Announcement

Collapse
No announcement yet.

Dynamic Balancing in the Home Shop

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Dynamic Balancing in the Home Shop

    In working on this T&CG, I'd like to remove a bit of vibration in the motor. Static balance isn't going to fix it; to much drag from the bearings and parts of the shaft are several thou out so static balance on anything but the bearings would seem to introduce error; besides i don't have a static balance with rails just the right distance apart to rest it right where the bearings would be.

    The proper way to do this, and any spindle or rotor is dynamic balancing; for the unfamilair that is measuring how much the element is out while running at speed, adding or removing weight in the right spot and trying again....but i can't recall there being content on dynamic balancing in the home shop?

    Any balancing experts here that can describe the process, or how it might be simplified for the home shop? I guess you need some sort actuator to measure the vibratory movement and way to monitor the shaft speed or angle?...and a V block bed for the bearings to sit in? I know high end commercial balancers can do everything but turn the lights out...but is there a doable way to simplify the things to the get worst of the imbalance out?

    Anyone accomplished this at home or have ideas how to go about it?

    thanks


    crap, another title typo, can someone add the "h" in home? thanks
    Last edited by Mcgyver; 05-04-2010, 05:42 PM.
    .

  • #2
    Iv never heard of doing it in the home shop. You should be able to find a motor shop that will do it for a reasonable price however (Cheaper then building your own dynamic balancer anyway)
    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

    Comment


    • #3
      The obvious way to do this for me is to attach an accelerometer to the frame and use the peaks from this to drive a strobe - a strong LED source will do - that illuminates the rotating item.

      You'd make marks on the shaft, or attach a degree disk to it. Then you'd build an amplifier for whatever signal the aceelerometer was giving. The LEDs would light up the disc to show you where the shaft was when it was slinging the frame on one direction. You'd take weight off that side and try again.

      I looked up transducers for this a week or so ago. I think my search came up with Nat Semi and Harris. I think I gave it up when I realised all the modern chips would be digital. My electronics is a bit old-hat now, so I gave up. I'm sure that deeper research would get you somewhere, even to the extent of suggesting some IC amps to do most of the job for you.

      It all depends on whether you're up to playing with opamps and little circuits. You wouldn't have to go past the breadboard stage to get something like this going, I'd have thought.

      I'm sure some experts will give you a better analysis than me.

      If I remember, I spent the rest of the hour reading the post about why you didn't need to balance grinding wheels, and left it at that.
      Richard - SW London, UK, EU.

      Comment


      • #4
        There is one method I heard from a man who used to work at the Alvis works. (makers of luxury cars and aero engines)

        He described some procedure which if I recall correctly went like this:

        The machine to be balanced was mounted on compliant mountings and a light metal disc fixed to the end of the rotating shaft.

        The machine was turned over by hand while scribing a circle on the disc centred on the axis of rotation of the machine.

        Then the machine was turned at speed and another circle scribed on the disc.

        Depending on how far out of balance the machine was the second circle would be more or less out of round and off centre. This would also indicate the heavy side etc. Both ends of the machine were done.

        Comment


        • #5
          When I had the Archer I had the engine dynamically balanced. Helped somewhat, but a 360-cubic inch opposed 4-cyl engine is just never going to be smooth...

          The process was to attach a 3-axis accelerometer at the front of the engine and another on the back. A non-contact photosensor was set up to note rotational position with a reflective dot on the prop spinner. The tech had a portable readout box. We ran the engine up, the tech said OK, and we shut her down. Bolted a few washers to the flywheel using the already provided holes (the readout box said where and how much). Started it back up, checked again, much better, done. Took about 30 minutes, cost $150 which is dirt cheap for anything aviation related.

          That said, Analog Devices makes a 3-axis MEMS accelerometer that would no doubt work fantastic for this application. Were I doing it, I'd get one of those (ADXL330, probably) and some kind of a once per rev index. I'd attach all 4 traces to my 4-channel scope and let her rip. The vibrations would be readily visible, and the relative position of the vibration to the index pulse would say how many degrees offset the weights would go.

          If your scope has FFT capabilities (mine does) you could sort out once-per-rev vibrations which are easily balanced out against other vibrations, probably caused by faulty bearings, that can't be balanced out.

          Comment


          • #6
            great ideas, rohart that sounds like my speed i'd seen it done, magic marker lines on the part and strobe light for small DC motors in china (he was the highest paid worker in the plant) but never clued in that its the accelerometer that fires the strobe! thank you. to get my feet wet i was thinking narrow project scope so i like this.

            Originally posted by fasto
            If your scope has FFT capabilities (mine does) you could sort out once-per-rev vibrations which are easily balanced out against other vibrations, probably caused by faulty bearings, that can't be balanced out.
            FFT is filtering of some sort? scope is a 465B analogue so probably not.....but....believe it or not i have an old spectrum analyzer, never used it, not to sure how to use it, but when it came up on kijiji for $100 machine vibration analysis was in the back of my mind. its a low frequency unit so should work just fine....that was bargooon of opportunity, first i need to get the basics lined up

            ....any sense of cost or recommendations on an accelerometer? cheap is good
            .

            Comment


            • #7
              If fast hasn't replied yet, I think yes - FFT is covered by a spectrum analyser. FFT means Fast Fourier Transform - in computing and signal analysis it does anyway - which basically means you can see the spectrum.
              Richard - SW London, UK, EU.

              Comment


              • #8
                Yes, FFT is conversion from time domain to frequency domain, which makes analyzing frequency signatures a piece of cake!

                The ADXL330 should work, www.analog.com is Analog Devices web site. You might even get them to send 1-2 samples. There's even a nice evaluation board prewired you can get. Analog will sell direct, if they won't send a sample.

