Screw Type Adjustable Dock Legs

[Rcdizy]

I want to make some screw type aluminum dock legs. The type that I could adjust the hight of the dock above the water with a ratchet wrench or a cordless drill with a socket on the end.


I would use square aluminum tubing.

What are the chances I could get 2 sizes of square tubing that fit somewhat tightly into each other for the telescoping action?

What grade of threaded rod would be best for freshwater corrosion resistance? could I use galvanized and coat it in waterproof grease to keep costs down?

Would Standard thread rod be good or would it have to her acme or other thread?

Would the lift nuts need to be brass or could I make them out of thick aluminum? (would they strip, or corrode/bind?) aluminum would be easier because I could have them welded in place

 

[RJSakowski]

I want to make some screw type aluminum dock legs. The type that I could adjust the hight of the dock above the water with a ratchet wrench or a cordless drill with a socket on the end.


I would use square aluminum tubing.

What are the chances I could get 2 sizes of square tubing that fit somewhat tightly into each other for the telescoping action?

What grade of threaded rod would be best for freshwater corrosion resistance? could I use galvanized and coat it in waterproof grease to keep costs down?

Would Standard thread rod be good or would it have to her acme or other thread?

Would the lift nuts need to be brass or could I make them out of thick aluminum? (would they strip, or corrode/bind?) aluminum would be easier because I could have them welded in place

McMaster Carr sells 1/8" wall thickness aluminum tubing in 1/4" increments. They maintain a +/- .015" tolerance. http://www.mcmaster.com/#standard-aluminum-hollow-tubing/=wyww3l.
Galvanized steel, aluminum, and moisture do not go well together. I would use stainless steel. I have used stainless fasteners with aluminum for marine applications for decades with no problems. http://www.mcmaster.com/#standard-stainless-steel-threaded-rods/=wywycy
You should be able to use standard threaded rod. I would think that 1/2-13 or 5/8-11 rod would work well.
You may have problems with aluminum nuts. Bronze nuts could be used; they could be welded with silicon bronze to stainless plates.

 

[Rcdizy]

McMaster Carr sells 1/8" wall thickness aluminum tubing in 1/4" increments. They maintain a +/- .015" tolerance. http://www.mcmaster.com/#standard-aluminum-hollow-tubing/=wyww3l.
Galvanized steel, aluminum, and moisture do not go well together. I would use stainless steel. I have used stainless fasteners with aluminum for marine applications for decades with no problems. http://www.mcmaster.com/#standard-stainless-steel-threaded-rods/=wywycy
You should be able to use standard threaded rod. I would think that 1/2-13 or 5/8-11 rod would work well.
You may have problems with aluminum nuts. Bronze nuts could be used; they could be welded with silicon bronze to stainless plates.

Thanks for the Reply!


Would the square tubing from McMaster car slide into each other?
Thanks.

Why would the aluminum nuts be a problem? even if I tapped them out of 3/4" ? the adjustment wouldn't see a lot of use...

316 stainless good for the rod?

 

[RJSakowski]

Thanks for the Reply!


Would the square tubing from McMaster car slide into each other?
Thanks.

Why would the aluminum nuts be a problem? even if I tapped them out of 3/4" ? the adjustment wouldn't see a lot of use...

316 stainless good for the rod?

I would check with McMaster on a sliding fit.

Online metals sells square tubing in both metric and inch sizes. http://www.onlinemetals.com/merchant.cfm?id=1645&step=2&top_cat=60

Regarding the aluminum nuts, aluminum can corrode under certain circumstances and when it does, the corrosion could lock thread. It would be fairly easy to set up an experiment. Tap a block of aluminum and insert a stainless screw. Use the same alloys that you would be using for your dock. Tie a string to it and throw in in the lake. See how it holds up.

316 stainless is considered marine grade

 

[janvanruth]

consider making the nuts out of blocks of delrin,
the rod i would think could be stainless, but galvanized would work, normal course threads should do the job
just make sure the galvanized rod at no point touches the aluminum, and thats where the delrin comes in....

 

[RJSakowski]

My concern about Delrin would be the shear strength of the threads. The legs need to support the weight of the dock and anyone standing on it as well as the rigors of wear and tear caused by waves and/or boats bumping into it. A larger thread size could be used, say 5/8-11 or 3/4-10, and a longer thread block.
If you design your legs so the drive system is above the normal waterline, that should help.