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"RED
LIONCONTROLS
Willow Springs Circle,
RD5,York,
Pa.
17402
(717) 767-6511
TWX:
510
657
4214
RLC
YRK
BULLETIN
NO.
TBS-001
Revised
9/10/79
SELECTING
TIME
BASE
&
INPUT
RATE FOR TACHOMETERS
Willow Springs Circle,
RD
5, York,
Pa.
17402 (717) 767·6511
(
A
digital
tachometer is an electronic counter with a
time
reference. Only
two
parameters
will
change the readout ob-
tained
at
a given speed:
1.
The time in seconds during which pulses are counted,
referred
to
as the Time Base
(TB).
2.
The numberofpulsestobe counted during one revolutionofthe
shaft, or Pulses Per Revolution
(PPR).
The
following
formulas can be usedtodetermine the neces-
sary
time
base
(TB),
or the numberofpulses required
for
each
shaft
revolution
(PPR)toobtain
the Desired Readout
(DR)
in
units per minute.
60
DR
TB =
PPRXRPM
60
DR
PPR
= TB X
RPM
Where:
60
= conversion
factor
changing
counts
per sec-
ondtocounts
per minute.
RPM
=speed
of
shaft
being sensed.
Example 1 - The
customer
wantstoindicate speedofa bot-
tling
machine
withamaximum
speedof300
bottles
per min.
They want
to
sense a60tooth
gear on a
shaft
which turns at
500
rpm when
the
machine
is running 300 bottles/min.
Calculate the correct
time
base.
60
DR
TB =
PPRXRPM
TB =
60
X 300 (Bottles/Minute)
60
500
(RPM)
TB = 0.6 seconds
On the same machine
calculate
the numberofteeth required
on
the
sensing gear
(PPR)ifthe
time
base
(TB)
is one second
(DITAK
II
A).
60
DR
PPR
= TB X
RPM
PPR
=
..§Q.
X 300 (BPM)
1 500 (RPM)
PPR=36
or, the gear
must
have36teeth
Example 2 - The
customer
wantstoindicate
speedofa
paper mill in feet/min. There is a 9 inch
diameter
rollover
which the web is running. A gear canbemounted on the end
of
the9"roll. Calculate the
time
base with a60tooth gear.
Roll circumference
=1f0
= 3.1416X9" =
28.27"or2.356feet
per revolutionofthe roll.
At 1 RPM,
this
is our Desired Readout, per minute.
60
DR
TB =
PPRXRPM
6R£DLION
CON1ROLS
TB - .§fL X 2.356 (FPM)
- 60 1 (RPM)
TB = 2.356 seconds
On
the same paper machineifa OITAKIIA with a one second
time base is purchased, calculate
the
numberofgear teeth
re-
quired.
60
DR
PPR
= TB X
'R'P'M"
PPR
= .§Q. X 2.356 (FPM)
1 1 (RPM)
PPR
= 141.36
You could use a
141
tooth
gear,ora Rotary Pulse Generator
with
141
pulses per revolution.
Example 3 - The
customer
wantstoindicate
speed on a
chicken processing machine with a maximum rateof70
birds
per min. He wants the tachometer
to
read in tenths
(70.0
BPM). The
output
shaft
is running35RPMat70
BPM.
With
a.
60
tooth
gear,
calculate
the
time
base. (Note:
700
input pulses
are required
for
a readingof70.0).
60
DR
TB =
PPRXRPM
TB =
..§Q.
X 700 (Birds/Minute)
60
35
(RPM)
TB
=
20
seconds
Normallyitis desirabletohave a
time
base in the rangeof1/2
to 4 seconds. With short time bases the reading
jumps
around
too
much tobeacceptable. With
time
bases above 4
seconds the
time
between updating the reading is
too
long
for
most applications. This
time
base is
too
long. Calculate
the
PPR
requiredifa
tachometer
with a 3 second base is
used.
60
DR
PPR
= TB X
RPM
PPR
=
60
X 700 (BPM)
3
35
(RPM)
PPR
= 400
A Rotary Pulse Generator with
400
PPR
is required.
A
simpler
and usually
better
optionisto
find a
shaft
running
at a higher speed
to
mount
the gear.
On
this
machine a
60
tooth gear can be mounted on the inputtoa50to
1 gear box.
The speed
of
this
shaftis50x35
RPM, or 1750 RPM.
Calculate the
time
base.
60
DR
TB
=
PPRXRPM
TB
=..2Q... 700 (BPMl
60
X 1750 (RPM)
TB
=
0.4
seconds
......
.
,.
Page 10
RED
LION
CONTROLS
57600-01
York, Pa.
I
CPH
II
IPM
II
.·MPS
I
I
FPH
II
LPM
II
RPS
I
I
IPH
II
.
MPM
il
YPS
]
I
MPH
II
RPM
Ii
I
DRAW
I
I
YPH
II
YPM
II
MINUTES
I
I
BPM
II
BPS
II
SECONDS
I
I
CPM
II
CPS
I
SPEED
DIFFERENCE
I
FPM
II
FPS
II
SPEED
RATIO
I
CPM
II
IPS
II
PERCENT
DRAW
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