General information on loadpin

Loadpin is a proven design element that has been used for more than 50 years in metrology. In older books it is also known as Vibrometer.

Today's designations are:

• Force measurement
• Load pin
• Force measuring bolt/stud
• Measuring axis
• Force sensing bolt/stud

In English-speaking following terms are used:

• Loadpin
• Load Pin (both spellings are used)

General

Measurement principles

There are basically two methods of measurement principles in load pin. The first and most popular priciple is with strain gages, The second ist based on measurement of magnetic field.

For a resistor based measurement Weatstone bridge is used out of:

• Strain gages
• Thin film applied

In magnetic measuring a transformer circuit is built. As an iron core the hole load pin is used. Through the strain of the load pin the magnetic properties of the material changes and thus the voltage at the secondary coil.

Force fields

In all datasheets rated load, working load, ultimate load and breaking load are given. The description of this information shall be made in this approach.


Nominal force Fnom
Specifies the force to which the rated accuracy applies. That means, if forces applied which are below the nominal force, than the test results meets the specified accuracy.

Working Load Fu
Specifies the force up to which the load pin can be applied permanently. In this load range increases the uncertainty of the measurements. The results are useful, but not in the specified accuracy.

Load limit Fgr
Specifies the force up to which the load pin may be charged once, without the changes in the force measurement.(Occurring the load limit may shift of the zero point on)

Breaking force Fbr
On exceeding this load fracture risk.

Operating constructions

Calculation of measuring loadpin force

Force indicated for transverse force transmission

The transmitted force may change direction. This figure can be used to calculate how great the resulting force is in relation to the angle.Angle between the measuring direction of the load pin and the applied force:

Fr ... resulting force

Fa … applied force
 
Fr = Fa * cos(α )

Resulting force in the event of looping

Loadpins are often used in lift systems or crane systems.Loadpins are assembled in pulleys as axles for these purposes. If the loadpin is not wrapped around 180° of the cable, the resulting force is less than the sum of the two cable forces.

Fr ...   resulting force

Fa … applied force

α ...  Angle between the measuring direction of the loadpin and the applied force

Fr = 2*Fa*cos(α )

Instructions for the correct assembly of load pins.

General

• Keep the certificates of loadpins with their documentation.
• Before assembling a load pin, check that the measuring range matches the requirements of the application.
• Make a note of the load loadpin serial number and place of installation in the documentation provided with it.
• Never use the cable to lift or handle the loadpin.
• Do not overstretch the cable of the loadpin.
• After the loadpin has been installed, you should avoid using welding equipment as the current flows directly through the cell or induction could destroy the cell. The risk can be reduced by connecting a flexible earthing cable( copper, about 1cm long) between the upper structure and the lower support of the cell.

It is always better to replace the load pin with a "dummy" in the adjacent area during welding work.

Wiring instructions

• If necessary, the connection cables must be protected by protective pipes.
• The cable of the loadpin must be laid separately from and at a reasonable distance from high voltage and power cables.

Ambient conditions

Temperature

The loadpin is normally calibrated for working temperatures of -10 to +40°C, at a limit temperature of -20 to +70°C. The loadpin can be calibrated for other temperature ranges upon request.

If sensors are used at temperatures of less than 0°C, they must not be cleaned with steam or hot liquids as this would cause condensation to form inside the loadpin.

Water, steam, etc.:

Loadpins are manufactured to a standard with a protection rating of IP 65 (other protection classes are available upon request.
Please ensure that loadpins are not used in situations where a higher protection class is required.

If the load pin is assembled in a recess a drainage pipe, bilge pump or other protective devices must be used.

Please ensure that the load pin is not soaked in water.

Mechanical assembly

You should always handle loadpins with care. For this reason, hammers should never be used - loadpin are precision measuring transducers. In order to ensure weighing accuracy, please make sure that the only force that is acting on load pins is the weight that is to be measured.

Other forces from the environment such as vibrations, impacts, wind energy and temperatures may falsify the measurement result or even destroy loadpin.

Please ensure that the loadpin is only acting on forces in one direction. The action direction is almost always indicated by an arrow on the load pin. If there is no direction arrow, please use the direction that is perpendicular to the groove of the axle holder. If you are unsure, please contact the supplier of the loadpin.

Mounting alternavives (Gallery)

For further information please click on the picture.

