Meeting & Exceeding Standards: Shipboard Equipment

October 13, 2020

Shipboard equipment and systems must withstand various degrees of shock and vibration from both their environment and their own internal components. The military has specified standards for testing the resilience of equipment and components to ensure they continue operating under rugged conditions.

MIL-S-901D – Shock Tests: High-Impact Shipboard Machinery, Equipment, and Systems

This standard establishes shock testing requirements for shipboard equipment as well as machinery and systems. These requirements ensure equipment and systems remain running even when a ship is under heavy weapons fire.

MIL-S-901D has three test categories (lightweight, medium, and heavy), two shock grades (A and B), three equipment classes (I, II and III) and three shock test types (A, B and C).

Test Categories

The lightweight test uses a shock machine with a weighted anvil plate no heavier than 550 pounds. Test equipment is attached to the anvil plate and struck with top, back and side hammer blows. The medium weight test involves a similar process with an anvil plate weighting up to 7,400 pounds. The heavyweight test is also known as the barge test. Test equipment is secured to a shock platform floating on water and explosive charges are set within 20-100 feet.

Equipment Grades

Test items are categorized as essential safety and combat equipment (Grade A) and equipment not essential to safety and combat (Grade B).

Equipment Classes

Test items are classed based on requirements to pass shock tests as it relates to mountings. Class I must pass without resilient mountings installed, Class II with resilient mountings installed, and Class III both with and without resilient mountings.

Shock Test Types

Type A tests assess the resilience of a principal unit – directly supported by the ship’s structure or a foundation attached to it – and items mounted in piping or duct systems. These units might include generators, switchboards, radio transmitters, missile launchers and valves. Type B tests assess subsidiary components of principal units, like a motor or power supply. Type C tests assess subassemblies of subsidiary components, like gauges, meters, relays and resistors.

MIL-STD-167-1 – Mechanical Vibrations: Shipboard Equipment

This standard establishes testing requirements for shipboard computers and systems against environmental vibration and vibration caused by internal components. It does not test propulsion systems or shaft vibrations. Tests are divided into two types (1 and 2).

Type 1: Environmental

Type 1 assesses the resilience of equipment when subjected to steady vibration. Equipment is secured to a shock and vibe table at the same points it would be attached to the ship. The equipment undergoes an exploratory vibration test, variable frequency test and endurance test.

The exploratory vibration test vibrates the table at frequencies from 4 Hz to 33 Hz at an amplitude of 0.010 ± 0.002 inch. The variable frequency test vibrates the table at the same frequencies by intervals of 1 Hz for five minutes each at varying amplitudes. Endurance testing vibrates equipment for at least two hours at the frequency that affected it the harshest during the previous tests. For mounted equipment, endurance testing vibrates for at least two hours at 33 Hz and equipment carries a load of 2.5g to simulate rough water factors.

Type 2: Internally Excited

Type 2 assesses the resilience of rotor machinery. Only one test is performed. Machinery is balanced, mounted and run at either a constant speed (for equipment with only speed) or the maximum speed and critical speeds (for equipment with varying speeds). Results are then checked by the allowable grade of unbalance for each varying rotor’s characteristics.

Sealevel Military Standards

Sealevel has experience in designing, manufacturing and testing products to meet and exceed MIL-S-901D and MIL-STD-167-1 standards. Sealevel is happy to work with customers to meet their specific needs.