OHAUS CKW3R User manual

SERVICE MANUAL
CKW Bases
Ohaus Corporation 19A Chapin Road, P.O. Box 2033, Pine Brook, NJ 07058-2033 [phone]

SERVICE MANUAL
CKW Bases
The information contained in this manual is believed to be accurate at the time of publication, but Ohaus
Corporation assumes no liability arising from the use or misuse of this material. Reproduction of this
material is strictly prohibited.
Material in this manual is subject to change.
© Copyright 2008 Ohaus Corporation, all rights reserved.
TM Registered trademark of Ohaus Corporation.

TABLE OF CONTENTS
CHAPTER 1 INTRODUCTION Page No.
1.1 Introduction................................................................................................................1-1
1.2 Service Facilities........................................................................................................1-1
1.3 Tools and Test Equipment Required.........................................................................1-2
1.4 Test Masses Required...............................................................................................1-2
1.5 Test Strategy .............................................................................................................1-2
1.6 Physical Description: Model CKW Bases..................................................................1-2
1.7 Specifications.............................................................................................................1-3
1.8 How Load Cells Operate............................................................................................1-4
CHAPTER 2 TROUBLESHOOTING
2.1 Troubleshooting.........................................................................................................2-1
2.1.1 Checking Load Cells for Trouble........................................................................2-1
CHAPTER 3 MAINTENANCE AND REPAIR PROCEDURES
3.1 Preventive Maintenance............................................................................................3-1
3.1.1 Preventive Maintenance Checklist .....................................................................3-1
3.2 Replacing the Load Cell.............................................................................................3-1
3.3 Setting the Overload Stops........................................................................................3-2
CHAPTER 4 TESTING
4.1. Testing.......................................................................................................................4-1
4.2 Load Cell Resistance Checks....................................................................................4-1
4.3 Calibration with an External Indicator........................................................................4-1
4.4. Consistency Check....................................................................................................4-2
4.5 Performance Tests ....................................................................................................4-2
4.4.1 Shift Test ............................................................................................................4-3
4.4.2 Full Load Test.....................................................................................................4-3
CHAPTER 5 DRAWINGS AND PARTS LISTS
5-1 CKW Base Scale.......................................................................................................5-2
APPENDIX A: GLOSSARY
A-1 Glossary.................................................................................................................... A-1
LIST OF TABLES
1-1 Specifications for CKW Base Scales.....................................................................1-3
2-1 Load Cell Output Readings...................................................................................2-2
4-1 Load Cell Resistance Readings............................................................................4-1
4-2 Load Cell Output Readings...................................................................................4-2
5-1 CKW Base Scale Parts List...................................................................................5-3
LIST OF ILLUSTRATIONS
1-1. Strain Gauge Load Cell.........................................................................................1-4
1-2 Downward force on the Platform bends the beam................................................1-4
1-3 Route of electrical Load Cell input and output.......................................................1-5
3-1 CKW Load Cell, Spacer Down Stop Nuts and Jam Nuts......................................3-2
4-1 Positions A, B, C and D are Centered...................................................................4-3
5-1 Exploded View of CKW Base................................................................................5-2
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TABLE OF CONTENTS
CKW Base Series Scales: Service Manual ii Ohaus Corporation www.ohaus.com

CHAPTER 1 INTRODUCTION
1.1 INTRODUCTION
This service manual contains the information needed for diagnosis and repair of Ohaus CKW
scale Bases: Champ Food Safe Bases, Item designations CKW R and L Series.
The contents of this manual are contained in five chapters:
Chapter 1 Introduction – Contains information regarding service facilities, tools and test
equipment, test masses, specifications, service strategy, and the mechanical and electronic
operation of the Base.
Chapter 2 Troubleshooting – Provides guidelines for evaluating the condition and
performance of a scale Base, and a standard troubleshooting methodology to follow, as well as
a diagnostic guide.
Chapter 3 Maintenance Procedures – Contains preventive maintenance procedures and
disassembly, repair and replacement procedures.
Chapter 4 Testing – Contains guides for operational and performance tests and adjustments.
Chapter 5 Drawings and Parts Lists – Contains exploded views of the CKW scale Base and
parts, identifying all serviceable components.
Before servicing the Base, you should be familiar with the Instruction Manual which is packed
with every Base.
1.2 SERVICE FACILITIES
To service a scale Base, the service area should meet the following requirements:
•Clean and level surface.
