Aerospace Bolts Support High-Temperature Applications Through Careful Engineering

A fair number of aerospace bolts end up on brackets near the engine, where they stay hot for most of a flight due to elevated temperatures. This post goes through what engineers look at when choosing one for a warm spot.

Key Takeaways:

• Quite a few aerospace bolts near the engine are made from nickel alloy.
• Engineers look for the strength figure at the bolt’s actual temperature.
• Plating is picked to suit the heat the bolt will see.
• The drawing usually names the alloy, so nobody has to guess.
• One alloy rarely suits every hot joint on an aircraft.

Anyone who has stood near an aircraft engine after a ground run will know how long the heat stays in the metal. Brackets and ducting in that area are held on with aerospace bolts that were picked with the heat in mind, usually after a fair amount of back and forth in the design office.

The material decision takes up most of that time, mainly because the temperature changes so much from one spot to the next. Purchasing staff ordering aerospace bolts for engine work will see the alloy and finish written on the drawing, and a few of them will check every certificate against it. The sections below go through how those choices get made.

What Heat Does to a Bolt over Time

Checking Strength at the Working Temperature: Most metals get a little softer as they warm up, and a bolt that is happy at room temperature will carry less once it sits at several hundred degrees. Engineers look up the strength figure at the working temperature for that reason. The datasheet usually has it, somewhere near the back.

Planning for Oxidation and Creep over Many Hours: Hot metal slowly picks up an oxide layer, and some alloys cope with that a lot better than others. Creep is the other thing people mention. A bolt held under load at high temperature for thousands of hours can stretch very slightly, and nickel-based alloys are popular near engines partly for this.

Picking Material and Finish for Warm Spots

Choosing an Alloy That Suits the Engine Area: Around the hottest sections, the drawing will usually call for a nickel-based superalloy. Titanium turns up a little further back where it is cooler, and it saves some weight too. Most designers check the temperature map for the nacelle before settling on anything.

Finishing the Bolt to Match the Heat: Cadmium plating turns up a lot on airframe bolts, though it is kept away from hot areas because of how it behaves once heated. Silver plating is used on some hot threads so the nuts still come undone at the next service. Somebody usually has a story about that, which is probably why.

Checking How Each Bolt Fits with Its Neighbours

Allowing for Expansion around the Joint: Metals grow at different rates as they heat up. If the bolt and the flange are made from different alloys, the clamp load can shift quite a bit once the engine is running. Engineers work that out on paper first, and then somebody checks it on a rig, which takes a while.

Matching the Bolt to Surrounding Parts: Unlike metals sitting together are checked for galvanic compatibility, and a washer or coating in between is a fairly normal fix. The same review picks up a handful of other items, usually in one meeting. A typical list looks something like the one below.

• The working temperature at that exact spot, not the engine average.
• Nut material, which is usually matched to the bolt.
• Any anti-seize compound the maintenance manual allows.
• How many heat cycles the joint goes through in a year.
• The torque value, which is set with the engine cold.

Sticking with the Specification Sheet

Following the Specification Sheet Closely: Every hot joint has a specification sheet somewhere, with the alloy and the temperature limit printed on it. A different grade might be sitting on the shelf, and someone will probably suggest using it at some point, but the design office has to agree before anything gets swapped. Most stores staff know that already.

Matching the Grade to Each Joint: A bracket by the exhaust can run a good deal hotter than one only a short way along the casing, so engineers look at each one separately. When the aerospace bolts turn up made to the right grade for every drawing, goods-in can usually book them in on the same afternoon.

Bolts That Stay Tight after Thousands of Hot Hours

Anyone working on an engine-side assembly can send the drawing across with the operating temperature marked on it and ask for a quote on bolts made to that grade. It saves a round of emails later on. A sample batch with its material certificates is usually the sensible next step, and most buyers ask for one anyway.