Loctite Assembly of Spool

My first reel design has a multi-part spool: anodized aluminum for the spool ends and copper alloy (bronze) for the shaft. I could have saved weight by using an aluminum shaft, but I could not reconcile myself to having aluminum as one element of a journal bearing.

So, how to fasten the spool ends to the shaft? A press fit would be simple, but requires tolerances of 0.1 thousandth on both shaft diameters and spool end holes. My capability is more like 0.5 thousandth. So I made the parts for my first reel with about 0.001 to 0.002 inch diametral clearance, then knurled the shaft. Assembly with an arbor press was easy.

While making the knurls, I found that 360 brass does not form well. So I changed the shaft to 544 bronze. It is also leaded and machines easily, and is probably more corrosion resistant.

Then my (mechanical engineer) nephew brought up the consideration of galvanic corrosion. Aluminum and bronze are widely separated on the galvanic scale. The anodize coating should protect the aluminum in many cases, but I am sure that the knurled ridges on the shaft break through the thin (though hard) anodize layer. My first reel has not corroded, but it is only a couple months old. I want to make more reels but cannot wait years to find out if I have a problem.

So I decided to try Loctite. It would allow the anodize layer to remain intact, and at the same time exclude electrolytes from the joint area. I consulted Loctite’s technical advisors and they recommended 3 alternatives, which are (in order of increasing strength): Loctite 243 (threadlocker), Loctite 2760 (higher strength threadlocker), Loctite 609 (retaining compound) with 7649 primer. I decided to try 609 first.

pull test, Loctite joint

Before committing real parts to the process, I made 3 sets of cylindrical test parts. These have the same mating surface diameter (7/16) and axial engagement (0.180) as the reel spool parts. They also have tapped holes so that eye bolts can be attached. Diametral clearance is 0.001 to 0.002, and the aluminum is anodized as described in a May 4, 2010 entry of this blog. I used the primer only the aluminum part. After 3 days of cure, I was ready to test the joints for strength. If the joints would hold 50 pounds or so, I would consider them adequate.

(Point of clarification: All three test items were glued with Loctite 609. I did not test either 243 or 2760 threadlocker.)

Pull Test Setup

Here is my pull test setup, on a post in my pole barn. I bought a 110 pound (50 Kg) spring scale on Ebay and used a large turnbuckle to apply the load. I put a small frame of drilled bars and bolts around the test article so parts would not go flying if the joint broke. All 3 test assemblies held at 100 pounds.

Later, I was able to easily break the joint with my arbor press, so rework of the assembly is possible.

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