Isopropyl & Dry Ice Cooling Bath for Bearings

[woodchucker]

Oiled up after bearing was shrunk on. I suppose you could do an oil/dry ice bath too if you could find an oil that wouldn't solidify. Would be a mess though!



These bearings are 6007ZZ deep groove radial ball bearings with ISO Group N (CN) internal clearance. Still working on the housing but aiming for ISO "H6" seats in the housing. These should be a tight clearance fit, retained by an internal helical snap ring for axial location.



Thanks! I was actually surprised how little headache I had getting the desired tolerances. Worst part was waiting overnight after roughing to get the steel back to ambient before doing the fine finishing. I use VCMT H01-AK aluminum specific ground inserts for the finish. They're wicked sharp and let you take the final dimension at <0.001" DOC on diameter without rubbing. Did the 3 pass method with roughly 0.005" stock to leave.

Apparently my lathe is leveled fairly well as I cut the entire mid section to the bearing seat diameter, then added the grooves and reduced diameters. The taper was 0.00010"/2" or there about.

Liquid nitrogen is a great method, but I found the dry ice to be very accessible. Bought mine at the grocery store for $2.50/lb. Only needed <1lb.

wow, can't get dry ice at the grocery store here. Only at ice places.. It would be nice though. I just had a procedure done where they froze some basal cell growth, used liquid nitrogen, the dr is a friend we play volleyball, and he swung the styrofoam cup that the liq nitrogen was in, and flakes went flying everywhere (outside the cup), I was seriously worried about getting splashed at that point, but he wasn't.

 

[pontiac428]

An acetone/dry ice bath and an isopropyl alcohol/dry ice bath will reach the same temperature. Which you use will depend on the compatibility of the part you're cooling. Isopropyl alcohol may provide a bit more of a barrier to flash rusting as the parts warm up due to the slower evaporation rate.

Evaporation rate is one thing, but aqueous alcohols (non-vapor state) ionize. Alcohols are corrosive for two reasons, one, they serve as an electrolyte like salt, and two, they are fairly nucleophilic. Process and file under good to know.

 

[macardoso]

Can you explain why you went with an m5 and m6 fits on these bearing seats? That is awfully tight for a CN clearance bearing and not normally needed when there is not high vibration. What is the bore on your spindle? It looks like it is in the 20mm range?

I'll admit first off that I'm not an expert on bearing design and this was the first serious design I've done where the fits were not copied from a machinery's handbook table.

I followed the SKF online design guide to select a standard seat of "k5" for normal to heavy loads (0,05 C < P = 0,1 C) on a 25mm and 30mm seat. I then used the tolerance modifier for bearing seats on a hollow shaft (my shaft has a through bore of 0.8552" (21.72mm) so the walls are very thin). This adjustment factor increased the fit up to "m5" to give an equivalent fit to a "k5" on a solid shaft (assuming I did the factors correctly).

The design load is 150lbf radial (belt tension under load at speed) and an operating speed of 10,000 rpm. No cutting forces should be transmitted to this bearing assembly.

 

[Ischgl99]

I'll admit first off that I'm not an expert on bearing design and this was the first serious design I've done where the fits were not copied from a machinery's handbook table.

I followed the SKF online design guide to select a standard seat of "k5" for normal to heavy loads (0,05 C < P = 0,1 C) on a 25mm and 30mm seat. I then used the tolerance modifier for bearing seats on a hollow shaft (my shaft has a through bore of 0.8552" (21.72mm) so the walls are very thin). This adjustment factor increased the fit up to "m5" to give an equivalent fit to a "k5" on a solid shaft (assuming I did the factors correctly).

The design load is 150lbf radial (belt tension under load at speed) and an operating speed of 10,000 rpm. No cutting forces should be transmitted to this bearing assembly.

Looks like you did your due diligence, that shaft wall is thinner than I thought and will definitely reduce the fit, so m5 makes a lot more sense now. K5 or k6 fits are common on solid shafts since the bearing inner ring will expand to the fit, but as you found out in your research, a hollow shaft will contract some when the bearing is installed reducing the interference.

 

[macardoso]

Looks like you did your due diligence, that shaft wall is thinner than I thought and will definitely reduce the fit, so m5 makes a lot more sense now. K5 or k6 fits are common on solid shafts since the bearing inner ring will expand to the fit, but as you found out in your research, a hollow shaft will contract some when the bearing is installed reducing the interference.

