Torque About Weighing Vessels
A
Vishay Nobel product story
Edited by the Engineeringtalk editorial team
May 5, 2000
Measuring weight may be complicated by attached pipework, vibration and elevated temperature. This application note explains how to deal with the problem.
While measuring the weight of a vessel is normally considered by many to be straightforward - mount it on three, four or six load cells and sum their outputs to give the total weight - sometimes life is not as simple as that.
This action may easily be complicated by attached pipework, vibration and elevated temperature.
In circumstances such as these many load cell installations have great difficulty in giving repetitive and accurate results due to the changes in the manner the loads are applied to the load cells.
Ways to reduce some of the changes in load application, free motion units, tiebars, rocking mounts, rubber-mounting etc.
are many and varied.
However, all of the above introduce extra elements into the weight path or create possible load shunts around it.
Very careful setting-up may allow for good performance at the time of installation, but the performance can degrade quite rapidly with time as components settle/stretch or distort with use.
So how is a vessel weighed that not only has vibration, pipework and elevated temperatures, but also has a large motor attached to a horizontal mixer shaft running down the axis of the vessel? To complicate matters the reaction torque from the motor is taken to an extra mounting point on the ground away from the vessel creating a very large and widely varying weight shunt error to ground on one side.
This is effectively a large force pushing up on one side of the vessel that would interfere with the weighing accuracy.
The solution supplied by Nobel Systems is to use the unique Nobel KIS family of Shear Force Load Cells to measure the load to the ground at ALL points including the torque reaction point.
By careful choice of KIS Load Cell capacities and appropriate Nobel signal conditioning it is possible to display not only the true compensated contents weight of the vessel, but a bonus - on a separate display - the Torque reaction Force generated by the Motor.
This Torque measurement along with weight measurement allows the vessel contents to be optimised for best efficiency.
The torque generated depends not only on the mass of the contents but the actual characteristics of the products being mixed at the time - low torque and/or weight (and thus contents) and throughput are reduced; too great and the motor can be burnt out or the mixer blades damaged.
Several systems have now been installed to this design and system accuracy has proven to be better than 0.05% of weighing range, but probably more importantly, the long-term repeatability of the systems have proved to be excellent.
The unique properties of the KIS load cells allow for them to take all of the potentially disruptive loading elements of the above system and ignore them to a very high degree.
This is because of the construction and loading method that allows for all loads, whether side, end, vibration, or even reversing loads as in the case of the torque reaction point, to be taken by the load cell without any external support or guidance.
The low deflection of the KIS assembly compared to most of its competitors gives the ability to offer greater accuracies when there are external influences on the vessel.
Nobel Systems also offers assistance in the design of pipework, including the use of flexible couplings, to ensure that the optimum installation is created.
Installation and maintenance of the KIS load cell assemblies is very routine, this giving significant short and long term savings that are often overlooked.
No 'cornering' adjustment is needed and in the unlikely event of a load cell needing to be replaced, a new one can be installed without the need to recalibrate the system.
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Contact Vishay Nobel
Tel +44 1274 782229
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