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  • Repairing shafts - MW article.

    I just had a read of the article in Machinists Workshop called Repairing worn or damaged shafts by J Randolph Bulgin.
    Well written and well presented but I have an issue with one of the 4 process described.

    Process No2 is called stubbing and in this example Mr Bulgin shows a worn end of a 1-1/2" shaft being completely cut away and a new stub being fitted by a threaded pilot.
    from the article:-

    "This example is a 1-1/2" diameter shaft and I drilled and tapped for 5/8"-18 stub. Obviously you cannot use a 1/4"-24 thread here, nor would you want to use a 1-1/4" thread."

    He then goes on to say you then screw this into the shaft and pin thru the thread with a taper pin.

    I'm very concerned over this method and feel that it could lead to either an accident or at least a shaft failure.

    Follow this logic thru and comment on it please if I'm wrong.

    A 1-1/"2 shaft has a surface area of 1.767 square inches.
    A 5/8" thread because of the undercut or thread depth is only 9/16" diameter and has a surface area of 0.248 square inches.
    That is only 14% [ fourteen percent ] of the original shaft.
    Add to this you have a sharp edge because of the undercut which leads to a stress point.

    Forget the pin at this point as it's behind the shoulder. All the stress is being taken on the one diameter of 9/16"

    If the shaft is subject to starting and stopping or reversing loads it then relys on the locking pin from becoming undone or at the least fretting loose.
    He does mention the use of left hand threads but that won't help cyclic loads.

    I can't see why he has to go that small and dismiss something like a 1-1/4" thread.
    At 1-1/4" allowing for the root diameter you will have 63% of the area.

    Fourteen percent is dangerous and many people reading that article and knowing no better could make the mistake of following these instructions.

    To bring this into perspective a 1-1/2" shaft equates to about a 15 horse power motor.

    John S.
    .

    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.




  • #2
    John
    Possible to see the whole article ?
    On line or...
    ? What is your #2 process
    i.e. How would you do it?
    please visit my webpage:
    http://motorworks88.webs.com/

    Comment


    • #3
      I've done this before only I just used a dowel pin and didn't bother to thread anything. Drilled both ends and inserted a dowel pin to keep everything straight and then welded it up. Even though there is a dowel pin in there it is essentially hollow, and stresses have nowhere to concentrate. It's actually sronger than the original shaft.

      Comment


      • #4
        Have repaired numerous shafts by all of methods and the stubbing method is normally used in our shop for things that take all of their load axially. Hydraulics cylinder rods and such. But some of the exceptions are converting spindles for drill heads into another of a usually shorter length. I never really cared for the split sleave method as I have had to straighten too many shafts that warped from the welding. Just as easy to weld and regrind. Too bad he missed the one we use for seal surfaces. Spray welding ceramic
        Forty plus years and I still have ten toes, ten fingers and both eyes. I must be doing something right.

        Comment


        • #5
          I've shortened axle shafts as Rusty describes but I didn't even use the pin. The stub is cut back to leave a 1/4" gap then the gap is welded up.

          <font face="Verdana, Arial" size="2">I can't see why he has to go that small and dismiss something like a 1-1/4" thread. At 1-1/4" allowing for the root diameter you will have 63% of the area.</font>
          That's not good either. You'd have to bore the 1 1/2" shaft to accept the 1 1/4" thread. You'd be left with about 1/16" of 1 1/2" diameter.

          Comment


          • #6
            I wasn't to thrilled with the threaded solution myself.I have always used a dowel and weld as Rustybolt mentioned.Either that or I turn the shaft down to 2/3 its diameter and cut it off twice its diameter long then make the new stub shaft with a shrink fit socket,shrink it on and run a small tig bead around for insurance.

            While I thought the article was good and the methods pretty well accepted I was also concerned about the axle shaft,some parts should be discarded and not rebuilt.I have fixed axles before,but I only cleaned the seal surface up round and shrank a sleeve over them,then substituted for a bigger seal ID and not reduced the shaft diameter or removed the radius in the shoulder.Those radius shaft shoulders are very important in distributing stress.
            I just need one more tool,just one!

            Comment


            • #7
              Nice analysis John. To get half the area I calculate the stub to be 1.06 inches.

              crunchy

              Comment


              • #8
                Yeah, I thought the article did NOT make enough of a deal of the weakening.

                And yeah, that axle bugged me too....big time. Way smaller section, WITH stress risers added, after "fixing".

                While those are useful methods, the whole thing reminded me way too much of the "drill hole and drip in solder" method of balancing grinding wheels, and a few other articles I have seen.


                BTW, not because the article was bogus....I don't think so. All that stuff works.

                I just didn't see as much mention of the trade-offs of strength as I thought was appropriate....

                [This message has been edited by J Tiers (edited 10-20-2004).]
                1601

                Keep eye on ball.
                Hashim Khan

                Comment


                • #9
                  Couplers work real well when there is enough room for it. Sprayed metal is very good but expensive. Have you priced out a pound lately? I prefer removing the shaft if possible and making a new one. JRouche
                  My old yahoo group. Bridgeport Mill Group

                  https://groups.yahoo.com/neo/groups/...port_mill/info

                  Comment


                  • #10
                    Not having read the article, puts one at a disadvantage for the exact case, but, I have stubbed many shafts ( including a broken crankshaft on a riding lawnmower)..
                    You are correct in your thoughts about the cross-section area...but in every case, the "design" of the shaft is equally important. The design considers the loads, and the support the shaft has.

