How do we make... parts smaller?

Posted on August 20, 2018 by Chuck Mahnken, Images by Amber Chafin

Engine components are chilled in liquid nitrogen to make them shrink. This makes it easier to install them in the engine.
Aiken, USA

In Alice in Wonderland, Alice was much too big to fit through the tiny door that led to the Queen of Hearts’ garden. Thanks to a bottle labeled “Drink Me,” she shrank down to 10 inches in size and continued her adventures. Here in the real world, when machinists want to shrink an engine part, they don’t have magical potions at their disposal. How-ever, they do have a few tricks up their sleeves to get the job done. 

The process of machining is defined as forming and cutting a piece of metal into a desired shape and size by machine tools. Travis Blystone, Machining Supervisor at the MTU Aiken Plant, likens the facility’s machine shop to a butcher’s shop. “A butcher takes a whole beef loin and makes tenderloins, T-bones, all sorts of different cuts. In machining, we start with a raw casting. Then we meticulously cut away the unwanted material—the fat and trimmings. The end product is lean, clean and high-quality,” 
says Blystone.

Machining and assembly require strict attention to detail. Incorrect dimensions can 
lead to big problems. Every part must be per-fectly snug. On the cylinder head and cubical part assembly line at Aiken, valve guides, valve seats and bearings must be “press fit” into their corresponding mating parts. Extreme cooling with liquid nitrogen ensures the perfect fit quickly and efficiently. 

Here’s a quick physics lesson: When temperatures are cold, the kinetic energy of a solid object is decreased, causing atoms to take up less space. As a result, the material contracts in size. Machinists use this principle to join parts every day. At the MTU Aiken Plant, the technique is utilized to assemble components for Series 2000, Series 4000 and Series 883 engines. For example, on Aiken’s cubical part assembly line, when a bearing is set into a portable tank of liquid nitrogen, it can reach a temperature of -320°F (-196°C) in less than 5 minutes. The bearing can shrink more than 10 microns in size. 

This reduction in size enables a machinist to place it into the flywheel housing quickly and easily. “They will pick up the bearing with an installation tool that lines it up, and drop it into the part,” says Blystone. “When it comes up to room temperature, the bearing expands larger than the hole, so it makes a tight fit. If you need to get the bearing out, you have to get a hydraulic ram and press it out, physically.” 

Ensuring strong connection is crucial to engine quality. Only after the part passes a close visual inspection can it move down the line. Any imperfection can lead to costly waste and production delays. “On the equipment rack, if we had a bearing that was a little bit loose, then pretty much the whole engine would have to be torn down,” says Blystone. “If we had a valve guide or a valve seat that came out, you’d have a cylinder head failure. It’s not good for the company, and definitely not good for the customer.”

The operative immerses a bearing bushing in a vessel filled with liquid nitrogen with the aid of a jig. The bearing bushing contracts when chilled...
... and can be inserted more easily in the equipment carrier where it then gradually expands.

Point of contact

Travis Blystone
+1 803 232 6060

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