Troubleshooting hydraulic failure on MIG fighter jets|
Towards the end of last year I told you about an advanced hydraulics self-study course I'm developing with my friend and colleague, Marian "Kalashnikov" Tumarkin (if you missed the Kalashnikov story, you can read about it in issue #58).
Just this week while we were finalizing the section on variable pump control, Marian told me about this interesting pump control problem he encountered as an engineer working in the former Soviet Union:
"At the time I was employed as a team leader by the Ukrainian Institute of Technology. We got a contract to investigate hydraulic system failure on the MIG-23 jet fighter. Pilots were reporting sudden loss of hydraulic pressure during flight.
Mechanics had found that the failure was the result of a broken spring in the pressure regulator (compensator) of the variable displacement pump. There were many attempts to replicate the spring failure in ground tests. However, all of them were unsuccessful. Similarly, all attempts to improve the spring's quality did not yield any positive results.
For my part, I had suspected from the beginning that the broken spring was a result of self-oscillation of the pressure regulator. However, it was impossible to prove my hypothesis during in-flight tests, because the pilot monitored hydraulic pressure downstream of a check valve. So even if self-oscillation did occur, he would not see any pressure oscillation.
We spent almost a year developing a computer model of the variable displacement pump with this pressure regulator. In the end, we were able to determine, with the aid of the computer model, the particular mode of pump operation which caused the self-oscillation. Two questions immediately arose:
- How to prove it in a ground test?
- How to fix the problem considering the sheer number of aircraft in service?
Using the computer model, we established that extra volume in the control chamber of the proportional spool increased instability of the regulator. Subsequently, we organized a ground test with a large pressure gauge connected to the control chamber of the regulator. In this test:
- we could monitor pressure upstream of the check valve; and
- add deforming volume to the control chamber.
After two hours of testing, we found the mode of pump operation, where the pressure gauge started jumping from zero to maximum and back to zero with a high frequency. After 10 hours of running the hydraulic system in this mode, we effectively destroyed the regulator spring - and the pressure gauge as well.
To find the answer to question 2, we simulated a variety of small design improvements to the pump regulator to improve stability. Finally, we found a simple solution: reducing the stroke of the proportional spool from 0.8 mm (0.03") to 0.3 mm (0.012") with a mechanical stop. The ground and flight tests confirmed that the problem was fixed."
If variable pump controls are mystery to you or you'd like to gain a better understanding of how they actually work and where to use them, then you'll profit greatly from Dr Tumarkin's expertise in our latest Instant Knowledge™ report: How to Understand Variable Pump Controls
"As a mechanic with more than 30 years experience, I think Industrial Hydraulic Control is
excellent. I use it as my hydraulics reference."
Find out more ...
Equipment Maintenance Supervisor
Oilfield Service Company
Is this problem destroying your hydrostatic transmission?|
Rupert Murdoch, the boss of global media giant News Corporation was a neighbour of ours where I grew-up. Not that my family was particularly well off. It's just that my father's farm happened to be situated close to a group of "sheep stations" the media mogul owned. But compared to the 300,000 acres Mr Murdoch controlled, Dad's land holding was modest indeed.
In 1981, just in time for the wheat harvest, Dad took delivery of a new combine harvester. It was one of many he owned over the years, but this one was different. It was the first I'd seen equipped with a hydrostatic transmission for the ground drive. The infinitely variable and step-less control afforded by a hydrostatic transmission was quite an advance over the mechanical gearbox with a variable speed input used in earlier models.
Anyway, in its second season the hydrostatic transmission gave trouble. Downtime during harvest was always guaranteed to elevate Dad's stress level to 11 out of 10. And that wasn't a pretty sight. I didn't know much about hydraulics then and looking back, boy do I wish a book like Insider Secrets to Hydraulics was available to me at the time.
Of course in the 25 years since, I've accumulated a bit of knowledge on hydrostatic transmissions. And an issue that is often overlooked and one that came up in a job I was involved in recently, is the combined effect of fluid compressibility and the 'accumulator effect' of conductors (the increase in volume of a hose or pipe as pressure increases).
When a hydrostatic transmission is subject to a sudden increase in load, the motor stalls instantaneously and system pressure increases until the increased load is overcome or the high pressure relief valve opens - whichever occurs first.
While the motor is stalled, there is no return flow from the outlet of the motor to the inlet of the pump. This means that the transmission pump will cavitate for as long as it takes to make-up the volume of fluid required to develop the pressure needed to overcome either the increased load or the high-pressure relief valve. How long the pump cavitates depends on the output of the charge pump, the magnitude of the pressure increase, and its influence on the increase in volume of the conductor and the decrease in volume of the fluid. This is illustrated in the following example.
A hydrostatic transmission operating the drill head on a drill rig is delivering a flow of 35 GPM at a pressure of 1000 PSI. A sudden increase in load on the drill bit instantaneously stalls the motor until sufficient pressure is developed to overcome the increase in load, which for the purposes of this example is 3000 PSI.
In order to increase system pressure from 1000 PSI to 3000 PSI, the transmission pump must make-up additional volume, due to the compression of the hydraulic fluid and the volumetric expansion of the high-pressure hose between the pump and the motor. But because the motor is momentarily stalled, there is no return flow from the outlet of the motor to the inlet of the pump. The only fluid available at the inlet of the transmission pump is 7 GPM from the charge pump, which is around 80% less than required!
In this example, the high-pressure hose between the pump and motor is SAE 100R9AT-16, 36 feet long. The volumetric expansion of this hose, due to the increase in pressure, is 9.7 in³ and the additional volume required due to compression of the fluid within this hose is 2.8 in³. Therefore the total, additional fluid volume required to increase the operating pressure from 1000 to 3000 PSI is 12.5 in³ (9.7 + 2.8 = 12.5).
To calculate the time taken for the operating pressure to increase from 1000 to 3000 PSI, which is equivalent to the length of time the transmission pump will cavitate, we divide the required make-up volume (12.5 in³) by the volume available from the charge pump per second (27 in³). In this example, the transmission pump cavitates for 0.46 seconds every time a sudden increase in load demands an increase in system pressure from 1000 to 3000 PSI (12.5 ÷ 27 = 0.46).
This problem occurs in applications where there are sudden fluctuations in load on the transmission. Typical examples include drill rigs, boring machines, and cutter wheels on dredgers. The solution involves increasing available charge volume - usually through the installation of an adequately sized accumulator in the charge circuit.
Editors Note: If you're reading this in Queensland, Brendan Casey will
show you how to reduce your operating costs by preventing unnecessary hydraulic failures, step-by-step, in a one-day,
Hydraulic Breakdown Prevention Workshop he's presenting on the Gold Coast on May 25, 2007.
Other dates around Australia in 2007.
So download the details, mark your calendar and plan to attend.
"This book has the potential to save many
organizations lots of m0ney. It should be on the bookshelf of every engineer, supervisor, planner and
technician that deals with hydraulic equipment... it's worth its weight in gold." Find out more
Alexander (Sandy) Dunn
Plant Maintenance Resource Center
Hydrostatic transmission fundamentals|
Produced by JI Case for training technicians on heavy-duty hydrostatic transmissions,
'Hydrostatic Transmission Fundamentals' explains axial pump and motor operating principles,
pump and motor rotating group composition and terminology, displacement control principles and typical
transmission charge and over-pressure protection circuits.
Find out more
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