KERN VHT User manual

KERN & Sohn GmbH
Ziegelei 1
D-72336 Balingen
E-Mail: [e-mail]
Tel: +49-[0]7433- 9933-0
Fax: +49-[0]7433-9933-149
Internet: www.kern-sohn.com
Service manual
Pallet truck scale
KERN VHT
Version 1.0
5/2009
GB
VHT-SH-e-0910

GB KERN VHT
Version 1.0 5/2009
Service manual
Pallet truck scale
CONTENTS
1HOW TO USE THIS SERVICE MANUAL............................................................ 4
2THE PRINCIPLE OF THE PALLET TRUCK SCALE........................................... 5
2.1 MECHANICS................................................................................................ 5
2.2 LOAD CELLS ............................................................................................... 6
2.3 INDICATOR ................................................................................................. 7
2.4 BATTERY..................................................................................................... 7
3POSSIBLE PROBLEMS...................................................................................... 8
4DESCRIPTION OF PROBLEM AND WHERE TO LOOK FOR CAUSES............ 9
5EXPLANATION OF POSSIBLE CAUSES AND HOW TO FIX THEM ............... 10
5.1 CHECK BATTERY ..................................................................................... 10
5.2 CHECK MECHANISM................................................................................ 10
5.3 CHECK LOAD CELLS................................................................................ 11
5.4 CHECK CABLING AND LOAD CELL BOARD ........................................... 12
5.5 CALIBRATE LOAD CELLS ........................................................................ 13
5.6 CHECK INDICATOR .................................................................................. 13
6PARAMETER SETTINGS ................................................................................. 15
7CALIBRATION INSTRUCTIONS....................................................................... 18
7.1 ZERO CALIBRATION ................................................................................ 18
7.2 SINGLE POINT CALIBRATION ................................................................. 19
7.3 MULTI-POINT CALIBRATION ................................................................... 20
8HOW TO ADJUST THE LOWERING VALVE.................................................... 21
9ERROR MESSAGES ........................................................................................ 22
10 DIMENSIONS OF PALLET TRUCK SCALE...................................................... 23
11 SPARE PARTS ................................................................................................. 24
2 VHT-SH-e-0910

Basic Information
Grundlegende Hinweise
The device must be repaired only by trained specialist staff or personnel with
professional formation (such as a repair-specialist accredited by law
concerning verification).
The service manual is obligatory for repair work.
After repair, original conditions of the device have to be restored.
Only original spare parts should be used.
Das Gerät darf nur von geschultem oder beruflich ausgebildetem
Fachpersonal (z. B. eichrechtlich anerkannter Instandsetzer) repariert werden.
Die Serviceanleitung ist bindend für Reparaturen.
Das Gerät muss nach erfolgter Reparatur wieder in den Originalzustand
zurückversetzt werden.
Es dürfen nur Originalersatzteile verwendet werden.
VHT-SH-e-0910 3

1 HOW TO USE THIS SERVICE MANUAL
This service manual helps you to solve technical problems with your mobile weighing
system.
In addition there is a calibration and parameter instruction.
Chapter 2 Explains the principle behind electronic weighing systems.
Chapter 3 Lists the most frequently occurring problems (A, B, C, D etc.). If your
problem is listed in this overview, remember the character and go to
Chapter 4.
Chapter 4 Connects the problem to possible causes (1, 2, 3, 4 etc.). The most
encountered cause is marked with a “Æ”. Remember the number(s) of
the cause you should check and go to Chapter 5.
Chapter 5 Explains the possible causes in more detail and describes possible
solutions.
Chapter 6 Explains the parameter settings.
Chapter 7 Explains the calibration procedure.
Chapter 8 Explains how to adjust the lowering valve.
!!! IMPORTANT !!!
Do not weld on the system with the load cells mounted. If the welding current passes
through the load cell, they will be destroyed.
Do not touch the contacts on the electronic boards. Static electricity will destroy
components.
4 VHT-SH-e-0910

