BRUEL & KJAER 2976 User manual

REVISION(S)
2976 Instruction Manual {BE 0654-12)
A minor revision based on the earlier Phase Indicator Instruction Manual (033-0654)
to comply with existing equipment. There are no changes which affect the use of
the instrument.
Dept.3, October 1984
:----------:: -.; :--~=---.- ------- -. ·n-s Q"1; 11n ;, Q · 1p ·Q r a - u11 ·9 -.-n:::;
Ch, 1 Input
Q Measure
Ch. 2 Input
Cl>
ning in their own bearings can be balanced
with only one trial
correction per plane.
+ BrUel & Kjaer

InstructionManual
Phase Indicator Type 2976
I
Channel
Selector
Ch, 1
'
Ch, 1 Input
Q
Phase Indicator
Type 2976
Trigger
Level
Battery
Power
On
Off
Set Filter
• Measure
Ch. 2 Input
Q
An auxiliary instrument for turning a serial
vibration analyzer into a highly effective
portable field balancing set. A tachometer
input (with non-contact photoelectric probe
supplied) provides a phase angle reference.
A liquid crystal
display reads out the mea-
sured phase with a resolution
of 1°.
Change-over switches are provided for se-
lection
of each of two vibration transducers
(for dynamic balancing), and for accurate
tuning
of the analyzer to the rotation fre-
quency. With the aid
of pocket-calculator
software available from B& K and a balanc-
ing set
incorporating the 2976, rotors run-
ning in their own bearings can be balanced
with only one trial
correction per plane.
+ Bri.iel & Kj~r

PHASE INDICATOR
TYPE 2976
October 1984

CONTENTS
1. INTRODUCTION AND SPECIFICATIONS (PRODUCT DATA).................................................. 1
2. CONTROLS...................................................................... ..... .............. ... ............ . .................. 3
2.1. FRONT PANEL. ....... . ...... ....... .. ................ ...... ....... . ................ .. ...... ..... .. .... 3
2.
2. REAR PANEL. .... ......... . ............ . .. . ...... . .. . .................................. .. .......... .... 5
2.3 . INTERNAL .. ........ . ........ ...... ......... ........ . .. . ......... ......... .. .. ........ ....... .... .. ...... 6
3. OPERATION............................................................. . ............................................................ 7
3. 1.
3.2.
3.3.
3.4.
3.5.
PRELIMINARY ...................... . ......... . ........ . ......... ... ................. . ............ .. ....
Environment ........ .......... . ....... .... .. ..... .............. . ................................ ........
Battery ... ...... ..... .. ... ..... ...... ..................................................................... .
Mounting . ....... ... ......................... ........................................................... .
SELECTION
OF OTHER INSTRUMENTS ..... .................. ........ .. .... ..... ......... . .. .
7
7
7
8
9
Vibration Meter....... ................ .. . ......... . .. ..... ................ ... .. . .......... . ............ 9
Tunable Filter ..... ......................... .. ....... .......... ........... .......... ........ . ....... .... 10
Accelerometers ... .. ...... ........... ....... .. . .................................................... . .. .
PREPARATION
FOR BALANCING ........... . ................ . ........ .. ......... .. .. .. .........
Connection of Instruments .... .. ..... ... ..... .. . ......... .... ...... ................ . ............. .
Vibration Analyzer ...... ...... .... ....... ...... .......... ............ ....... ........... ..... ......... .
Tachometer Probe ........ . .. ....... ..... .. ........... ........ .. . ...... .......... .. ....... .. ...... ....
10
10
10
11
12
Use of the "Set Filter" Facility on the 2976 ........ . ................................ . ...... 14
Selection of Trial Masses .......... . ............................................. .. ................ 15
BALANCING PROCEDURE ........................................................................... 17
DIFFICULTIES IN ACHIEVING ACCEPTABLE BALANCE ................................ 18
Interfering Vibrations .. ..... ....... ............ .................................. . ...... .. ......... ..
Speed
Variations . .......... ....................... . ....... . ............. . ....... ... ............... ....
Phase
Instability ........... ....... .................. .. ........ . ...................................... ..
Nonlinearity ...... .. . ......... .......... .. .. ...... .... ..... ............ ........... ...................... .
18
19
19
19
4. SERVICE AND REPAIR ......................................................................................................... 21

