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Generally, in a pressure drop control valve that uses a piston and a spring (for example, in jet engines to control fuel flow), there is always potential for error in terms of the height of the spring. As we know, a shim is usually used to fix this. However, I was wondering if there are alternatives to such a procedure. Are there other ways to correct the height on a valve spring, or perhaps produce the spring in a way that reduces the need for a correction method like shimming?

I was thinking maybe laser correction could be a viable method for correcting a spring that is off by a few thousands of an inch? It is extremely accurate, but then the main disadvantage is cost. Are there other methods or technologies that exist or are perhaps being worked on at the moment?

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Shimming is always attractive for fit adjustment as it is fairly straightforward to do ie you can measure any gap with a feeler gauge and select the appropriate thickness of shim to suit.

The downside of manufacturing parts to fit exactly is that you then need to match individual parts to their location throughout their life and you then need a different system if you want to compensate for wear. Equally if you need a replacement part you need to report the exact size you need and get it made to fit as opposed to just ordering a standard part and having an existing known method to compensate for any variation.

Again if you know that the springs will always be a bit short you know you will always be filling some gap and its just the gauge of the shim which varies however if you are aiming for an acceptable fit out of the box there is always a risk that it will be slightly too long in which case a shim will no longer fix the problem.

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  • $\begingroup$ That makes sense, I guess although shimming is the most convenient way to fix a spring that's too short, the main disadvantage is that it cannot be used for a spring that's too long and it can only be used in certain systems, i.e. it's not very versatile. $\endgroup$
    – AkThao
    Oct 30 '16 at 14:45
  • $\begingroup$ How about if they could make a spring that has the same tensile strength needed to function properly in a valve but is made of a different material. Something that's not necessarily stainless steel but can be moulded far more easily. A material that upon measuring, can easily be 'resized' without the need for a shim or without a new spring having to be made? Such a material would save time, money and resources, as a shim would not be needed. And it doesn't matter if the spring was too long or too short, it could simply be altered to the exact correct length. $\endgroup$
    – AkThao
    Oct 30 '16 at 14:50

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