Insane Hydraulics

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Hydraulic Troubleshooting - Slow Hydraulic Motors

A couple of years ago I got an "S.O.S." call from a client who provided field assistance to agricultural equipment in his area. For several days he'd been "struggling" with the hydraulic system of a big tomato harvester and the problem remained no matter what he did. He knew these machines from head to toes and had successfully assisted many clients in the region for years, and yet this one didn't respond to any of the usual tricks.

The owner was complaining that the tomato transporting belts, driven by hydraulic motors, were running slow. Sounds simple, doesn't it?

For those of you who are not familiar with this type of equipment a short explanation: normally such harvesters have multiple transporting belts directly driven by gerotor hydraulic motors that are connected in series. This one had seven Danfoss OMRs, six of which were equipped with a speed regulating valve:

The series connection spares space, hoses, and pump size, as these belts are mostly horizontal and don't require much torque. It is the last motor in the series that produces the most torque because it drives the vertical belt that lifts tomatoes and dumps them into a trailer. Very often manufacturers don't even install drain lines on the motors, which was not the case with this machine, though - all the drain lines were there, just the way I like it!

Although I am yet to find a Portuguese farmer who is completely satisfied with the speed of his machinery, the belts indeed were running slower than they should. The mechanic had applied the good old "let's replace the old stuff with new and see what happens" principle, but it didn't work - all of the components had been replaced, the pump had been inspected and tested in our workshop showing no signs of wear, and the damned belts remained slow.

Simple things first - pump actuation, is it belt driven? No, it is a directly connected gear pump with a spline shaft. O.K., moving on, although the client had verified the relief and the by-pass valves, I verified them again (trust no one, remember?). I even connected the pump directly to the first motor, by-passing all the valves (don't try this at home, kids!) - nothing changed.

Then I disconnected the drain line of the first motor to check for excessive leakage - the oil was trickling from the open hose end as it should as the motor was new. (By the way, the mechanic had verified the drain lines of all the motors and did not detect any excessive drain flow) At this point, I was sure that at least the first motor had a good volumetric efficiency, and as it was connected directly to the pump I decided to use it as a flowmeter, (the mechanic had a digital flowmeter at the site, but he claimed it was broken, because when he tried to get a flow reading at the pump's outlet, the flowmeter showed very high flowrate deviations, sometimes two or three times as big as the pump's nominal flow).

A small strip of reflective tape on the motor shaft plus an optic pick-up rpm gauge and voila - a simple and effective flowmeter without any oil on my clothes (wish it was always this easy). The machine started and then I saw something that was definitely not normal. As the diesel RPM was climbing, so was the flow, until a certain point, and then the flow increase (the hydraulic motor RPM) would seem to stop being linear with the diesel RPM, in fact, it seemed to remain constant. When the engine RPM went from 1000 to 2000, the flow was to double, and this was not happening. Why?! Quick verification of the diesel flywheel speed confirmed that the machine's tachometer was accurate, and then the owner added that the belts were slower in the morning and when the oil was at normal temperature the speed improved a little.

At this point, I was pretty sure about the origin of the problem. And how about you? Any ideas? I promise you already have all the information to indicate the most probable cause of the malfunction!

O.K. Let us state the obvious:

A) The first motor has good efficiency, hence the motor's speed is directly connected to the oil flow passing through it. When the motor's speed stops increasing it can only mean one thing - the oil flow stopped increasing. So the main reason for the belts being slow is insufficient oil flow.

B) The pump is a fixed displacement type and was proven to have good efficiency, the mechanical part that moves the pump is o.k., and the pump's outlet is connected directly to the first motor of the series with no oil deviation whatsoever.

In this case, the only way to make the insufficient oil flow come out of the pump is to not let the sufficient amount of oil enter the pump. And this can only mean one thing - a deficient (partially obstructed) suction line!

This explains all of the symptoms - belts running slow, oil flow rate stopping to increase at a certain point (when the pressure in the inlet was dropping below the evaporation point and the pump was turning hydraulic oil into a nasty fluid/vapor mixture). This also explains why the speed improved with hotter, and therefore more fluid oil. It even explains why the mechanic couldn't get a stable reading from his turbine-type flow-meter - the presence of vapor pockets in the fluid was causing the light turbine wheel to jerk instead of moving with constant speed.

And by the way - my "improvised flow-metering solution" faired much better under these conditions - the vapor pockets were imploding on entering the motor, and its speed generally corresponded to the amount of oil passing through it.

Another possible explanation I could think of at the moment was air entering the suction line, but it had to be a pretty fair amount of air, and this possibility was quickly discarded when I examined the oil coming out of the first motor - it was clean. Were it to have a significant amount of air bubbles in it, it would come out as a beer-foam-colored mixture. So I was left with only one theory - a strangulated suction line, which in that case was a 3-meter long R1AT 1.1/4'' hydraulic hose.

However, when I presented my theory to the client, he was rather sarcastic about it, and he told me that it was impossible for the suction line to be obstructed. He claimed that the tank had been thoroughly cleaned and the suction hose was brand new.

To prove my point I would need a vacuum gage in the pump inlet, but I didn't have one with me, and the mechanic (to my surprise) did not know what a vacuum gage was.

As you can imagine finding a vacuum gauge in the middle of nowhere is a difficult task, but I was lucky to spot an old air compressor in the barn we were working at, and since it had a 5-bar pressure gauge, I told the owner that I could devise a primitive vacuum gauge out of it to check my theory.

What I did was pretty simple - I opened the gauge and carefully moved the needle so that its resting point stopped in the middle of the scale, thus allowing it to read negative (in relation to atmospheric) pressures. Then I tested it with my mouth... Don't laugh now, your mouth can create around half a bar of negative pressure, so it is a good way to test vacuum gauges in case of doubt. Have you ever let a beer bottle get stuck on your tongue? Try the same with a vacuum gauge and check its reading! Only... don't let anyone see you do this!

Then I introduced the client to the "suction line theory", and showed him how the suction gauge worked. Luckily I had all the necessary fittings in my toolbox to connect the gauge to the pump's inlet. The machine started, the arrow jumped down right away, and when the RPM climbed to 1500, the gauge was showing around 0.7 bar negative, which was wa-a-ay too low!

Although the proof was staring at his face, the man was still reluctant about my theory, so I offered him to test a similar suction line mounted next to this one. And, of course, when the machine started, the pointer hardly moved from its place no matter what the engine speed was, (like every decent suction line should be). Hurray!

As it was getting late, no further work was done to the machine. I advised the client to inspect the suction line and the tank to see where the strangulation point was. Unfortunately, I never knew what exactly had strangulated it. I heard they solved the problem by replacing the hose. A week later the mechanic bought two vacuum gauges from us.

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