 |
March 2000 Tech Feature | www.virtualindian.org |
| Home / Features / Babbit Bearings |
| |
March 2000 Tech Feature | www.virtualindian.org |
| Home / Features / Babbit Bearings |
| Babbit
is a fairly soft metal used as bearing material in our 4 cylinder Hendersons,
Indians and Aces.
Babbit is also used in vintage automobiles, airplanes, and machinery. It has an advantage in that it is soft enough not to score a shaft but hard enough to handle engine load. It has the ability to hold oil and imbed any hard metal particles in its surface so as not to score the shaft. Two types of most common babbit is lead based and tin based. Lead based babbit has the wrong properties for 4 cylinder motorcycles. I use tin based babbit as it has better temperature resistance, less distortion and a higher load bearing ability. Rebabbitting old shells is a fairly precise procedure involving temperature control of the molten babbit, molds and proper cooling. If any one is neglected problems occur ranging from small bubbles, miniature cracking, folding and or breaking away of the babbit from the shell during use. Firstly the old babbit is removed from the shell. I never reuse old babbit for bearings but keep it and make soft hammers. Removal of the old babbit can be done in one of two methods, either by dipping in a hot pot of old babbit or with a torch (see picture 1). After removal of the old babbit the shells are cleaned, polished and degreased (see picture 2). The cleaned shells are now ready for retinning. Tinning can be done by one of two methods. Either by dipping or a tinning compound. The tinning compound is a modern approach and is less labor intensive but achieves the same result. The shells are heated and the compound brushed on and melts to cover the entire inside bearing surface (see picture 3). A coating is placed on the outer bearing so the compound will not stick to it. Not enough heat and the tinning will not be thorough, too much heat and the flux will begin to burn and possibly trap itself underneath the tin causing future babbit separation and bearing failure by the possible blockage of and oil hole or oil groove. I no longer touch the inner bearing surface after this process to aviod any contamination of the suface by dirt, oils or fingerprints. I use my heat treating furnace to melt the babbit and hold the molten babbit at about 700F. The shells are prepared on the molds and are sealed with an asbestos putty. This helps hold the shells in place and stops any leaks. If not the molten babbit will run out just as fast as it is poured in. The molds are heated so the hot babbit does not freeze on a cold mold. I build up the edge of the bearing with putty as well to fill it higher than the shell as during the cooling the babbit will shrink and suck in the extra on top. The skim, or dross, is drawn to one side in the pot of molten babbit and is then laddled at a steady rate into the molds (see picture 4). The molds are now cooled at a specific rate and the bearings are then removed. The bearing shells are now ready for rough machining (see picture 5). After the bearings are rough machined on the lathe, they are then ready for redrilling of the oil holes, recutting oil grooves, finish reaming, align boring and fitting.
|
Picture 1. Removing old babbit. 
Picture 2. Complete set of Henderson Deluxe bearings including trans, rods, crank and clutch hub. 
Picture 3. Tinning the bearing shells. 
Picture 4. Pouring babbit. 
Picture 5. Note: For illustration purposes shells are separated. Normally machined first, I like to leave babbit approx. .001" projected above shell. This provides a tight fit. Too much can produce a cramped condition on the crank. Rob Olsen can rebabbit your Indian Four, Ace and Henderson bearings. Rob can be reached at: ssc@vianet.on.ca |