02-07-2017, 02:55 AM
Ron,
The math is straightforward: aluminum grows right at .000012" per inch per degree. That's "twelve millionths of an inch — PER INCH… per degree.
An easier way to think about this is that aluminum grows: "12 parts per million". If you have a 2" bore, it will grow 24 millionths per degree.
Steel is half that. Right at 6 millionths per degree.
These numbers may sound intimidating or confusing if you're not used to working with them, but it's not a big deal. Let's say you have a 1-3/8" OD bearing you want to put into an aluminum front wheel hub. You take 1.375 (the size of the bore) and multiply it by 12. That equals 16.5. So the hub will grow 16 1/2 millionths per degree over ambient. That means that every 60° gets you .001" (16.5 millionths x 60 = just about .001"). If you carefully measure your bearing AND your bore at room temperature, and it has say... .0015" interference, you know that you need to heat the hub 90° over ambient to get the bore and the bearing to be "equal" in size. It's always good to have .002" to make a bearing nice and easy to drop in, so you need another 120° to get you that. 90° + 120° = 210° (over ambient).
So... on a 70° day, you will need to heat the hub to 280° (in this hypothetical situation) to have .002" clearance when you are ready to drop in the bearing.
I typically recommend NOT bothering to chill bearings -- steel only grows (or shrinks) half as much, so that same 1.375" bearing will only move around 8 millionths per degree. A good freezer will go down close to 0°F, but that only buys you a 70° drop at best, and that's only a bit over half a thousandths ---- not much!
The other thing I would recommend is using a regular oven not a toaster oven (if possible). Having an oven where the hub can sit and "soak" in the heat for a period of time is always the best way to make sure the piece is actually at that temperature. I have a dedicated oven that has 3/4" aluminum plates on both shelves, and once the oven has been on for a while at say... 300°F, those plates are up to temp, and you can just set your aluminum parts (hubs?) on the plate and it will be up to temp fairly quickly because the heat will transfer from the big plate into your small part very well.
Pete
The math is straightforward: aluminum grows right at .000012" per inch per degree. That's "twelve millionths of an inch — PER INCH… per degree.
An easier way to think about this is that aluminum grows: "12 parts per million". If you have a 2" bore, it will grow 24 millionths per degree.
Steel is half that. Right at 6 millionths per degree.
These numbers may sound intimidating or confusing if you're not used to working with them, but it's not a big deal. Let's say you have a 1-3/8" OD bearing you want to put into an aluminum front wheel hub. You take 1.375 (the size of the bore) and multiply it by 12. That equals 16.5. So the hub will grow 16 1/2 millionths per degree over ambient. That means that every 60° gets you .001" (16.5 millionths x 60 = just about .001"). If you carefully measure your bearing AND your bore at room temperature, and it has say... .0015" interference, you know that you need to heat the hub 90° over ambient to get the bore and the bearing to be "equal" in size. It's always good to have .002" to make a bearing nice and easy to drop in, so you need another 120° to get you that. 90° + 120° = 210° (over ambient).
So... on a 70° day, you will need to heat the hub to 280° (in this hypothetical situation) to have .002" clearance when you are ready to drop in the bearing.
I typically recommend NOT bothering to chill bearings -- steel only grows (or shrinks) half as much, so that same 1.375" bearing will only move around 8 millionths per degree. A good freezer will go down close to 0°F, but that only buys you a 70° drop at best, and that's only a bit over half a thousandths ---- not much!
The other thing I would recommend is using a regular oven not a toaster oven (if possible). Having an oven where the hub can sit and "soak" in the heat for a period of time is always the best way to make sure the piece is actually at that temperature. I have a dedicated oven that has 3/4" aluminum plates on both shelves, and once the oven has been on for a while at say... 300°F, those plates are up to temp, and you can just set your aluminum parts (hubs?) on the plate and it will be up to temp fairly quickly because the heat will transfer from the big plate into your small part very well.
Pete