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  • Plain Bearing Question - max speed?

    Hey guys, I need some opinions.

    Right now, I'm working on a transmission for a go-kart. This is entirely a "pet project". Its just for fun and I don't like spending money on things that are purely for fun. Luckily, I've got ALOT of junk lying around

    So, I've cobled up a simple four speed manual transmission but I don't have any 5/8" ball bearings handy. I need 5/8" because one of the shafts is an odd-ball splined shaft and its too hard to turn the bearing surfaces smaller and too complicated to reproduce at home. BUT ... I do have bronze bushings with a 5/8 ID. My question is regarding the maximum speed for a bronze bushing.

    I'm figuring the shaft isn't likely to ever spin faster than 2000 rpm, and 1500 is much more likely. So, with 2000 rpm that gives me a velocity of 328 fpm. I've got an input of about 10 ft-lbs in torque and I'm figuring maximum load on the bearings would be 100 lbs. This is way larger than it will be practically speaking. Its more like 50 lbs, but in the spirit of "over engineering", I'll call it 100 lbs. Each bearing is .625" long, so that gives me a pressure of 255 psi.

    Now, I don't know anything about these bearings. They are not sintered and they have a pretty dark cast to them, but that really doesn't help any According to McMaster-Carr, I think 660 bearing bronze would be ok for this application. It's max speed is listed as much higher than my 330 but the combined load and speed is a bit on the high side.

    Anyway, I guess I'd like to hear your opinions. How likely is it that a craftsman ride-on lawnmower with a 6 speed transmission from the '80's would use 660 for the bronze bush bearings, and how well do you think said bearings will work in a transmission? My transmission will be spinning many times faster than the lawnmower transmission ever would have, and if they used a low speed bearing I'm in for trouble I think.

    I'd really like to have ball bearings in there just for reduced friction since I don't have much HP, but oh well. Next time I make a big purchase at Enco, I'll pick up a bunch of chineese ball bearings for these types of projects.

  • #2
    Is adding a ball bearing outboard an option? Maybe this is a dumb question since I am struggling to figure out just where the bearing goes.

    I think the issue is not one of speed in your case. From what I read, you are in the ballpark there. The issue is of load and lubrication. A plain (also plane-- for Evan) bearing is really a hydrodynamic bearing....a bearing that does its job just by floating the mating shaft on an oil film. These films squish out under load. The solutions are either lots of area reducing the force/unit area that would squish the oil out (think big lathe headstock bearings), or pressure lubrication (think automobile engine).

    Paul
    Paul Carpenter
    Mapleton, IL

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    • #3
      old rule of thumb

      An old rule of thumb for bushings was 50lbs of 500 RPM. If either of those number went up the other had to go down. Probably should check a farm supply store or auto parts store for an inexpensive ($5) ball bearing for that application. From my experience (personally and with 3 kids) that go kart will take a beating and be needing enough repairs without having to constantly replace a bushing.

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      • #4
        I have been using 660 bronze bearings I made almost exactly the same size on the countershaft of my 3 hp shaper drive. They are turning about 1000 rpm for hours on end and have been running now for at least 100 hours no problem. As long as they have oil they will hold up to much higher rpms than that.

        Surprising to many people is that a lot of electric motors use steel on steel. The bearing is hardened and the shaft is not. They last for 1000s of hours.
        Free software for calculating bolt circles and similar: Click Here

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        • #5
          Interesting... I didn't realize that about electric motors, either.

          Paul - This is a "custom" built tranny. I salvaged some pieces from the six speed but I'm making a new case and a different shift mechanism to handle shifting at high rpm (thats what destroyed the old one) so I could redesign my box to use some ball bearings.

          Kfq2d - Yep... these are really just for me and my friends to play around with, but I have the occassional niece and nephew who will drive them too. They will probably be much easier on things than I am

          <edit> - It sounds like I'll just need to make sure they get plenty of oil and I'll give it a shot with these bearings and see what happens. They had been run in grease and I considered adding a grease zerk. I've been worried the past couple of days on how I'm going to get enough oil to the bushings and still prevent oil from going everywhere. I'm going to need a seal on the output shaft, I think.

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          • #6
            Originally posted by Evan
            Surprising to many people is that a lot of electric motors use steel on steel. The bearing is hardened and the shaft is not. They last for 1000s of hours.
            Which electric motors use steel on steel? Every machinery motor I've been into (up to 5 HP) has had radial bearings.

