Page 221 - Improving Machinery Reliability

192 Improving Machinery Reliability

Table 3-22
Backlash Values for Solid or Forged-Steel Gears

Center Normal Diametral Pitch
Distance,
Inches 1 -2 2-3 3-4 4-8 8-1 2 12-20

to 6 midmax .010/.016 .008.014 .008.014 .006/.012
6-10 .014/.022 .0 12/.018 .010/.016 .010/.016 .008/.014
10-15 .0 161.024 .014/.022 .0 I2/.0 I8 .0 121.0 18
15-20 .0201.030 .018/.028 .016/.024 .014/.022 .012/.020
20-25 ,0221.032 .020/.030 .01 W.026 .0161.024 .014/.022
25-30 .0241.034 .022/.032 .020/.030 .018/.028 .016/.026
30-35 .026/.036 ,0241.034 ,0221.032 .020/.030
35-40 ,0261.036 .026/.036 .024/.034 .022/.032
Notes: 1. Iiiclrides gear- blank temperature 50°F above housing.
2. Inchides centrifugal growth up to 25,000 fpm.
3. Backlash is a minimrtrii of 1% times that required by (1) and (2).

When backlash figures do appear too tight, we can perform the following simpli-
fied calculations to verify the adequacy of a vendor’s backlash values.
Radial stresses in solid rotating discs, psi:

where
p = weight density, lblin3 (.280 for steel)
g = 386 inlsec2
p = Poisson’s ratio (0.30 for steel)
r, = outside radius of disc, in
r = variable radius at which stress is to be found
v = peripheral velocity, inlsec
From Hooke’s law, for uniaxial loading:

sr in
Strain E = -I- ; ___ - -
(L) 3Ox1O6 -in

Maximum growth in radial direction:

6 = (E) (center distance, in)

Decrease in backlash due to maximum growth in radial direction:
A x1 = (2) (6 tan e)