Quieting Wire Springs

Question:

Our company makes window balances. One of the parts of this balance is a metal spring. When the spring is compressed and expanded it can make a "boing" sound. The spring is compressed when the window is raised and expanded when the window is lowered. Is there a powder coating process that can help quiet these springs? Springs are made from phosphorous-coated music wire and hard drawn galvanized steel wire. I appreciate your thinking about this. J.W.

Answer:

I guess you can expect the springs to make noise when you use music wire to make them. Being a former guitar player, I am familiar with these wires (or strings as the case may be). Your question has prompted flashbacks of me with long hair playing rock and roll in the '60s and '70s. The memory of my appearance wasn't a pretty sight, but it was fun to remember a more carefree time, and the music wasn't that bad either. Thanks for the trip down memory lane.

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There are several powder materials that can help. I recommend a thermoplastic powder coating material to handle the expansion and compression of the spring over the long term without cracking. Materials like Teflon, nylon, PVC, etc. can eliminate the "musical boing" from the spring. However, to be an effective sound-deadening device, the powder coating will need to be applied at a high film thickness (greater than 5 mils). This means that the method of application should be an electrostatic or non-electrostatic fluidized bed. Further, the lower melt temperatures of these thermoplastic materials will prevent the spring material from annealing. Nobody likes a limp spring because the wire was annealed.

The real problem will be applying the coating to the wire spring without locking the coils together. In order to accomplish this, you will have to apply the powder to an expanded spring or to the wire before the spring is coiled. Either of these methods will coat the spring without fusing the coils together. If you purchase the spring already made, then you can ask your supplier to perform the coating operation. However, if your company forms the spring, then coating the raw wire after drawing but before coiling will be the simplest process. In this case, the wire would pass through an induction coil to heat the wire and then pass through a fluidized bed to apply the powder. The last step would be water quenching the coated wire to harden the powder material. This methodology is currently employed in powder coating both pipe and rebar. Since induction coils are very efficient at heating steel wire, be sure to use a low setting. Remember you want to attract and flow the powder coating without annealing the wire. In this case, a wire temperature of less than 400F is preferred. Good luck with your new process.