RotorWay Engineering Report #1

 

Red lettering is actual RotorWay comments from their engineering report

RotorWay's first engineering report, dated November 18th, 2003 is a titled "Stress Analysis of Rotorway International 180.hp Secondary Shaft E23-6125."  The engineering report appears to be written by a knowledgeable engineer, however, no engineer will stand behind work.  RotorWay has yet to name the engineers behind this document, making it very difficult to engage in a productive conversation.

The engineering report is a 30 page document with a large number of color graphs, making its computer scanned file size rather large.  For that reason, Pro-Drive is unable to provide these document electronically.  Please contact RotorWay for a copy of this document.

Most of the report is a discussion of the forces, torques, and resulting stresses applied to the secondary shaft.  One notable section is section "5.1 - Belt and Chain Loads."  In that section RotorWay's engineers make statements such as "tension in belt/chain"  and "total side load from belt or chain, lbs." 

These statements are contradictory to the typical rhetoric from RotorWay about the increased loading due to the belt.  Hopefully, everyone agrees that a belt tensioned to the specifications provided by Pro-Drive, will have approximately the same tension as a chain system.  RotorWay's own engineering report never makes a distinction between the loading on the chain and belt.

Besides, if the belt had higher loading, wouldn't the bearing typically run hotter?  It is typical for a belt drive system to run an average of 50 degrees F lower then a chain installation, counter intuitive to higher loading.

The main point of confusion in the RotorWay engineering report is in section "7.2 - Endurance-Limit Modifying Factors."  In this section the endurance limit is calculated for the secondary shaft material.  The endurance limit is the stress at which failure will occur at some point in the parts life, typically between 1 million and 10 million cycles (8 hr to 80 hr at 2200 RPM).  Therefore, for the part to last longer then 80 hours, the shaft should operate with a stress less then the endurance limit.  (Technically, it will need to be a little less then the endurance limit due to the mean stress caused by the torque.)

What is important is the equation used to calculate that endurance limit, shown as:

Se=Ka*Kb*Kc*Kd*Ke*(61,100)

In this equation the "K" constants modify the original endurance limit (61,100psi) to a new acceptable endurance limit.  Most of these values are completely acceptable, Ka = .875, Kb=.841, Kc = 1, and Ke=1.  However, the Ke, the "miscellaneous factor" can be between 0.24 and 0.90 for a fretted part.  However, RotorWay's engineers chose a Ke of 0.80.  That means, in the range of 0.24 to 0.90, RotorWay feels that the influence of fretting corrosion only reduces the strength of the shaft to 80% of its original strength!

The problem is that fretting corrosion can severely damage the secondary shaft and reduce the endurance limit significantly.  All of the failed parts that have been reviewed by Pro-Drive have had fretting, and the parts shown on Jack Kane's website show severe fretting also. The report can be summarized in the last paragraph, as follows:

""The situation with regard to press fits is complicated by the simultaneous presence of stress concentration and fretting corrosion. The relations governing fretting corrosion are not well understood at the present time." Reference 2 [RotorWay's Reference 2] list a stress concentration factor of, Kt = 1.95, on bending stress to account for a collar pressed onto a shaft.  It also list fatigue factors of, Kf=2.0 to 3.9, depending on the configuration and the material.  This report applied a Kt = 1.95 to the bending stress to get a max alternating stress of +/- 34.8ksi.  This is below the +/- 35.1 ksi design allowable that included Ke=.8 and indicates that any factor larger then 2.45 could create a life problem."

In section "6.2.2-Affect of Upper Bearing Interference Fit on Shaft Stress" RotorWay's engineers say that "Reference 2 [RotorWay's Reference 2] reports fatigue factors that are as high as 3.9 for cases where fretting corrosion is significant..."  That means, RotorWay knows that the "Kt" could be as high as 3.9, much higher then the 2.45 listed that could create a life problem. 

Depending on how much fretting corrosion your shaft has, if it has suffered enough fretting damage  to reduce its endurance limit to 73% of its original value, your 35mm shaft could break in less then 80 hours.

1. "Mechanical Engineering Design, edition 5" by J.E. Shigley and C.R. Mishke, published by McGraw-Hill 1989.

2. ASME Handbook, Metals Engineering-Design, McGraw-Hill Book Company, 1953

3. "Metal Fatigue in Engineering" by H.O. Fuchs and R.I. Stephens, John Wiley & Sons, 1980

 


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