The noise comes at the instant of shifting because the propeller is lazily turning in the water and the engine's drive gears are spinning at 264 rpm.
The isolator, made by over-molding ring-shaped plastic parts onto eight titanium rods, mounts on a sleeve that mates to the propeller shaft. When the engine shifts into gear it absorbs energy as the titanium rods fixed within the plastic rings deform with up to a 15° arc as the ring nearest the drive gears turns faster than its counterpart at the propeller end. Mercury Marine R&D materials engineer Scott Olig says the rods act as springs damping transient vibration and high shock loads.
The ring-shaped parts made from Delrin 150 acetal from Du-Pont Engineering Polymers, Wilmington, Del., also give the titanium rods corrosion isolation from other metal elements of the drive system and protect the drivetrain against damage in the case of a severe propeller impact.
Both parts have molded-in races designed to shear under severe loads, helping prevent the transfer of destructive impact forces to pinion gears, drive gears, or other drivetrain components, explains Olig. After an impact, the boater can easily and quickly install a spare isolator hub.
"DuPont also provided key computer-aided finite-element-analysis assistance," points out Olig. "Their analysis gave us crucial data for designing the molded parts and for simulating the non-linear force deflection curves of their interaction with the titanium rods during operation. "Combining Delrin acetal with titanium rods let us design a part that functions in ways neither the plastic nor metal could alone," says Olig. The Delrin-equipped isolator will be on display at DuPont's
DuPont Engineering Polymers, (800) 441-0575, plastics.dupont.com