Page 237 - Improving Machinery Reliability

208 Improving Machinery Reliability

Making the generally accepted assumption that the alternating torsional stress will
not exceed 20% of the steady stress, we obtain

z, = (0.2) (10,900) = 2,180 psi

Bending Moments Assessed for Gear Coupling

There are three relevant bending moments" caused by a gear coupling when trans-
mitting torque with angular or parallel misalignment:

Moment caused by shift of contact point. This moment acts in the plane of angular
misalignment and tends to straighten the coupling. It can be expressed as

where T is the shaft torque, D, is the gear-coupling pitch diameter and X is the
length of the gear tooth face (see Figure 3-86).

Moment caused by coupling friction. This moment acts in a plane at a right angle
relative to the angular misalignment. It has the magnitude

where p is the coefficient of friction.
Moment caused by turning torque through a misalignment angle. It acts in the
same direction as the friction moment MF and can be expressed as

MT = T sin c1
The total moment is the vector sum of the individual moments:

A gear coupling suitable to transmit 19,600 HP at 6,400 rpm is assumed to have a pitch
diameter D, = 9 in and a face width of 1.3 in. Using a coefficient of friction = 0.3

- ..-. ____ __________ ~~
*An additional moment, due to overhung weight of coupling, is usually negligibly small and is omit-
ted from the bending moment analysis for both gear and diaphragm couplings.