Sections Telescopic   booms   may   be   a   pinned   boom, full-powered  boom,   or  a  combination  of  both.  A “pinned  boom”   means  sections  are  pinned  in  the extended  or  retracted  position.  A  “full-powered  boom” means  sections  extend  or  retract  hydraulically.  Some models  have  a  full-powered  main  boom  with  a  pinned boom tip section. Read the operator’s manual for the proper operation of the type of boom that is equipped on the crane you are assigned to operate. On a full-powered boom, the sections are extended and retracted (except for the base section) by hydraulic cylinders, called extension cylinders. The cylinders are mounted  parallel  to  the  boom  center  line  within  each section. The   boom   extension   cylinders   on   most telescopic  booms  have  sequencing  valves  that  allow  the sections to extend (telescope) by equal amounts. These cranes  usually  have  a  single  telescope  control  lever  in the  cab. However,  on  cranes  not  equipped  with sequencing valves, the operator will have to extend each section equally. (The crane will have two or three boom telescope control levers in the cab, each controlling only a  single  boom  section.)  If  the  boom  sections  are extended unequally, the most fully extended section of boom  could  bend  to  uneven  stresses.  Additionally,  the load chart will be invalidated for determining rated capacity  of  the  crane.  Boom  sections  that  are  marked off in equal increments, as shown on the boom in figure 12-27, make it easier for the operator or signalman to make  sure  each  section  is  extended  equally. When a load is placed on a telescopic boom, the load weight on the boom causes the hydraulic rams within the boom to stiffen up and slightly curve. As the load is removed  from  the  boom,  the  rams  return  straight. Because of this, do not extend the boom while it is under load. Read the operator’s manual for boom extension information. Hoisting  Mechanism The hoisting mechanism for a telescopic crane is a hydraulically  powered  hoist  drum.  The  hoist  drum  is mounted  behind  the  boom  on  the  crane  house  or revolving   turntable. Some  hydraulic  cranes  are equipped with two hoist drums: one for the main hoist and the second for the auxiliary or whip line. House Assembly The house assembly is a revolving unit that supports the  boom. Some  small  hydraulic  cranes  have  the operator’s  cab  and  counterweight  attached  to  the revolving  unit. OPERATOR’S CAB.—  The  telescopic  crane  will have hoist, swing, and boom control levers similar to the  cab  of  the  lattice  boom  crane.  Control  lever(s) is/are also provided to extend and retract the boom. The  hoist  system  does  not  require  foot-controlled brakes. When the hoist control lever is returned to the neutral  position,  the  hydraulic  system  holds  the  load in  place. POWER SOURCE.—  The  power  for  a  telescopic crane comes from hydraulic fluid. In most cases, the main carrier engine drives the hydraulic pump that supplies the hydraulic fluid to hydraulically controlled components.  Power  is  diverted  to  hydraulic  motors  or cylinders by the valve body at the operator’s control station.  The  hydraulic  power  provides  positive  control of all crane functions. COUNTERWEIGHT.—  The  counterweight  on  a telescopic crane provides greater stability when lifting loads.  When  you  are  performing  near-capacity  lifts  at high boom angles using a telescopic crane, about 60 percent  of  load  weight  is  placed  on  the  outriggers  away from the load. When you are performing the same lift with a lattice boom crane, about 60 percent of the load is placed on outriggers close to the load. CRANE ATTACHMENTS The crane is a versatile piece of equipment that can   be   equipped   with   various   attachments   to perform  a  number  of  different  operations.  These attachments include a hook block, a clamshell, and a dragline. HOOK BLOCK A crane that is rigged with a hook block is the primary unit for lifting an objector load, transferring it to a new place by swinging or traveling and then placing the load. Figure 12-28 shows an eight-part line rigged hook  block. The number of parts of a line rigged on the hook block is important for figuring the capacity of the crane. Most crane load charts show the rated capacity of the crane for different parts of the line; for example, a crane that is capable of being rigged with a eight-part line is rigged with a six-part line. The eight-part line gives the crane a greater lifting capacity; therefore, you must check the load chart for the six-part line capacity to avoid  overloading  the  crane. 12-16