Overspeed Trip Devices, Their Types, and Functionality

When operating a diesel engine of a vehicle or marine vessel, there is a chance that uncontrolled acceleration may occur. Uncontrolled acceleration can be highly detrimental to the engine assembly, potentially leading to mechanical failure or accidents. With the use of a component known as an overspeed trip, diesel engines can be safer to operate with the restriction of acceleration, even in the case of a failed diesel engine governor.

A rapid acceleration of a diesel engine can occur due to various reasons, one being the sudden removal of an engine load. For a marine vessel, this can occur when a propeller lifts over water during poor weather conditions, resulting in excess power. Additionally, the failure of a clutch or the fracturing of an engine drive shaft can also cause the sudden removal of an engine load. In other instances, overspeeding may also result from a stuck fuel control shaft.

Whenever a trip occurs, it will require manual resetting. This is due to the fact that an automatic reset may cause the engine to enter an overspeeding condition again if the issue is still present. Additionally, testing must be conducted on a regular basis to ensure proper functionality. To promote more guaranteed functionality, some engines will implement two overspeed trips that are independent from one another. One is an electro-pneumatic type, featuring a trip setting for 15% above nominal speed. The other device, meanwhile, is a mechanical type that has a trip setting for 18% above nominal speed.

Mechanical overspeed trip devices are fairly simplistic, often featuring a weighted spring loaded bolt that is set into the engine’s rotating shaft. As centrifugal force occurs, it is exerted on the bolt fitting. Once centrifugal force surpasses the preset spring force due to speeds exceeding the 15% rated speed threshold, the bolt will be ejected. At this time, the bolt will collide with a latch, resulting in the release of a plunger that stops the flow of fuel supply to the engine.

For electro-pneumatic overspeed trips, a spring loaded pneumatic cylinder is situated at each fuel pump. During standard operating conditions, the force exerted by the spring will maintain the positioning of the rod, preventing the operating end from making contact with the pin. When a trip condition occurs, a solenoid valve will force air into the cylinder, allowing for the piston to overcome the force of the spring. This will cause the piston rod to pull the pin of the fuel rack, moving it to a “no-fuel” position to stop the flow of fuel.

While mechanical and electro-pneumatic overspeed trip devices are quite common, electronic types are another option that one may use as well. With such devices, three magnetic pickups are monitored for overspeed protection. With their design, such devices excel in high speed applications such as steam or gas turbines. Additionally, electronic overspeed trip devices provide an increased amount of reliability over their mechanical counterparts.

With the proper implementation of overspeed trip devices, one can better protect their engine and all of its related components. At Cogent Purchasing, we are your sourcing solution for bearing engine components, plug overspeed trip parts, aircraft products, and so much more. Take the time to explore our vast catalog of parts at your leisure, and our team of industry experts are always on standby to assist you through the purchasing process however necessary. If you find particular items that you are interested in, take advantage of our RFQ service to quickly request quotes for your comparisons. Call or email one of our representatives at your earliest convenience and see how Cogent Purchasing can serve as your strategic sourcing partner for all your operational requirements.


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