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    This article describes technical causes behind the split multi-section gear pump failure, described in the "Travelift Adventure" post, and also touches a couple of gear pump related myths.

    Brief malfunction description:

    A five section Parker gear pump, four sections of which were feeding independent Sauer Danfoss PVG 32 proportional valves, split the bodies and spat out the seals after ten minutes of operation, causing notorious agitation of standers-by and appearance of Grey hair on the head of the attending technician (details here).

    Before going into the main causes of the failure, I would like to talk a little about the pressure induced forces that exist in "traditional" gear pumps, as well as to touch a couple of gear-pump related misconceptions I come across fairly often.

    In a common single section gear pump (like this one, for example) you have two end plates, one with a shaft sticking out of it  and the other one blind, and a body in between, with all the gears, seals and what not inside. The "sandwich" is held together with bolts, which either are screwed into the threaded holes of the body, or pass through the whole sandwich and secure the parts with nuts on the other end.  Each of the plates has an area exposed to the outlet pressure which produces force directed to separate the plates from the body, and therefore creates tension stress in the screws. Let us pass on, now, to the myths, starting with

    Gear pump myth number one.

   
Not a myth, actually, but a misconception I often see around workshop population, and which concerns gear pumps that use passing screws. With the threaded body and short screws the understanding of separating forces is straightforward and normally causes no difficulties  - the internal pressure acts on the plate, creating a force which equals the exposed area times outlet pressure. If we had to calculate the internal stress of the screws, it would be this very force that we'd use for the calculations. However in case of passing screws (diagram, internal parts are excluded for the sake of simplicity) it is often misunderstood that, as the upper plate creates an "area times pressure" force in one direction, and the lower plate creates an equal force in the opposite direction, the resulting screw tension force is doubled. This is wrong - the tension stress of passing screws is the same as with the threaded body design, and if we were to calculate it, we'd still use the same one area times pressure, just like with the threaded body design.

    Gear pump myth number two.

    
This misconception boils down to believing that in multi-section gear pumps, held together with long threaded rods that pass through the whole assembly,  the pressure induced separating forces of every section add up, causing a much bigger tension stress in the screws/rods than in the case of a simple pump. Once again, let us look at the diagram - the section with higher pressure creates higher separating force, which is then transmitted to the screws through the end plate and the body of the section with lower pressure. The pressure inside this section relieves the central body from some of the stress due to the separating force acting on the endplates, but it doesn't add up! If we were to calculate the tension stress of the screws in a multi-section pump, we'd have to use the same one area times the highest  pressure.

   Now that we are clear about gear pump pressure induced separating forces, let us go back to the malfunction and its

    Main Causes:

    1.
The first and most obvious cause, of course, is the four threaded rods used to hold the five bodies together However, the same rods had been used for ages in single section pumps without a single problem. So what went wrong?

    Two mistakes were made during the rod choice - the rods were threaded through the whole length, and the rod grade was too low.  The separating forces of a multi-section pump do equal to the ones of a single pump, however due to the fact that multi-section pumps use far longer rods, the same amount of tension force results in a higher rod length increase, which is why the rods suitable for single section pumps can be not suitable for multi-section pumps.  Rods that are threaded though the whole length are a poor choice for these units, because the thread reduces the effective load bearing area of the rod, making it "thinner". For large multiple gear pumps the use of high grade steel rods, which have threads only at the ends, is imperative.

    2. The second cause was the presence of high pressure spikes, which caused the rod lengthening and the consequent "foda-se, caralho!" (Portuguese for "unpleasant") situation. To trace the origin of the spikes let's look closer at the operation of the PVG 32 valves used in this system.

    On the section view of a PVG 32 pump side module you can see that the relief function is accomplished by the compensator spool 6. When the pressure exceeds a set level, the small poppet of the valve 1 lifts from its seat, venting the left side of the compensator to tank and causing it to dislocate to the left, thus connecting the inlet to the tank port. The relief function is there, but like in case of any spool type relief valve, it is relatively slow.

     In that hydraulic system, each PVG valve had a ON/OFF module, controlled by a PVEO 24v solenoid. These modules were used to operate the four hydraulic winch motors. Due to let-us-get-rid-of-what-we've-had-for-years reasons 100 liter per minute spools were chosen for the sections, which exceeded the pump supply, and so when the valve was in ON position, the compensator spool closed the P to T passage completely. When the solenoid was de-energized, the distributor spool would return to central position in about one tenth of a second, which was faster than the pump section compensator could open the P to T passage, so whenever the ON/OFF solenoid was shut-off there was a pressure spike in the P line, caused by the combination of  "slow" compensator spool and the fixed displacement gear pump unstoppable flow. By the way, the choice of a high flow spool can be gracefully explained by the desire to lower the spool pressure drop and therefore the consequent heating, as the winching function required no flow control and was to be operated for long periods of time.

    There are several ways to address the problem. The most obvious is installing pre-charged accumulators at the pumps' outlets. It is, probably, the best technical solution, with the drawback of being expensive and requiring regular maintenance. Another, cheaper solution lies in installing additional fast acting pressure relief valves in order to clip the spikes. Yet another - using a longer pressure line hose which will damp the surges due to the hose accumulator effect. Another one is to limit the distributor spool travel (or, alternatively, apply a smaller flow spool) to the point where the compensator starts compensating. In this type of valve, flow regulation is done by means of metering the pressurized fluid to tank, from P to T, so when the compensator meters flow, there is already a small passage open between the inlet gallery and the tank line, making the  high pressure surges unlikely. Yet another solution would be to find a way to dampen the distributor spool movement (by installing orifices in the pilot lines, for example), thus giving the compensator time to open.

    In that particular case there were four fast acting SUN relief valves readily available from the old installation, with all the fittings, pipes and everything, so I opted for installing them directly at the pump's outlets. Even when they were adjusted 40 bars above the PVG relief setting, they'd still spit a stream of oil from the T line each time the solenoids were disconnected. The pump hoses were also increased a little.

   Lessons to learn:

    - Multi-section gear pumps should be assembled with high grade steel rods with thread only at the ends.
    - Sauer Danfoss PVG valves, as well as other proportional valves based on the same design, under certain conditions can create high pressure spikes in the inlet line. These include: the combination of flow saturation condition (when the compensator spool closes the P to T passage completely, which can happen when a spool flow rate is higher that the pump flow, or when several spools are actuated at the same time, demanding more flow than the pump can deliver) and a fast release of the spool/spools to the neutral position (which is the case of the PVEO on/off solenoid).
    - Such pressure surges can be aggravated in fixed displacement pump circuits that use rigid (steel) piping.
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