Loose blades

[acunningham]

I was in the Deutsches Museum in Munich today - it's excellent, and I can definitely recommend it.

They have quite a few turbojet engines with cut away sections so the interiors can be seen, and in a few cases (such as the RR Pegasus) touched. I noticed that the compressor and turbine blades on the Pegasus were all slightly loose, and could be rattled by hand. Does anyone know why this is? The only reason I can think of is that with thermal expansion at normal operating temperatures they become tight.

[jpoth06]

You are quite correct, thermal expansion is the answer. I believe early developments of the jet engine had solid turbine/compressor wheels and as a result thermal expansion warped the blades. Hence the development of individually mounted blades with a tolerance to allow for expansion.

Sometimes you can here the result of cool blades on a windmilling engine when the blade shrouds (sometimes referred to as 'clappers') 'clap' together as it passes TDC.

[lomapaseo]

Loose means many things until you describe the surfaces.

Basically blades are loose to permit easy assembly in a cascade fashion without stack up of tolerances causing interference.

They become extremely tight in their attachments as soon as they start spinning. In spite of this they can still move imperceptively against their atttachments in order to dampen out vibrations that might cause fatigue.

The noise that you may hear when hand turning a rotor full of blades is typically due to the blade rocking in its slot at very low speed.

When you start adding things to the blade like dampners, snubbers, shrouds etc. you have to be very careful not to interfere with the set of the blade at full speed and temperature, thus these devices often create tinkling noises, when present, when turning the rotor by hand.

[nitpicker330]

Quite a few of the older Jet Engines "rattle" whilst the wind blows them around on the ground. Quite normal, as the posters explain above.

[Daz801-82]

Conversely, the stg 5 & 6 hp compressor blades on R-R 524, 22B and 535C engines are "cemented" in using high temperature sealing compound and a lockplate crimped to hold the blade against the dovetail.

Daz