Modifying outer bearing races

I have set out to modify the the outer bearing races of 10X22 bearings for us in a 2011 Proi 2.5 cc speed engine. I want to change the existing that uses a 10X22 outer race with the polyamid cage and 9 balls to a cageless near full compliment ceramic ball rear bearing and a cageless front bearing. Both the rear and the front bearings become magneto bearings. The rear bearing taking the axial thrust and the radial forces of the crank assembly and the front bearing taking the radial forces and controls the amount of total endfloat between the two.

I made a fixture of sorts, that holds the outer ring while it gets turned. I made the inner bore of the shell holder slightly less than 0.01mm smaller than the outer diameter of the bearing shell. I used castor oil as a lubricant to assemble the ring into the fixture and had a clamp ring to aid in the assembly. Then I bored out to the diameter of the ball race to the centreline from one side and then diamond polished it. There is a ring inside that allows the extraction of the outer race. Some assemble tooling was required and some more tooling required to measure the position of the crankshaft to the front bearing seat. Other tooling to measure the position of the inner race to the outer race position as well. Instead of making shims for the outer race of the front bearing, I turned the end of the race shorter by the measured amount.

I have some pictures of the tooling and the shaft and case. The project worked better than I thought, and had a RPM gain of 300 to 400 rpm. I did not think that the cages could be producing enough drag to make that much difference. The idea of the project was to get a longer life from the integral bearing crank shaft, and a longer life from the front bearings. Im not sure on the front bearing life just yet as it has only done test bench running. The other engine with the home made outer bearing shell of the same design but using 2.5mm ceramic balls is lasting very well so far. This one uses 1/8 ceramic balls, so I expect this set up to last a longer time.

Fixture and Modified 6X15 outer shell. Turned using Kyocera insert ccgt060202mp-ck pr1425 on a 10mm shank boring bar at 500 rpm 0.1mm cuts 0.2mm diameter at a time.

Crankshaft polished with balsawood mounted onto 1/8 shaft and 3000 grit diamond paste. These cranks have the inner race as part of the crank shaft, also know as an integral bearing crank.

This is the crank and modified outer race assembled with ceramic balls in place, awaiting the heated case to be placed over the assembly. It is the easiest way to get the rear outer bearing shell into the case.

And when the case has been assembled.

Care has to be taken as if the shaft moves back in the case a few mm, the balls will fall out.

The tools to assemble the bearings with and the to get the front inner assembly together etc.

You can see the rear bearing puller and the assembled front inner race in the clear assembly housing. The hollow aluminium bar in the centre at the bottom, pushed the inner race and it's compliment of balls into the front outer race.

Impressive work.

Neil

Hi Neil

A lot of work for a small increase but that's what competition is about, did you mean Profi engine but made a typo ?

Emgee

Thanks Emgee, yes Profi, I am not a good speller, I will edit it. The main reason was to get longer life from the shaft bearing. The added rpm was an unexpected bonus.

Shining like silver, very well done.

Michael W

Hi NL, as a bearing man myself I was interested in your activities in which you clearly have some knowledge. Just wondering what speeds you operate and what were the failure modes that you suffered from that made you make these alterations ?

I also don't think the cage change alone would account for the increase in speed that you have gained. Possibly more to do with changes in the level of pre-loading of the assembly at the operating conditions ?

Gerry

Hi Gerry, the failure is in the crank shaft, it will deform and wear, once it gets a little bit worn, they deteriorate very rapidly. If you are careful and replace the outer race and steel balls when they just start to get some noise in them, polish the crank, you get more life from them. They last longer just by replacing the steel balls with the G3 ceramic balls. With my older engine, it has worked quite well, so tried this on the newer engine that uses 1/8 balls instead of the smaller 2.5mm balls. In all cases, they use a C3 clearance bearing, but more specifically use a clearance of Ø 0.02mm to Ø0.03mm. I like the higher end of Ø 0.03mm clearance between the inner and outer race ball clearance. The end float and loading position is still the same. In reality, model engines set the bearings so that they are in effect being used as Radial contact bearings instead of the regular deep groove position of running in the middle of the race. These engines do about 39300 to 40200 rpm. If the engine is only making power to fly the model at 275 to 280 Km/h they last a lot longer than when they are making power to be doing 290 km/h or faster.

Neil

Hi Gerry, the failure is in the crank shaft, it will deform and wear, once it gets a little bit worn, they deteriorate very rapidly. If you are careful and replace the outer race and steel balls when they just start to get some noise in them, polish the crank, you get more life from them. They last longer just by replacing the steel balls with the G3 ceramic balls. With my older engine, it has worked quite well, so tried this on the newer engine that uses 1/8 balls instead of the smaller 2.5mm balls. In all cases, they use a C3 clearance bearing, but more specifically use a clearance of Ø 0.02mm to Ø0.03mm. I like the higher end of Ø 0.03mm clearance between the inner and outer race ball clearance. The end float and loading position is still the same. In reality, model engines set the bearings so that they are in effect being used as Radial contact bearings instead of the regular deep groove position of running in the middle of the race. These engines do about 39300 to 40200 rpm. If the engine is only making power to fly the model at 275 to 280 Km/h they last a lot longer than when they are making power to be doing 290 km/h or faster.

Neil

Impressive!!

cheers

Bill

Great work Neil, may I ask, is the crankshaft heat treated, ie is the bearing area hardened?

Ian S C

I did not make the crank, only polished the bearing area that had some wear on it. The crank is very hard where the bearing area is. On previous damaged shafts, they are 60 Rc.

The rpm gain of 300 to 400 sounds interesting, I have seen modified ball races that have crowded balls but this was to stop the damage to the engine when the cage failed. I have always used "plastic" cages and have yet to have a failure. To me the ball races loose the smooth feel quite quickly but they are being run at the top end of their performance range just over 40k in my 3.5cc engine and I doubt they get enough lubrication being a two stroke and the rules only allow 80/20 fuel.

Thanks for that Neil. Bearing life is a rather relative sort of thing, industrially one looks at thousands of hours, where as with a motor like this one might look to expect a hundred or two hours, at extreme use it would be much less, and still considered a good life.

Ian S C