•Away from magnetic fields such as motors or large transformers.
•Free of vibrations such as fork lift trucks close by, large motors, etc.
•Stable and level work surface.
•Not exposed to direct sunlight or radiating heat sources.
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CHAPTER 1 INTRODUCTION
1.3 TOOLS AND TEST EQUIPMENT REQUIRED
1. Common hand tools
2. Standard electronics tool kit
3. Digital Voltmeter (DVM) capable of reading from 1 mv to 50 V dc.)
4. Megohmetter (50 Volt dc maximum test voltage)
5. Desk magnifier on a stand
6. Ohaus Indicator: CKW-55, or equivalent commercial indicator
7. A calibrated Torque Wrench N.m. – Ft/Lbs
1.4 TEST MASSES REQUIRED
The masses required to test the Ohaus CKW Base when connected to an Indicator must meet
the requirements of ASTM Class 4 or OIML F2. Mass values 3kg to 30kg,depending on the
Base model. The full scale rated capacity of the Base should be used for Span calibration.
1.5 SERVICE STRATEGY
The Load Cells used in Ohaus Bases are non-repairable. The repair method for the Base is
direct replacement of Load Cells with connecting cable and hardware.
Each base contains the following basic replaceable assemblies: Platform, Load Cell with
Connecting Cable, and Down Stop and mounting hardware. There is an exploded view drawing
of the Base and associated parts list in Chapter 5. This service manual contains sufficient
information to isolate the problem, replace the component, test and restore the Base to its
original factory specifications.
1.6 PHYSICAL DESCRIPTION: MODEL CKW BASES
Construction Platform – Fabricated stainless steel
Scale Base – Formed and welded base, 304 stainless steel
Load Cell – Stainless steel
Overloading Corner Loading – 100% of full capacity
Safe Overload – 150% of full capacity
Ultimate Overload – 300% of full capacity
Operating Environment Designed to meet OIML 3000e and NTEP 5000d requirements
to operate at a temperature range from –10°C to 40°C /
14°F to 104°F, 0 to 90% relative, non-condensing humidity
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CHAPTER 1 INTRODUCTION
1.7 SPECIFICATIONS
Complete specifications for the Ohaus CKW scale Bases are listed in Table 1-1. When a base
has been serviced, it must meet the specifications listed in the table. Before servicing the base,
determine what specifications are not met.
TABLE 1-1. SPECIFICATIONS FOR CKW SCALE BASES
Model CKW3R CKW6R CKW15L CKW30L
Capacity (MAX) x
Readability (d) 3 kg x 0.0005 kg
6 lb x 0.001 lb 6 kg x 0.001 kg 15 lb x 0.002 lb 15 kg x 0.002 kg 30 lb x 0.005 lb 30 kg x 0.005 kg 60 lb x 0.01 lb
Maximum Resolution 1:6000 1:7500 1:7500 1:6000
Capacity (MAX) x
Approved Readability (e) 3 kg x 0.001 kg
6 lb x 0.002 lb 6 kg x 0.002 kg 15 lb x 0.005 lb 15 kg x 0.005 kg 30 lb x 0.01 lb 30 kg x 0.01 kg 60 lb x 0.02 lb
Approved Resolution
(n max) 1:3000 1:3000 1:3000 1:3000
Shift Tolerance new ±1 ±1 ±1 ±1
Shift Tolerance Maint. ±2 ±2 ±2 ±2
Base Dimensions (WxDxH) 254x254x57.5mm
10x10x3.5in 305x305x57.5mm 12x12x2.26 in. 305x305x69.5mm 12x12x2.74 in.
Load Cell Excitation 5 to 15 DC/AC
Load Cell Rated Output 2mV/V
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CHAPTER 1 INTRODUCTION
1.8 HOW LOAD CELLS OPERATE
Load Cells are devices that convert force into a signal. The CKW Base has a Strain Gauge
Load Cell, which is made from a metal beam with holes drilled in it. Strain gauges are affixed to
the beam at the top and bottom to measure changes in the beam due to deflection. The gauges
are bonded very securely to the metal, where they sense very small deflections in the metal
caused by the load being applied to the cell. Because the signal levels are very small, the
circuit is protected from all outside influences such as moisture, physical damage, or electrical
interference.
