TechArchive

Original article date: January 2000

Manufacturers of rotational filling machines are increasingly moving towards using weighing as a means of improving accurate fill control. Alan Chapman, managing director of Tedea-Huntleigh, examines the design criteria and the pitfalls.

Typically, rotational gravimetric filling machines use strain gauge load cells that move rotationally with the carousel. As a result, the load cells must be capable of working under difficult dynamic conditions including vibration, angular acceleration and centrifugal forces – all of which can adversely affect performance.

To compound the situation further, the load cells can be subjected to significant side forces caused by bottle jams and other process mishaps. Finally, chemical spillages and cleaning fluids can damage the load cells and degrade performance.

Standard load cells fall well short of the requirements for such equipment. Despite the use of elaborate electronic filter mechanisms and additional mechanical protection, performance usually falls below expectations.

To address the problem, Tedea-Huntleigh has developed a family of special load cells for rotary weighing which:

• Significantly reduce dynamic errors – even at speeds of up to 15rpm
• Provide first settling times of under 300ms
• Can withstand extreme side loads of up to 1000% of rated capacity
• Can withstand chemical spillages and washdown.

The dynamic errors in rotary weighers arise from several effects, the most significant being centrifugal forces and vibrations. The magnitude of these errors is proportional to speed of rotation, to the distance of the filling station from the centre of the carousel, and to the height of the bottle platform above the weigh cell.

Angular acceleration, which occurs primarily during starting and stopping, also adds to the dynamic errors. The 1400 series of load cells incorporates three independent design concepts, each of which addresses problems from one error source such that the combined design significantly reduces dynamic errors.

Settling time:- the throughput of any rotary machine is proportional to the speed at which the load cells can measure and transmit a stable weight signal. A typical undamped load cell will take up to 1s to settle before a reading can be taken, greatly restricting the filling speed. The well tried and tested method of viscous damping is used to reduce settling times and it has been shown to be more effective than alternative methods.

However, the standard damping method does not work effectively in the case of rotary weighing and a new variation of viscous damping has been conceived and implemented, which provides settling times of less than 300ms.

Environmental considerations:- these are extremely important for load cells used in rotary filling machines. Not only is such equipment subjected to regular cleaning and washdown, but also to chemical spillages. The 1400 series is available in two versions. The 1410 is designed to be installed underneath the filling table where the design of the carousel can be utilised to provide full environmental protection. The lower profile 1430, which is designed to be installed on top of the carousel, is housed in a stainless steel enclosure and load is applied through a shaft which is sealed with an elastomeric diaphragm. A breather pipe is fitted to compensate for pressure differences between the inside and outside of this enclosure and all external metal parts are stainless steel.

Overload and side load protection:- load cells in rotary fillers can be subjected to very high side forces if problems arise in the process. These forces may be caused by bottle jams or when pieces of broken bottles become lodged in the equipment and the load cells must be capable of withstanding both radial and tangential forces.

The resistance to side loads is dependent upon the rated scale capacity and the height above the cell at which the load is applied. Both the 1410 and the 1430 can withstand side loads of over 1000% of rated capacity for forces applied 100mm above the cells. In addition to side load resistance, both models have built-in bi-directional overload stops.

• Tedea-Huntleigh

January 2000