Saturday, June 18, 2011

Propeller Lapping and Locknut Retention

     In a previous post, I discussed fitting the flanges to the propeller shafts with lapping compound. The propellers have to be fitted the same way. For me, this was easiest to do after the shafts were in the boat, since the shafts are well supported and accessible. I use the same procedures for the props as for the flanges, so I won't repeat it all here, except to reiterate that the goal is not less than an 80% contact pattern between the shaft and propeller hub.

     Before the lapping, I had made up two 3/8" brass keys for the props. These have to be fitted to the shaft, and the prop hubs, before installation. They should NOT be an extremely tight fit. I dress the keys on fine sandpaper on a flat surface, until the keys just slide into the prop hubs, and also the shaft keyway, without binding. A slight amount of drag is ideal. You're looking for a snug fit, with no play, but something that is still smooth enough to assemble by hand. Also, the corners of the keys need to be profiled until they match the radius that is machined into the shaft keyway. If the corners bind, the key will not fit properly, and will prevent proper installation of the prop.

     Also before lapping the props, the hub and shaft should be completely deburred. Any sharp corners or burrs from machining need to be removed, and all corner edges broken with a fine file or stone. Sharp corners lead to stress riser fracturing or cracking, and tiny burrs can break off during assembly, and lodge between the prop and shaft, preventing a proper fit. It is always good practice to smooth and deburr machined edges and sharp corners, so break out the files and deburring tools, and make sure everyhting is smooth before proceeding further.

     Once the prop is lapped to the shaft, the shaft and propeller need to be thoroughly washed, cleaned, and dried. Thoroughly means thoroughly. Lots of soap and water, followed by a drying with compressed air or clean, lint-free cloths. Then clean and dry them again. At this point, I recommend a mock-up assembly. Place the prop on the shaft, and seat it fully without the key installed. Mark the shaft to indicate the position of the prop hub, then remove the prop. Now, place the key into position, and remount the prop on the shaft. It should be in the same position as before, relative to the marks you made. If it is not, it indicates a fitment problem with the key, and this needs to be rectified before proceeding further.

     Now, finally, it is time to final mount the prop. Clean everything! (yes, again!). I use a light film of motor oil on the shaft and prop hub to facilitate assembly. I have seen all sorts of concoctions for prop mounting, involving oil, grease, Rectorseal, etc. I don't recommend anti-seize, due to the fine metal content, but the final choice is up to you. I like light oil because it can flow easily out from between the prop hub and shaft, but enough remains behind at the microscopic level to help prevent crevice corrosion, and it helps with prop removal down the road. At any rate, lube the components, position the key, and slide the prop onto the shaft. Next, lube the threads, and the rear face of the hub, and thread the nut onto the shaft. If your setup uses a bearing washer ahead of the nut, be sure to use it. Otherwise, tighten the nut to about 80% of its final torque specification, and go have lunch. While you're away, any excess oil trapped between the shaft and prop will flow out, allowing the hub to fully seat. After lunch, torque the nut to final specification.

     Now, use some solvent, and clean the rear face of the prop nut, and the shaft threads, until they are clean and dry. My method at this point is to mix up some 5 minute epoxy, apply it to the threads, and quickly torque down the locknut. The epoxy will act as thread-locker, and keep the locknut from backing off. But, when you need the nut off, it will break free.

     At this point on my Sea Ray, once the locknut is installed, it is followed by a cotter pin. Here's the problem - the cotter pin doesn't actually secure the nut in position; it just prevents the nut from falling off the shaft. If Sea Ray had used a castellated nut, I wouldn't have a problem. Instead, the cotter pin bore is a good 1/4" behind the nuts. Sure, it may prevent losing the nut, but by then, the prop is loose on the shaft, the shaft and hub are getting damaged, and the shaft may even break. The solution takes a page from the aviation industry in the form of stainless steel lockwire. I machined the locknuts as shown below, and lockwired them in position. They can't loosen now...
     Notice that the lockwire is secured and twisted in such a way that the nut can't loosen. Next post, I'll talk about shaft alignment, and also shaft seals and brushes.

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