Fig. 30. Erection of cable way tower.
Fig. 31. Erection of a cableway tower.
A recent very ingenious mounting of cableways in Germany is illustrated and described in Engineering News for November 20, 1913. Briefly, the cableways are mounted on masts instead of towers, and some range of lateral position is secured by manipulation of the side guys which stay the mast. Practically the same scheme was tried some fifteen years ago with a cableway used on some fortification work at Tybee island, Savannah. It is possible that this scheme may be developed so as to become quite useful in certain situations, where only a limited lateral range is necessary, or where a cramped situation at the terminals might preclude the use of ordinary towers. It should be far less expensive than grading off a bench and constructing traversing towers. The masts might be of steel, trussed as required for the length, and mounted on a foundation suitable for such a concentrated load.
In figuring upon a cableway installation the required capacity in two respects should be carefully borne in mind - first, the maximum load which it may be desired to handle, second, the yardage per month, or in other words the number of trips at the proper average working load, which is usually assumed as much less than the maximum.
The heaviest load may be in connection with moving some item of the construction plant, or it may be some valve, machine or casting entering into the permanent work. A very ordinary safe capacity for a cableway is 20 tons; loads up to 35 tons may be safely carried by slinging them between two cableways working side by side.
Cableways with two cables close together and one carriage have handled 35 to 50 tons when especially designed for such service.
In the case of an occasional special load which it is desired to pick up from or deposit at some lateral distance from the cable, lateral guys or even another cableway may be used to pull the load over. Depending on the weight of the load it may be pulled over till the fall line is 15 deg. or 20 deg. out of plumb laterally. The operation, however, should be conducted by some person familiar with cable-ways generally and that one in particular. Generally speaking, ordinary traveling towers on the same tracks may be brought near enough together so that the two cables may be safely Siamesed on a maximum load provided the load is slung below the cableways a distance equal to the height of the towers; lighter loads may be lifted much higher. By using some special sling or spreader, or if the load is such that an end may be carried by each cableway, the hooks may be kept far enough apart so as to favor the conditions materially. The number of trips per hour depends very largely upon the celerity of the attendants who hook and unhook the skips. In connection with the possible number of trips per hour or per day, and the cubic yards possible to handle, following are some records of actual performance:
With the two Lidgerwood Cableways at the Wachusett Dam the expert rigger attained a record of 235 trips in 10 hours, or a trip each 5 minutes 6 seconds. Subsequent performances of the regular crew never exceeded 185 trips per 10 hours, or a trip each 6 1/2 minutes, and the average performance required by the practicable rate of construction was less than that. For instance, a day's timing of a cableway showed 86 trips in 627 minutes; average trip 7 1/2 minutes; ten shortest trips in 36 minutes, ten shortest consecutive trips in 52 minutes. At the Roosevelt dam during the month of March, 1909, two Lidger-Wood cableways in twenty-nine eight-hour night shifts took onto the dam 7900 cu. yd. of large stone (number of trips not known), and also did the necessary moving of derricks on the masonry. Obviously they could not be and were not worked at night up to their daytime capacity. In twenty-eight eight-hour day shifts they made 5003 trips with concrete mortar and spalls. This was an average of one trip each 5.37 minutes, without making any allowance for some unavoidable small miscellaneous services. The average day trip was approximately as follows: hoist 75 ft., convey 350 ft. to 400 ft., lower 125 ft. and return with an empty skip. The materials thus handled during the month were sufficient to build 18,328 cu. yd. of masonry, of which 50 per cent, was stone and 50 per cent, mortar and concrete.
At the Cross River dam two 1250-ft. span cableways handled in one month material for 18,500 cu. yd. of cyclopean concrete masonry composed of 33 per cent, large stone and 67 per cent, concrete, supplying six derricks, the performance of which was 10.8 cu. yd. per derrick per hour. The maximum ten-hour record for the two cableways was 257 2-cu. yd. buckets of concrete and mortar and some stone, amount not stated; or assuming 275 trips, it would be equivalent to about four and one-third minutes per trip.
At the Olive Bridge were four cableways of 1534-ft. span. The monthly record for the four cableways, eight-hour day, was 35,300 cu. yd. of masonry, of which 25.3 per cent, was stone and 74.7 per cent, was concrete. The eight-hour record for the four cableways was 404 2 1/2-cu. yd. batches of concrete plus 160 cu. yd. of concrete blocks and 400 cu. yd. of stone. Assuming that 3 cu. yd. of blocks or stone were carried per trip, it would mean a trip each three and one-fourth minutes or 591 trips in 1920 minutes. The record for one cableway was 226 batches of concrete and 57 cu. yd. of stone, say 245 trips or at somewhat better than a trip each two minutes. This maximum day's record for one cableway may have been for a cableway where the trip was shorter than the average trip for the four. Hourly or even daily records, although interesting, are of less value than monthly records in estimating the amount of work that a cableway may be expected to accomplish. The foregoing records (disregarding the Wachusett dam where the methods were so different) show a remarkable agreement at about 9000 cu. yd. per month per cableway.