Fun With Pinball

The Restoration and Photographic Study of a 1934 Rock-Ola World's Series Pinball Machine (continued)

The Tilted Field Disc Problem

Once the Field Disc, Die Cast for Mechanism and all of the related parts and hardware are reassembled a problem may present itself.  I first read about it on the Buckwerx web site.  Apparently with age, wear and/or fatigue the Field Disc can end up not being level with the playfield.

Level Field Disc Tilted Field Disc

This game was no exception.  Because the Field Disc wasn't level, balls that came down the Hit Runway could hang up on the edge of the Field Disc instead of falling into the hole in the Field Disc.  Somehow the Field Disc, its shaft and the 608 Bearing for Field Shaft under the playfield were no longer in alignment.

But when the 608 Bearing for Field Shaft was removed from under the playfield there was plenty of room for the Field Disc shaft to tilt in its hole in the main casting so the Field Disc could be make level if it could be held in a new position.

Straight Field Disc Shaft Tilted Field Disc Shaft Straight & Tilted Field Disc Shaft

Seen from below the Field Disc shaft can move quite a bit forwards and backwards.  (Click on the images to enlarge and compare them.) When the Bearing is mounted over the end of the Field Disc shaft it tilts the Field Disc towards the lower part of the playfield which can prevent balls from rolling onto the Field Disc.

Level Field Disc Tilted Field Disc

When the Bearing is mounted such that the Field Disc is level with the playfeld the back mounting hole does not align with pivot in the Casting.  When mounted with a machine screw in the Casting the Bearing pulls the Field Disc shaft towards the back of the playfield which tilts the Field Disc forward as shown earlier.

Alignment Mismatch Corrected Bearing Corrected Bearing Mounted in Place

The solution is to cut away part of the Bearing so that it can be mounted away from the pivot in the casting in a position where the Field Disc is level with the playfield.  I added a thin washer under the machine screw to better clamp down on the Bearing.

Not enough clearance around the pivot Slightly more clearance on the pivot Original & Modified Field Segment Bushing

When I reassembled everything with the Bearing mounted in its new position and tested the mechanism I found that while it worked, it was sluggish.  It behaved as if the spring tension on the Field Segment was not providing enough torque to turn the Field Disc reliably.  My initial reaction was that perhaps I had the wrong spring installed on the Field Segment.

On further inspection I realized that the bushing in the Field Segment was binding on the Bearing at the pivot point.  The pivot in the casting wasn't much taller than the bushing in the Field Segment so when the machine screw was tightened it clamped down on the Field Segment.  I gave the bushing in the Field Bearing a light pass with a file and some fine sandpaper on both sides to make it slightly shorter.  The 1st photo above shows how even the pivot and the Field Segment bushing were.  The 2nd photo above shows the pivot rising just above the Field Segment bushing to avoid any binding.  This was all the difference it needed.  After reassemby the mechanism worked smoothly and reliably.

Modified Bearing Installed Patent US1975374 Fig 3

This is the assembled Field Disc mechanism with the modified Bearing holding the Field Segment shaft so that the Field Disc is level with the playfield. The drawing on the right comes from the original patent and shows the assembly as it would be seen from above the playfield.

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