Loadcell Manufacturing

2.  SPECIFICATIONS:  

(See tables "A" and "B").  Figures quoted as minimum or maximum values are guaranteed.  Typical figures, or minimum and maximum values shown in parentheses, are representative of a typical unit and are provided for guidance only.   


3.  INSTALLATION:  

3.1 Handling:  

The three parts of each loadcell are held together only by a film of grease.  It may be helpful to supplement this during installation with a strip of masking tape around each joint if handling is likely to be difficult.  The parts of each loadcell form a matched set and should not be interchanged with parts from other cells.
 
 

3.2 Mounting Blocks:  

Mounting blocks should be accurately flat and level, and sufficiently rigid not to deform under load.  The areas in contact with the loadcell should preferably be ground flat to ensure uniform contact with the cell (which is essential for good linearity).  Care should be taken to avoid trapping dirt between the loadcell and the mounting blocks.  It is usually unnecessary to fix the loadcell in place but this may be achieved if required by three pins inserted into (or welded to) the mounting blocks (in this case one or both mounting blocks should be adjustable to allow alignment of the loadcell).



3.3 Constraining:  

Suitable constraining of the structure is essential to minimise side-loading, translation, and tilting, of the loadcell.  However, some translation due to thermal expansion and tilt due to deflection under load are inevitable.  Where these have a preferred direction (usually along a diameter of the structure) this should be aligned with the cable entry of the loadcell.

   

3.4 Cables:  

The loadcell cable contains two screened twisted pairs of wires.  Each pair comprises a black and a white wire and is identified by numerals printed on the white wire at nominally 50mm intervals.  Pair one is for the loadcell supply while pair two provides the output signal.  Each pair is individually screened to minimise cross-talk.  The cable has a third screen to intercept electrical interference.  The screens are not connected to the loadcell casing.   In multiple loadcell systems the corresponding wires from each cell should be connected together in a water-tight junction box with a single cable (usually of larger cross-section) to the indicator.  It is strongly recommended that the inner and outer screens should be kept separate.  At the indicator the outer screen should be connected directly to the chassis as close as possible to the loadcell connector and the inner screens should be connected to the analogue "0v" line.   The single orange wire in each loadcell cable is connected to the casing for convenience in testing insulation.  These should not be connected in the junction box.   In routing the cables, care should be taken to avoid power cables and to ensure that the loadcell cable will not become immersed in water.

   

3.5 Environmental Protection:  

Both loadcells and cables should be protected from immersion in water and from the direct incidence of sunlight or precipitation (either of which may create abnormal temperature distribution within the loadcell and affect the temperature compensation).  See also the rate of change of temperature specification.   Loadcells may be damaged by arc welding or by nearby lightning strikes.  Contrary to popular belief, failure results from a rise in local earth potential causing break-down between the straingauges and the billet rather than from the flow of current through the loadcell.  Shunting the loadcells with an earth strap is therefore ineffectual.  Temporary protection can be achieved by completely disconnecting the loadcells from the indicator and from each other.  Permanent protection is best achieved by insulating each loadcell from the ground and from the structure, and earthing each cell casing to the indicator with a substantial conductor.

 

3.6 Maintenance: 

The only maintenance necessary is to grease each loadcell at least annually and more frequently if significant motion of the load bearing plates occurs.  The grease should be suitable for extreme pressure applications.  It is preferable to grease the loadcells under minimum load conditions.

Loadcell Repairs.

Electronic Transducers can repair most makes and capacities of loadcells.