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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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