Geek1945
Well-Known Member
Most 2 stroke engines connecting rods have caged needle bearings CNB, on some Rotax engines the piston wrist pin are full fill bearings FFB. Normally FFB bearings cause verbal praise spoken during disassemble scattering everywhere. If CNB remain intact they are simple to install and remove as a cartridge.
So why would anyone replace a CNB with a FFB? Since CNB are separated by the cage each bearing carry's a higher load which is not the case with FFB. Additionally the rotating CNB is riding against a non-rotating cage, despite of this CNB have a slight reduction in friction allowing higher speeds. It must be mentioned needle bearing are normally found in 2 stroke engines. Most 4 strokes use sleeve bearing which have considerably more surface area and are pressure lubricated. Since the CNB cage restrains all sides and ends of the bearings they are also shorter. CNB and FFB are always the same diameter only their length is different.
The life of any bearing is dependent on three factors assume lubrication is the same. First is bearing size, a bigger bearing can carry higher loads. Second the bearing load where less is better. Third is speed where slower is better.
Since FFB all rotate at the same speeds like vehicle tires the wear is minimal and since more and longer bearings are carrying the same load as CNB the wear is less but there are more bearing surfaces which does produce slightly more friction. One must remember a shaft and rod surfaces are also involved more bearing surface mean less pressure on both on shaft and rod. The result of replacing CNB with FFB is longer bearing life. Since engine acceleration is gradual and infrequent in aircraft the FFB have no noticeable effect except longer life (see EAA video on 2 strokes).
Working with loose needle bearings, which are cheaper than CNB, require some different methods. Loose needles can be easily placed on 3M Scotch tape like bullets and greased since the tape holds the bearings one only need to cut off the correct number apply some stick glue and wrap the around shaft or rod opening. Both tape and glue will be expelled in first minute of engine start up.
Another method is applying a strong magnet to hold the needles during assembly and removal a small amount of assembly grease works too. Of course using these methods on an assembly line is impractical and not cost effective. Besides with careful use CNB will suffice yet, with the load of a propeller you need all the bearing carrying load you can get especially with the torsional loading of a 5'+ propeller.
With today's friction reducing coatings of Moly, Ceramic, and Teflon treating bearings and friction surfaces one can expect extended 2 stroke life. Along with proper maintenance 2 stroke engines should be just as reliable and overall cost per hour as 4 strokes considering acquisition costs.
So why would anyone replace a CNB with a FFB? Since CNB are separated by the cage each bearing carry's a higher load which is not the case with FFB. Additionally the rotating CNB is riding against a non-rotating cage, despite of this CNB have a slight reduction in friction allowing higher speeds. It must be mentioned needle bearing are normally found in 2 stroke engines. Most 4 strokes use sleeve bearing which have considerably more surface area and are pressure lubricated. Since the CNB cage restrains all sides and ends of the bearings they are also shorter. CNB and FFB are always the same diameter only their length is different.
The life of any bearing is dependent on three factors assume lubrication is the same. First is bearing size, a bigger bearing can carry higher loads. Second the bearing load where less is better. Third is speed where slower is better.
Since FFB all rotate at the same speeds like vehicle tires the wear is minimal and since more and longer bearings are carrying the same load as CNB the wear is less but there are more bearing surfaces which does produce slightly more friction. One must remember a shaft and rod surfaces are also involved more bearing surface mean less pressure on both on shaft and rod. The result of replacing CNB with FFB is longer bearing life. Since engine acceleration is gradual and infrequent in aircraft the FFB have no noticeable effect except longer life (see EAA video on 2 strokes).
Working with loose needle bearings, which are cheaper than CNB, require some different methods. Loose needles can be easily placed on 3M Scotch tape like bullets and greased since the tape holds the bearings one only need to cut off the correct number apply some stick glue and wrap the around shaft or rod opening. Both tape and glue will be expelled in first minute of engine start up.
Another method is applying a strong magnet to hold the needles during assembly and removal a small amount of assembly grease works too. Of course using these methods on an assembly line is impractical and not cost effective. Besides with careful use CNB will suffice yet, with the load of a propeller you need all the bearing carrying load you can get especially with the torsional loading of a 5'+ propeller.
With today's friction reducing coatings of Moly, Ceramic, and Teflon treating bearings and friction surfaces one can expect extended 2 stroke life. Along with proper maintenance 2 stroke engines should be just as reliable and overall cost per hour as 4 strokes considering acquisition costs.
