NATIONAL BUREAU OF STANDARDS REPORT

9947

FAILED MCCAULEY PROPELLER HUB

To

Bureau of Aviation Safety

National Transportation Safety Board

Department of Transportation

Washington, D. C.

<NBS>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards’ was established by an act of Congress March 3, 1901. Today, in addition to serving as the Nation’s central measurement laboratory,

the Bureau is a principal focal point in the Federal Government for assuring maximum application of the physical and engineering sciences to the advancement of technology in industry and commerce. To this end the Bureau conducts research and

provides central national services in three broad program areas and provides central national services in a fourth. These are: (1) basic measurements and standards,

(2) materials measurements and standards, (3) technological measurements and

standards, and (4) transfer of technology.

The Bureau comprises the Institute for Basic Standards, the Institute for Materials

Research, the Institute for Applied Technology, and the Center for Radiation Research.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the

United States of a complete and consistent system of physical measurement, coor- dinates that system with the measurement systems of other nations, and furnishes

essential services leading to accurate and uniform physical measurements throughout

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following areas of science and engineering:

Applied Mathematics—E iectricity—Metrology—Mechanics—Heat—Atomic Physics—Cryogenics’—Radio Physics2 —Radio Engineering'-'—Astrophysics2—Time

and Frequency.2 THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to methods, standards of measurement, and data needed by industry, commerce,

educational institutions, and government. The Institute also provides advisory and

research services to other government agencies. The Institute consists of an Office of

Standard Reference Materials and a group of divisions organized by the following

areas of materials research

Analytical Chemistry—Polymers—Metallurgy — Inorganic Materials — Physical

Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides for the creation of appropriate opportunities for the use and application of technology within the Federal Government and within the civilian sector of American industry. The primary functions

of the Institute may be broadly classified as programs relating to technological meas- urements and standards and techniques for the transfer of technology. The Institute

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1 Headquarters and Laboratories at Gaithersburg:, Maryland, unless otherwise noted ; mailing address Washington, D. C. 20234.

2 Located at Boulder, Colorado 80302.

1 Located at 5J285 Port Royal Road, Springfield, Virginia 22151.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

3120412

NBS REPORT

9947

FAILED MCCAULEY PROPELLER HUB

By

T. R. Shives

Engineering Metallurgy Section

Metallurgy Division

To

Bureau of Aviation Safety

National Transportation Safety Board

Department of Transportation

Washington, D. C.

IMPORTANT NOTICE NATIONAL BUREAU OF STAN

for use within the Government. Be and review. For this reason, the p whole or in part, is not authorize Bureau of Standards, Washington,

the Report has been specifically pr<

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

accounting documents intended

bjected to additional evaluation

sting of this Report, either in

Jffice of the Director, National he Government agency for which

es for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Hardness : A diamond pyramid hardness traverse was made on a transverse

section through the failed hub socket near the crack origin. These

values and the approximate Brinell equivalents are given below:

Distance from edge

of section adjacent

to threads

DPH

10 kg

A PP r °*- BHN

500 kg

1 mm 162 170

2 160 138

3 161 139

5 162 170

7 163 171

9 168 177

1

1

168 177

12 168 177

13 167 173

The hardness i

s

relatively uniform and in all cases is well above the

mini mum Br i ne 1

1

hardness of 125 r-^n 1 for aluminum alloy 2017-Tb as specified in ASTM Spec i f i cat i on d277“67 6

The blade retention nut was specif ied to be aluminum alloy 7^75 in

the T -6 condi t i on

.

Brinell hardness measurements as f 0 1

1

ows were taken

on the back of the nut

:

Measurement BHN,-^^ . 500 kg

1 137

2 173

3 1 40

7 173

5 177

6 173

All measurements were above the minimum Brinell hardness of 1 35,-^ , 500 kg as specified in ASTM Specification B277-67.

Meta 1 lographi c Examination : The fracture originated at a fatigue crack

near the root of the innermost thread in blade socket No. 2 and extended

across the socket (fig. l). The fracture surface is shown in figures 3

and 4. The fracture was sectioned adjacent to the fatigue crack origin

and examined meta 1 1 ographi ca 1

1 y . Figure 5 shows a representative area of

the material in the unetched condition near the crack origin. The

material is clean and shows little or no porosity. Figures 6 and 7 show

I he mi cros t rue Lure of the hub socket adjacent to the c rack 01 i g i n and

away from the origin, respectively. There appears to be nothing

abnormal or detrimental in the 2014-T6 heat treated structure shown.

The smaller grain size observed immediately adjacent to the fracture

origin may have resulted from relatively faster cooling after forging

in this area. No unusual material condition that may have contributed

to crack initiation was found at the origin.

Thread Examination : There was a good deal of fretting in the threads

of all three blade sockets as well as i n the threads of the retention

nut from socket No. 2 (figs. 8 and 9 ). (The other two retention nuts were not available for examination.) Fretting occurred adjacent to,

but not directly at the fatigue crack origin.

