Lightweight Shock Machine (LWSM) Testing per MIL-S-901D MIL-S-901 is the military specification covering shock test requirements for shipboard machinery, equipment, systems and structures. Anyone who produces an item for military use in any of these shipboard categories will likely need to complete this testing. The purpose of this testing is to verify the ability of shipboard installations to withstand shock loadings which may be incurred during wartime service due to the effects of conventional or nuclear weapons. These requirements grew out of the Navy’s experiences during World War II and the subsequent underwater explosion (UNDEX) experiments. In many cases, it was found that critical ship systems could be disabled by the shock effects of nearby weapons explosions that were not a direct hit on the ship itself.
The current “D” revision of MIL-S-901 was released on 17 March 1989 and has subsequently had clarifications issued in the form of a letter from Naval Sea Systems Command (NAVSEA). The most recent letter was dated 21 November 2012 and was titled “Cost Avoidance and Clarification of MIL-S-901D Shock Test Requirements for Shock Qualification of Equipment on Lightweight Shock Machine”. A similar letter was issued covering testing on the medium weight shock machine. The information that follows is based on the aforementioned documents. The MIL-S-901 specification defines not only the test methods, but also the equipment required to perform the test. The equipment defined by this specification consists of three machines (or platforms) that cover the categories of test based on equipment weight (see Categories below). Since the majority of tests for small equipment are carried out on the Lightweight Shock Machine (LWSM), the following considerations are tailored toward the use of the LWSM equipment.
When required to perform a test per MIL-S-901, the equipment for test must be classified according to four criteria as outlined in the specification. These are Category, Grade, Class and Type.
There are three categories (criteria 1 above) of tests based on the size and weight of the equipment to be tested:
Equipment is classified according to one of the following grades according to its intended use:
Equipment is then classified according to one of the following three classes according to its mounting configuration:
Tests are then classified according to the following types according to equipment level of assembly:
Once equipment is classified according to Category, Grade, Class, and Type, it must meet a number of other criteria in order to be successfully tested on the LWSM. The equipment must be able to be accommodated by one of the standard fixtures fitted to the LWSM and defined by the MIL-S-901 specification drawings. There are a total of six standard fixtures defined as follows:
For Base Mounted equipment, the smallest mounting platform that comfortably accommodates the test item should be used. For all fixtures, the practical size limitations cannot be exceeded. This means not only that the equipment service mount must be able to be attached to the required fixture mounting surface, but also cannot be mounted too closely to the main attachment bolts that secure the fixture to the LWSM. This would effectively defeat the mode action of the fixture that serves as the shock input source to the Unit Under Test (UUT). Likewise, for many test items the mounting spacers called out in Figure 7 of MIL-S-901 must be used if the item contains large flat surfaces in order avoid reinforcement of the bulkhead plate by the test item thereby defeating its natural response action. Also, items with resilient mounts or other flexible elements that have capability of deflecting 1.5 inches or more cannot be tested on the LWSM, but must be subjected to Medium Weight or possibly Heavyweight shock testing. Finally, the operating modes or positions of the test item must be considered in order to determine the number of orientations that the item must be tested in. Some clarification for this aspect of the testing was provided in the November 2012 letter from NAVSEA. These are some of the aspects to consider for testing on the LWSM. Of course, there is much more detail in total that is contained in the MIL-S-901 specification and clarification letters. Most test labs that conduct LWSM shock testing will be glad to provide support in determining the total test requirement. As always, the NAVSEA authority has final authority over the conduct of MIL-S-901 tests and is also a good source of information for anyone required to conduct these tests.
*MIL-S-901D, Shock Tests, H.I. (High Impact) Shipboard Machinery, Equipment, and Systems, Requirements for. 17 March 1989.
*NAVSEA Letter, ser 05P1/462,. Cost Avoidance and Clarification of MIL-S-901D Shock Test Requirements for Shock Qualification of Equipment on Lightweight Shock Machine. 21 Nov 2012.
NTS maintains MIL-S-901 lightweight shock testing machine certified by the United States Navy (CDNSWC)