Attachment points to
Platform
Strain Gauges
Beam
Printed Circuit Board
completing Wheatstone
Bridge circuit Holes drilled in Strain
Gauge Load Cell
Figure 1-1. Typical Strain Gauge Load Cell.
The strain gauges are wired into a Wheatstone Bridge Circuit. (See Figure 1-2.)
Force
Wheatstone Bridge Circuit
Strain Gauge
+
+
Strain Gauge –
Strain Gauge
Strain Gauge
–
Printed Circuit Board (PCB )
completing Wheatstone
Bridge circuit
Figure 1-2. Downward force on the Platform bends the beam, causing two gauges to stretch
and two to compress in opposition, changing the electrical resistance of the circuit.
At zero load, all strain gauges are unstressed. Weight placed on the Platform bends the
beam, causing two gauges to stretch and two to compress in opposition, changing the
electrical resistance of the circuit.
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CHAPTER 1 INTRODUCTION
1.8 HOW LOAD CELLS OPERATE
The difference in the output signal before and after the mass was placed on the platform is
taken, interpreted and displayed.
Figure 1-3. Route of electrical Load Cell input and output through Wheatstone Bridge circuit.
A Load Cell is a force sensor, which receives a voltage (excitation) from a regulated power
source in the Indicator, and sends back a low-voltage milli-volt (mV) relative to the force applied.
The Load Cell signal is read by the Indicator, which converts it to a numeric value and shows it
visually. This output value increases as weight is loaded to the Load Cell.
For example, a 500kg Load Cell with 2mV/V output and 5V excitation would have a change of
0.02mV per kg change in the load, that is, 10mV/500=0.02mV per kg. At a load of 250kg the
signal voltage would thus increase by 5mV from the value measured at no load.
Tension
Tension
Compression
Compression
+Excitation
(Supply)
–Excitation
(Supply)
INPUT(volts)
LOADCELL
2 mV/V
Strain
Gauges
4 strain gauges
connected as a
Wheatstone
Bridge
OUTPUT (milli volts)
+Sig
–Sig
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CHAPTER 1 INTRODUCTION
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CHAPTER 2 DIAGNOSTIC GUIDE
2.1 TROUBLESHOOTING
This section of the manual provides guidelines for evaluating the condition and performance of a
Base unit, and a standard troubleshooting methodology to follow.
2.1.1 Checking Load Cells for Trouble
1. Visual Check:
– Remove any shipping material from between the upper and lower Housings, if
present.
– Clean the unit before evaluating any mechanical problems. In some cases,
debris may have accumulated inside the Base Housing. Make sure there is no
buildup of any foreign material.
– Examine the Base Housing for dents, bent Platform or signs of physical abuse
that could cause the Base to malfunction. Make sure that the proper Platform is
supplied with the Base. Replace all damaged parts. See Chapter 5 for parts
identification.
– Remove the Platform from the Base. Check that the Down Stops are not
touching the Top Plate. (See Figure 5-1, Chapter 5.) This would restrict
movement, causing improper operation of the Base. If the Down Stops are
improperly set, adjust them. (See Section 3.3.)
– Check the metal surfaces of the Platform. All surfaces should be parallel.
If the platform is deformed, it should be replaced.
– Check the cables leading to the Load Cell for cuts, abrasions or other signs of
excessive wear and tear.
– Check for a bent or twisted Load Cell: Place the top surface and then each of
the sides of the Load Cell on a flat surface, to see if it rests flat and even. A gap
indicates a bent or twisted Load Cell.
– Examine the Load Cell for corrosion due to high humidity or exposure to
chemicals.
– A Load Cell that is even slightly bent or corroded should be replaced.
2. Perform a Resistance Test, to determine if the Load Cell is severely damaged or a
short circuit to the frame has occurred. (See Chapter 4 for details.)
3. Perform an Output Voltage Test:Measure the no load, 50% load and full load
output. The reading should meet the Load Cell specifications. (See Chapter 4.)
– The Load Cell output should be very close to linear over its capacity range.
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CHAPTER 2 DIAGNOSTIC GUIDE
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CHAPTER 3 MAINTENANCE AND REPAIR PROCEDURES
3.1 PREVENTIVE MAINTENANCE
Ohaus scales should be carefully handled, stored in a clean, dry, area, and cleaned periodically.
Follow these precautionary steps:
– When a scale has had chemicals or liquids spilled on it, all exterior surfaces should be
cleaned as soon as possible with warm water on a damp cloth.
