Control Line Speed - 2006/09
Ceramic-bearing use in model engines
Dave Mark <[email protected]>
Many items are taken into consideration in an effort to increase the power output of an engine used in CL Speed. I have covered a few of them in the past.
I wrote about increasing the clearance between the shaft and front housing with the goal of reducing rotating drag in the seal area of our front-intake engines. This is an old idea that was used with great success long ago on engines such as the Fox .29X.
The Fox had a sleeve bearing on the crankshaft. The premier change that made this engine into a screamer was to increase the clearance between the shaft and the case. The case did not last long after the clearance was increased, but longevity was not the goal.
Along came the SuperTigre engines with dual ball bearings on the crank. This increased the level of precision tremendously. The bearings supplied in the engines had ball cages made from brass. The brass proved susceptible to corrosion from the acids formed by the high‑nitromethane fuel that was used at that time. These brass retainers would often come apart and send bits of brass throughout the engine with disastrous results. Speed fliers then went on the hunt for replacement bearings that were better than the supplied bearings. This pushed them into the confusing world of the ball‑bearing distributor.
If a person approached the bearing distributor with the inside diameter, outside diameter, and thickness of the bearing he or she wanted to replace, the dealer would present a list of many different ball bearings that ranged from low in price to outrageous. The dealer would then ask what class bearing, end play, load, rpm, and cage type were required. Many modelers made the mistake of thinking that more expensive must be better. They would buy spindle bearings, take them home, install them, and find that their engines ran slower with these than with the standard bearings.
Things are a little easier today because many of the engines used in Speed come with bearings that have the correct fit and a form of plastic ball retainer that is far superior to what was available in the past.
Advantages of hybrid ceramic bearings
Modelers interested in power output next looked at bearings that had ceramic balls. Hybrid ceramic bearings have several notable advantages:
- Ceramic or silicon‑nitride balls can be finished to better than 0.15 micro‑inch Ra, and roundness is often better than 0.00001 in.
- This precision results in a coefficient of friction on steel that is about 70% lower than steel‑on‑steel, which translates to roughly 30% lower internal bearing friction, lower cage wear, lower internal temperatures, and reduced raceway wear.
- Ceramic balls are very hard (about 78 on the Rockwell C scale), which helps resist wear caused by the unconstrained motion of balls traveling in a bearing race.
Because the outer and inner races are different diameters, the balls do not roll in an orderly circular fashion; they roll, skip, and slide in the x, y, and z planes. This motion is a major source of heat and wear in the bearing.
Variability and the importance of assembly tolerances
Despite the advantages, results varied: one modeler would install replacement bearings with ceramic balls and see an increase in rpm, while the next would see no increase. The problem was assembly tolerances used for mass‑market sales. One set might be assembled with the correct clearance, the next slightly too tight or too loose.
Bearing manufacturers typically match inner and outer races and then slightly vary the diameter of the balls to arrive at the desired clearance. Therefore, to achieve a correct C3-plus fit, it is necessary to measure the steel balls in an existing bearing and then select the correct ceramic ball size.
Greg Settle’s work
Aurora, Colorado modeler Greg Settle experienced these inconsistencies and investigated the process of assembling ceramic bearings. Greg has been a modeler all his life, was an F‑105 pilot in the Air Force, and is persistent when solving problems. He decided to fly Speed, focused on the 1/2A Profile Proto event, and within two years took first place in that event at the 1999 Nats. He then built a .21 Sport Speed model and passed the 150 mph mark, making him a member of the North American Speed Society’s 150 Club.
As Greg researched bearing suppliers, Cerbec emerged as a source of high‑quality ceramic balls. Greg built fixtures and acquired measuring equipment needed to disassemble steel bearings and reassemble them with Cerbec balls. He stocks an average of 10 sizes of ceramic balls for each nominal‑size steel ball. Cerbec sells balls in lots of at least 100 pieces, so Greg maintains thousands of balls in stock to cover the various sizes used in our engines.
Greg’s service is used by modelers in Pylon Racing, RC boats, RC car circle‑track racing and drag racing. Bearings and engines he has worked on have been used in models that have won and set records in various contests.
This process will not revitalize a worn bearing. Best results are obtained if the balls are changed in new bearings that are equipped with a TH9 bearing cage. These TH9 cages are fragile and can be broken when attempting to disassemble the bearing. Greg has made many special fixtures that support the cage during disassembly and assembly.
TH9 cages are not available as an aftermarket item. If you have a bearing equipped with metal cages, it is not a candidate for ball replacement. In that case you would need to purchase a replacement bearing with the correct (plastic TH9) cage and then have the balls replaced. The total cost is still less than buying ready-made ceramic bearings available to modelers today.
Bearing removal and installation
Please keep in mind that removal and installation of ball bearings in modern engines is a precision job.
- Bearings should never be removed by pounding them out of a cold case. This will ruin the bearings and often score the case, ruining the fit.
- To remove bearings: place the case on a metal sheet and heat it in an oven to 275°F (about 135°C). Leave it in the oven at least 15 minutes. With many engines, the bearings will then easily drop out of the case. If the bearings will not move, the heat can be raised to a maximum of 350°F (about 177°C). It is unwise to heat an aluminum case higher than this. If the bearing still will not move, use a puller.
- To install bearings: again, heat the case to 275°F and place bearings on a fixture that pushes against the edge of the bearing from the outer ring. Do not drive bearings in with a hammer or press them into a cold case with a drill press.
If a modeler is uncomfortable removing and installing bearings, Greg will perform this task for an additional fee.
Contact
Greg Settle 1194 Lewiston St. Aurora, CO 80011 Tel.: (303) 343‑8778 E‑mail: [email protected]
MA
Transcribed from original scans by AI. Minor OCR errors may remain.
