Yesterday was an unusually successful one, one that lifted our spirits over an evening meal. It was marked partly by progress but, more importantly, by confirmation that potential failure points had been eliminated. If that all sounds rather cryptic, well, it is, but let me explain.
Our problems
Over the preceding few weeks, we have worked, in part, on our hybrid drive system. This consists of a small John Deere 4045 diesel engine connected via a clutch to an Esco Power PHT drive. This device transmits power to the Twin Disc gearbox and the Praxis DC electric motor. A more elaborate description can be found HERE. Testing of our drives has revealed some disturbing problems:
rough running at idle, resulting in excessive torsional vibrations
noise from the mechanical coupling between the PHT drive and the electric motor
mechanical noise from the PHT drives themselves
improper plumbing of the oil cooler leading to oil starvation at the PHT drive
vibration of the Praxis DC motors on their cradle as distinct from the vibration of the engine itself.
Working through potential solutions
Upon further investigation, we found several instances of poor machining and fit between the coupling and both the PHT power output flange and the splined electric motor shaft. These issues were causing the excessive vibrations and noise we observed during testing. Parts were sent off to Istanbul for re-machining and were returned in a much-improved condition, with the motor and coupling now having a slight interference fit and being concentric.
We examined the design of the assembly. The motor and gearbox were supplied by 6 relatively soft, flexible mounts. Engine/PHT drive/Gearbox form a rigid beam over these mounts. An additional aluminum beam supports the motor and is mounted onto and above the gearbox. Whereas it did appear to be a substantial arrangement, the heavy DC motor was, in fact, supported in 2 points only (see illustration on left). It was effectively a hinge joint for small displacement low-frequency vibration. The mechanical coupling provided no axial constraint, leaving the heavy motor to flap in the breeze.
Our modification involved fabricating two 12mm steel side plates (see photo on right) that were used to locate the motor support beam to the PHT drive casing at 4 additional points. We effectively fixed the motor support beam, preventing further movement. This may sound complicated and ordinarily it would be a simple design analysis task. However, due to time constraints and limited local resources, we relied on experience and a bit of luck. We also oversaw the replacement of the E-Motors and took more care with both axial and angular alignment of the assembly, ensuring it was well within the manufacturer's tolerances. But the question remained, would it work?
Proof is in the testing
The engines were first run up at idle and allowed to stabilize. Tickover was adjusted from 600 to 750 RPM, resulting in much smoother running and eliminating a major source of excitation to any additional vibration.
Engines were shut down, and we then referred to the E-Motors. These were run up through the speed band to an eventual 1000 RPM (at the motor). We noticed quite a noise from the PHT drive, traced to the oil pressure bypass valve, as the system was cold and operating on relatively viscous lubricant (screwdriver behind the ear - stethoscope trick). This reduced as the oil temperature increased with operation.
We also checked that oil flowed as the pump had [previously been run dry. This was visual check by disconnecting the oil feed line to the gearbox, we also took oil samples. The revised plumbing was now functional.
We checked the flexible coupling's smooth operation by driving it from both ends, and again, it was good.
Lastly, we checked the motor for vibration (using a glass of water), and then Praxis engineer Marco brought out a small vibration meter for a more scientific measurement. It's all good again!
So, our diesel-electric drives are now running. Vibration problems are behind us, alignment is good, and the throttle, clutch, and gearboxes all work as planned. Motors can drive the shaft line and be driven as generators to charge the power batteries in isolation or when driving the shaft line. And we have two of those.
I think we can call that a good day. The application to ASBAS, the Free Zone Authorities, for sea trials will go in on Wednesday, we will leave the Free Zone Marina on Monday next.
Chris Leigh-Jones
As an aside, I received the following photographs from our son, Jeff. He runs a yacht delivery company (PolusYachts.com) and was doing so for a small yacht sailing west down the English Channel. In light winds they were motoring constantly until ....... the prop shaft snapped. Cause traced to improper fitting of the engine exposing a weak point in the shaft line. Fatigue failure ensued. Luckily, and with quick thinking, he secured the prop with the shaft still in its gland and called for a tow. The perils of the sea and a very relieved owner.
