IMPULSE•G+/VG+ Series 4 Technical Manual
November 2022
Page 108
5.2.21 Snap Shaft Detection
Snap Shaft Detection is designed to detect a broken or loose coupling by monitoring for a speed deviation between
rotating shafts on a drive train. Ideally, mount one encoder on the motor, which drives the gearbox and one encoder
on the last rotating part of the drive train, usually near the drum if used for a hoist motion. It is required that a
second brake be mounted on the drum shaft. The encoders are wired into two separate PG-X3 encoder option
cards, with channel 1 (CN5-C) being the high speed shaft input and channel 2 (CN5-B) being the low speed shaft
input. The VFD will use channel 1 as the flux vector feedback. The software monitors and compares the speed of
both encoders. A gear ratio is entered into parameters C11-12 and C11-13. In an example, if the gearbox ratio is
46.34 : 1, then you would program C11-12 to 4634 and C11-13 to 100. If the difference in speed is greater than the
value in C11-10 for a period longer than the setting of C11-11, a “Snap Shaft” is displayed on the keypad display
and the VFD will perform the action at snap (C11-09).
Parameter U01-30 should be monitored during operation to obtain the exact speed difference in Hz between the
two shafts. The low speed shaft speed is normalized internally by multiplying the speed of the gear ratio. The value
of C11-12 should be adjusted at start up such that U01-30 approaches 0.0.
Table 5-37: Snap Shaft Parameter Settings
Parameter
Display
Function
Range
Default
C11-08
Snap Shaft
Determines if snap shaft detection is enabled.
0, 1
0
0 Disabled
1 Enabled
C11-09
Action at Snap
Action taken at detection.
A setting of 0 will set the brake and display a fault.
With a setting of 1, the VFD will continue to run.
0, 1
0
0 Brake/Fault Out
1 Alarm Only
C11-10
Delta Speed
Difference in speeds of the two shafts normalized
by the gear ratio.
0.0–E01-04 Hz
1.0
C11-11
Delay Time
Gear backlash time in milliseconds
0–2000 ms
250
C11-12
Gear Ratio Num
Gear ratio numerator
1–65535
10000
C11-13
Gear Ratio Den
Gear ratio denominator
1–65535
10000
