"RED
LIONCONTROLS
Willow Springs Circle,
RD
5,
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 number
of
pulses
to
be counted during one revolu-
tion
of
the
shaft, or Pulses Per Revolution
(PPR).
The
following
formulas can be used
to
determine the neces-
sary
time
base
(TB),
or the number
of
pulses required
for
each
shaft
revolution
(PPR)
to
obtain
the Desired Readout
(DR)
in
units per minute.
60
DR
TB =
PPR
X
RPM
60
DR
PPR
= TB X
RPM
Where:
60
= conversion
factor
changing
counts
per sec-
ond
to
counts
per minute.
RPM
=speed
of
shaft
being sensed.
Example 1 - The
customer
wants
to
indicate speed
of
a bot-
tling
machine
with
a
maximum
speed
of
300
bottles
per min.
They want
to
sense a
60
tooth
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 =
PPR
X
RPM
TB =
60
X 300 (Bottles/Minute)
60
500
(RPM)
TB = 0.6 seconds
On the same machine
calculate
the number
of
teeth required
on
the
sensing gear
(PPR)
if
the
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
have
36
teeth
Example 2 - The
customer
wants
to
indicate
speed
of
a
paper mill in feet/min. There is a 9 inch
diameter
rollover
which the web is running. A gear can
be
mounted on the end
of
the
9"
roll. Calculate the
time
base with a
60
tooth gear.
Roll circumference
=1f0
= 3.1416X9" =
28.27"or2.356feet
per revolution
of
the roll.
At 1 RPM,
this
is our Desired Readout, per minute.
60
DR
TB =
PPR
X
RPM
6R£DLION
CON1ROLS
TB - .§fL X 2.356 (FPM)
- 60 1 (RPM)
TB = 2.356 seconds
On
the same paper machine
if
a OITAK
II
A with a one second
time base is purchased, calculate
the
number
of
gear 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,
or
a Rotary Pulse Generator
with
141
pulses per revolution.
Example 3 - The
customer
wants
to
indicate
speed on a
chicken processing machine with a maximum rate
of
70
birds
per min. He wants the tachometer
to
read in tenths
(70.0
BPM). The
output
shaft
is running
35
RPM
at
70
BPM.
With
a.
60
tooth
gear,
calculate
the
time
base. (Note:
700
input pulses
are required
for
a reading
of
70.0).
60
DR
TB =
PPR
X
RPM
TB =
..§Q.
X 700 (Birds/Minute)
60
35
(RPM)
TB
=
20
seconds
Normally
it
is desirable
to
have a
time
base in the range
of
1/2
to 4 seconds. With short time bases the reading
jumps
around
too
much to
be
acceptable. 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
required
if
a
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
option
is
to
find a
shaft
running
at a higher speed
to
mount
the gear.
On
this
machine a
60
tooth gear can be mounted on the input
to
a
50
to
1 gear box.
The speed
of
this
shaft
is
50
x
35
RPM, or 1750 RPM.
Calculate the
time
base.
60
DR
TB
=
PPR
X
RPM
TB
=..2Q... 700 (BPMl
60
X 1750 (RPM)
TB
=
0.4
seconds
......
.
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