USES:
• Dynamic field balancing of
rotating machinery
(together with a vibration
analyzer)
FEATURES:
• Liquid crystal display of
vibration phase
• Photoelectric Tachometer
Probe MM 0012 included
The Type 2976 is designed to be
used
with vibration-measuring sys-
tems for static (single-plane) and dy-
namic (two-plane) field balancing of
rotating machinery . It provides an
immediate display of the phase be-
tween each measured bearing vibra -
tion signal and a reference signal
from a tachometer probe . By calcula-
tion using this phase indication and
the measured bearing vibration le-
vel,
the required correction
mass(es) and position(s) can be de-
termined with a high degree of accu-
racy. The Phase Indicator includes
facilities for easy and correct tuning
of the filter normally used in a bal-
ancing system . It forms part of Field
Balancing Set Type 3517, and can
be
fitted into a Portable Vibration
Analyzer Type 3513 . A second Ac-
celerometer (included with Type
3517) would then be required for
two-plane balancing.
The 2976 is equipped with a liq-
uid crystal display which indicates
either the vibration phase angle (in
degrees),
or an E for unsatisfactory
tachometer triggering . A trigger
lamp is provided which flashes
every ti me the tachometer probe is
triggered, to facilitate setting up the
angle datum mark on the rotor .
2-044 0249-2A
BP0249
• Built-in rechargeable NiCd
cells
• Accelerometer Change-
over switch for dynamic
balancing
• Set Filter /Measure switch
for accurate tuning of
vibration analyzer to
rotation frequency using
tacho signal
The Tachometer Probe MM 001 2
supplied contains an infra-red light
source and is triggered by a con-
trast mark on the rotor . It is pow-
ered from the 2976 through its con-
necting cable . If a magnetic probe is
required, this can be obtained separ-
ately . Full details of both the
MM 0012 and the MM 0002 Mag-
netic Probe are given in separate
Product Data sheets . The 2976
may also be electrically triggered
from a suitable tacho source.
A functional diagram of a vibra-
tion analyzer adapted for balancing
by the addition of the 2976 is
shown in Fig.1. The tunable band-
pass filter incorporated in the anal-
yzer removes unwanted vibration
components, for accurate phase
and amplitude measurement of the
component at the rotation fre-
quency .
As a signal can undergo a phase
shift when it is passed through a
band-pass filter, it is very important
to keep this filter accurately tuned
to the rotation frequency, even if
this drifts between measurements.
The 2976 therefore incorporates a
Set Filter / Measure switch, which
in the Set Filter position supplies a
type 2976
Phase Indicator
Channel
Selector
Ch.1
Ch.1 Input
Q
Phase lndQtor
Type 2976
18 -1
constant-amp! itude signal at the ro-
tation frequency to the filter and vi-
bration meter, in place of the vibra-
tion signal . The filter can then be
tuned for 1 80° phase indication,
signifying precise filter tuning .
The phase-locked loop used to der-
ive the constant-amplitude signal
from the tacho signal also provides
a convenient way to generate 1° ti-
ming pulses (at 360 times the rota-
tion frequency).
A change-over switch is incorpor-
ated for the connection of two vibra-
tion transducers used for dynamic
(two-plane) balancing .
The 2976 includes its own built-
in NiCd batteries, and is provided
with two 7-pin DIN DC power sock-
ets of the type found on most B & K
portable instruments, enabling it to
share a battery charger (such as
ZG 011 3) with one of the other in-
struments in the balancing set.

