Cable Car Turnaround
At three termini in the San Francisco cable car system, the cars must be rotated by hand by two crew members on large turntables in order to reverse direction.
To initiate rotation, one crew member lifts a switch, and both push on the car or on the turntable. This must be done with the car on the turntable for part of the arc, and off the turntable to realign the tracks to receive the next incoming car. Risk factors include high force, awkward postures, and contact stresses on various parts of the body. Mean forces (in lbs., shared by two people) were measured:
To initiate rotation - 147 lbs.
and to maintain rotation 78 lbs.
This exceeds generally accepted guidelines of 45-55 lbs per person. In fact, the US Military (MIL-HDBK-759B, 1992) calls for a limit of 25 pounds under low traction conditions, which exist when the stone surface is wet or sandy. The San Francisco cable car system is considered to be a National Monument, and physical design changes are highly restricted.
Initially, vertical and horizontal handles were attached to the turntable to reduce shear forces on the knee. This introduced contact stresses on the upper body. In phase two, shoulder and back pads were installed, allowing operators to effectively use their upper body weight to help rotate the turntable. The pads are designed to be comfortable for operators ranging from the 5th to 95th percentile in stature.
In addition, the stone surface underfoot was scored to increase the traction surrounding the turntable.
Evaluation of Intervention
- Reduces shear forces on the knee
- Virtually eliminates contact stresses
- Pads are positioned to reduce awkward postures
- Substantial push forces still required
Semi-Quantitative Evaluation of Intervention
- Reduction of Identified Risk Factor
- No New Risk Factors Introduced
- Productivity not Reduced
- Low Cost
Other Possible Interventions
- Add a crew member at each end of the line to help move the turntable
- Install motors to rotate the turntable
Submitted by Ira Janowitz, UC Ergonomics Program; The San Francisco Municipal Railway