Since you seem to know bearings well, do you agree with my "H6" tolerance on the housing bores? I don't want the bearings to spin in the bores if I make them too loose.

 

[Beckerkumm]

6007 bearings have a 35mm bore but we seem to be talking about 25 and 30mm. Am I misreading something? The reason I ask is that a 6007 running at 10K rpm is nearing it's limit for a standard clearance ABEC 1 bearing. Are you running SKF Explorer which are generally ABEC3? I haven't looked up the speed limits on those bearings but extra precision is helpful when reaching high speeds at normal clearance.

I can't help with the housing fit but given the speed and bearing size, there will be expansion towards the housing. Light use probably won't generate enough heat to matter but in a production type setting, I can see issues with a stamped steel cage 6007 in that application if the fit is too tight.

Dave

 

[macardoso]

6007 bearings have a 35mm bore but we seem to be talking about 25 and 30mm. Am I misreading something? The reason I ask is that a 6007 running at 10K rpm is nearing it's limit for a standard clearance ABEC 1 bearing. Are you running SKF Explorer which are generally ABEC3? I haven't looked up the speed limits on those bearings but extra precision is helpful when reaching high speeds at normal clearance.

I can't help with the housing fit but given the speed and bearing size, there will be expansion towards the housing. Light use probably won't generate enough heat to matter but in a production type setting, I can see issues with a stamped steel cage 6007 in that application if the fit is too tight.

Dave

That was a typo. The two bearings are 6205 and 6206. There is a 6007 in the machine but not in this assembly. Had that number stuck in my head.

I'm definitely pushing these bearings close to the limiting speed. If I wanted a better quality bearing that will run smoother and cooler at high speed, what would I look to buy?

 

[Beckerkumm]

Spindle bearings where you need to be concerned about tooling runout and radial load at higher speeds represent a design trade off. Older machines ran larger high load, precision bearings with either a machined brass or phenolic cage with an oil drip system, similar to lathe headstock bearings. Shielded bearings are rare with higher than ABEC 3 precision or stamped steel cages. I assume they are available but probably at more cost than is desirable. If you are replacing bearings, I'd run what you have and see if they heat up. A swap to ABEC 3 Explorer bearings may or may not make an improvements as will swapping to a C3 clearance or a slightly looser fit on your spindle. If the bearings heat up, the cheapest replacement would be an Explorer C3. The original bearings could have been C3 as well as most are not marked. An SKF JEM bearing is C3 as standard and often it is easier to find C3 bearings than CN. I'm like you in that I prefer CN for spindles but I try to stay well under the speed limit as that is really the top end rather than the normal operating range of the bearing.

Dave

 

[woodchucker]

sometimes a looser fit is better for something that heats up. The clearance can be desirable. Where you want to use C2 bearings would be for lets say your mill table, you need the fit to be tight to avoid more backlash. Your spindle, the rpms on most mills are below their max and you want low run out.

Automotive due to the high heat C3.. it allows room for expansion.

just my opinion, not extremely knowledgeable, but thought I would share a differing opinion and let you know that I have no expert knowledge.

 

[Ischgl99]

Since you seem to know bearings well, do you agree with my "H6" tolerance on the housing bores? I don't want the bearings to spin in the bores if I make them too loose.

I want to make sure I am understanding what this part does, I took a look at the parts manual on the Grizzly site and it looks like you are making a new part to drive the spindle spline that will be connected to the motor? If so, then yes an H6 fit would likely be proper for the application since you will have a point load from the motor torque on the housing. Since you are using a CN bearing, I would not want to have interference in the fit that would compress the outer ring after installation, and the point load should keep it from spinning in the housing.

The limiting speed you referenced in a comment above can be higher or lower than what is stated in the bearing catalog and is based on the actual load conditions. The number in the catalog is for a standardized condition and there are adjustment factors for your actual conditions. Do you have a way to install a temperature sensor in the housing so it directly measures the outer ring? The limiting speed assumes 70C outer ring temperature. The standard also assumes a load of 5% of the static rating of the bearing and an ambient temperature of 20C.

Once you get this running, I would run it in in stages to see how the temperatures go to make sure you don't seize a bearing. If the temperature runs away too quickly, it can lock the bearing and do a good bit of damage. If you find the temperature is getting too high, you might need to use C3 bearings in this application.

The Explorer bearings Beckerkumm mentioned are similar to FAG X-Life bearings that are made to P6 standards and are smoother running and can carry a higher load than standard bearings without much added cost, so if the size you are using is available in that series, it is a good idea to go with those.