-
-
PHASE INDICATOR 2976
"Ch . 1
a Input"
"To r-- --- ---- - -- --- - --- 7
I Conditioning Preamplifier section I
-"Channel Preamplifier j
~
2 Piezoelectric "
Ch . 2
Selec t or"
Input"
"Input"
I
-I
Accelerometers a
Input"
-.
"To
I
-
Charge
I
--Ra
nge
Ext . -Integration Filters H- L+--
. I
Preamplifier--
Selection -
"From Preamplifier Output
" Measure"_
Filter
"To
-I I
-
~
Input "
Ext .
I I
Filte
-~- -
-- ------- -- - - ---- J
lnpu
r
t"
acho
"T
lnp
(e
f
MM
ut"
.g.,
or
0012)
9k
"Fr
ACO
(Filte
om
utput
re~)"
-
--
~
. -
r -------- -- - - - -
-- -7
"Set Filter "
Tachometer ""1..Jl_J1_
I frequency I
I Frequency I •,
I -divider
~
I
~
-+ 360
I
Phase -locked loop
I
Phase
I I
-+
Ref.
Input
I I
vco -10 clock
I
..
I -
Phase
-Integrator (360 X
Phase
I detector -ta
ch o -I
-+
measuring
-freq.) I
Input
I -
I I
I
IH
L _ _ _ ______________ j
-
TUNABLE
BAND-
PASS
FILTER
f\
L+-- -
-
" Fi lter
" Fi lter
Input "
Output"
E.G ., TYPE 1621
.;;-
VIBRATION METER,
E.G., TYPE 2511
"
To
Ext .
Filter
Output"
Meter 0
circuit
"Reco
Outpu
(AC)
-+-
l'
rder
t"
801 000
Fig. 1. System diagram of a field balancing set incorporating the Type 2976 Phase Indicator
Tacho Input:
Connector: 3-pole BNT socket, accepting
plug JP 0315 or standard BNC plug
JP 0035
Supply for MM 0012: +6 V DC approx.
Trigger level: ±1 V, front-panel adjus-
table,
with preset position at +80 mV
Trigger range: 60 mV to 50 V peak-peak
(50 V max. peak). Internally -selectable di-
vide-by-1
00 attenuator for use with
large- amplitude electrical trigger signals
Input impedance: 1 00 kO
Trigger lamp: Lights when satisfactory
triggering is obtained from Tacho input
Channel 1 and 2 Inputs:
Connector: Mi
niature 10 - 32 UNF screw -
locking coaxial sockets, accepting plug
JP 0012 , for connection to vibration pick-
ups.
Conditioned Vibration Signal Input (From
Preamplifier Output):
Connector: Standard BNC socket accept-
ing
plug JP 0035
Function: Connected to To Ext. Filter In-
put when Measure is selected
Phase-Measuring Input (From AC Output,
Filtered):
Connector: Standard BNC socket accept-
ing
plug JP 0035
Input range: 25 mV to 3 V RMS
Input impedance: 1 MO
Rotation Frequency Range:
2 Hz to 1 kHz (120 to 60 000 r/ min)
Output to Filter (To Ext. Filter Input):
Connector: Standard BNC socket accept-
ing
plug JP 0035
Specifications 2976
Set Filter signal: 0, 222 V Pk - Pk square
w13ve at rotation frequency (fundamental
component 0,1 V RMS approx.), output
impedance 1 kO
Measure signal: Same as signal at From
Preamplifier Output socket
Selection: By front -panel switch
Selected Channel Output (To Preamplifier
Input):
Connector: Standard
BNC socket accept-
ing plug JP
0035
Function: Carries low -level vibration sig-
nal
from the pick-up selected by the front -
panel
Channel Selector switch to the in-
put of the vibration analyzer
Phase Display:
Measurement range: 0° to
360 °
Resolution: 1°
Accuracy: ±2°
Type: 4- character 1/ 2 in ( 12 ,7 mm) liq-
uid crystal display
Error indication: Shows E for error if Ta -
cho signal is aperiodic, or i
nsufficiently
stable
Power Supply:
Source: I
nternal 6 V rechargeable NiCd
battery
Operating-time: From one charge, ap-
prox.
10 hours operation with MM 0012
connected, 25 hours without
Battery life: 1000 standard charge /dis-
charge cycles
minimum
Charging-connectors: Two standard 7-
pin DIN sockets
wired in parallel and ac-
cepting plug
JP 0703 , Battery Charger
ZG 0113 or Power Cable AO 0042
Charge ratings: Constant charging cur -
rent 36 mA at voltages from +7 to + 20 V
DC
Charging-time: 20 hours approximate ly
from complete discharg e
Electromagnetic Compatibility: Complies
with U.S. F CC requirements for Class B
Computing Device
Dimensions:
Height:
132 ,6 mm (5. 22 in)
Width: 69 ,5 mm (2, 74 in)
Depth: 200 mm (7, 87 in)
Weight:
970g
Accessories Supplied:
0,6 m 7-pin DIN Power Cable AO 0042
Photoelectric Tachometer
Probe MM 0012
3- conductor coaxial BNT cable
(3 m long)
for MM 0012 AO 0158
2 BNC signal cables (0,6 m long)
Low -noise BNC signal cable
(0,6 m long)
6 V Ni
Cd battery
2 spare 200 mA fuses
Accessories Available:
Magnetic Tr ansducer
Battery Charger
Piezoelectric
Accelerom eter
AO 0133
AO 0197
QB 0024
VF 0012
MM 0002
ZG 0113
Type 4370
Magnetic card- programmes for executing
static (1-plane) and dynam ic (2-plane)
balancing
calculations on Hewlett -Pack-
ard and Texas
Instruments program -
mable pocket calculators :
2-plane, HP-
67 and HP- 97
.2 -plane, Tl - 59
1- plane , Tl - 59
1- plane , HP - 67 and HP- 97
1- plane or 2- plane , HP-41
WW9002
WW9007
WW9009
WW9010
WW9021

