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Figure 3-15.  Typical Worm and Worm Gear Mechanism
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TM-9-254 Gearcase Transfer M548 M548A1 (
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Backlash
TM 9-254 3-7. Worm and Worm Gear Mechanisms - Continued d. Fitting  Plunger  to  Housing.    The  plunger  must  fit  in  the  housing  without  any  side  play.    If  it  fits  loosely,  the wormshaft will have side play which will appear as backlash on the micrometer dial.  In order to facilitate handling of a new plunger when fitting, select a piece of brass or steel rod, a little larger in diameter than the spring hole in the plunger, and turn a slight taper on the end of it.  Then force the plunger tightly on the rod.  The rod must not turn within the plunger.  Lap the  plunger  in  the  housing  so  that  the  plunger  will  be  free  enough  to  move  smoothly  up  and  down  when  a  slight  finger pressure is exerted against it. e. Lapping Worm and Worm Gear Mechanism . (1) Before lapping is started, always clean the parts thoroughly and examine the teeth for nicks, burrs, and sharp  edges.    If  a  new  worm  is  to  be  installed,  place  the  worm  in  a  lathe  chuck  and  with  the  lathe running  at  slow  speed,  file  a  slight  radius  on  the  corners  of  the  worm  thread.    Whether  a  new  or  old worm is used it is always best to check the gear mesh before  lapping  is  started.    Wipe  a  fine  coat  of Prussian blue on the worm thread and assemble the instrument.  Turn the worm over the entire range of movement and then disassemble.  Check for high spots and bottoming.  Scrape off the high spots  If an old worm is bottoming in the worm gear, set the worm up in a lathe and turn off about 0.010 inch for the outside diameter of the worm thread.  Check to insure that the worm is not bottoming on the worm gear. (2) If  a  new  worm  bottoms  in  the  worm  gear,  then  the  worm  gear  teeth  are  worn  excessively  and  a  new worm gear should be installed.  When the bearing of a new worm in an old worm gear is checked, it may be found that the new worm is riding on the corners of the worm thread, because the old worm had a larger radius on the corners.  Do not increase the radius on the new worm.  Examine the corners of the worm gear teeth.  If there is a visible ridge, scrape it off with a three-cornered scraper. (3) The wormshaft must fit in the semicircular bearing surface of the plunger without any side play.  If side play  is  present,  this  will  appear  on  the  handwheel  as  backlash.    Before  lapping  in,  relieve  the  center portion of the semicircular bearing as shown in figure 3-16.  This is done so that the wormshaft will wear itself in deeper, reducing the possibility of developing side play. (4) Apply  a  thin  coat  of  fine  lapping  compound  to  the  worm,  gear,  seat,  socket,  and  plunger’s  bearing surface.  Mount the wormshaft mechanism in the housing and adjust ring so that there will be no end play  in  the  shaft.    When  lapping  worm  and  worm  gear  mechanism,  better  results  will  be  obtained  by removing the spring behind the plunger and installing a solid plug.  In this manner, any minute high spot which  might  be  rolled  over  will  be  removed,  increasing  the  perfection  of  the  fit.    Lap  in  by  turning  the wormshaft  a  few  times  in  each  direction.    Remove  wormshaft  mechanism  from  housing  and  inspect mating surfaces between shaft and plunger; between worm and  gear;  between  seat,  ball,  and  socket. The lapping process is complete when fine abrasive marks appear across the entire mating surfaces of the items being lapped together. 3-14

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