The Engineering Metrology Section of the National Bureau of

Standards made measurements of the minor diameter and the thread depth

of the threads on the retention nut and the threads of one of the intact

blade sockets (socket No. 3)*

The minor diameter of the socket was measured at three places in

each of two planes: the 0° plane which is parallel to the drive shaft,

and the 90° plane which is in the plane of the blades. These measurements are as follows:

Minor diameter, Minor diameter,

Locat i on 0 ° plane 90° plane

Entering end

Center

Back

5.251 in

5.252

5.254

5.238 in

5.240

5.232

The permissible range for the minor diameter as calculated from

specifications on drawing No. E-3^97 (Propeller hub) of McCauley

Industrial Corporation is as follows:

Minimum diameter 5*2294 in

Maximum diameter 5-2350 in

All but one of the six measurements were greater than that allowed

indicating that the over-all minor diameter of socket No. 3 wa s too

great. No meaningful data could be obtained on the minor diameter of

the failed socket. No. 2.

The thread depth in socket No. 3 was determi ned to' be 0.0947 in.

The tolerances specified by McCauley drawing E-3497 are O.O966 to

0.0930 in. The thread depth, therefore-, meets specifications.

- b -

The retention nut pitch diameter projected to the reference plane

(O.O87 ' n below nut face) is 5.320 in with the tolerances being 5-324

to 5.322 in (McCauley drawing C -3493 ; nut, blade retention). The

thread depth measured O.O966 in with tolerances being 0.0900 to 0.0930

in. The thread depth on the nut meets specifications, and while the

minor diameter measured slightly less than specifications, the

measured value does not differ sufficiently from specifications to be

considered significant.

The threads over a large portion of the blade retention nut were

rather badly damaged. Figure 10 shows the thread profile in a damaged

area as well as in an area where the threads are intact. The thread

profile appears satisfactory in the undamaged area.

The threads of the failed hub socket were also damaged badly as is

shown by the profile of two threads adjacent to the fracture (fig. I la).

Figure lib shows the thread profile of socket No. 1 which was not

damaged and the threads appear to be satisfactory. In figure 12, the

thread nearest to the fracture is shown at a higher magnification.

The material has been deeply etched to show flow. The top of the thread

has been severely deformed. Other damaged threads exhibited deformation

of this nature.

Cone I us i ons : The material in the hub meets specifications for chemical

composition of aluminum alloy 2014 and it also meets hardness specifications for the T-6 condition. The blade retention nut meets hardness

specifications for aluminum alloy 7075"T6. The microstructure of the

hub material appears to be satisfactory. Examination of the thread

profile of undamaged threads of both the hub and retention nut indicate

that the threads are satisfactory. It is the opinion of this

investigator that the hub and nut damage (deformation) observed occurred

subsequent to failure, and therefore did not contribute to the fracture.

The oversize minor diameter found in measuring socket No. 3

indicates a possible mismatch between this socket and its related

retention nut. It is conceivable that the mismatch is responsible for

the fretting observed in the threads of socket No. 3. Inasmuch as fretting was also observed in the threads of sockets No. 1 and No. 2,

it would seem that a mismatch condition was also present in these sockets.

Fretting is indicative of cyclic stressing and it is possible that

this stressing could have initiated the fatigue crack that propagated

to cause the failure of the hub in socket No. 2. Fatigue c'rac-ks m^y be

associated with fretting, and their origins, as in this case, are sometimes found not directly in a fretted area, but adjacent to it.

USCOMM-NBS-DC

Figure 1. McCauley Propeller Hub as received. Arrow on photograph points to approximate

location of fracture origin

on inside of hub. X 3/8

Figure 2. Blade retention nut from blade

socket No. 2 as received. X 2/3

Figure 3» Fracture surface of failed hub socket,

No. 2. X 1

Figure 4. Origin of fatigue crack in failed hub

socket, No. 2. X 10

Figure 5 . Representative longitudinal area adjacent

to origin of fracture. Unetched. X 100

Figure 6. Longitudinal microstructure immediately

adjacent to origin of fracture. Keller's

etch. X 100

Figure 7. Microstructure of longitudinal section

away from fracture. Keller's etch.

X 100

Figure 8. Threads of failed hub socket showing

f rett i ng. X 2

I

Threads of blade retention nut showing

f rett i ng. X 2

Fi gure 9*

Figure 10a. Thread profile of

damaged threads of

blade retention nut. X 7

Figure 10b. Thread profile of

undamaged threads of

blade retention nut. x 7

Figure 11a. Thread profile of hub

socket No. 2 showing

two damaged threads

adjacent to fracture. X 18

Figure lib. Thread profile of hub

socket No. 1 showing

undamaged threads. X 18

Figure 12. Thread adjacent to fracture in hub

socket No. 2 showing deformation.

Keller's etch. X 200