have been misquoted.

Take it easy. I didnt quote you. So I didnt misquote you. I did however misinterpret what you said. Im sorry to have offended you.

Your reservoir to reservoir timed pumping plan is fine. I have just always considered the experimental overhead required by the sheer magnitude of fluid needed and ample reservoirs together with plumbing and appropriate valving controls, to be prohibitive to most owners. It is much easier to sample pump delivery within the integrated system. Thats why I gauged the pump outlet. This lets me know that the relief valve is not leaking since I am able to achieve 2500 relief pressure even when hot at 700rpm idle. Relief pressure rises to about 2550 at 2500rpm. This small difference tells me the relief is functioning and well designed. Raising the loader at full speed takes 1000psi - - far from relief cracking. Where else can the fluid go? We are looking for, in my case, 4gpm bypassing unused to the return line at a pump pressure of 1kpsi. And we are looking for this in a system that will achieve relief pressure at low idle when hot. Im betting that when the fluid is pumped it all shows up in the lift cylinders when valved there. I will concentrate on the suction circuit before I go any further.
I have asked Mahindra for pump specs without answer at this point. There really would be no reason to look for more pumpcapacity than its cu-in/rev times rpm can theoretically produce. I see no reason these specs are not available.
larry

 

[Dougster]

Take it easy. I didnt quote you. So I didnt misquote you. I did however misinterpret what you said. Im sorry to have offended you.

Sorry to have taken it so personally, but the notion that an economical, high-accuracy hydraulic system flow test was somehow just not practical hit too close to home. Such high-accuracy, contractual-grade fluid system performance tests were my specialty for over 18 years and I assure you this is a very doable exercise. If we are through talking about loader speed and focusing in now on hydraulic system flow, there is only one way anyone is ever going to get an accurate answer: Run a high-accuracy system flow test. Anything else at this point is just smoke blowing in the wind.

Again, no dog in this fight so I don't even belong here. I'm going away for good this time. Call it professional pride... I just wanted to correct the record on what I had said.

Dougster

 

[bindian]

I drove my 6520 4WD this morning. After poking around it different gears awhile, I reved it up to 1500, sat the loader bucket flat on the ground and timed the bucket to full height. It was right around 6 seconds, about like the 6520 4WD I timed a couple of weeks ago.
hugs, Brandi