Posted: 3/20/2025 5:36:59 AM EDT
[Last Edit: targetworks][Edited]
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I have little in the way of experience with using bearings, so I could use some guidance. I need to fit an aluminum shaft into two pillow block bearings. The shaft diameter is slightly larger than the ID of the bearings - as best as I can measure the shaft OD is about 0.002" greater than the bearing ID. More specifically: The pillow block bearings claim to have an ID of 1 inch or 25.4mm - and my digital caliper suggests that the ID may be 0.999". I have an extruded aluminum rod, nominally 1 inch OD, which my caliper suggests may be closer to 1.001". How should I go about fitting the bearings to the shaft? For example, if I were to use a heat gun to heat up the bearings, could I expect to increase the bearing ID sufficiently to allow for the insertion of the shaft? And, if using such an approach, how hot would I need to get the bearings, and would I need to be concerned about degrading the grease which, I assume, is packed inside the bearings (because each bearing has what appears to be a zerk fitting on it). The spec sheet (below) says prelubricated temp range up to 120 degrees C - but I'm not sure if I can get away with heating the bearing inner surface above that point for a short period of time in order to try and fit the bearing on the shaft. Alternatively, should I consider reducing the OD of the aluminum shaft, either by somehow polishing it (with Scotchbrite?) or by attempting to skim it down by taking a light pass on a lathe? I have also considered the possibility of using emery cloth rather than a cutting tool on the lathe, but that may be problematic both due to safety concerns with using emery cloth on a spinning workpiece, as well as possible concerns about getting abrasive particles onto the ways. I will need to locate one of the bearings at about 6 inches from one end of the shaft, while the other bearing will need to be mounted near the other end of the shaft. Any suggestions for how best to proceed would be appreciated. The bearing in question: Attached File |
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Can you freeze the aluminum shaft? That might shrink it enough to insert it into the bearing... Eta: If the bearings have a zerk fitting and are preloaded with grease, heating them might be ok and then push all of the heated grease out and replace with new? Some sort of press or clamp setup? Round or bevel the end to get it going and press the bearing on? Not a metal worker or mechanic, just a dumb carpenter. I have replaced and serviced the bearings on my trailer but thats about it... |
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I could probably freeze the aluminum shaft. And I could probably use a hot plate (with a temperature setting knob) and a non-contact IR thermometer to verify the temperature rather than playing with a heat gun (although I might have trouble reading the bearing temp with an IR thermometer due to the shiny bearing surface). I don't have any kind of fixture or press available. |
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| You can use emery on it. Just be very careful. Hold each end between your index fingers and thumbs. It won't take much time (seconds) to polish it down to size. I would uses something like 600 grit. My only concern is how round is the shaft and how concentric do you need it to be. |
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Originally Posted By JKrammes: You can use emery on it. Just be very careful. Hold each end between your index fingers and thumbs. It won't take much time (seconds) to polish it down to size. I would uses something like 600 grit. My only concern is how round is the shaft and how concentric do you need it to be. My application does not require a high degree of concentricity or precision in the rotation of the shaft - it will not be spinning continuously at a high speed, rather I will be occasionally/intermittently rotating an antenna assembly through an angle of at most approximately 90 degrees (between limit switch stops), using a gear motor (a repurposed windshield wiper motor) at a slow speed of at most a few rpm - so, while I'd prefer to avoid obvious eccentricity in the rotation, it does not need to be dead-on. |
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Originally Posted By Postal0311: If a high level of precision is not required, a few minutes (total) with an extra fine grit sandpaper will do it. Are you suggesting something like a sanding sponge without even turning it on a lathe, and rather just manually touching up the surface? I don't have a good intuitive feel for how much effort is required to skim off a couple of thou... |
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Aluminum has a thermal expansion coefficient, at room temp, of around 1.3e-5 inch/(inch * deg F), so to change the diameter by 1 part in 1000 (or shrink a 1" dia shaft by a thou), you need to lower the temperature by 77 degrees. If you're off by 1 thou, your freezer should work, maybe with a little gentle heat on the bearings. If you're off by two thou ... You're looking at dry ice, or preferably liquid nitrogen, to get it cold enough, assuming the expansion ratio stays th same at low temps. If you go that route you will want to be very careful handling the cold rod and the materials you used to chill it. |
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OP, sanding/polishing a couple thou off the OD of your shaft is probably gonna be a LOT more than you think. I wouldn't be too skeered of it. BUT, if it were me, I'd try the heat it and freeze it approach first, if that don't work, ok no blood no foul. Keep us posted. |
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| I only know enough to add, if you go with too much of an interference fit, you can bind the bearings. I've done this on two recent projects. Had to tear it back apart and take a couple of spring passes. |