– Do not leave a mass on the scale when the scale is not in use.
– Allow time for the base to stabilize after moving it from an area which is at a different
temperature than the area where it is to be operated. Allow one hour for each 5°F
(2.7°C) temperature change before using the base.
3.1.1 Preventive Maintenance Checklist
The scale should be inspected and checked regularly, as follows:
1. Clean the base.
2. Remove the Platform to inspect and clean the area beneath the Platform.
CAUTION
DO NOT USE CHEMICAL CLEANERS OR SOLVENTS OF ANY TYPE.
SOME CLEANERS ARE ABRASIVE AND MAY AFFECT THE SCALE’S FINISH.
3. Check the Load Cell cable for broken or damaged insulation.
4. Make a visual inspection as detailed in Chapter 2.
3.2 Replacing the Load Cell
A Load Cell that is even slightly bent or corroded should be replaced. The Load Cell may also
need to be replaced because of instability, or because the scale does not calibrate or repeat, or
because it is physically broken.
1. Remove the Base Platform and disconnect the Base from the Indicator.
2. Remove the Load Cell Mounting Bolts and washers that secure the Top Frame to the
Load Cell.
3. Set the Top Frame and the Load Cell Spacer aside. (See Figure 3-1, next page.)
4. Remove the bottom Load Cell Mounting Bolts, washers and spacer. The Load Cell
assembly can now be removed from the Lower Frame.
5. Install the new Load Cell following steps 1 through 4 in reverse order.
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CHAPTER 3 MAINTENANCE AND REPAIR PROCEDURES
3.3 Setting the Overload Stops
The Overload Stop gaps must be checked and reset if the Load Cell is replaced. This
procedure requires two test masses: one equal to the base’s capacity, and one equal to 150%
of its capacity. (To check capacity, refer to the Specification Table in Section 1.7.)
There are six screws in the Lower Housing of the Base. Each is capped with a round Overload
Stop Nut and a Jam Nut. (See Figure 3-1.)
To adjust the Overload Stops:
1. Apply 100% load at each corner stop, one at a time; adjust the round Overload Stop Nut
and lock it with the Jam Nut when the Upper Housing just touches the Overload Stop
Nut.
2. Repeat this for all four corners.
3. For the two sets of Overload Stop Nuts and Jam Nuts located in the center of the Base,
use a load of 150% of the Base’s capacity placed on the center of the Platform. When
the Overload Stop Nuts are properly set, lock the Jam Nuts in place.
4. Test the Base with Indicator connected to see if full capacity can be achieved.
Overload Stop Nut
Jam Nut
Load Cell Spacer
Load Cell
Figure 3-1. CKW Load Cell, Spacer Overload Stop Nuts and Jam Nuts.
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CHAPTER 4 TESTING
4.1 TESTING
Before and after servicing a CKW Base, conduct Load Cell Resistance Checks, Calibration
using an external Indicator, and Performance Tests to confirm that the base meets
specifications. Allow time for the base to stabilize after moving it from an area which is at a
different temperature than the area where it is to be operated. Allow one hour for each 5°F
(2.7°C) temperature change before using the base.
The following tests require a Digital Multimeter, an Indicator and hand tools. The Digital DC
Voltmeter should have a reading range from 1 mV to 50 V full-scale. It should be capable of
reading differences of one micro-volt per increment. The indicator can be either the Ohaus
CKW-55, or an equivalent commercial indicator.
NOTE:
Make sure the test area is free from drafts and that the base rests on a level and
vibration-free surface.
4.2 Load Cell Resistance Checks
Use an ohm-meter to measure across each pair of wires in turn, and compare the results with
Table 4-1. The Load Cell must be completely disconnected from the Indicator and at no load
when these tests are made.
In addition to the four resistance elements which make up the Wheatstone Bridge, there are
commonly one or two resistors in the excitation lines. The resistance across the excitation wires
is usually the highest resistance measured across any two wires.
If the resistance readings are in the range specified, skip to the next section. If they are
incorrect – for example, outside the expected range, open circuit or short-circuit across any two
wires. If the Load Cell is defective, replace it. (See Chapter 3.)
