TM 9-243 SAFETY WARNING Never  get  under  a  load  that  is  only supported   by   a   jack.      Any   jack   is subject  to  failure  and  personal  injury could result. Safety  is  a  very  important  factor  in  the  use  of  jacks  and trestles   and   cannot   be   overemphasized.      Often   the nature    of    their    application    makes    them    inherently dangerous.    Rated  capacities  should  not  be  exceeded. Jacks are usually lifting, pulling, spreading, or supporting something  which  can  become  lethal  if  a  mishap  occurs. Most  accidents  occur  when  the  load  is  not  stabilized. These,   devices   are   designed   to   exert   force   in   one direction,(through  the  center  axis  of  the  portion  of  the jack which contacts the load.) All jacks should be inspected prior to use, to insure they are  safe  for  the  intended  application.    Check  for  rated capacity,   previous   wear   or   damage   and   manufacturer deficiencies. The following safety factors should be adhered to: 1. Keep fingers away from all moving parts. 2. When  jacking  up  vehicles,  make  certain  no  one  is under  the  vehicle  to  be  raised.    Set  the  hand  brake firmly  and  block  the  front  wheels  if  a  rear  wheel  is being   changed.      Block   the   rear   wheels   if   a   front wheel   is   being   changed.      Place   blocking   or   other supports  under  the  vehicle  when  it  is  raised  to  the desired height to prevent it from dropping if the jack fails. 3. Make  certain  that  hydraulic  jacks  are  filled  with  oil and that there are no visible oil leaks before using. 4. Any    new    or    repaired    jack    should    be    carefully inspected by the operator prior to use. 5. Overloading   can   be   hazardous   to   the   jack,   the operating  personnel,  and  the  load  in  event  of  jack failure. 6. Be aware of the capabilities of the jack, especially its load capacity. Special considerations for trestle safety: 1. Follow   the   instructions   in   the   respective   technical manuals regarding the size, placement, and methods of using trestles. 2. Insure  the  trestles  are  level,  standing  with  all  four legs  on  a  hard  surface,  and  that  the  pawl  is  fully seated,   both   before   and   after   applying   the   load. Insure the weight is distributed as intended. 3. Use trestles only in pairs. 4. Avoid  shaking  and  bumping  of  the  vehicle  or  load. ALWAYS   check   to   make   sure   all   trestle   legs   are firmly  resting  on  the  floor  when  the  work  area  has been unattended. 5. As   many   tires   as   possible   should   remain   on   the vehicle, and on the ground. 6. Tires remaining on the ground should be chocked to prevent  movement,  especially  when  loosening  any portion of the driveline. 7. Trestles  supporting  a  load  should  not  be  used  for leverage  while  attempting  to  pry  or  pivot  yourself  or a subassembly under a vehicle. 8. Trestles    should    be    extended    only    as    high    as necessary. 9. A secondary support method, such as wood blocks, a   floor   jack,   A-frame,   or   crane   should   be   used   if there    is    a    question    regarding    the    stability    of    a maintenance procedure.  DO NOT use a cut off 4x4, 6x6,  or  railroad  tie  that  is  standing  on  end.    This  is extremely    unstable    and    SERIOUS    INJURY    OR DEATH    may    result.        Wood    blocks    must    be    of sufficient  strength  to  support  the  load.    Do  not  use concrete blocks. Change 3  20-4.1/(20-4.2 blank)