154 MODEL AVIATION
There are many good
features that make
hybrid ceramic
bearings worth using.
[[email protected]]
Control Line Speed Dave Mark
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 being the reduction of 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 bearing supplied in the engines had
ball cages made from brass. The brass proved
to be 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, outside diameter,
and thickness of the bearing he or she wanted
to replace, he or she would be presented with
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 would make
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.
Modelers interested in power output next
looked at the use of bearings that had ceramic
balls. There are many good features that make
hybrid ceramic bearings worth using. The
surface of a ceramic or silicon-nitride ball can
be finished to <0.15 micro-inch Ra, and the
roundness is often better than 0.00001
tolerance.
This precision results in a coefficient of
friction on steel that is 70% lower than steelon-
steel, which translates to a 30% lower
internal bearing friction, lower cage wear,
lower internal temperatures, and reduced
raceway wear.
Another desirable feature of the ceramic
balls is that they are very hard, measuring 78
on the Rockwell C Scale. This extreme
hardness 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, but they roll, skip,
and slide in the x, y, and z planes. This motion
is the major source of heat and wear in the
bearing.
But one modeler would purchase a set of
replacement bearings with ceramic balls,
install them, and see an increase in rpm. The
next person would see no increase. What was
going on here?
The problem was in the assembly
tolerances used for mass-market sales. One set
would be assembled with the correct clearance
and the next would be slightly too tight or
loose.
Aurora, Colorado, modeler Greg Settle
was having all the aforementioned problems.
The inconsistencies he found in purchased
ceramic bearing sets caused him to look into
the process of assembling ceramic bearings.
Greg has been a modeler all his life. He
was an F-105 pilot in the Air Force and is
extremely persistent when he sets his sights on
solving a problem. He decided to fly Speed
and focused on the 1/2A Profile Proto event.
Within two years of starting, he took first
place in that event at the 1999 Nats.
Then he 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. Only 29 people have
passed this mark since the event was started.
As Greg researched bearing suppliers,
Cerbec came up as the source of the highestquality
ceramic balls. Take a look at the
company’s Web site at www.cerbec.com for
an in-depth discussion about ceramic bearings.
Greg dove in, built fixtures, and acquired
the measuring equipment needed to
disassemble steel bearings and then assemble
them with Cerbic balls. He pointed out that
bearing manufacturers match inner and outer
races and then slightly vary the diameter of the
balls to arrive at the desired clearance. As a
result, it is necessary to measure the steel balls
in a bearing and then select the correct
ceramic ball that will produce the desired C3
plus fit.
Greg stocks an average of 10 sizes of
ceramic balls for each nominal-size steel ball.
Cerbic sells the balls in lots of a minimum of
100 pieces. This results in Greg’s having
thousand of balls in stock so he can cover the
various sizes used in our engines.
Greg’s service is used by many modelers
in disciplines such as Pylon Racing, RC
boats, and especially RC car circle-track
racing and drag racing. Bearings and engines
Greg has worked on have been used in many
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
bearing cages are fragile and can be broken
when an attempt is made to disassemble the
bearing. As a result, Greg has made many
special fixtures that support the cage during
disassembly and assembly.
The TH9 cages are not available as an
aftermarket item. This means that if you have
a bearing that is equipped with metal cages, it
is not a candidate for ball replacement. In this
situation it would be necessary to purchase a
replacement bearing with the correct cage and
then have the balls replaced. The total cost is
still less than the price of ceramic bearings
that are available to modelers at this time.
Please keep in mind that the 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, thus ruining the fit.
A case should be placed on a metal sheet
and heated to 275° in an oven. It should
remain in the oven for 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 the highest
temperature of 350°. It is unwise to heat an
aluminum case to higher temperatures. A
puller should be used to remove the bearing if
it will not move at 350°.
Bearing installation should also be done
with great care. They should never be driven
in with a hammer or pressed into a cold case
with the use of a drill press. The case should
again be heated to 275° and bearings placed
on a fixture that pushes against the edge of
the outer ring.
If a modeler is uncomfortable with the
idea of removing and installing bearings,
Greg will perform this task for an additional
fee. Those who are interested in this type of
project for their engines should contact Greg
Settle at 1194 Lewiston St., Aurora CO
80011; Tel.: (303) 343-8778; E-mail:
[email protected]. MA
Ceramic-bearing use in model engines
09sig5.QXD 7/26/06 9:01 AM Page 154
Edition: Model Aviation - 2006/09
Page Numbers: 154