2.1. FRONT PANEL
BATTERY:
CH. 1 INPUT:
CH . 2 INPUT:
CHANNEL SELECTOR:
PHASE:
2. CONTROLS
Phase Indicator
Type 2976
Phase
I
IS 1)
Trigger
level
Channel
Selector
Power
Ch . 1
On
·:.
•+1V Off
Norm
Tacho Input Battery
Set Filter
® •
..
Measure
Ch. 1 Input Ch. 2 Input
~
~
Q
Fig.2.1 . Type 2976 front panel controls
Moving-coil indicator whose pointer leans to the right (thus: /) when
the instrument is switched on provided the battery voltage is sufficient
for correct operation.
Pair of coaxial sockets accepting miniature screw-locking plugs
JP 0012 and accelerometer cables such as AO 0038 and AO 0122,
for connection of two vibration transducers. Connects to the change-
ov·er contacts of the CHANNEL SELECTOR.
Two-way paddle switch for selecting CH. 1 INPUT or CH. 2 INPUT. The
selected
channel is fed to the rear panel socket labelled TO PREAMPLI-
FIER INPUT.
Four-character liquid crystal display (LCD) indicating phase. The first
character will display "E" (for "Error") if the voltage at TACHO INPUT
is
insufficient, absent, excessively phase-modulated, non-periodic or
outside the frequency range of the instrument. The entire PHASE dis-
play will be blank if the TACHO INPUT voltage is correct and the input
voltage at the rear-panel socket labelled FROM AC OUTPUT (FILTERED)
3

V
V
Tacho
Input
+ f ~ From AC Output (Filtered) ~
0 _ 7\. (Pha5E: · measur ing ~
't"' '-./ input) A '-./
V+ 0.22
0
2 f-1
1
To Ext . Filter Input _J
("Set Filter")
_1
POWER:
SET FILTER/MEASURE:
TACHOINPUT:
TRIGGER LEVEL:
4
0° 90° 1 80 ° 270° 360°
t
Indicated
phase
(bl 1 Slope 2 set to " 2"
Phase
0° 90° 180° 270° 360°
t
Indicated
phase
(a) 1 Slope 2 set to " 1"
801088 / 1
Fig.2 .2. Diagram of 2976 relative phases
is less than 25 mV RMS. The remaining characters indicate phase
angle, from 0° to 360°, with 1° resolution and ±2° tolerance . The
angle displayed is the phase of the first positive-going zero-crossing at
the phase-measuring input during each cycle of the TACHO INPUT fre-
quency. See Fig.2.2.
Two-position paddle switch for switching the instrument On or Off.
Two-position paddle switch for setting the centre - frequency of the tun-
able band-pass filter and checking the speed stability of the rotor to be
balanced.
"Set Filter": A square wave of 0,222 V peak-peak (fundamental com -
ponent 0, 1 V RMS, source impedance 1 kO), at the TACHO INPUT fre -
quency, is fed to the rear-panel socket labelled TO EXT. FILTER INPUT.
When the filter centre-frequency is equal to the frequency of the TA-
CHO INPUT signal, this will result in a PHASE indication of 180°, and
the indication on the vibration meter will be maximum.
Measure: The signal at the rear-panel socket labelled FROM PREAM-
PLIFIER OUTPUT is fed
to the TO EXT. FILTER socket. This is the nor -
mal position of the switch for unbalance measurements .
Three-pole BNT socket, for connection of Photoelectric Tachometer
Probe MM 0012 or alternative tacho source. Accepts plug JP 0315,
coaxial cable AO 0158 (for MM 0012), or standard BNC plug JP 0035.
The innermost conductor (tip) carries the tacho signal . The intermedi-
ate conductor (ring) carries +6 V DC (approx.) from the internal battery
to power the MM 0012 (maximum current 80 mA). Tacho trigger
range 60 mV to 50 V peak-peak (50 V max . peak). Trigger level ±1 V,
front-panel adjustable , with preset position at +80 mV . Input imped-
ance 100 kO.
Knob for setting the positive-going voltage level on the TACHO INPUT
waveform, which is defined as 0° for the PHASE display. Continuously
adjustable between -1 V and +1 V. Click-stop "Norm" position (fully
clockwise) sets trigger level to +80 mV. The adjacent yellow lamp
lights when satisfactory triggering is achieved . On delivery the TACHO

