TM 5-3895-373-20 15.20.    ADJUST AND ALIGN MAIN AND EXTENSION SCREED PLATES -Continued B. ADJUST Continued. 2. INSTALL   CRIBBING   BENEATH   EXTENSION SCREEDS. Screed operations present a crushing hazard. Stay clear of screed during  screed  operations.  Failure  to do so may result in death or serious injury. NOTE Cribbing   is   used   under   the   screed only  as  a  safety  precaution.  Do  not lower  the  screed  onto  the  cribbing. Cribbing may have to be repositioned during alignment procedure  to  allow  for  placement  of straightedge and to view measurement points. a. Place    cribbing    beneath    extension    screeds. Leave   leading   edges   and   trailing   edges   of screed    plates    clear    of    cribbing    to    make measurements. b. Ensure  weight  of  screed  is  not  supported  by cribbing.    Cribbing  is  being  used  as  a  safety precaution only. 3. SET  MAIN  SCREED  CROWN  ADJUSTMENT TO NULL POSITION WITH SCREED RAISED. a. Lower  hinged  steps  on  main  screed  per  TM  5- 3895373-10. b. Remove thumbscrews (6) and crown adjustment wrench (7) from main screed. c. Use crown adjustment wrench (7) to loosen hex nut (8). NOTE The main screed plate has a positive lead  crown  that  cannot  be  nulled  by crown    adjustment.    Sized    spacers between main screed plate and main screed   frame   on   leading   edge   of main    screed    hold    the    crown    in position.    Only    main    screed    plate trailing edge crown can be nulled by crown adjustment. d. Determine   if   a   positive   or   negative   crown   is present  by  reading  crown  pointer  (9)  on  main screed. e. To null a positive crown on main screed trailing edge,  pull  crown  adjustment  wrench  (7)  down toward  screed  trailing  edge  until  crown  pointer reads  approximately  zero  and  no  resistance  is felt on wrench. f. To null a negative crown on main screed trailing edge,   push   crown   adjustment   wrench   (7)   up toward  screed  leading  edge  until  crown  pointer reads  approximately  zero  and  no  resistance  is felt on wrench. g. Place   straightedge   along   underside   of   main screed trailing edge. Have a second person turn crown   adjustment   until   trailing   edge   of   main screed plate is flush with straightedge. h. Tighten  hex  nut  (8)  on  crown  adjustment  with crown adjustment wrench (7). i. Replace  crown  adjustment  wrench  (7)  on  main screed and secure with thumbscrews (6). GO TO NEXT PAGE 15-138