Unshakable: Shock-resistant electric motors

Shocks, which place a sudden, intense load on the system, are particularly challenging for motors. When designing shock-resistant motors, it is essential that the housing and components are highly stable to prevent parts from breaking off or the motor from failing.

A motor’s shock resistance is defined by the g-forces it must withstand when subjected to shock. Vibrations, jolts, and impacts can generate high g-forces—sometimes many times the force of gravity. Depending on the application and industry, electric motors must be designed to be fail-safe so that these forces do not cause components to come loose or impair functionality.

Shock Resistance Categories

BEN Buchele classifies shock-resistant motors into three categories—Standard, Medium Load, and High Load—and designs the motors to be correspondingly robust. In the standard range, which covers loads up to 15g for a duration of six or twelve milliseconds, no special measures are necessary. The company’s gray cast iron motors, including all components such as bearings or flanges, are already designed to be sufficiently robust to safely withstand the stress.

In the medium range from 15 to 25g (for the same duration), higher-strength screws are used to hold all components together. Additionally, the winding—the heart of the motor—is treated and protected with improved materials so that the increased shock loads do not affect the individual wires.

Measures for motors requiring shock resistance exceeding 25g are evaluated on a case-by-case basis. Typically, ductile iron is used for housings and components; this material consists of spheroidal graphite and, due to its structure, can better absorb and dissipate sudden impacts without fracturing. High-strength steel components are also sometimes used. Additionally, special bearings, such as cylindrical roller bearings, can be selected to ensure greater stability in these applications. All specifications must be taken into account when selecting materials for such loads: operating mode, runtime, speed, etc.

Example Industries: Marine and Steel Industry

In the marine sector, shock-resistant motors are frequently required, both for mooring and anchor winches as well as for pump motors on or below deck. Shock resistance is specified by shipping companies based on certain standards to ensure safety on board in the event of an emergency, such as shelling or similar incidents. The requirement is that no components may come loose and fly around when subjected to shock; maintaining the motors’ functionality is of secondary importance. For smaller vessels, such as patrol boats or supply ships, the shock resistance requirements are lower (approx. 15 to 20g); however, at the customer’s request, BEN Buchele also manufactures motors for these vessels using ductile iron to ensure enhanced safety. For large frigates, shock resistance of up to 100g is often required, in which case an individual assessment is conducted to determine which materials, screws, and bearings are suitable for the respective motor.

In the steel industry, the company supplies, for example, shock-resistant motors that drive large forging hammers for steel processing. Here, the issue is not isolated shocks but continuous impacts that are constantly transmitted to the entire system and thus also to the motors. The g-forces here are specified at 30g, so the highest precautions are necessary: ductile iron for all enclosing components, reinforced bearings, and, if necessary, higher-strength screws. In applications with continuous shock loading, it is important not only to prevent the motor from breaking apart but also to ensure the motor’s long-term functionality.

Shock Tests

At the customer’s request, the electric motor manufacturer conducts shock resistance tests on motors. One example is a test conducted for a marine equipment supplier, in which a size 225 motor was subjected to shock testing in collaboration with a certified testing institute to assess the effects on critical areas—such as the castings, bolted joints, or steel components.

Normally, shocks act on an object from three directions, so classification societies specify the maximum g-forces from three directions: vertical, transverse, and longitudinal. In this example, 39g was required vertically, 26g transversely, and 20g longitudinally—a medium to high load. The motor was designed using ductile iron and steel components. BEN Buchele built a test frame on which the motor was suspended and rotated into all positions via a rotating mechanism to apply shocks from the three different directions. This meant that the motor was raised on the frame and dropped forcefully to simulate the g-forces. The test once again yielded valuable practical insights that confirmed the design approach. The company also conducts shock tests for other sizes and g-forces upon customer request.

Images: BEN Buchele Elektromotorenwerke GmbH