2.2. REAR PANEL
EXTERNAL POWER SUPPLY:
FROM AC OUTPUT
(FILTEAED):
FROM PREAMPLIFIER OUTPUT:
TO EXT. FILTER INPUT:
INPUT is
AC-coupled, but it may be altered to DC-coupling by means of
the internal AC/DC switch.
External Power Supply
.iv.iv
~ Serial no .
Madei1Den11ark 8 22460
Fig.2.3. Type 2976 rear panel controls
Pair of 7-pin sockets, wired in parallel, accepting standard DIN plugs
JP 0703. When the 2976 is installed in a Set (Type 351 3 or 351 7),
one socket is connected to Battery Charger ZG O 11 3 and the other to
the EXT. POWER SUPPLY socket on Tunable Band Pass Filter Type
1
621 via a short 7-pin DIN cable AO 0042. The connections for both
sockets
are indicated on the panel. Charging current 36 mA at be-
tween+ 7 and +20 V DC.
Standard BNC socket accepting plug JP 0035, for feeding in the signal
whose phase angle is to be indicated on the PHASE display. Is nor-
mally connected to the AC recorder output of the vibration measuring
instrument. Voltage range 25 mV to 3 V RMS. Input impedance 1 MO.
The phase angle indicated is the first positive-going zero-crossing in
each cycle of the TACHO INPUT frequency. See Fig. 2.2.
Standard BNC socket accepting plug JP 0035, for feeding in the con-
ditioned vibration signal. Is normally connected to the external filter in-
put socket on the vibration meter. When the SET FILTER/MEASURE
switch is set to Measure, the signal is passed directly to the socket on
the 2976 labelled TO EXT. FILTER INPUT.
Standard BNC socket, accepting plug JP 0035. Carries the signal se-
lected by
the front-panel SET FILTER/MEASURE switch. Is normally
5

810444
Fig.2.4. Type 2976 internal switches
connected to FILTER INPUT on the rear panel of the Type 1 621 Tun-
able Band Pass Filter.
TO PREAMPLIFIER INPUT:
Standard BNC socket, accepting plug JP 0035, carrying the signal se-
lected
by the front-panel CHANNEL SELECTOR switch. Is normally
connected to the transducer input of the Type 2511 Vibration Meter
via a low noise BNC cable AO O 197.
2.3. INTERNAL
1 SLOPE 2:
ATT/DIR:
AC/
DC:
6
Three miniature slide switches are mounted at the bottom of the right-hand circuit-panel
inside the 2976. Access is gained to them by turning the large screw at the centre of
the bottom of the front panel about half-a-turn anticlockwise, and sliding the bottom
panel off towards the rear. When replacing the bottom panel, turn the thumbscrew fully
anticlockwise and check that the panel is locked in position.
Determines whether 0° phase corresponds to the negative-going or
positive-going edges of the TACHO INPUT signal. The instrument is de-
livered with the switch set to 2.
1: Negative-going edge (black-to-white transition with MM 0012) .
2: Positive-going edge (white-to-black transition with MM 0012).
Setting this switch in the Att (1) position inserts a 40 dB (divide-by-
100) attenuator in the TACHO INPUT, for use with signals which may
exceed 50 V peak. The instrument is delivered set to position Dir (2),
the direct position.
In the "AC" (1) position (as delivered), the TACHO INPUT signal is AC-
coupled. In the "DC" (2) position it is DC-coupled.

3. OPERATION
3.1. PRELIMINARY
3.1.1. Environment
3.1.2. Battery
Phase Indicator Type 2976 may be used over a wide temperature range extending from
-10° to +55° C (+14° to +131 ° F). Operation will not be affected by relative humidity le-
vels between 0 and 90 %, provided there is no condensation.
The 2976 is supplied fitted with a rechargeable NiCd battery pack OB 0024 of nominal
voltage 6 V. The procedure for installing a battery in the Phase Indicator is as follows:
1. Unscrew the rear-panel screw adjacent to the FROM AC OUTPUT (FIL TE RED) socket,
and slide off the right-hand side panel.
2. Unscrew the two screws at the top edge of the printed circuit panel, and hinge the
panel down.
3. Slacken the single screw through the battery clip (located in the top of the cabinet)
sufficiently to enable the battery to be slid in with its smaller terminal to the rear,
and the brown plastic insulation-strip wrapped round it.
4. With the battery in place, tighten the screw and connect the two battery leads to
their respective battery terminals.
5. Secure the printed circuit panel back in place with its two screws, replace the side
panel and secure that with its single screw .
The battery supplied has a life of more than 1000 charge/discharge cycles . One charge
is sufficient to power the instrument for approximately 10 hours with a Photoelectric Ta-
chometer Probe MM 001 2 connected, and 25 hours with a passive probe or an electri-
cal TACHOMETER INPUT signal. When POWER is set to "On", the pointer of the BAT-
TERY indicator will lean to the right if the battery voltage is sufficient.
The charging current required by the battery is 36 mA DC into pin 3 of either of the EX-
TERNAL POWER SUPPLY sockets on the rear panel . The charging current will be inde-
pendent of voltage for any applied voltage between +7 and +20 V DC. Charging from full
discharge takes approximately 20 hours. Normally, however, the instrument is not likely
to be switched on for longer than a working day of 8 hours, and it can then be fully re-
charged overnight .
7

