Internal Bearing Clearance

[Challenger440]

So the munchkins left a reel of fishing line out in the yard which I of course noticed right before it was sucked under the deck. I was reaching for the PTO disengage when it went under. Nonetheless, the fishing line was wrapped tightly around one of the mandrel shafts. I took the mandrel apart and the lower bearing is hurting.

So question is, what is the proper internal clearance for a mandrel shaft bearing(thinking normal or C3)? It is a 6204 bearing, but I can't find anything that recommends the ideal size.

I'm a noob to bearing nomenclature and sizing, any info is much appreciated.

 

[matt_i]

I don't know for sure. Its probably .0001" or .0002" radial.

6204s are cheap, I like SKF if I can source them, I'd recommend a new one. Main thing is to get the proper shield/seal/open combination.

Mandrel = blade spindle-shaft? (v belt sheave on top, blade underneath)

The good news is that being a mower deck its not a high precision/close tolerance bearing application.

Aside but related: I drove over an innocuous plastic shopping bag (the kind thats well less than .001" thick) on the highway once, somehow it stuck on the axle housing of the truck and then gripped the driveshaft. That material, when hot, must have been stretched to a quarter mile long, but wrapped around the pinion shaft 4750 times and melted itself to the seal. All of the sudden I'm dripping axle fluid/EP gear oil on the ground...and got to build some special tools to remove the yoke and replace the seal....

 

[OccupantRJ]

The SIZE is 6204. If you are speaking of tolerance, there are bearing tolerance charts on the Internet. More important in your case is what type shields/seals are in the bearing to keep dirt out. Most auto parts, electric motor shops and mower shops may have this bearing in the $10 range. They may have a cheap version and a more expensive version.

 

[bwringer]

I guess I don't really understand the reason for the clearance question.

The bearing is a 6204, probably 2RS (Rubber Seals, 2 sides).

This is a bog-standard 20mm bore x 47mm OD x 14mm thick metric bearing, used in all kinds of machinery from mowers to motorcycle wheel bearings.

 

[Whitworth]

Are you trying to build up the shaft?

Don’t fully understand internal clearance. If referring to the inner diameter of the bearing, and looking for an interference fit, then range of 0.010 mm to 0.025 mm over nominal size. Example; 20.017 mm would be a press (interference) fit. Varies a bit based on size of bearing, and brand of bearing.

If slip fit, just under nominal, maybe 0.007 mm undersized. Using millimeters because most bearings are sized as such. Easy to convert to inches.

Most mower equipment the bearings will be press fit. In some rare cases the inner race is held with a nut or similar.

If the shaft is undersized due to spun bearing, etc, I usually resort to a Loctite product such as 609 bearing retainer compound.

 

[pcmeiners]

C3 is normal clearance, anything tighter as in c1 is for precision fit such as lathes, medical equipment(robotics), clearance above C3 are often used when high heat is involved. Aside from getting the correct seals, stay clear of Chinese bearings, get a well known brand.. SKF, Timken, NSK, Nachi, NTN etc... Generally bearing without a clearance marked on the box/bearing are C3. Clearance primarily refers to bearing/raceway clearance. If a bearing has a known clearance replace with the same clearance. Likely >95% of bearings are C3.

 

[Challenger440]

The original bearing is chinesium so I was hoping to upgrade the bearing with a known brand as doing so will be even cheaper than replacing the mandrel shaft as a unit. For replacement parts, the bearing and mandrel shaft are sold as a unit with the bearing pressed on, similarly the one I took off was pressed on.

The chinesium bearing doesn’t designate CN, C3, etc.

Per SKF, C3 is not nominal clearance, C3 is slightly larger than nominal. See here (also open up their linked table 1): http://www.skf.com/us/products/bear...ing-data/radial-internal-clearance/index.html

If I had to guess, I’d say the factory bearing is normal clearance but I see a plethora of C3 bearings around and very few designated as CN or some other descriptor for normal. So I was hoping to glean some grain from those who know much more than I.

 

[Whitworth]

I wouldn’t waste any additional $$ on brand name bearings for a mower deck; the dirt, filth and moisture are harsh and the short lifespan in that environment doesn’t matter/don’t care if it’s Chinese.

 

[metlmunchr]

C3 is the most common bearing. The class refers to internal clearance between the balls and the inner and outer race, and in general the precision of the bearing. It does not refer to the size of the bearing bore.

Common applications for ball bearings use a press fit on the rotating element and a slip or light push fit on the fixed element. A press fit on both ID and OD is unusual and requires very tight tolerances on both bore and shaft to prevent wrecking the bearing in no time.

If you had to press the original bearing off the shaft then its likely the shaft wasn't damaged in the bearing seat area, so a new bearing can be pressed in place with no additional work.

Matt's story about the plastic bag is typical of the damage those things can do when they get in the wrong place, yet when used for their intended purpose, will barely hold a box of corn flakes without ripping apart.

 

[Challenger440]

Metlmunchr, so much good information!! Thank you! I missed that the clearance designations were for the clearance befween the balls and races, makes sense.

 

[larry4406]

The mower deck spindle bearings have always pissed me off, at least on my MTD 54" mower which is same as Cub Cadet.

The spindle housing has a zerk fitting for greasing the spindle assembly yet the bearings inside have seals on them. The bearing seals prevent the introduction of grease to the bearings. So the only thing the grease does is wet the shaft perimeter and its fit to the bearing as near as I can tell and fill the void so as to prevent the accumulation of water. False sense of security is obtained by greasing the spindles, at least this has been my experience.

I think the designs are flawed and that the bearing seal on the inner sides of the bearings should be removed or not present at all so as to allow the introduction of grease. Bolens does this on their mower spindles.

Lastly, the self tapping bolts that secure the spindle "ears" to the deck often seize in the aluminum housing and snap off or the ear breaks off. If the bolt snaps off in the hole, you can drill these out and then use a through bolt with nut and lock washer. If you are so lucky to get the self tapping bolt out, use never seize when reassembling. You will likely be back sooner than you think.

 

[pancho400cid]

The more interference between the shaft and the bore, the more internal clearance the bearing needs to keep from binding internally when pressed onto the shaft or outer bore, especially once it heats up in service.

"C-3" internal fit class is sometimes called "electric motor bearings". As said it's a comparatively loose internal clearance as motors tend to have a fair amount of interference between bearing and shaft.

As said the bearing width, bore and OD are the same for the different fit classes of a given bearing. The only way to know the internal fit class is by info stamped on the box or bearing itself.

 

[TractorJeff]

Sometimes an over zealous person over greases them pushing the seals out Ruining them anyways! I read on the Internet years ago( so it has to be true!) that they leave the Seals inside to prevent over greasing from pushing out the outside seal causing premature failure.

 

[matt_i]

If the question was regarding the fit between the shaft and inner race, I always have used the .001" of interference per 1" of shaft dia with success. The outer race is typical push/light tap fit.