Concrete Mixer Torque Application

Company: AMEC / BAA 
Location: Terminal 5 Development,
Heathrow Airport 

Reported Requirement: 

Large scale concrete production is critical for large scale developments such as those at Terminal 5 Heathrow. Being able to control and monitor the quality of concrete being produced was being disrupted due to the consistency of power being supplied. 

Controlling the power going into the mix is critical in terms of controlling consistency, and this was not possible due to fluctuating power levels from the national grid. 

The existing system used a measurement of the electrical input power (KW) to determine the viscosity of the mix, large fluctuation in the input voltage and the inherent inaccuracies of measuring power on large motors in this way made control of the process difficult. 

A more consistency measurement of the mix viscosity was therefore required. 

Project Scope: 

To accurately measure the viscosity of the mix the most direct and accurate measurement in an active process is to measure the input torque to the mixer. This technique has been proven in other mixing processes from China Clay to Bio-Reactors. 

To install a torque transducer in line with the drive was assessed to be uneconomic as this would require major structural works to the mixer and the mountings of the motor and gearbox. The drive line was then assessed to isolate a suitable component or length of shaft on which the torque could be measured. The drive line was very short but did have a coupling hub with and internally splined drive. A short length of this hub had a true mechanical path that transmitted the torque from the gearbox to the mixer. Using standard Datum Series 430 components the coupling was converted into a torque transducer. 

ADVANTAGE: No mechanical change to the drive line, no extra cost, minimal process disruption. 

The concrete mixer is power by a motor that drives a spindle shaft within the mixing drum to mix the concrete. The shaft is connected by the motor via a large coupling, by measuring the torque on the coupling; it would provide insight into the force being applied to the concrete mix. Understanding this applied force within the mixing process would provide key information of mixture viscosity and therefore allow concrete quality and control without the need to control the input power. As long as the coupling controlling the shaft could be monitored, data could be provided in the viscosity of the mix, thus controlling quality and efficiency of concrete production.

Concrete Mixer Torque Solution: 

Concrete production for the T5 Heathrow project was identified as being an inefficient cost under normal circumstances, and a cheaper alternative was required. 

Under normal circumstances, concrete mix is controlled by measuring the amps during the mix process and using the amps to understand when the correct mix consistency has been achieved. If the power being supplied to the motor is fluctuating and not at a consistent level, it can be difficult to keep costs at a controllable level.

One way of controlling the power fluctuations was to switch to known power supply in order to control mix quality. In the case of Terminal 5, this was the decision taken and a generator was supplied at the concrete production facility. This alternative, although resolved the issue, proved to be an expensive option, and alternative options were tabled. 

Datum Electronics were contacted to see if we could provide any assistance. Datum Electronics specialise in Torque Measuring Solutions and Instrumentation, and advised that the measurement of torque within the mix process, could be an alternative to control and monitor concrete production. 

Torque Measurement in essence is a very simple mechanical process, in its most basic meaning it is a measure of the ‘force’ being used in turning (or attempting to turn) something. When a force or ‘torque’ is applied to a shaft; the shaft twists (by a very small amount). This twisting produces a ‘stretch’ in the material of the shaft, and it is this process that allows a measurement of force in the mixing process. This measurement is achieved by the use strain gauges bonded to the shaft, measuring the strain induced in the shaft by the applied torque or ‘force’.

The input torque to a mixer is directly related to the viscosity of the mixture. Therefore … (In theory), torque measurement is the ideal principle in controlling the mix consistency, not just in concrete production, but also many other forms of mixers. 

The problem was how to apply these principles within the environment at Terminal 5. 

Theory into Practice: 

The concrete batcher facility used by AMEC at Terminal 5 Heathrow has a double input spindle for each batcher. The difficulty with this application was how to gain access to one of the shaft couplings, not only to install the gauges on the coupling, but also to get the signal off the coupling and logged into a display unit in the batcher control room. 

The primary challenge was to measure the torque transmitted into the mix without a major redesign of the mixer gearbox and drive. We looked at the project and offered a solution that was cost effective and involoved minimum disruption to the mixer itself. A large proportion of the solutions we propose solve the problems of :- 

(a) Finding a mechanical component in the drive line suitable for measuring torque. 

(b) Finding a position/engineering a solution to transmit the torque data from the shaft. 

The ideal solution is to locate a component that can be (removed/substituted/spare part) of the drive line that can be instrumented in laboratory conditions and calibrated before reinstallation to the drive line. This form of solution has the advantage of a fully sealed sensor installation and the possibility of direct calibration of the torque. 

Because of the nature of the environment, it was far easier to take the coupling out of its housing and assemble the torque measuring equipment at Datum Electronics. 

A solution was provided where Datum System 430 Electronics were installed onto the shaft along with an inductive power coupling. A stator unit was engineered to fit the envelope around the coupling this provides power for the shaft unit and acts as a receiver for the torque signal. Torque and Shat speed signals are transmitted from the stator as RS485 data to the control room. The data is displayed and recorded in the control room as either torque or power. 

We designed a system whereby the coupling could be re-installed with a completely separate stator unit fitted concentrically around the coupling. This stator unit was vital because this provided power to the strain gauges and received data from the gauges. 

This signal was converted and sent to the display panel mounted in the control room. 

Torque Measurement: Concrete Production 

The Torque monitoring solution provided the user with data relating the consistency and performance of the concrete mix. 

Data supplied from the shaft provided raw data and indication of how the mix was performing as it was being mixed. 

The data transmitted from the shaft show the sharp rise in torque as the load is added to the mixer and the drop off in torque (viscosity) as the mix becomes more consistent. Skilled operating staff are able to “tune” the mix be the addition of fluid to attain the desired consistency in the shortest time. The repeatability of the data from the torque signal has allowed a reduction in the time required. 

Analysis of the logged data shows that not only does the data show the change of viscosity but it also shows the consistency of the mix in term of the lumps within it. (Figure 2) 

Once the mixture was at the right consistency as indicated by the data, it could be dropped and the next load made ready. 

The display of data for the operator was optimised to show the exact performance of the mix 

Due to the fluctuations in the mix process, the data was smoothed in a software programme giving an averaged torque level allowing the user to know the exact performance of the mix. 

Following successful proving trials on the first mixer a second system has been ordered and installed. 

Once the first unit was installed, a second Torque measuring systems was requested for the second batcher due to the successes of the first batcher. 

For more information on all Torque Measurement solutions, Transducers, sensors and instrumentation, please contact us for more information. 

For more information on all Torque Measurement solutions, Transducers, sensors and instrumentation, please contact us for more information. 

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