Page 222 - Improving Machinery Reliability

Machirieiy Reliability Audits and Reviews 193

where

8 = pressure angle

Decrease in backlash due to thermal expansion:
A x2 = (2a)(AT) (C tan e)

where
a = temperature coefficient of expansion (6.5 x idin for steel)
AT = maximum anticipated temperature rise, OF
C = center distance, gear-to-pinion, in
‘Total decrease in backlash = A x, + A x2

Mesh Lubrication

Industry practices indicate that some flexibility can be conceded on nozzle place-
ment. As an example, for pitch-line velocities between 5,000 and 15,000 fpm, the oil
spray could be directed to either the incoming or outgoing side of the mesh. Howev-
er, the most reliable means of mesh lubrication would be to spray oil into the outgo-
ing mesh side because it would allow maximum cooling time for the gear set, and
would apply cooling oil at areas of highest temperature. Above 15,000 fpm, about
90% of the oil should be sprayed into the outgoing, and only about 10% into the
incoming mesh. This assures the application of cooling oil in the high-temperature
areas, and lubricating oil in the high-contact stress areas. As for total oil quantity
sprayed into the gear teeth, we could use one of two rules of thumb:
1. For every inch of face width, 5 gpm of flow would be required. This guideline
does not consider the effects of speed on horsepower ratings and heat loss, but
appears to be valid for a wide range of gears.
0.42 x gear HP lost
2. Oil flow, gpm =
1.4
Application of this formula should limit gear temperature rise to approximately
4(a°F, even on less-than-average-efficiency gears. While the highest flow value may
not necessarily be optimum flow for a given mesh, it should nevertheless govern the
sizing of the lube system.

Why “Property Rated” Gears Still Fail

The design of the gear teeth for desired strength and durability is generally given
close attention by both the purchaser and the manufacturer, and service factors are
applied according to AGMA 421.06 to take care of expected overload conditions.