2 THE PRINCIPLE OF THE PALLET TRUCK SCALE
The pallet truck scale consists of the following components:
1. Mechanics
2. Load cells
3. Indicator (module)
4. Battery
2.1 MECHANICS
The mechanical part is the pallet truck chassis with a special fork construction.
For the weighing system the pallet truck fork consists of two parts:
1. The basic chassis
2. The upper forks
Between the lower / basic fork construction and the upper fork, sensors / load cells
are mounted that will sense the force placed on the upper fork.
2
1
It is most important that all forces pass through the load cells and that no parts of the
basic fork, pushrod or load wheel holder of the pallet truck have mechanical contact
with the upper fork (weighing part).
VHT-SH-e-0910 5

2.2 LOAD CELLS
One of the two most important components of weighing pallet truck are the load cells.
A load cell is a rectangular steel component, built in the special pallet truck chassis,
in the lower part of the fork. The load cell has a deformation 0.07mm under a load of
1000 kg. This deformation depends on the weight of the load. In the load cell,
electrical resistor, called strain gauges, are glued to the load cell body.
Strain gauge
The electrical resistance of the strain gauges in the load cells changes linearly with
the deformation of the load cell.
The load cell is connected with four wires to the indicator. Two wires, black and red,
supply an input voltage to the load cell. This voltage to the load cells is called
excitation. The output voltage of the load cell, called “signal”, green and white wires,
depends on the strain gauge resistance change.
With a maximum load on the load cells (example: 4 x 1000 kg), the signal voltage
(caused by the change in resistance) is approximately 0.01 Volt. The signal voltage
at 100 kg is only 0.0002 Volt. The signal change level with 1 kg weight change is only
0.000002 Volt. When working with such small electrical signals, the cabling has to be
100%. Therefore, the signal is transmitted from the load cells to the indicator via a
special cable. All connections have to be soldered and humidity or corrosion of
contacts will cause instability in the reading.
6 VHT-SH-e-0910

2.3 INDICATOR
The indicator operates like an ohm/voltmeter. The change in resistance is measured,
translated and displayed as a weight. The indicator is a multitude more accurate then
a multimeter but in a very limited voltage range. It measures micro voltages at
extreme high accuracies, independent from temperature changes and distortion of
radio frequencies.
Because of the extremely small signal levels the board is very sensitive to humidity
and contact with chemical substances that will influence the solder contacts.
2.4 BATTERY
The power supply takes place via an 12V interchangeable battery module
(VHT-A01).
VHT-SH-e-0910 7

3 POSSIBLE PROBLEMS
The most frequently occurring causes of malfunctioning of the weighing
systems are:
•Supply: Worn out battery, fuse, or power supply
•Bad or broken contacts
•Jammed or worn out cabling
•Loose screws or bolts
•Mechanical problems: the upper part of the fork has contact with the chassis or
lifting mechanics of the pallet truck due to deformation, wear or accumulated dirt.
•Moisture in the electronics or load cells
These problems cause 99% of all possible failures and will result in the
following problem descriptions:
A. Nothing works. The display is dead, even when pressing the keys.
B. The indicator is unstable; the weight readout goes up and down.
C. Different weights are displayed for the same load when weighed several times.
D. The weighing system is accurate with lower weights but inaccurate with higher
weights.
E. The weighing system is inaccurate, however, the inaccuracy is linear, meaning
the error is always the same percentage of the weighed load.
F. Without a load, a positive or negative weight is displayed and it is not possible to
reset the display to 0kg. The scale reacts normally when a load is placed on the
scale.
G. An error message indicates over- or underload, with or without load on the scale.
H. The display does not react to loads being placed on or removed from the scale.
I. Keys do not work.
J. Segments fail on display.
For error messages displayed on the indicator, please refer to chapter 9.
8 VHT-SH-e-0910

