To: alosaurus and Got the T shirt
Here is the definitive answer regarding conductive tyres (tires) made by Dunlop and I assume other manufacturers of aircraft tyres (tires).
Dear Mr. Zuckerman
Tyre conductivity has been a contentious issue for some years now.
During the late 1930's Dunlop started advertising it's tyres as helping to
dissipate static electricity.
The US military included a statement requiring a level of conductivity for
all tyres in MIL-T-5041, which has been a governing spec for most tyres for
many years. It has always been difficult to measure due to the weather and
other environmental factors. (Two measurements taken thirty minutes apart
or even two hundred yards apart can be significantly different.)
Due to the introduction of new materials in tyres and wheels during the last
fifteen years, the conductivity of the tyres has been reduced considerably.
Most airframe manufacturers now require an aircraft to be earthed using a
cable connected to the ground during re-fuelling etc.
This matter was discussed in length at an industry meeting a couple of years
ago and it was decided to remove the conductivity from the latest
specification MIL-PERF-5041-J due to the difficulty in measuring it and the
introduction of hard tethering of aircraft.
When retreading, it is left to the approved retreader to use the compound
that best suits his equipment and provides the best performance. At Dunlop
we normally retread using the same compound as is used on the new tyres.
I hope this answers your questions, if not please don't hesitate to contact
me.
Best Regards
Ray Fletcher
Programme Manager
Dunlop Aircraft Tyres Limited
Phone: 44 (0)121 627 7227
Fax: 44 (0)121 377 7150
Website:
www.dunlopaircrafttyres.com
Regarding the damage to the bearings it is not the surge of static build-up that causes the most damage it is when the wheels are rolling and the balls in the bearings are moving in relation to the inner and outer races. As the ball rotates a new point of contact is made and as the ball moves away from the original point of contact a small spark is generated and this etches both the races and the balls. I once saw a demonstration where this condition was illustrated using a flashlight battery as the DC power source. Helicopter transmissions are also susceptible to this spark erosion especially in the gears. As the gears rotate a spark is generated causing etching of the mating surfaces.
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The Cat