Acetal resin enables quieter marine transmission

Edited by the Engineeringtalk editorial team May 3, 2006

An innovative propeller hub isolator developed by Mercury Marine delivers quieter, smoother gear shifting for marine engines.

An innovative propeller hub isolator developed by Mercury Marine delivers quieter, smoother gear shifting for marine engines.

The isolator, called Flo-Torq IV, will be on display in the DuPont booth (2632) at NPE 2006 in Chicago, from 19th to 23rd June 2006.

"The hub isolator virtually eliminates the noise, vibration and harshness effects perceived as 'shift clunk' when shifting from neutral into gear", said Scott Olig, Materials Engineer, R and D, at Mercury Marine.

The component consists of two ring-shaped plastic parts joined by eight titanium rods.

The plastic parts, made of DuPont Delrin 150 acetal resin, are overmoulded onto the rods.

The isolator is mounted on a sleeve that mates with the propeller shaft.

When the engine is shifted into gear, the isolator functions as a torque spring, absorbing the difference in rotational energy between the shaft of the propeller, which at the instant of shifting is turning lazily in the water, and the engine's drive gears, which are spinning at 264rev/min.

Energy is absorbed as the titanium rods fixed within the acetal parts are deformed by up to 15 degrees of arc as the acetal part nearest the drive gears turns faster than the acetal part at the propeller end.

The rods act as springs, damping transient vibration and high shock loads.

The parts made of Delrin, serve as rotational direction leaders for the titanium rods, and they also help damp energy.

In addition, they provide corrosion isolation of the titanium rods from other metal elements of the drive system.

The acetal parts also have another important function: protecting the drive train against damage in the case of a severe propeller impact.

Both parts have moulded-in races that are designed to shear under severe loads, helping prevent the transfer of destructive impact forces to pinion gears, drive gears or other drive train components, according to Olig.

After an impact incident occurs, the boater can easily and quickly install a spare isolator hub.

"By combining Delrin acetal resin with titanium rods", said Olig, "we were able to design a part that performs functions neither the plastic nor metal could do alone".

Mercury Marine manufactures the Flo-Torq IV isolator at its plant in St Cloud, Florida.

It is currently used on production models of supercharged four-cylinder, four-stroke outboard engines up to 200hp.

In the future the company plans to adopt it for all of its four-stroke engines, including six-cylinder models with up to 275hp.

"DuPont provided valuable assistance during development", said Olig.

"A key element of assistance was computer-aided finite element analysis".

"It gave us crucial data for designing the moulded parts and for simulating the nonlinear force deflection curves of their interaction with the titanium rods during operation".