Fleet Maintenance Reduction Program

Navy Shock Test Description

Many manufacturers and representatives are interested in selling equipment to the U.S. Navy for use onboard Navy ships.  They are often “Shocked” to learn that there are a number of qualification standards that must be passed before the Navy will even consider a shipboard test.  The main hurdles are shock, vibration, and EMI (electromagnetic interference).

This discussion is about Navy shock testing as defined in the Military Standard (MIL-S-901C).

MTTC recently assisted a company in qualifying an expansion bolt (as a replacement for form-fitted bolts) for use in equipment mounts and large shaft flanges.  The tests must, to the best of our ability, reflect real-world conditions.  Hence, these bolts were used to connect shaft portions (emulating propeller shaft applications).  It was mounted in a cage and held at each end with high strength bearings.  See Figures 1 and 2.  Just prior to the shock test, a pneumatic motor was used to spin the shaft up to 100 rpm, reflecting the normal rotation speed of a specific ship’s propeller.

Figure 1

Figure 2

Figures 3 and 4 show the actual installation of the expansion bolts on the flanges.  Although it is difficult to see, this is a 2-piece flange.  Since it is mid-way between the support bearings, it will see the maximum stresses when the test occurs.

Figure 3

Figure 4

Figures 5 and 6 show the actual hammer.  It weighs over 1000 pounds and is lifted to a point that is parallel to the ground (minimum stroke) to over 5 feet in the air (maximum impulse).    Figure 5 shows a 5-foot lift.  The hammer weight and height are designed to impart between 100 and 110 g’s (g being the force of gravity at the earth’s surface).

Figure 5

Figure 6

Figure 7 shows the laboratory technician in the yellow t-shirt as he has just released the hammer.  The blur in the hammer pit is the hammer starting to move down.  At the bottom of the stroke, it will impact the test stand’s anvil and, with great noise, the equipment will be subjected to very high stress levels.  In addition to the initial impulse, the test stand resonates so you see forces in both vertical directions (up and down) for several seconds.

Figure 7

In this particular case, the bolts were subjected to 3 shots, of increasing intensity in the vertical (z) direction.  Depending on the equipment, the system to be tested also has to be shocked in the two perpendicular horizontal directions (x and y).  That would, therefore, involve a total of 9 hammer strikes.

Don’t kid yourself – 100 g’s is a massive shock.  As previously mentioned, the bolt test involved using 2 bearings that would enable the shaft to be rotated during testing.  During these particular tests, on the highest level shock, our test stand’s bearing failed when the upper bearing shell split.  Figures 8 and 9.  The expansion bolt held, however so the test was successful.

Figure 8

Figure 9

Just for fun, it ought to be mentioned that, for very large equipment like guns, missile launchers, generators and so forth, the above-mentioned test stand is not large enough to accommodate the systems under test.  For Heavyweight tests, a barge, floating in a lake is used.  The equipment is solidly mounted on the barge and a large explosive charge is placed in the water several feet from the barge.  When it is detonated (figures 10, 11 and 12), the entire barge with equipment is subjected to a shock similar to a mine or torpedo strike.  This is not a quiet science!

Figure 10	Figure 11
Figure 12

If you have equipment that you would like to qualify for military use, be prepared to face testing such as this.  If it has any electronics or electrical components, EMI testing will be required.  Vibration testing is also usually required to reflect long-term use on vibrating ship hulls.  If a fastener can come loose, the vibration test will identify it.