Moulded bearings promise a longer lifetime

Edited by the Engineeringtalk editorial team Jun 10, 2005

Sarnatech BNL has helped GAF Materials Corp to significantly improve the performance of its Master Flow rotary roof turbines with a suite of custom-designed plastic ball bearing assemblies.

Sarnatech BNL, a leading designer and manufacturer of innovative plastic bearing solutions, has helped GAF Materials Corp, North America's largest roofing and ventilation manufacturer, to significantly improve the performance of its Master Flow rotary roof turbines with a suite of custom-designed plastic ball bearing assemblies.

Allowing a continuous flow of air through a building's attic space, GAF's rotary roof turbines help provide the ventilation crucial for a long-lasting roof system and avoidance of structural damage.

In earlier turbine models, the upper and lower cowl bearings had a tendency to disassemble in very high winds.

To overcome this problem, BNL has designed the new parts to assemble against the natural pull-apart direction, making it almost impossible for them to break apart - even in hurricane conditions.

The BNL bearings have been tested to perform at speeds of up to 200rev/min.

Moulded from a specialist grade of acetal available only in Europe, the bearings are quiet, low friction and lubrication free.

The low friction and free running design of the BNL bearings enable the rotary roof turbine to revolve as slowly as 3rev/min, making it effective even in very light winds which makes a significant difference to air flow on a still day.

Competing products with higher friction bearings require stronger winds to begin rotating.

BNL's unique fully moulded bearing raceways - manufactured using a technique perfected during the company's 35 year history - result in exceptional wear rates, giving the GAF roof turbine a far greater life expectancy than that offered by competitor products that use bearings with machined raceways.

Tests conducted by BNL show that the new fully moulded bearings for GAF will outlast similar fully machined bearings by up to 700%.

The superior wear rates achieved by fully moulding a bearing raceway are due to the existence of a fine crystalline layer which is formed as a result of the rapid cooling of the hot plastic on contact with the tool.

This fine crystalline boundary layer is very hard, but as the tool fills, it insulates the core and subsequently, the plastic cools more slowly, giving more time for the crystal structure to grow with larger, less resistant crystals.

BNL has found that the thicker the fine crystalline region, the greater wear resistance the surface (ie the raceway) of the part has.

Competitors typically machine a raceway into this surface as a secondary operation, resulting in the removal of the hard crystalline layer and therefore significantly reducing the wear rate.

BNL's specialist in-house design expertise has resulted in greater added value to the bearings through the integration of additional features such as clips on the caps making them easy to snap into place, and an upper bearing that includes a bayonet fastening feature, meaning a simple "push in and twist" movement is all that is needed to install and secure it to the roof turbine.

Another integral snap feature on the bearing inner, secures the bearing to the central turbine shaft.

These special cost reducing design features remove the need for additional heavy and expensive metal screws and bolts that are time consuming to assemble.

"We have enjoyed working with Sarnatech BNL for many years", said Steven Boehling, Engineering Manager at LL Building Products, a wholly owned subsidiary of GAF Materials Corp.

"The company's technical ability and execution of this project enabled us to market a greatly improved product that is taking away market share from the competition", he said.

"BNL delivered an integrated bearing solution that was superior to designs from several other manufacturers and stayed within our budgeted cost targets".