NOTE: A common rule of thumb is to never swing or perform work with a revolving unit over the cab or front of the truck. Some  truck-mounted  units  are  equipped  with outriggers  mounted  on  the  rear  that  increase  the  stability of the truck. These outriggers are normally hydraulically actuated and controlled from the cab of the truck and provide a much larger and more rigid base than tires. The  advantage  of  truck  mounting  over  track mounting is its capacity for rapid movement from one job to another. The boom can be placed easily in the boom rest for traveling and then driven down the road at 25 to 35 miles per hour. This is better than the slow laborious job of trailer loading, securing, hauling, and unloading  a  track-mounted  excavator. The truck-mounted excavator suffers from a lack of maneuverability  compared  to  the  track  mounting, because it requires a large area to turn around or to sidestep.  Additionally,  an  important  weakness  is  the  ease with which it can get stuck. Constant care must be exercised to keep away from soft ground during or after it rains. Also, tire damage can occur when working in garbage dumps or a rock quarry. Self-Propelled  Wheel  Mounted The self-propelled single-engine unit has a two- range transmission, enabling it to travel between 3 and 28 miles per hour. Maneuverability on the job is subject to the same limitations as the truck mounted, except the short  wheelbase,  and  in  some  models,  four-wheel steering allows it in tighter places. The  self-propelled  model  has  front  axle  oscillation lock levers. These levers are used to lock out the front axle from oscillating up or down, holding the axle rigid and level with the main chassis. The lock lever is used to help stabilize the excavator when working over the side. NOTE: When reading, make sure the oscillation lock levers are up in the oscillate position, allowing the axle freedom to oscillate up or down. The self-propelled model has a set of outriggers used to increase the stability of the unit. These outriggers are hydraulically actuated and are controlled from the cab and provide a much larger and more rigid base when the  revolving  unit  is  placed  in  the  working  position. When traveling, always check the travel route for weight, height, and width limits, make sure the boom and steering selector are placed in the travel position, and the swing brake is engaged. Do not travel with the 9-24 boom over the side of the excavator, and if traveling off of the road, do not travel faster than 5 miles per hour. NOTE: After 2 hours of highway travel or every 50 miles, whichever occurs first, stop the machine to let the tires cool for 1/2 hour. Heat damages the tires and can cause tire failure. ATTACHMENTS All  hydraulic  excavator  attachments  are  made  of three strong structural members, such as the boom, the dipper stick, and the bucket (fig. 9-58). The structural members are hinged to each other, and the boom is hinged to the revolving unit. Movement at each hinge is controlled  by  two-way  hydraulic  cylinders. Boom The boom is normally concave towards the ground that  allows  space  to  pull  the  bucket  closer  to  the excavator,  permits  deeper  digging  without  interference from the travel unit, and enables the operator to see past it more easily when it is raised. There are two holes for connecting the boom cylinder rod eye to the boom (fig. 9-59). The top hole is for maximum digging depth, and the bottom hole is for maximum dump height. Be sure to  read  the  operator’s  manual  for  instructions  on  the boom height-depth adjustment. The outer end of the boom is usually prolonged into a two-piece bracket, in which the dipper stick is held by a heavy hinge pin or pins. Dipper Stick The dipper stick is usually one-piece, but some models  may  hydraulically  extend  and  retract  by  a telescoping boom. The dipper stick hydraulic crowd cylinder is either connected on the top or on the bottom of the dipper, and the bucket and bucket dump arms are connected at the end. If  the  dipper  stick  hydraulic  crowd  cylinder  is mounted on the top, extending the cylinder forces the bucket in towards the machine, known as “crowding.” Retracting the cylinder forces the bucket outward, known  as  “extending.”  When  the  cylinder  is  mounted underneath the boom, retracting the cylinder crowds the dipper stick, and extending the cylinder extends the dipper  stick.