Page 185 - The Tribology Handbook

Flexible couplings B4

COUPLING EFFECTS
Effect on critical speeds N,, = Critical speed
without m2
Since couplings are fitted at the end of machine shafts,
they constitute an overhung mass. Overhung masses Ncw = Critical speed
withmz added
reduce the lateral critical speed of rotors. If a machine is
operating near its critical speed the overhung mass of the
coupling needs to be considered. 1 .o
0.5 -
0.9
-
0.8
Ncw
Nc 0
0.7

0 0.05 0.10 0.15 0.20 0.25
-
L2
L1

Gear coupling effects
The axial Ioacls that may be applied by gear couplings to the coupled machines can be estimated from the tooth contact
forces generated from the torque transmission multiplied by the likely coefficient of friction. This will normally have a value
of about 0.15 but if the surface of the teeth becomes damaged could rise to 0.3.

C pattern
misalignment

Moments on the sleeve
balance out
F
--
Z pattern or
parallel offset
misalignment
Moments on rhe sleeve
add up.
Additional lateral forces F arise Rotor
bearing
spacing

Bearing load at coupling end of the machine
= - [*+ (L+2ai 1
Direction of The angle between the direction MR
relative offset of offset and the direction of the L b
of far end of ~~~~l~~~~ load is 8 where:
bearing
coupling bearing load 8 = 30"-45' typically MR = 0.16T for straight tooth couplings.
0.12T for barrelled tooth couplings.
Figure 4.7 Lateral bearing loads generated by gear couplings
B4.7