                A 465B is a great scope, but might not work so well at the low frequencies because it's analog. 60Hz is a bit of a bear, the traces dim out quite fast so you've really got to crank the brightness. If you're good at using it you can pull the signals out. I'm hesitant to use my 453A at these low freq's because the tube's 40+ years old and I don't want to toast the phosphor.

                If your specturm analyzer goes down to 20 Hz you'll be OK. Mine (HP 8591E) bottoms out at 9kHz so it's of no use at these frequencies, that's why I use the TDS2044's FFT analyzer.

                You could try using the 456B in X/Y mode, instead of X/T mode. You'd hook up the two axes of the accelerometer that are perpindicular to the spindle to the two (normally) horizontal inputs. The, set it for X/Y which makes the top input drive the X axis and the bottom input drive the Y axis (normally driven from the timebase).

                The 465B has a secret, sneaky third channel, which I'd hook up to the index pulse. That's the external trigger input. You've only got 2 possible input scales so you might need a voltage divider (x1 and x10). Then press the trigger view button (third one down? been a long time.)

                I'm not sure this will work, but X/Y should produce a circle showing the disturbances. The trigger view trick should work even using the scope in X/T mode.
                Last edited by fasto; 05-04-2010, 07:50 PM.

                Comment


                • #9
                  Well, this is offered as a crazy shot in the dark but ought to be worth some interesting comments.....

                  Remember the gizmos sold to mount behind the rim of your car tires in order to maintain 'balance'? They were a circular tube, filled with oil and ball bearings. The idea was that the bearings would 'settle' in the tube at the point of needed weight application as the wheel spun. If you could mount a tube to your motor shaft and 'see' where the balls settle in during rotation (use an auto timing light synced to the shaft??), could you then determine were counterweights are required?

                  Disclaimer: I have no idea if those wheel balancing devices worked, and yes, I know if the car was stopped the balls would fall to the 'bottom' of the tube, unbalancing the tire.

                  Comment


                  • #10
                    accelerometer supplier

                    Sparkfun.com sells the ADXL335 accelerometer mounted to a breakout board.

                    http://www.sparkfun.com/commerce/pro...oducts_id=9269

                    Comment


                    • #11
                      If you're not into actually building an electronic balancer device, you are stuck with some form of redneck testing. I'm guessing the bearings can remain on the rotor when it's out of the motor. If so, make up some soft cradles to carry the bearings. Find a point at each end of the shaft where the surface runs concentric with the bearing- probably a point very close to the bearing. Then run the rotor up to speed using a drill with a rubber disc in it, like a flexible backing for sandpaper. The rotor should be vibrating because its off balance. Carefully bring the point of a marker to just touch those spots at each end of the shaft. You might want to get someone to help with this, since the rotor won't want to keep spinning for long while you put the drill down and get the marker ready.

                      The more it is out of balance, the shorter those marks should be, and where the center of each mark is, that's where the imbalance is vectored. Use some tape added to the rotor opposite those points, and that will affect the balance. Some experimentation will be needed now to see how much tape needs to be put on to reduce the vibration to as low as you can get it. Then you would peel the tape, weigh it, then either add that much to the light side, or drill out that much from the heavy side.

                      The important thing to start with is finding the precise spots where the imbalances are centered. Both ends will be out, and will interact, so it's not a walk in the park but it can be done.
                      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                      Comment


                      • #12
                        My old GE test manual has a way that was used at the works...

                        accelerometers, basically a coil and magnet with weight, were put on each bearing, and a reading of amplitude was taken on each.

                        IIRC, a test weight was put on one end, roughly judged from the amplitude at that end. It would either make it better or worse.

                        if worse, it was moved around 1/4 turn. Again better or worse. if not better, it was moved again, 180 from original.

                        That procedure found the light side, and there were some calculations given to find the right weight.

                        Same at other end.

                        If it is of interest, I can find it and provide more details.....
                        1601

                        Keep eye on ball.
                        Hashim Khan

                        Comment


                        • #13
                          Maybe this helps?


                          Nick

                          Comment


                          • #14
                            Another method (without using transducers etc) is to make adjustable balance weights for both ends of the shaft. Small clamp-on aluminium discs with holes tapped for steel grub screws as weights will do it for minor imbalance.

                            Fit the discs with a weight installed, run the motor and see if vibration has decreased. Ideally, you'd use a vibration monitoring probe, but just feeling it wil get you a long way.

                            If vibration has increased, move the weights to the opposite sides. Trial & error, but it doesn't take long. Once you're happy, use Loctite on the screws to prevent them flying out.

                            Variations of this have been used for years - Jubilee hose clips on vehicle prop shafts etc.

                            Ian
                            All of the gear, no idea...

                            Comment


                            • #15
                              Originally posted by Pherdie
                              Well, this is offered as a crazy shot in the dark but ought to be worth some interesting comments.....

                              Remember the gizmos sold to mount behind the rim of your car tires in order to maintain 'balance'? They were a circular tube, filled with oil and ball bearings. The idea was that the bearings would 'settle' in the tube at the point of needed weight application as the wheel spun. If you could mount a tube to your motor shaft and 'see' where the balls settle in during rotation (use an auto timing light synced to the shaft??), could you then determine were counterweights are required?

                              Disclaimer: I have no idea if those wheel balancing devices worked, and yes, I know if the car was stopped the balls would fall to the 'bottom' of the tube, unbalancing the tire.
                              I bought a used car years ago that had those exact things on all the wheels. It took me a while to figure out it was the cause of all the strange vibration issues. Removed those and thightened a couple loose shocks, no more issues.... I am sure if one always drove fast enough it might sorta work, but not my experience.

                              Comment

                              Working...
                              X