Repair of Orbit Limit Switch Plastic Belt

During the fall of 2011, Remote Handling discovered that a plastic toothed belt had stripped in the Cyclotron Bridge orbit limit switch mechanism.  This toothed belt is supposed to wrap around the stationary outer race/chassis of the center-post bearing on the Cyclotron service bridge.  The belt meshes with a smaller spur gear, which connects through a gear reduction system to a potentiometer and a limit readout.  The purpose of this limit switch is to prevent multiple continuous orbits of the bridge in one direction around the Cyclotron, which would twist and stress the trolley electrical wires.  Below is a picture showing the stripped belt condition as of Sept 23, 2011 (photo by Travis Cave).

During November 2011, Maico Dalla Valle and Travis went through the drawings of the service bridge assembly and ordered a replacement belt from the original manufacturer (still in business).  Maico removed the central bearing assembly, disassembled the bearing, replaced the belt, re-lubricated the bearing, and reassembled the unit into the service bridge.  The belt that was installed by Maico was installed with one less link / tooth than the previous belt, to obtain a tighter fit, so that fewer shims had to be used to fit it snugly against the outer race / chassis.  After assembly back into the service bridge, Maico commented that a much better mesh was achieved with the pinion. and the system was much easier to turn by hand.  This was likely due to re-lubrication of the centre post bearing and and improved  mesh / fit of the pinion with the belt.  A photo of the serviced assembly with the new belt is shown below:

Orbit Counter System - Operational Improvement (mid-Jan to Feb 7 2012)

During the 2011 winter shutdown, there had been some observations of inaccuracy in the orbit counter mechanism used for the automated radiation survey of the Cyclotron.  These observations were made by both Remote Handling and RPG staff.  The purpose of this orbit counter is to provide an angular position measurement of the service bridge relative to a known reference point in the Cyclotron vault, within a certain degree of accuracy and precision.  It appeared that the reading coming from the counter wheel that makes contact with the outer sector surfaces and follows the service bridge along it's orbit was skipping at various suspect locations around the perimeter of the Cyclotron. This skip, at it's worst, was deemed to be sizable enough to invalidate the automated survey of the tank.  Several e-mails and reports were generated by multiple parties in Remote Handling and RPG about the problem (attached).  The problem was not repeatable each time.  A photo of the counter wheel riding along the perimeter sector surfaces is shown below (taken with RH outrigger camera):

Various theories on the problem and potential solutions were discussed, including:

- Warped aluminum perimeter sectors protruding a small amount into the path of the orbit wheel, causing the wheel to skip as it passes over these bumps (see photos below)

- Insufficient traction between the counter wheel and the perimeter sector surfaces that the wheel rides on.

However, after the center post bearing had been serviced by Remote Handling, and during the routine service bridge work for the 2012 shutdown (copper blockers, shadow shields, photo survey), the following observations were made by RH staff:

- The orbit counter was operating smoothly and accurately and showing no sign of the suspect skips along it's path, throughout the entire range of orbit speeds and along all outer sectors of the Cyclotron.  In fact, according to Don Jackson, he could not remember the orbit counter ever working so well.

- The images from the outrigger cameras were noticeably more stable, showing less signs of structural vibration from the service bridge during orbit.

In my professional opinion, I believe this marked improvement in the orbit counter stability is due to the cleaning, lubrication and re-fitting of the center post bearing assembly described in the previous section above.  Don Jackson could not remember the center post bearing ever being serviced.  I have certainly not been able to find any written record of such service.  If that bearing was poorly lubricated and not rotating smoothly, then vibrations, seizing, and sticking would have result during rotation.  Any skip, jump, or vibration from such a sticky bearing would be amplified across the 30+ foot lever of the service bridge to the point at which the orbit counter wheel contacts the bridge sector.  These vibrations could likely have contributed to false orbit counter readings.  See photo below:

In any case, the orbit counter system has been observed to be operating normally and adequately by RH staff for several weeks of the 2012 shutdown.  I recommend that an automated tank survey is attempted as normal procedure, with both RH and RPG staff present to observe and document the behaviour of the orbit counter.

Thanks,

Grant Minor, Remote Handling