Hey everyone, Cody here. Today, I want to talk about something we’ve tackled on a few projects before—bonding leadwires to surfaces that are going to be exposed to high centrifugal forces. Think about the kind of stress we’re talking here—like what we see on rotating components, such as compressor and turbine rotors, drive shafts, or even wheels and gears. If you’ve ever worked on a big or high-speed component, you know that centrifugal forces can get intense. We’re talking several hundred thousand times the force of gravity. For example, at a 3-inch radius on something spinning at 50,000 RPM, you’re looking at over 200,000g. Yeah, it’s serious business.
When dealing with such high centrifugal forces, it’s crucial not just to properly bond the strain gage itself but also to make sure those leadwires are locked down tight. The environment and operating temperature play a huge role in how you select your materials—leadwire, solder, anchoring, and protective coatings all need to be chosen carefully.
Choosing the Right Materials
For long-term use in temperatures ranging from -100°F to +400°F, I always go with M-Bond GA-61 for both adhesive and coating. It’s reliable up to +500°F in the short term. But if we’re dealing with lower temperatures (up to +200°F), M-Bond AE-10 or AE-15 can step in as a substitute.
Now, let’s talk about the actual process of bonding those leadwires.
Step-by-Step Leadwire Bonding Procedure
Step 1: Surface Prep
First things first, you’ve got to clean that surface where the leadwires will be routed. It’s the same kind of prep you’d do before bonding a strain gage.
Step 2: Masking
Next, slap some MJG-2 Mylar Tape down to keep the adhesive right where you want it—no need for a mess.
Step 3: Applying Adhesive
Now, grab a spatula and spread a thin, uniform layer of GA-61 Adhesive. If you’re using AE-10/15, a brush works well, and you won’t need as much heat to cure it. After applying, pull off the Mylar Tape and cure the setup for an hour at +350°F.
Step 4: Sanding for Bonding
Once the coating is cured, lightly sand it with SCP-3 400-grit Silicon-Carbide Paper or GC-5 Pumice Powder. You want to aim for a dull, matte finish—this will make it easier to bond the next layers of adhesive.
Step 5: Leadwire Selection
Pick your leadwires carefully. Stay away from vinyl or Teflon insulation in high-g fields—it’s just too risky. The conductor could pop right out of the insulation under those extreme forces. Instead, go for film-insulated leadwire like 130-AWN or 134-AWQ. If you follow the procedures, bare-copper or tinned-copper leadwires can also work.
Step 6: Soldering
Now, it’s time to solder. Use solder with a melting point of +430°F or higher, and keep those joints small and neat. Make sure to clean off all the flux afterward—this is crucial for a strong, long-lasting bond.
Step 7: Cleaning
Once soldering is done, clean the leads thoroughly with M-Prep Neutralizer 5A, and dry them out completely—use a hot-air gun if you’ve got one handy.
Step 8: Routing the Leads
Secure the leads along the surface at 1-inch intervals using wire or Mylar tape. It’s essential to keep everything tidy and in place.
Step 9: Additional Adhesive
Apply a small amount of GA-61 between the hold-down points. Work it in and around the leadwires to prevent air bubbles. Cure just enough to solidify the resin—about half an hour at +300°F.
Step 10: Final Coating
Remove the anchoring wire or tape, then apply the final coating. Cure it for two hours at +350°F or for one hour at +375°F. If you need to build up more layers, you can sand lightly between coats until you’ve got the thickness you want.
Step 11: Finishing Touches
If you need a smooth airfoil surface, apply and cure additional layers of coating, sanding between each one to make sure everything bonds well. When you’ve got the thickness you want, use a grinder or sand by hand to get the contour right. A final thin coat will give you that smooth, glossy finish we’re all after.
Fiberglass Cloth Method
If you’re looking to save some time, the Fiberglass Cloth Method can significantly speed up the bonding process. But keep in mind, this might affect the stiffness and resonant frequency of thinner structures.
I’ve used this method on a few projects, and it works like a charm, especially when time is of the essence.
Step 1: Surface Prep
Just like before, start with a clean surface.
Step 2: Leadwire Prep
Follow the same steps for leadwire selection, attachment, and cleaning.
Step 3: Fiberglass Cloth Prep
Cut your FGC-1 Fiberglass Cloth to size, and if needed, add an extra insulating layer if your leadwires need it.
Step 4: Resin Application
Place the cloth on a clean surface, and work GA-61 (or AE-10/15) into it until the weave is completely filled. Trim it if necessary, but make sure to clean your tools right away.
Step 5: Masking and Wetting
Mask the surface and apply a thin film of adhesive.
Step 6: Applying the Cloth
Press the coated cloth onto the surface.
Step 7: Leadwire Positioning
Position the leadwires on the cloth, keeping them separated and wet with adhesive.
Step 8: Overlaying with Cloth
Add another layer of treated fiberglass cloth over the leads.
Step 9: Clamping
Cover the assembly with Teflon Film, Silicone Rubber, and a pressure plate. Tape everything down and apply enough pressure to keep the shape while it cures.
Step 10: Curing
Cure GA-61 for two hours at +350°F or AE-10 at +150°F for an hour.
Step 11: Final Touches
Once it’s cured, remove the clamping materials. If you need more resin, sand the surface lightly, coat, and cure again. You can shape the surface to your desired contour. A final thin coat will give it a smooth, glossy finish.