POLE DERRICK

Various types of light-hoisting equipment are sometimes used on construction projects. A typical example is the POLE DERRICK, also known as a DUTCHMAN, shown in figure 6-13. This device is often powered by means of a handoperated or engine-driven winch. It can be set up readily in the field and moved about from job to job.

Figure 6-13.\Pole derrick, or Dutchman.

The pole derrick is essentially a gin pole constructed with a sill and with knee braces at the bottom. Also, guys usually are installed fore and aft. The pole derrick is suitable for lifting loads of 1 or 2 tons (0.9 or 1.8 metric tons). Since it is light in weight and has few guys, the device can be moved readily from place to place by a small crew.

OTHER HOISTING EQUIPMENT

The two sources of power you will use in hoisting are your work force and machine power. Of the two, machine power is more uniform. On a single vertical line, a crew member of average weight can pull with a force of 100 pounds (45 kilograms), while on a single horizontal line, the same crew member can pull with a force of only 60 pounds (27 kilograms). When you get several crew members on a single line, there is no way to measure the actual strength each crew member puts into the combined pull. When you have to use a lot of crew members, you will not be able to get enough personnel on a vertical line because of limited space. In this case, you should change the line to a horizontal pull by using a snatch block as a fairlead.

Machine power is much more predictable. In fact, all cranes have lift tables that show you their lifting capacities on the basis of a single-line pull. The power from winches and other hoists is also figured on a single-line pull.

As you already know, you can change your advantage by reeving different types of purchases. Always make the mechanical advantage fit your source of power. With some purchases, you have the extra feature of being able to increase mechanical advantage without increasing friction loss. A good example of this is the luff upon luff, which has twice the mechanical advantage of a threefold purchase, while the friction loss of 60 percent is the same with both. Because the friction loss remains the same on a luff upon luff, the use of it saves wear and tear on equipment.