TABLE 4-1. LOAD CELL RESISTANCE READINGS
RESISTANCE READINGS IN OHMS
Unit Ex+ to Ex–S+ to S– Ex+ to S– Ex+ to S+ Ex– to S+ Ex–to S–
CKW 387 ± 10 350 ± 4 297 ± 10 297 ± 10 262.5 ± 10 262.5 ± 10
4.3 Calibration with an External Indicator
1. Install the Load Cell. (See Section 3.2.)
2. Connect the excitation, signal shield and sensing wires (if provided), to the Indicator.
Calibrate the Indicator to read the load in the units required, for example, kilograms or
grams. (Consult the Indicator instruction manual for calibration procedures.) For
calibration mass weights, see Table 1-1, in Chapter 1.
3. Check that the Indicator is reading correctly over a range of values.
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CHAPTER 4 TESTING
4.4 Consistency Check
To check the system prior to installation, make the following measurements. (Also make these
checks if the Load Cell will not calibrate, or if the Indicator does not read correctly, or is giving
unstable readings.)
1. Using a voltmeter, measure and record the excitation voltage supplied by the Indicator.
Most Ohaus indicators supply 5 volts dc for the excitation voltage.
2. Using an mV meter, measure and record the signal voltage with no load on the base.
CAUTION:
IN THE NEXT STEP, DO NOT OVERLOAD THE BASE BEYOND FULL
CAPACITY RATING.
3. Using an mV meter, measure and record the signal voltage at the loads shown in
Table 4-2.
4. Compare the measured output readings with the theoretical values from the Load Cell
data. (See Table 4-2.)
TABLE 4-2. LOAD CELL OUTPUT READINGS (in mV with 5V Excitation)
Model/Capacity Zero Load 50% Load 100% Load
CKW3/3kg 4.3 ± 2.0 Zero + 2.3 Zero + 4.7
CKW6/6kg 1.6 ± 2.0 Zero + 2.8 Zero + 5.6
CKW15L/15kg 1.7 ± 2.0 Zero + 2.4 Zero + 4.8
CKW30L/30kg 1.3 ± 2.0 Zero + 2.7 Zero + 5.4
4.5 Performance Tests
Accurate performance of the CKW Bases is determined by two performance tests. The
displayed readings are compared with the tolerances listed in Table 1-1.
NOTE:
The following performance tests are used to evaluate base operation before and
after repairs. The base must meet the requirements specified in each test as
well as the specifications listed in Table 1-2. Before proceeding with the
following tests, the base should be calibrated. (See Section 4.3.)
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CHAPTER 4 TESTING
4.5.1 Off Center Load (Shift) Test
The Off Center Load (Shift) Test is made before and after a Base has had the Load Cell
replaced and adjustments have been made that affect its performance. The Off Center Load
Test verifies that all sections of the Base weigh within specified tolerance.
If the Base does not pass the Off Center Load Test, verify that the Overload Stops are properly
set. (See Section 3.1.)
If the Off Center Load Test cannot be passed, the Load Cell must be adjusted or replaced.
Prior to starting this test, the Base must be connected to a properly functioning Indicator, with
capacity and readability values set according to the specifications for the Base under test.
1. Place test weights equal to one third of the Base's capacity sequentially at each of
the positions A, B, C, and D, as shown in Figure 4-1. Note the Indicator reading at
each position. (For Base capacity, see Table 1-1.)
2. Check the variation between mass weights and indicator reading against specified
Shift Tolerance in Table 1-1.
A B
C D
Figure 4-1. Positions A, B, C and D are centered
at each quarter of the base platform.
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CHAPTER 4 TESTING
4.5.2 Adjusting Off Center Load
If the Off Center Load (Shift) is excessive, perform adjustment as follows:
Top view of Load Cell
Side view of Load Cell
3
2
1 4
Figure 4-2. Off Center Load adjustment points.
1. Place the test weight in the center of the Platform.
2. Tare the indicator.
3. Move the weight to point A and record the reading.
4. Move the weight to point B and record the reading.
5. Move the weight to point C and record the reading.
6. Move the weight to point D and record the reading.
7. If the reading at point A is negative, file at points 1 and 4 AT AN ANGLE.
8. If the reading at point B is negative, file at points 1 and 2 STRAIGHT ACROSS.
9. If the reading at point C is negative, file at points 2 and 3 AT AN ANGLE.
10. If the reading at point D is negative, file at points 3 and 4 STRAIGHT ACROSS.
Note: It is not recommended that you try to adjust more than –5 counts if the beam has
been filed already. If the beam has not been filed previously, you can adjust –10 counts.
Remember, when filing you are weakening the beam. File a little at a time.
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