4 DESCRIPTION OF PROBLEM AND WHERE TO LOOK
FOR CAUSES
A. Nothing works. The display is dead, even when pressing the keys.
1Check battery and charger.
6Check the indicator.
JIs the safety fuse still good, or is there a bad contact?
B. The indicator is unstable; the weight readout goes up and down.
2Check the mechanism of the fork construction.
4Check the load cell cables and the load cell calibration board.
6Check the indicator for moisture or loose contacts. (5.6.2) up to (5.6.6)
3Check the load cells.
JAre the screws tight?
C. The same load is lifted several times, but different weights are
displayed.
2Check the mechanism of the fork construction.
3Check the load cells.
JThe weighing parts may not have contact with fixed, non weighing parts.
D. The weighing system is accurate with small weights, however, it is
inaccurate with heavy weights.
2Check the mechanism of the fork construction.
JBecause of the bending of the loading part of the system, the weighing part
has contact with the rigid part of the system.
E. The weighing system is inaccurate, however, the inaccuracy is linear.
This means that with a load of 100 kg the inaccuracy is 1 kg, with 500 kg it is
5 kg, and with a load of 1000 kg the inaccuracy is 10 kg.
JSee calibration procedure.
F. Without a load on the forks a negative or positive weight is displayed. It
is not possible to reset the display to zero. The system works normally
once a weight is placed on the unit.
5See calibration procedure.
JThe zero setting has a restricted capacity. Calibrating the system will bring
the zero setting to within the restrictions.
H. The display does not react to loads being placed on the scale.
6Check the indicator. Only 5.6.1 and 5.6.4.
4Check the cabling.
3 Check the load cells.
I. Keys do not work.
6Check the indicator.
J. Segments fail on display.
6Check the indicator.
VHT-SH-e-0910 9

5 EXPLANATION OF POSSIBLE CAUSES AND HOW TO
FIX THEM
5.1 CHECK BATTERY
•Check the battery
Empty battery can be replaced with an 12V interchangeable battery module
(VHT-A01).
5.2 CHECK MECHANISM
5.2.1 Check whether weighing is obstructed mechanically.
Weigh yourself while standing on the extreme corners of the system. Your
weight must be identical on all corners. The difference between the weights
maybe up to one graduation.
If this is the case, it may be concluded that all load cells are working and the
mechanics are fine with small loads.
After finishing this test, load the weighing system with a normal or heavy load.
Using the tare key, the display is reset to zero. Now, with this weight on the
scale, load the corners of the weighing system by placing your own weight on
top or side of the weight that is on the scale. Each corner should give an
identical read-out plus or minus 1 or 2 kg. If the read-outs are not identical and
show a difference larger than 2 graduations, see 5.2.2.
5.2.2 Try to determine where there is interference between weighing and non
weighing components.
It may be necessary to dismantle the system to see where the interference
occurs. Often there are traces on the paint that indicate the most likely place. If
there is doubt, assemble the weighing system with paper placed between the
weighing and non weighing parts. After loading the system to its full capacity
and maybe by moving (shacking) the load side ways, traces on the paper will
show where the friction might be.
Determine what can be done mechanically in order to end this friction. e.g.
align the load cells and the upper fork shoes, allow less moving space for the
lifting mechanics in the forks like the push rod and the load wheel holder. It
might be necessary to align the rods of the load wheels to prevent them from
touching the load cells. It can be that with an unloaded truck it is not possible
to see that friction does occur. Therefore see if the rods can touch the load
cells by pushing them towards the load cells. Clean the inside of the forks.
5.2.3 Are the bolts tightened correctly?
The load cells are mounted with bolts. These must be tightened correctly.
Check if all bolts are tightened firmly.
5.2.4 Are the fork shoes mounted freely? (Always check this!)
Underneath the pallet truck you can see whether the fork shoes are mounted
correctly and do not interfere with the frame (basis of the forks) at the sides.
Especially welding of the load wheel axel fixture can interfere. Sometimes
pieces of wood or stone can cause friction between weighing and non-
weighing parts.
10 VHT-SH-e-0910