 

• 
  Loadpin with cross-hole
• 
  Loadpin with both-sided splint protection
• 
  Loadpin with both-sided splint protection
• 
  Loadpin with welded on anti-twist protection and splint
• 
  Loadpin with head and splint
• 
  Loadpin with locking pin
• 
  Loadpin with torque support
• 
  Loadpin with head and nut
• 
  Loadpin with sunk-in distance sleeve
• 
  Loadpin with sunk-in distance sleeve
• 
  Loadpin with welded on torque support

 

Causes of faults

Because of its robust and simple design, the loadpin needs no maintenance if it is properly assembled. It is guaranteed to continue operating for many years. However, faults may occur as a result of the issues mentioned above.

The most common causes of faults include:

• Overloading or other mechanical stresses that exceed the limit values of the load pin.
• Welding operations carried out near to the loadpin
• Overheating
• Moisture in the loadpin as a result of sudden temperature fluctuations (washing the load pin using a jet of hot water or a steam cleaner)
• Chemical effects
• Connection cable damage

Testing the loadpin at the place of installation (this applies to versions without an amplifier only)

The values, which must be measured using a load pin that is in correct working order, can be found on the test certificate and/or type plate and/or in this manual, as can information about the colour coding of connecting wires and pin assignment.

Loadpin can be tested as follows:
(The values given are for a standard version load pin.)

Resistance measurement of the load pin bridge, while disconnected from the amplifier. The resistance (input resistance) must be approximately 375 ohms between the supply wires.
Between the output wires of the bridge, the resistance (output resistance), must be approximately 350 ohms.
Test the resistance between the body and the connecting wires of the loadpin. The value, which must be measured using a multimeter, must be more than 3000 MOhm.

Using the loadpin that is connected to the amplifier, when the loadpin is not loaded the mV output of the bridge must be approximately 0 mV. If this output is more than 10% of the maximum output signal, the load pin must be replaced.
Test the output at different load levels in accordance with the test certificate (see example below).

Example: sensitivity of the load pin: 2 mV/V bridge supply of the load pin: 10 V

Measured value of bridge output without load (0%) of the load cell: approximately 0 mV. measured value of bridge output at nominal load (100%): approximately 20 mV

Measured value of bridge output at 0.5 nominal load (50%): 10 mV. Values for other loads are calculated accordingly.

When ordering a load pin, always specify the type and serial number of the defective loadpin.

Force transmission

A description of the mounting situation is given on the second page of every data sheet. The way in which the loadpin should be loaded is described in this schematic representation.The supplied loadpin has been calibrated in exactly this configuration. No other load can change the calibration or lead to damage to or the destruction of the load pin.

Output arrow

 

The output arrow indicates the direction in which the loadpin emits a positive signal.

Reversal of the output signal

Output signal	Possibility of signal inversion
Passive signal (mV/V)	Exchange +Us and -Us
Voltage output (±10V)	With galvanically isolated inputs, GNDA and Ua can be exchanged.
Current signal (4..20mA)	No change of direction is possible

If the direction of force deviates from the prescribed direction,
measuring distortions will occur and the loadpin could be destroyed.

Attaching loadpins

A force loadpin must be attached in order to secure its alignment. Axial displacement must be secured first and rotation second. Proper securing is essential for obtaining accurate results. An axle holder pursuant to DIN15058 is used for this as standard.

Axle holder assembly:
Axle holder grooves are designed pursuant to DIN15058 in most cases. This standard recommends using one axle holder for diameters of up to 100mm and two axle holders per force loadpin for larger diameters. For best results, leave a small gap between the axle holder and the axle holder groove. This will allow the loadpin to bend freely in the outer support.
Gap B should be approximately 0.2 mm.

Experimental setup

A two-axis loadpin (diameter: 40mm) was charged with 100kN. The load was applied at different angles. As can be seen in the table, the forces resulting therefrom in the x and y direction. The resulting force and the angle were determined.

Results: Download .xls

Electronics

Loadpins can be supplied with integrated electronics or as passive sensors with strain gauge bridge. The "Electrics" section is intended for loadpins with integrated electronics only.

Thanks to a digital filter, the integrated electronics have a stable, low noise, zero point stable output signal, particularly at low frequencies of 5 to 105 Hz. The resolution on the analogue output is 4096 parts.

