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Bell Rotor Hub Tension Torsion Straps – A Side-By-Side Comparison Says It All

( 02/19/2019 ) Written by: Conor Marr

A head-to-head comparison of LORD-made Bell 206 Tension Torsion (TT) Straps with a competitor’s offering led to alarming results. The surprise came from the numbers – and the staggering difference between them.

Simply put, we subjected a LORD 206 TT strap, and a competitor’s 206 TT strap to a series of tests commonly used to gauge part acceptability. While some of the differences between the LORD strap and the competitor strap were benign, the head-to-head ground-air-ground (GAG) cycle test revealed a worrisome difference in the competitor part. When subjected to a GAG test using the overspeed centrifugal force (CF) condition, the competitor part lasted less than 1/5th the cycles of the LORD strap!

The TT strap is a flight critical part that attaches the blade to the rotor hub of a helicopter. These elegant parts take the entire centrifugal force of the rotor blade while allowing pitch input, or feathering, of the blades, all in a compact package. To accomplish this task successfully, and more importantly, with reliable safety, the design and materials are both highly engineered and quality controlled.

The GAG cycle test we conducted is configured to mimic the forces the blade exerts on the strap to react the centrifugal load during operation. A GAG cycle represents some of the most severe changes in loads on the part. It starts with a low load representing low rotor RPM, then increases the load to full or overspeed rotor RPM, resulting in a high load on the part.  While this test does not directly correspond to the operational life of the part, it is used as a screening method to detect low life parts. 

The difference between the two straps is evident. The competitor strap lasted far fewer GAG cycles than the LORD part under the same conditions. This low cycle result from the competitor part is coupled with clear manufacturing and quality differences between the competitor’s own parts, including stray wires extruding through the protective casing.

When choosing which part to hold the blades on your aircraft, to give you consistent, quality, and reliable performance, the choice is clear.

Experts in noise, vibration and motion in flight-critical components and systems, LORD is the original STC and PMA holder, designer and manufacturer of the TT strap which is available for more than 20 aircraft.

 

 

ABOUT THE AUTHOR MORE BY THIS AUTHOR
Conor Marr

Title:  Manager, Sustaining and Aftermarket Engineering at LORD Corporation

I lead two teams in the Dynamic Systems group: Sustaining Engineering interfaces with the Erie ,Dayton, Cambridge Springs, and Bowling Green plants for any production decisions that update the form, fit, or function of the design. Aftermarket Engineering works to improve current products in the field, both LORD generated, or competitor generated, and directly serve the end user to provide increased life, easier inspection, or increased performance.

 

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