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Trailer Bearings (swing arm) Help

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  • Trailer Bearings (swing arm) Help

    I've made some swing arm bearings for my boat trailer.
    these were made about 2 years ago ......when I had only just got my bridgeport and was first starting to venture into machining.
    I've made them out of stainless of unknown grade 400 serries I think.
    what I want to know is ...should I bush theses bearings with bronze or brass or something
    the ss-steel pin (304) will rotate only 15 degrees.
    I've heared a lot about, that you should have soft metal rubbing on hard metal in the case of plain bearings like this.
    but as it only turns 15 degrees and does not rotate at high rpm .....will this set-up work.
    meaning I wont end up with a "wear situation" here.
    btw ...stainless because it's going in salt water

    In the pictures you see first the main mount thats just about ready to be bored..... about 1.25 inches soon as I tidy it up.
    then the inner bearing,then the outer cap with the whole in the centre for a grease nipple..along side these is the shaft that will pass thru the whole lot......with a duplicate set of bearings on the other side of the mount.
    swing arm is not in the picture but is 2x2 inch solid.
    boat (YACHT) with trailer is about 1.5 tons.

    all the best.mark

    [This message has been edited by aboard_epsilon (edited 05-20-2005).]

  • #2
    Don't use 300 series SS in salt water. It is very susceptible to chloride induced stress cracking. Also the same applies to the parts you have already made. Be certain they aren't 300 series.

    <font face="Verdana, Arial" size="2">Stress Corrosion Cracking
    Stress corrosion cracking (SCC) is one of the most common and dangerous forms of corrosion. Usually, it is associated with other types of corrosion, which creates a stress concentrator, which leads to cracking failure. Nickel containing stainless steel is especially susceptible to chloride induced-SCC. The point of maximum susceptibility occurs between seven and 20 percent nickel. This makes Types 304/304L, 316/316L, 321, 347, etc., very prone to such failure.</font>
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    • #3
      nope they are slightly magnetic..
      so that means they are 400 grade .
      how is a circular 304 bar going to get stress cracks .oh and they will only be dunked in salt water for an hour or so when launching..then imediatly repumped with grease to force out anything thats crept into them.
      all the best.mark

      [This message has been edited by aboard_epsilon (edited 05-20-2005).]


      • #4
        The magnet test isn't a reliable indicator. 300 series will become magnetic if work hardened. The round bar is just as susceptible to SCC as any other shape. The higher the load the more severe the problem is. The chloride ions creep into microscropic intergranular cracks and can't be washed out.

        <font face="Verdana, Arial" size="2">Many cold drawn and/or polished bars have a noticeable amount of magnetism as a result of the previous cold work. This is particularly the case with grades 304 and 303, and much less so for the higher nickel grades such as 310 and 316. Even within the chemical limitations of a single standard analysis range there can be a pronounced variation in the rate of inducement of magnetic response from cold work.</font>

        <font face="Verdana, Arial" size="2">Under the combined effects of stress and certain corrosive environments stainless steels can be subject to this very rapid and severe form of corrosion. The stresses must be tensile and can result from loads applied in service, or stresses set up by the type of assembly e.g. interference fits of pins in holes, or from residual stresses resulting from the method of fabrication such as cold working. The most damaging environment is a solution of chlorides in water such as sea water</font>

        I don't want to hear of your boat overtaking you on the motorway.
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        • #5
          I would be less concerned about stress cracking in occasional immersion in salt water, especially if rinsing and cleaning after use is followed.

          I would be more concerned about the parts galling together and becoming one part. Stainless is particularly susceptible to this, and I would include a bronze or engineering plastic bushing in the housing to prevent this.
          Jim H.


          • #6
            Thanks guys
            all I have is this 304 for the shafts
            and no materials for the bearings sleaves.
            so what I've got there will have to do for now .
            will keep an eye on it ..and pull apart to monitor them regularly......and if the right materials are found cheaply enough they will be substituted.
            and it wont be that hard to sub them either because I've made everthing modular.
            I asked and you guys filled me in on what to do.
            I know at present I cant do exactly what you guys say.....because of lack of money to buy new materials.
            but as soon as I find the right stuff at the right price ..your thoughts will be implimented.
            all the best..mark


            • #7
              Mark,what size are those pins? And did I read right that the swing arm is 2x2" solid?

              If that is the case your design is so over built it should survive a nuclear attack

              1.5T is nothing,here a boat trailer for that capacity would have swing arms made from 1-1/2" x 3/8 mild steel flat bar,hot dipped galvanized if it's fancy

              As to the SSC issue,I would not worry.I have made tons of stainless parts for saltwater use over the years,nearly all were 316 or nitronic 50 none ever had any trouble with SSC,even the ones with barnacles attached.
              I just need one more tool,just one!


              • #8
                316 is much more resistant than 303 or 304 because of the different nickel content. 303 and 304 are definitely not recommended for salt water use.

                When I worked on float planes they didn't allow use of 300 series at all for parts like rudder pins.

                [This message has been edited by Evan (edited 05-20-2005).]
                Free software for calculating bolt circles and similar: Click Here


                • #9
                  We use 304 a lot,but only on parts exposed to salt spray,it pits too quick submerged which allows barnacles and other sorts of marine growth to occur rapidly.

                  The real shocker is when the piping is handling concentrated chorine.

                  One application is the chlorination units in our city water works.
                  A 1"NPT brass ball valve will last 2 weeks tops.316 will last three months.

                  The only thing that really lasts is solid nickel with lapped seats,but they cost $460 each Even then they only hold up for maybe two years.
                  I just need one more tool,just one!