Metals have their place. And so do plastics. But sometimes they work best when used together. That's the case with an innovative propeller hub isolator developed by Mercury Marine engineers for some of the company's outboard engines. "The hybrid component does things neither the plastic nor metal could do alone," says Kevin Anderson, manager of materials science for Mercury Marine R&D.
Called the Flo-Torq IV, the isolator consists of two annular acetal (DuPont Delrin) parts overmolded onto eight titanium rods. Mounted on a sleeve that mates with the propeller shaft, the hub isolator acts as a torsional spring to absorb the difference in rotational energy between the propeller and drive gears when shifting out of neutral. As Anderson explains, the titanium rods deform by up to 15 degrees of arc as the acetal part nearest the drive gears turns faster than the acetal part near the propeller.
That deformation of the titanium absorbs no small amount of rotational energy. Anderson points out that the drive gears are spinning at 264 rpm during engine idle, while the undriven propeller spins slowly in the water. "When shifted into gear, the propeller engages with the spinning gears, creating a loud 'clunk' and noticeable vibration through the boat," Anderson says.
Past hub designs were rendered either in stainless steel or in acetal, with the plastic going into lower horsepower engines. But neither of these designs actually did anything about the drivetrain vibration. The new design, by contrast, virtually eliminates shift clunk, Anderson reports.
And it does more than ward off vibration. The propellor hub has been designed to act as a spring only for the first 10 degrees of prop rotation. After that, it transfers all
the torque from drivetrain to propeller — about 260 in-lb of torque when fully loaded.
There's no doubt boaters like the smooth, quiet shifting. For engineers, though, the nice thing about it is how it combines the best of metals and plastics into a single component. Titanium alone wouldn't offer the rotational range, plus it would have been too pricey. The polymer alone didn't have the modulus needed to maintain a constant spring force over time or absorb enough energy in higher horsepower drivetrains.
Together the two materials meet these goals and more. Anderson adds that the acetal also protects metals near the titanium from galvanic corrosion and acts as a safeguard against propeller impacts. "The plastic part, which is easily replaceable, shears away by design to save the more expensive propeller," he says.
So far, the new hub isolator has been a big success for Mercury Marine. It's already in service on the company's supercharged 4-cylinder, 4-stroke outboard engines up to 200 HP. "There haven't been any warrantee claims," Anderson notes. In the future, the company plans to extend the design to engines up to 275 HP.
Mercury Marine's propeller hub isolator was the subject of a technical paper at the recent Structural Plastics Conference and New Product Design Competition. For more information on this event, visit http://rbi.ims.ca/4928-539.
For more discussion on metal plastic hybrids join our Materials Forum at http://rbi.ims.ca/4928-540.