Two separate chains are used, one as a load chain and the other as the hand chain. The hand chain drives a pocketed chain wheel sheave that in turn drives the spur gears. The spur gears drive a single load sheave. One end of the load chain is attached to the swivel load hook. The chain then passes over the load sheave. The other end is attached to the hoist frame. A me- chanical brake, consisting of a ratchet and pawl or friction disk unit plate, is used to sustain the load. To lower, pull the operating chain in the reverse direc- tion,  slipping  the  friction  brake  or  releasing  the ratchet-pawl  device. The worm-geared hoist and the spur gear chain hoist operate in the same way except the gearing is reduced with the use of a worm gear drive. The lead (pitch) of the worm gear makes the hoist nonoverhaul- ing. Lever-Operated  Chain  Hoist One  of  the  widely  used  pieces  of  lifting  equip- ment is the ratchet hoist. In more common terms it is usually called a come-along (fig. 2-22). The ratchet hoist (come-along) has an operating handle similar to a ratchet wrench, hence its name. It is normally light in weight and comes in a variety of sizes, depending on the job to be done. A hoist has a ratchet and pawl or a friction disk brake incorporated in its mechanism to hold the load when the handle is released. Ratchet hoists are reversible so the load may be  raised  or  lowered.  Ratchet  chain  hoists  come equipped  with  the  load  chain  either  of  the  roller sprocket (bicycle) or link chain type. Load  chains  should  be  lubricated  and  should show no indication of binding. Do not exceed the load Figure  2-22.—Lever-operated  chain  hoist  (come-along). rating of the hoist or use extensions on the ratchet lever. Inspect load chains for wear, reduction in link bar diameter, and increase in unit lengths. Use the manufacturer’s data sheet or manual for reference. PIPE  PATCHES While not a tool in the normal sense of the word, pipe patches are an integral part of the ABF’s tool- room. There are miles of pipes in a fuel system, and damage to this piping can result from battle, accident, or merely the wear and tear of daily use. The ABF should know how to locate and repair piping system troubles. Repairs to piping are classified as permanent or temporary.  Permanent  repairs  are  made  when  the  time and the material are available. Temporary are made when  the  correct  material  is  not  available  and/or  the system  cannot  be  secured  for  the  time  needed. Temporary repairs are usually made by securing some  type  of  patch  over  the  damaged  section  of  pipe. The  material  used  for  the  patch  depends  upon  the  type of piping that is being repaired. A good general rule is to make the temporary patch from the same type of material  that  is  used  for  the  flange  gaskets  in  the system. Back up the patch with a piece of sheet metal, and secure the sheet metal to the pipe with bolted metal  clamps  or  similar  devices.  A  sealing  compound may be applied between the patch and the pipe to help seal  the  patched  area. Jubilee  pipe  patches  (fig.  2-23)  are  frequently used to stop leaks in piping. These patches may be obtained from standard stock or they may be fabri- cated on board ship. When making up a patch of this type, be sure to reinforce the flange so it will be strong enough to hold against the pressure of the system. The main disadvantage of the Jubilee patch is that assorted sizes must be stocked since each patch is manufac- tured to fit only one size of pipe. Temporary repairs to some piping systems also may be made by using plastic patching materials. The materials  required  for  plastic  patching  are  furnished in a special kit. Emergency  Damage  Control Metallic  Pipe  Repair  Kit Most water and fuel lines can be easily repaired and service restored to the system in as little time as 30 minutes by using the emergency damage control metallic pipe repair kit, often called a “plastic patch. ” 2-14