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Originally Posted By User55645: Where are you in CT and how long is the shaft? I have a lathe and, if it's not too large, I can help you w/ it. Thanks for the offer. I may be OK as far as access to a lathe is concerned. I am in Litchfield County. The shaft is approximately 22 inches long. I do have access to a suitable lathe at a local makerspace - it's 'old iron' (a restored Logan 820) but it should be adequate for this task - I will be using it at least to drill a centered hole to accommodate the motor drive shaft and an encoder via a tailstock drill bit (that is, if one of the drill chuck keys from my home shop will fit into the tailstock chuck - because the original chuck key is currently 'missing in action'). While I am at best an occasional hobby machinist, I did at one time (about a dozen years ago) go through a series of 'Intro to Machine Tool Technology' courses run by a local community college, where I had hands-on access to a decent-sized manual lathe (and a manual Bridgeport, etc.). While that training is a bit 'stale', I believe that I retain enough knowledge to safely handle a task like this one. But from time to time I may have questions, for example, I'm wondering whether I may need to be concerned about tightening down a three-jaw chuck on the workpiece possibly deforming the aluminum - and if so, whether it may be a good practice to use a thin brass shim between the chuck jaws and the aluminum workpiece. Also, that lathe does not have a 'steady rest', and it will not accommodate a 1-inch diameter workpiece all the way through the headstock chuck and coming out the other end, because it looks like that bore is at most 0.75" diameter - so it will be necessary to have almost the entire length of the workpiece sticking out of the headstock chuck. I'm hoping that I can use an indicator to get an acceptable centering for the drilling - and then if I do decide to try and turn the piece to remove a thousandth of an inch, I can probably at least use a dead center in the tailstock to better hold it for that operation. |
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Originally Posted By targetworks: Thanks for the offer. I may be OK as far as access to a lathe is concerned. I am in Litchfield County. I'm also in Litchfield County so don't hesitate to contact me if need be. You should be fine w/ that lathe, even w/o a steady, but the offer still stands. |
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Originally Posted By User55645: I'm also in Litchfield County so don't hesitate to contact me if need be. You should be fine w/ that lathe, even w/o a steady, but the offer still stands. Originally Posted By User55645: Originally Posted By targetworks: Thanks for the offer. I may be OK as far as access to a lathe is concerned. I am in Litchfield County. I'm also in Litchfield County so don't hesitate to contact me if need be. You should be fine w/ that lathe, even w/o a steady, but the offer still stands. Thanks again - I will certainly keep your offer in mind. |
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Your calipers have small flats on the knife edge ends, and can't make precision measurements of inner diameters, only approximate. A couple of thousandths will come off the shaft easily by shoe shining with the shaft in a vise and abrasive paper. A couple thousandths too much, too. But your application can tolerate a hair of clearance. |
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Originally Posted By AeroE: Your calipers have small flats on the knife edge ends, and can't make precision measurements of inner diameters, only approximate. A couple of thousandths will come off the shaft easily by shoe shining with the shaft in a vise and abrasive paper. A couple thousandths too much, too. But your application can tolerate a hair of clearance. Yes - I am not well equipped for making precise measurements, but luckily my application should not require maintaining close tolerances. |
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Stick it in a vise, emery finish off the area needed. Rotate 1/3 of a turn. Emery again. Repeat until bearing fits. Make sure to let it rest or throw it in the fridge or something to bring the temp back down to ambient for a while before measuring or test fitting. It's not difficult and a little elbow grease will get the job done. I'd probably rule out taking a cut on the lathe unless you have good equipment and some experience. Mounting the part and only skimming a thou or two is harder than you might think. |
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I managed to skim down one end to 0.999" by rotating it in the lathe (at less than 100 rpm) and using three grits of emery cloth - 80, 120, and 600. It was harder and more time-consuming to do that than I expected - maybe because it's 6061 alloy. I only need to inset the bearing a couple of inches on that end - but I will need to reduce the diameter of the other end for a full 6 inches to position the other bearing, which is likely to be difficult for me to accomplish with emery cloth, that is, to get a uniform reduction in the diameter. I guess that the only part where the diameter will be critical will be the area where I will be mounting the bearing - so as long as the section of the rod leading up to that is slimmed down enough to have an easy sliding fit I should be OK. But using the lathe at my makerspace, between the lack of a steady rest and having a too-small bore diameter which prevents me from sliding the end of the rod close to the headstock chuck, I will be totally unable to center drill the end of the rod to accommodate an 8mm diameter shaft to drive the rod from the windshield wiper motor. @User55645 it would be great if I could take advantage of your offer of help - I will send you a DM, thanks... Attached File |
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Originally Posted By Boostinjdm: Less than 100 rpm? Why bother with the lathe at all? I don't remember for sure, but I probably would have been spinning that at 800-1000rpm and even that is probably slow for proper machining. There is no steady rest on the lathe - centering the aluminum rod in the headstock chuck is difficult as a result. Having the end of a two-foot long rod whipping around eccentrically at 800-1000 rpm while polishing it with a hand-held strip of emery cloth is a non-starter. |
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Thanks to some awesome help from @User55645 I am now able to fit both bearings on the aluminum rod, the rod has been center drilled to fit an 8mm drive shaft, and holes have been drilled to permit pinning the shaft. I can now move onto the next stages of the project, which include items such as (in no particular order):
And that's just the mechanical stuff - I also have a number of electronics development, prototyping, and assembly tasks, along with microcontroller interfacing and firmware development tasks, and much much more to do before it is complete. Here's a photo showing a test fitting of the bearings and the drive shaft: Attached File |
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