            Now, I don't know anything about these bearings. They are not sintered and they have a pretty dark cast to them, but that really doesn't help any According to McMaster-Carr, I think 660 bearing bronze would be ok for this application. It's max speed is listed as much higher than my 330 but the combined load and speed is a bit on the high side.
            You probably want to calculate the PV rating for the bearing -- it's the product of speed times pressure:

            So 328 sfpm x 255 psi is a PV rating of 83,640 -- very high!

            The Max PV constant is listed for all the bearing materials in McMaster, MSC et al... 660 Bronze has a Max PV rating of 75,000, which would imply that you'd need to move up to something like Aluminum Bronze. But you've also overestimated several of your parameters, so you might consider derating the pressure or the RPM to put yourself back into bearing bronze.
            Last edited by lazlo; 07-22-2008, 03:27 PM.
            "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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            • #7
              Yep, I got 83,000 but the PV rating isn't the maximum load times the maximum speed, its the maximum speed AND load. For my application, the heaviest loads are when it is spinning very slowly. My figure of 100 lbs might be close to the truth if all of the torque generated by the engine was soaked up in the horizontal force pushing the gears apart in the transmission. This would mean that the shaft would be essentially stopped.

              Basically, I'm trying to say that if I were to come up with a function that related force to rpm, the maximum on the curve would be under 75,000. For instance, that 2000 rpm corresponds to about 6000 on the engine and lawnmower engines, in my expierence, don't make much torque at 6000 rpm

              So I checked to make sure that my maximum speed and my maximum load were within the specs for the 660. I think 660 would be ok, now the real question is: is it 660?

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              • #8
                Originally posted by Fasttrack
                Yep, I got 83,000 but the PV rating isn't the maximum load times the maximum speed, its the maximum speed AND load.
                The textbook definition of PV is the product of pressure and velocity. So in your case, the Max PV is as I calculated above: 83,640, which exceeds the SAE 660 specs

                There is also a separate maximum pressure and maximum velocity for each bearing material. The maximum pressure rating for 660 is 4,000 psi, and the maximum velocity rating for 660 is 750 sfpm.

                Edit: I just re-read your middle paragraph Fastrack, and I think I understand what you're trying to say: that the max pressure is not at the max rpm. Good point
                Last edited by lazlo; 07-22-2008, 04:07 PM.
                "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                • #9
                  edit:
                  oop! LOL - I just read your edit!

                  Yep, the numbers that I gave were the maximum speed and the maximum pressure. They don't both occur at the same time. The PV rating assumes that those numbers and loads occur at the same time.
                  Last edited by Fasttrack; 07-22-2008, 04:15 PM.

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                  • #10
                    Sure, as long as you don't exceed the max velocity (sfpm) or the max pressure, you're good to go.

                    Take some pictures!
                    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                    • #11
                      Originally posted by lazlo
                      Sure, as long as you don't exceed the max velocity (sfpm) or the max pressure, you're good to go.

                      Take some pictures!

                      Sometimes max pressure is the only thing one needs to consider --- I recall a Kawasaki 750 turbo (factory unit) plain bearing - 200,000 RPM's

                      Reason being is their totally floating the shaft on oil - no ifs ands or buts, so plain bearing is the description but its actually a very sloppy but precision fit and they just stoke it with high pressure lube from a major center channel, it floats like an air bearing.

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                      • #12
                        Which electric motors use steel on steel? Every machinery motor I've been into (up to 5 HP) has had radial bearings.
                        Many fractional hp motors use steel bushings. The starter motor I was using on my ebike was steel bushings but they are also used on continuous duty motors. Steel shaft bushings are common as dirt on low end consumer equipment.
                        Free software for calculating bolt circles and similar: Click Here

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                        • #13
                          Originally posted by Evan
                          Many fractional hp motors use steel bushings. The starter motor I was using on my ebike was steel bushings
                          A starter motor is ultra low duty cycle, so I'm not surprised that it would have steel on steel bushings. But even cheap PC fans and treadmill motors have sleeve bearings.
                          "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                          • #14
                            Whatever. It's common. There isn't a source of replacement parts I can point to since they aren't intended to be replaced and the manufacturers don't exactly brag about it. Steel on steel is also used in a lot other places such as the counter shafts in dryer pulley assemblies. By using a really hard bearing on a soft shaft they hold up pretty well if they run dry. The golden rule for plain bearings is hard against soft.
                            Free software for calculating bolt circles and similar: Click Here

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                            • #15
                              Fasttrack use what you got and have fun Now . If it where`s out fix it then. Will probably wear something else out first. Ball bearings would be nice but if you aint got you aint got so build with what you have it will work for a long time
                              Every Mans Work Is A Portrait of Him Self
                              http://sites.google.com/site/machinistsite/TWO-BUDDIES
                              http://s178.photobucket.com/user/lan...?sort=3&page=1

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