                    I like the "failed" shaft to provide the stub if possible ! so you will remove more stock than broke off it .
                    Lets look a a bent crank, where heat and straightening efforts fail. the crank is supported at the bearing journals and the bent end turned down to a stub ( radius those inside corners !)of 2 to 3 times stub diameter in length if possible. Select a steel of more tensile than the shaft .
                    I use 130,000 TS for a 60,000 TS shaft.
                    So my "New" shaft needs only to be one half of the cross-section area of the stub.. OK
                    now thread the end of the (old)stub for about 1/2 diameter in length...I like it to be a normal thread..a little smaller than the stub diameter and ONLY at the tip.
                    Now bore the new shaft for a shrink (.0015 per inch)fit and tap the bottom to match the stub. Stuff the crank in the lathe with aluminum shims on the jaws and light chucking.
                    heat up the shaft ( 500 degrees temp difference gives .003 per inch growth)
                    I freeze the crank first to help this .
                    now turn on the lathe and take the shaft and ram it in.. the threads will pull it tight and then it will lock on..the jaws should allow it to slip so still be careful...and fast !
                    Last step is to turn the new shaft to size and shape..and it will last..
                    The new material has shrunk on the old and compresses it, which adds to its strength.

                    In many cases, the "joint" should be in the middle of a bearing journal, or wherever it will see the least flex !
                    If flex is a problem, use a longer stub, as well as higher strength steel

                    just my thoughts

                    Comment


                    • #11
                      I Forgot to say, in the case of the 1.5" shaft, the numbers would show a 1.188 stub diameter to match the higher strength steel.
                      I would use a 1" thread about 3/4" long and the stub would be anywhere from 3 to 4 1/2" inches long.. no pin is necessary with the freeze fit

                      Comment


                      • #12
                        My biggest concert with the article is the fact that all he had done was to make a new stub up with a short thread on it and screw this into the old shaft.
                        In the cases he used he had a 1-1/2" shaft and the stub had a 5/8"-18 thread on it about 1" long direct up to the shoulder.
                        At this transition point with the undercut you would have a stress point.

                        The main fault is there is no support given to the new stub, it relies on the thread to pinch the two vertical faces together which can give no radial load support at all.

                        Imagine a large pulley and belt load on this shaft, remember we are talking 15 horsepower sized shaft.
                        The load is trying to pull the shaft sideways and bend at the joint which puts the 9/16" diameter undercut under compression on one side and under tension on the other.
                        Now rotate the shaft 1/2 a turn and the forces are repeated on opposite sides.
                        Now image that shaft rotating at running speed being bent by the radial loads backwards and forwards.
                        A bit like trying to hold an end mill in a drill chuck. Try it with a pencil, doesn't last long does it

                        If he had made a stepped shaft with a diameter of say 1-1/8" for 2" long then gone down to his 5/8" thread no radial load would have been expected to be taken by the thread.
                        You could have had a nice parallel fit on the 1-1/8" with radius at the transition point to prevent a stress point.

                        Radial loads would have been take care of by the larger supported 1-1/8" diameter not wanting to bend.

                        In fact if you go one step further the thread isn't even needed.
                        By the time you get a decent sized stub inside the shaft, loctite and a cross pin this isn't going anywhere.

                        A couple of bands of straight knurl to raise the surface up about 10 thou will improve the grip by 100% especialy if you get one situated underneath where a bearing fits as this will prevent the shaft from belling out that bit.

                        The biggest enemy of stepped shafts are stress points, Boeing's have more books on this than in my library , that's how important it is.
                        Most shafts break from these points in the first place, either at steps or a favourite is the bottom of deep sharp side woodruff keyways that peel apart.

                        Shafts going thru rotors on electric motors are held in place with just a straight knurl up to about 15 / 20 Hp, after that they start fitting keys.
                        That gives an idea of what load a straight shaft and knurl can manage.

                        Here's a DC armature with the long shaft snapped at the step because it was a sharp corner.



                        This is about a 7HP motor out of a fork truck, these are powerful little beasts.



                        This is the new stub made and knurled where the bearing fits on the original armature, note the amount sticking inside to give support and the radius in the end of the armature to match the one on the new stub.



                        Finished job. This was loctited and pressed in, no thread, no cross pins, just pressed in.
                        There is an alloy cooling fan to go on where the keyway is and then the main drive bearing.
                        By just going to a simple stepped shaft, no threads you cut the time down possibly by half for a job such as this.
                        This job was done about 4 or 5 years ago and believe me if it had failed whey would soon be knocking on the door.

                        John S.
                        .

                        Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



                        Comment


                        • #13
                          John,

                          The shaft you repaired is pressed into the armature? Is that a common practice or just in the UK? (Canada here). I am curious as i have never sceen any evidence to lead me to the conclusion that it is a press fit

                          Rob

                          Comment


                          • #14
                            It is at least pleasing and reassuring to me to know that someone is reading this stuff. Differences of opinions is what makes this world go around.

                            Comment


                            • #15
                              I'll think about this article as I walk in under a electric crane next time.



                              Yep, I go around when I can and not under the load. Sometimes the brake is on the "other end" of the motor and the shaft sees all the load braking.

                              In a instance like this please don't pin, thread or leave a 5/8" threaded shaft to do the load. I have throwed things in the river I thought were not safe. They make a pleasing splash.

                              And as I practise my martial arts training destroying wooden ladders that are not safe I remember it also. (ladders that have been altered, cut down and splints installed)

                              SOmetimes the contractor thinks he is saving money by using repaired instead of fixed. Sometimes he is not.

                              John, you repair motors for a living? what are your views on life threatening situations where a repaired motor can fail?

                              ------------------
                              David Cofer, Of:
                              Tunnel Hill, North Georgia

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