Configuration of loadpin electronics

The load pin with integrated amplifier supplies an analogue output signal of -10.0 V to +10.0 V or of 4-20mA. The display in the unloaded condition can be combined with the tare function (Tara) at 0.0 V or 4 mA, or to other factory preset values.

If both compression loading and tensile loading are displayed, a voltage output of ±10V is recommended.

Tare function (tara)

Applying a control pulse to the "tara" input automatically synchronizes the output signal to 0.0 V or 4 mA. The control pulse must be high for at least 1s, then low for at least 100 ms

Please note: no high signal must remain on the tara input when you switch on.

Scaling function (scale)

The loadpin is available with a scaling function as an option. Owing to a high level on the "scale input", the measuring signal that is currently applied is scaled to 10.0 V (20mA).

Prior to the scale function being activated, the tara must be applied.

Procedure: the sensor is subjected to a mechanical stress at 100% of the load. Applying a control pulse to the "Scale" input automatically synchronizes the output signal to 10.0V. The control pulse must be high for at least 5s, then low for at least 100 ms.

Please note: when switching on, no high signal must be present on the scale input.

Configuring the scale function (Setup mode 1)

The end value can also be scaled at less than 100% of maximum load.

The proportion of the calibration load on the maximum load can be set in Setup Mode 1 in increments of 5%.

Procedure:

1. Switch off operating voltage;

2. Create the scale input(grey) on the operating voltage(high potential);

3. Switch on operating voltage:

4. Disconnect the scale input from the operating voltage(high potential) - (Setup mode 1 is now active).

5. Re-applying the high potential to the scale input (for 2s) raises the threshold by 5%.

   Applying the high potential to the tara input (for 2s) lowers the threshold by 5%.

   The output signal now shows the voltage that will be displayed after the scale function has been activated.

   Example: If a voltage of 1.0 V is applied to the output (in Setup mode 1), calibrate to 10% of maximum load.

   If a voltage of 9.0 V is applied to the output (in Setup mode 1), calibrate to 90% of maximum load.

6. Switch off operating voltage;

7. Switch on operating voltage. The measurement amplifier is now back in normal operating mode..

Threshold (open collector)

The threshold switch reacts when the threshold is crossed. The threshold set in delivery condition is 90% of the measuring range. Above 90% of the measuring range, the threshold output is switched to earth. If elongation falls below 88%, the output switches to high impedance.

Configuring the threshold function (Setup mode 2)

The threshold switch of the threshold can be set in increments of 5%.

Procedure:

1. Switch off operating voltage;

2. Create the tara input on the operating voltage (high potential);

3. Switch on operating voltage:

4. Disconnect the tara input from the operating voltage (high potential); (Setup mode 2 is now active).

5. Re-applying the high potential to the scale input raises the threshold by 5%. Applying the high potential to the tara input lowers the threshold by 5%. In Setup mode 2, the output signal show the voltage at which the threshold will be triggered.

   Example: If you see an output voltage of 1.0 V on the output, the threshold value will be triggered at 10% of the maximum load and reset again at 8%.  If you see an output voltage of 9V on the output, the threshold value will be triggered at 90% of the maximum load.

6. Switch off operating voltage;

7. Switch on operating voltage. The measurement amplifier is now back in normal operating mode.

Safety instructions

All safety instructions must be observed during assembly, commissioning and operation of the loadpin. Only appropriately qualified personnel must be allowed to work with the loadpin. Failure to comply with the safety instructions may lead to serious injury and/or material damage. Before commissioning, please check to ensure that the load pin is suitable for your application. The instructions given in this document and on the test certificate must be observed.

Used cables

	Cabel Diameter	Bend Radius	Temperature Range
STC31V	2mm
24-4	4mm
4x0,14 PUR	5mm
2x2x0,25 PUR	6mm
3x2x0,25 PUR	6,5mm
SAC 4P/5P/8P	5,9mm	59mm	-40°C .. +80°C solid laying

Can-Bus

There are loadpins with integrated Can-Bus and external amplifiers, which are connected to the loadpin.

CAN-USB Adapter

Software for controlling the measuring amplifier:

Explosion protection Atex

On customers side the areas of application were divided in three zones.

There are three ways to use the explosion protection:

• Protection via Zener barrier
  
  
• Protection via capsulation
• Protection via admission of electronics integrated in a sensor.

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