5.4.2 Adjustment of Contact Mark and Backlash of Bevel Gear Pair
When impact or noise is generated due to excessive wear or abnormal contact mark in tooth surface during
the use of the spiral bevel gear pair, or when a new bevel gear pair is replaced, the contact mark and backlash
should be readjusted and checked regularly.
(1) Inspection of backlash
There are two ways to check the backlash: one is dial indicator measurement. When people are measuring,
place the contact of the dial indicator on the tooth surface of the big end of the big bevel gear, fix the small bevel
gear, and swing the big bevel gear according to the rotation direction. At this time, if the reading of the dial
indicator is (0.14~0.3) mm, the backlash is correct. Another method is to put a lead piece, with length of (15~20)
mm and thickness of 0.5 mm, or a fuse with "∽" shape between the non-gearing face of the gear pair, and turn the
gear pair, and then the measured thickness of the lead piece or fuse after being squeezed at the big end is the
normal backlash there. This value should be (0.1~0.25) mm. For accuracy, three points should be measured on the
same circumference of the gear to take the average value.
(2) Inspection of tooth surface contact mark
Contact mark of tooth surface is checked by staining. Before inspection, the big and small bevel gears should
be cleaned and dried, and a layer of even red lead oil should be coated on the tooth surfaces on both sides of the
big spiral bevel gear, and the gear pair should be rotated in the positive and negative directions, and then the mark
stuck on the tooth surface of the small bevel gear is contact mark. The ideal contact mark should be distributed
near the small end in the middle of the working tooth. The mark is allowed to be spotted, but the length is not less
than 60% of the tooth length and the height is not less than 50% of the tooth height.
See Table 5-2 for the adjustment methods of backlash and tooth surface contact mark.
During adjustment, the axial movement of large and small bevel gears will change the backlash and contact
mark. If the requirements for contact mark and backlash are contradictory, the contact mark should be ensured to
be correct. In the case the adjustment range of backlash can be appropriately enlarged, especially when gears and
bearings are worn, but the backlash should not be less than 0.1 mm.
In the normal use of tractor, as long as the contact mark is normal and only the backlash increases, no adjustment
is needed. However, when the tractor is overhauled or a pair of central transmission gears or bearings is replaced,
careful adjustment must be made to ensure the backlash and contact mark.
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