3.1.3. Mounting
8
Fig. 3. 1. Battery Charger ZG O 113
The charging current is best supplied by Battery Charger ZG 0113 (Fig.3 .1 ). When the
2976 is used together with Tunable Band Pass Filter Type 1621 (as in the Portable Vi-
bration Analyzer Type 3513, for example), and rechargeable batteries are installed, it
can share a ZG 0113 with the Filter and have its batteries recharged at the same time.
The ZG O11 3 is plugged into one of the EXTERNAL POWER SUPPLY sockets on the
2976, and a 7-pin DIN cable AO 0042 connected between the remaining socket and
EXT. POWER on the rear panel of the 1621. This arrangement may extend the charging
time of the Filter batteries very slightly, but not by more than 10 %.
To gain access to the fuse installed in the Phase Indicator section, take off either side
panel as for installing a battery (step 1 above). The fuse is a slow-blow 200 mA type,
(B & K no. VF 0012), and is located at the top rear of the instrument.
Note: Make sure that only fuses with the required rated current and of the specified
type are used for replacement. The use of mended fuses and the short circuiting of fuse-
holders is prohibited.
The 2976 is designed to fit into the B & K Module System, which encompasses perma-
nent instrumentation racks and the carrying case KA 2000 (used to house the Portable
Vibration Analyzer Type 3513, Fig.3.2).
Fig. 3.2. Installing Type 2976 in Portable Vibration Analyzer Type
3513
To mount the 2976 in the space at the right-hand end of the Type 3513, proceed as fol-
lows :

1. Turn about half-a-turn anticlockwise the large screw at the bottom centre of the
front - panel of the 2976 .
2. Slide the bottom panel off towards the rear and unscrew the four rubber feet from
the bottom panel .
3.
Turn about half-a-turn anticlockwise the large screw at the bottom centre of the
front-panel of the Tunable Band Pass Filter Type 1 621 .
4 .
Slide the 1621 out as far as its rear-panel connections will allow (without discon-
necting them) .
5. Repeat steps 3 and 4 for the Vibration Meter Type 2511 .
6.
Using a short-handle screwdriver, unscrew the accelerometer box holder DZ 9302.
7. Fasten the bottom panel of the 2976 in the space next to the bottom panel of the
1 621 and facing the same way, using four screws YV 3322.
8. Disconnect the Battery Charger ZG 0113 from the EXT. POWER socket of the 1621
and connect it to either of the EXTERNAL POWER SUPPLY sockets at the rear of the
2976.
9. Connect the remaining EXTERNAL POWER SUPPLY socket to EXT. POWER on the
1621 using the short 7-pin DIN cable AQ 0042 supplied.
10 . Connect TO PREAMPLIFIER INPUT on the 2976 to INPUT on the rear panel of the
2511 using a low noise cable AO 0193.
11 . Make the following interconnections to the 2976 rear-panel sockets indicated, using
cables AO 0133 :
FROM PREAMPLIFIER OUTPUT:
TO EXT. FILTER INPUT:
FROM
AC OUTPUT (FILTERED):
TO EXT. FILTER INPUT
(2511)
FILTER INPUT ( 1621)
RECORDER OUTPUT (2511 ), set to "AC"
12. Slide each of the three instruments on to their bottom panels in the carrying-case,
locking each one by turning the large screw clockwise, and checking that it has en-
gaged properly.
3.2. SELECTION OF OTHER INSTRUMENTS
The 2976 is a specialized instrument . The vibration measuring instruments with which
it is used are of more general application. Do not attempt to use the 2976 to balance
machinery without first obtaining adequate familiarity with the uses of the other instru-
ments and the directions for operation given in their instruction manuals.
3.2.1. Vibration Meter
The 2976 is normally used with . Vibration Meter Type 2511.
9

