92nd Street Bridge Emergency Pinion Replacement

92nd Street Bridge Emergency Pinion Replacement (Chicago, IL)

Type: Double Leaf Bascule Span
Location: Chicago, IL
Client: Chicago Department of Transportation

Multiple ship impacts with this structure over the years had resulted in extreme radial misalignment between the rack and pinions for one of the bascule leaves resulting in tip loading of the pinion teeth.  Over time this resulted in heavy wear and eventually breakage of the pinion teeth. SBE personnel were engaged to develop an interim solution that would allow the bridge to continue to operate with the pre-existing rack radial misalignment.  The scope of work involved taking precise measurements of the rack radial misalignment and design of a unique pinion to mate with each rack.  The pinions used heavily modified addenda and dedenda to achieve satisfactory engagement and minimize tip loading and tooth bending loads. Work also included preparation of drawings and specifications for the new pinions and weld repairs to the existing racks, strain gage testing to determine the span drive machinery operating loads, shop inspection of the refurbished components and site inspection of the refurbished components after installation during start up testing.

The strain gage testing determined that as the leaf opened there was excessive imbalance and vertical alterations to the CG location were warranted.  SBE personnel provided balance calculations and recommended weight changes to reduce the maximum span drive machinery loads by a factor of eight.