The Smoke Got out

[reddog552]

Cutting 3/4 " ply today, I cut one piece walked away from saw herd a Loud screech,turned around Smoke flying every where. I shut dowm the saw and investigated. Smoke defiantly from motor.Tried to spin saw a lot of resistance and catching. Removed Brushes they looked fine. Removed outer cover & field coils.Armature looked great no discoloring Commutator was fine.The end bearing had spun to the point the inner dust cover had separated. The outer cover whitch supports the bearing shows some wear. Where can I get this bearing, Outer cover may need to be replaced too? Anyone ever see this falure before any advice?

[dbhost]

What saw / model motor is this?

[mpc]

Start by looking along the outer edge of the bearing for a number-letter string. Odds are this is a standard bearing that you can order at a local auto machine shop, bearing supply store, Fastenal, McMaster-Carr, etc. Few devices use customized bearings. Those numbers describe the size of the bearing, if it's sealed or not (plastic/rubber between the metal races), type of use, etc. and are an industry-wide standard.

If you can't pull the damaged bearing off the motor shaft, look online or in the Yellow Pages (remember those?) for an automotive machine shop. They have big presses and pullers to remove and install bearings properly; most will charge you just a few bucks to remove or install a bearing. The last time I needed one, my local machine shop charged me six bucks to remove a VERY tight bearing and then another six bucks to install the replacement. Took them just a few minutes each time.

Some tricks for removing bearings yourself:
* Ideally a puller or press tool is used to separate the parts. Such tools apply a constant/steady force between the bearing and whatever it's mounted to instead of using a hammer-like series of blows to separate the parts. Hammering tends to deform the pieces...

* Gear pullers, which you can rent for a refundable security deposit at many auto parts stores, can remove bearings. They typically destroy the bearing in the process... but if you are replacing a bad bearing what does it matter if you damage it even more?

* For bearings-on-a-shaft assemblies like your pictured BT motor+bearing that won't fit in their presses, many machine shops remove old bearings by a) ripping the outer part off the inner part, allowing the ball bearings to fall free and then b) using a metal cutting chisel to cut the inner race. Once it's a "C" shape around the shaft, instead of a closed circle, it'll fall off. Careful cutting with a metal cutting saw blade can do the same.

* Bearings like the one pictured should have the installation force applied to the INNER ring. Basically, whatever part of the bearing is a tight-fit to the mounting surface is where installation force should be applied. You NEVER want the installation force to pass through the ball bearings themselves or be applied to the plastic/rubber part between the two races.

* For installation, a bearing press - which is basically a giant hydraulic jack/vise - is what a machine shop will use. For the bearing in your pics, the motor itself would be held while force was applied to the inner ring of the bearing to drive it onto the motor shaft. For at-home installation: stand the motor shaft upright (I assume there is a metal shaft all the way through the motor) and position the bearing on the end of the shaft. Use a socket from a socket wrench set that matches the diameter of the bearing's inner race to apply the force. Push down on the socket - maybe using a board as a hand grip - and see if you can drive the bearing onto the motor shaft. If not... you could tap the socket with a hammer but this is somewhat a "last resort" technique. If the press-fit is really tight... wrap the motor in a plastic bag and stick it in the kitchen freezer for a while to shrink the metal... and warm the bearing to about 100 degrees F and try again.

mpc

[reddog552]

Saw is a BT3000 . Baearing came off easly with a puller.Found the bearing "6200z' online for 6 bucks.My worry is that the bearing spun in the plastic cover, Smoke was plastic melting. the bearing seems to be a snug fit in plastic cover.My question is how tight of a fit is it normaly.My 5yrs as a motor assembler came to good use on this 1.

[All Thumbs]

Saw is a BT3000 . Baearing came off easly with a puller.Found the bearing "6200z' online for 6 bucks.My worry is that the bearing spun in the plastic cover, Smoke was plastic melting. the bearing seems to be a snug fit in plastic cover.My question is how tight of a fit is it normaly.My 5yrs as a motor assembler came to good use on this 1.

Don't know about the saw's motor but I've disassembled some routers where the bearing fit into the plastic cap isn't all that snug.

[capncarl]

I have repaired small motors with sloppy bearing fits by coating the bearing bore hole with epoxy and pressing the assembly in and letting it harden, and never had a failure. These were only fractional horsepower motors though, not the screaming BT motor. Considering the price of the motor and the ease of the fix I'd give epoxy a try. Maybe bench test it so your time assembling the saw is not wasted if it fails.
capncarl

[mpc]

Bearings should in general be a tight fit to metal shafts, or when pressed into metal cups. Otherwise there will be some relative rotation between the two non-lubricated parts. You want all rotation to happen inside the bearing. I would expect the bearing needs to be a somewhat snug fit into the plastic too or otherwise you'll have the outer race of the bearing slowly, or not so slowly, rotating against the plastic making a lot of heat. Not good. If the new bearing isn't a snug fit in the plastic housing, epoxy is an option. If the gap is really large, a thin wrap-around of shim material may be called for. Wrap almost all the way around the bearing - you don't have to make the ends butt together perfectly. 350 degrees out of 360 is enough. They are harder to find but many bearings are available in a "few thousands oversize." Something like 5 or 10 thousands oversize to repair situations exactly like this.

mpc

[capncarl]

Just out of curiosity I spent a few minutes searching for oversized bearings. I do know that some exist but I could not find them. Any changes in the bearing size changes something else. I.E. the 6200 bearing you have is 30mm od X 10mm id X 9mm wide. Going to a larger od bearing changes the width, whereas you couldn't get motor closed. An outside bearing sleeve is offered for the 30 mm bearing by USA TOLLEANCE RING, #AN30X9S, 1.259 / 1.257 (31.97 mm). A word of caution, if your bearing od bore is not properly fitted for a sleeve there is a good chance the motor bearing housing will split when you press in the new bearing resulting in a ruined motor. This sleeve, new bearing and machine work could result in more cost than the motor is worth. I would recomend carrying the motor to an electric motor rebuild shop and see if they have any repair options.
capncarl