3.2.2. Tunable Filter
The normal filter for use with the 2976 is the Tunable Band Pass Filter Type 1 621, and
this is usually the best choice because it can be tuned continuously. It is therefore al-
ways possible to tune it for zero phase shift at the rotation frequency, unless the latter is
unsteady .
When used together the 2511 and the 1 621 constitute a vibration analyzer, and the
combination will be referred to as such from time to time in this Manual.
3.2.3. Accelerometers
For two-plane (dynamic) balancing, vibration measurements are made in two measuring
planes. Two vibration transducers are mounted on the housings of the bearings in which
the rotor to be balanced is rotating. Normally two Piezoelectric Accelerometers should be
used.
Strictly speaking, the choice of Accelerometer is not critical since the rotor can be bal-
anced with uncalibrated transducers of different sensitivities . This may be acceptable in
cases
where the only requirement is to balance the rotor as well as possible. Normally,
however, it is required to know the residual unbalance after the rotor has been bal-
anced . This can be determined only by measuring the vibration at the bearings, using cal-
ibrated instruments. It is therefore most convenient to use two Uni-Gain® Accelerome-
ters, both of the same type, such as Types 4370 (Fig.3.3), 4384 or 4371 .
For
single-plane (static) balancing, only one Accelerometer is needed. This should be con-
nected to CH . 1 INPUT.
Fig .3.3. Accelerometer Type 43 70
3.3. PREPARATION FOR BALANCING
3.3.1. Connection of Instruments
10
Unless the 2976 has been installed in a Portable Vibration Analyzer Type 351 3, as de-
scribed in section 3.1 .2, it should be connected up to the other instruments to be used

according to Fig.3.4. The interconnection details are listed in Table 3.1. There are seven
interconnections to be made.
Phase Indicator
V~ration To External
From Preamplifier 2976 To Preamplifier Tunable
Band Pass Filter 1621
Input eter Filter Input
Output Input
r.i·· '------~0 ~, . ,- 1==~:;.__ ____ _.
From AC Output
(Filtered)
Cable
'"----------------=1~- 1,-------_.._------d•ia .. · ... .. !. 1--------------1
Filter_.,,,,. · • ~ """-.Filter To External.,..,.... ., . · ,·- '-Recorder
0
0
Measuring
Plane
1
AO 0122
AO 0158
Input
Output
Photoelectric Tachometer
Probe MM 0012
Rotor
to be
balanced
Filter Output Output (AC)
0
0
Measuring
Plane
2
Fig.3.4. Field balancing system incorporating 2976
Accelerometer 1 Accelerometer 2 Phase Indicator 2976 Vibratior, Meter 2511 Filter 1621
801087
AO 0038 or AO 0122
Output
CH. 1 INPUT
AO 0038 or AO 0122
Output
CH . 2INPUT
AO 0197 (low noise) TO
PREAMPLIFIER INPUT
INPUT
AO 0133 FROM PREAMPLI- TO EXT. Fl LTER
FIER OUTPUT
INPUT
AO 0133 TO EXT. Fl LTER
FILTER INPUT
INPUT
AO 0133 FROM AC OUTPUT RECORDER
OUTPUT
(FILTERED)
set to" AC"
AO 0133
TO EXT. Fl LTER
FILTER OUTPUT
OUTPUT
810005 / 1
Table 3. 1. Interconnection details for setting up a balancing set incorporating the 29 76
Before the system can be operated, an eighth interconnection must be made. This goes
to the TACHO INPUT on the front panel of the 2976. Normally it is made to the Photoe-
lectric Tachometer Probe MM 0012 using 3 m cable AO 0158 (both supplied). However,
the Magnetic Probe MM 0002 may be used instead. If an electrical tacho source is avail-
able from the rotor to be balanced, this will usually found to be most convenient. Details
of electrical requirements are given in Chapter 2. Internal preset switches which enable
these to be modified are described in section 2. 3.
On the Vibration Meter, the EXT. FILTER switch should be set to "In".
3.3.2. Vibration Analyzer
Calibrate the Vibration Analyzer in accordance with the directions given in its instruction
manual(s). Set it to measure RMS vibration displacement.
11

Mount the two Piezoelectric Accelerometers on the bearing housings of the machine to
be balanced. One Accelerometer should be located as close as possible to each of the
two bearings. To obtain the maximum vibration signal, the axis of each accelerometer
should be radial to the rotor axis, and should lie parallel to the direction of minimum
bearing stiffness (see Fig.3.5). If the rotor is to be balanced in only one plane, only one
Accelerometer need be mounted, and often the choice of bearing is not critical. Choose
the bearing closer to the centre of mass of the rotor. Refer to the booklet "Measuring Vi-
bration", or the Instruction Manual for the Vibration Analyzer, for guidance on Acceler-
ometer mounting methods.
Right
0
--- Bearing support
Rotor
axis
790031
Fig.3 .5 . Diagram illustrating right and wrong Accelerometer-moun-
ting positions for field balancing
3.3.3. Tachometer Probe
12
Select a convenient position on the rotor to make a contrast mark for triggering the Pho-
toelectric Tachometer Probe MM 0012 . The MM 0012 functions by projecting a beam of
infra-red light at the rotor surface, and generating a voltage related to the proportion ref-
lected back. The 2976 is triggered by a periodic change in this voltage, caused by a con-
trasting mark on the rotor passing through the beam .
To
make the mark, first paint a band right round the rotor circumference, in the plane
where the mark is to be, using matt black paint. Place a 10 mm (0,4 in) square piece of
clean, reflective adhesive tape with sharp, clean-cut edges, across the band . The
MM 0012 incorporates an infra-red filter to minimize the effects of visible light, particu-
larly artificial light containing mains-frequency flicker. However a further improvement
in immunity, where necessary, can be obtained by the use of a contrast mark made from
a patch of retroreflective tape such as Scotch lite 52 70 . Another trick for triggering in dif-
ficult situations is to illuminate the surface of the rotor under the Probe with light from a
torch or battery-powered hand-lamp.
The Photoelectric Tachometer Probe MM 001 2 should be treated with care, and its infra-
red window (at the tapered end) kept clean. It must on no account be permitted to con-
tact the rotor while the latter is in motion. If it is difficult to prevent the Probe becoming
contaminated by dirt, it should be replaced by the Magnetic Probe MM 0002.
The mechanical details of the MM 001 2 are shown in Fig.3 . 6. It should be mounted on
a stiff bracket by means of screws through the two mounting slots in its body. These
slots enable it to be adjusted so that its axis is radial to the rotor axis (Fig.3 .7). The opti-
mum position for the tip of the Probe is at a distance of about 4 mm (1 /6 in) away from
the surface of the rotor . The easiest and most versatile support for the MM 001 2 is a
magnetically clamped stand (Fig.3.8).

770521
Fig. 3. 6. Mechanical details of Photoelectric Tachometer Probe
MM0012
Fig.3. 7. Example of a typical mounting arrangement for Photoelec-
tric Tachometer Probe MM 0012
Fig.3.8. Use of a magnetic dial-gauge stand for mounting Photoelec-
tric Tachometer Probe MM 0012
13

With the 2976 switched on, set TRIGGER to Norm . If the nature of the machine to be
balanced permits, inch the rotor round by hand at least one revolution. Check that the
yellow TRIGGER lamp flashes once when the contrast mark passes the probe, and does
not flash at any other point in the rotation. This can help elucidate any tachometer prob-
lems at an early stage.
3.3.4. Use of the "Set Filter" Facility on the 2976
14
Check that the batteries of all the instruments are in sufficiently good condition for meas-
urement, in accordance with the directions given in their respective instruction manuals
(section 3.1 .3 for the 2976). Check that there is no possibility of the rotor fouling any of
the cables connecting the instruments to the machine.
Run the machine up to speed to check the operation of the Tachometer Probe and set
the filter centre-frequency. (If it is required to minimise the number of machine runs, the
initial measurements can be performed during this run.)
When the machine is up to speed, check that the yellow TRIGGER lamp is illuminated,
indicating correct triggering. If the machine speed is low, the lamp will flash at the rota-
tion frequency, in synchronism with the passage past the Tachometer Probe of the con-
trast mark. If the lamp does not light, adjust TRIGGER until it does light, and try to set
the knob approximately in the centre of the range where correct triggering is obtained.
Set SET FILTEA/MEASURE on the 2976 to Set Filter.
On the 1 621, set the controls as follows:
FILTER:
SWEEP CONTROL:
FREQUENCY RANGE:
23%
Manual
The range containing the rotation frequency
Tune the centre-frequency of the tunable filter to obtain a maximum indication on the
meter of the vibration analyzer. The maximum indication should be approximately 10 on
the SA 0183 scale. If a maximum is obtained at about 3 or lower, it is because the filter
has been tuned to a harmonic. Tune the 1 621 to a lower frequency until the correct
maximum is found. If no maximum can be obtained, suspect a mistake in the system in-
terconnections .
Precise tuning of the filter is obtained by reference to the PHASE display of the 2976.
This will indicate "180" when the filter is exactly tuned to the rotation frequency. If in-
stead it indicates "E" for "Error", look for one of the following problems:
1. The TRIGGER lamp is not lit (owing to unsatisfactory triggering at the TACHO IN-
PUT).
2. The TACHO INPUT signal suffers from phase modulation (jitter) due to fast or slow
variations in rotor speed.
3.
The TACHO INPUT signal suffers from aperiodicity due to the presence of more than
one contrast mark on the rotor .
4. The Photoelectric Probe is responding to reflections from flickering or fluctuating
light sources, or specular reflections from steady light sources, which cause spuri-
ous triggering.