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With rising energy prices and more stringent regulations by the day, it is becoming increasingly important to investigate all options for reducing the energy profile of all machinery. A properly installed new belt drive will initially give 95% to 97% efficiency. Over time, however, energy is lost due to slippage, flex bending of the belt over the pulleys, etc. Ultimately this loss disseminates as heat, which in turn affects the performance of pulleys, shafts, couplings and associated bearings.
In most of the drives, maintaining optimum belt tension during the course of service by reducing the tension decay itself would lead to a substantial energy saving. In terms of design, by and large, the wrapped belt and the Raw Edge Cogged (REC) belt remain the work horses in industrial applications, as these belts could be interchanged without needing to alter drive design.
A year long study by PIX Transmission looked at the energy saving potential of REC over the wrapped belt under similar drive conditions. It also explored the relative advantages of REC over wrapped belt, in terms of belt re-tensioning requirements by studying the belt tension decay with time under controlled conditions. Finally the survey analysed and quantified the economic advantage of REC over the wrapped belt. In all the studies, the belt was mounted over the drive, driven and tensioner pulleys, applied the required tension by moving the tensioner pulley upward till the specified tension is reached. The belt was allowed to run for one minute without applying the load on the generator so that belt seats properly in to the pulley grooves. The belt tension was re-adjusted and then the tensioner pulley was locked on its position. The test was started and after reaching the testing speed, the load was gradually applied on the generator within a minute.
Energy consumption
The energy saving study was calculated by applying a 6kW load on the generator and the cumulative power consumed by the motor with the wrapped belt A42 was recorded in the data acquisition system for every 30 seconds, up to 150 hours. Then a REC belt, AX42, was tested under similar conditions, and the data were recorded in the system. The summary of power consumption at 24 hours interval, up to 150 hours, is given in the table above:
Belt tension decay
In this study a wrapped V belt was mounted on the fixture, applied 50 kg of tension and allowed to run under 6kW load in the generator. The tension decay with time was recorded for every 30 seconds using the DAQ, with the testing condition set in such a way that the rig gets tripped whenever the belt slippage exceeds the preset
limit of 4%. In such a case the belt was allowed to cool down, belt tension was re-adjusted to the original lever and then the testing restarted till the belt failed to transmit power.
The tension decay study for an REC belt under similar conditions as that of wrapped belt was done till the belt failure. The tension decay in terms of percentage with respect to the original for wrapped as well as REC belts is given in figure below. From this graph the following inferences can be drawn:
- A significant portion of tension decay takes place with in 24 hours of starting the testing;
nearly 30% in wrapped and around 20% in the case of REC belt. Such a sharp decay in belt tension could be attributed to the bedding of belts in to the pulley groove.
- The reversal in the rate of decay is attributed to the heat induced thermal shrinkage of polyester cord. The shrinkage force of polyester cord tries to counter act the tension decay by working against the belt growth.
- The performance of an REC belt under laboratory testing conditions seems to be almost 3.0 to 3.5 times better than the corresponding wrapped belt.
Belt slippage with tension
The effect of belt tension on slippage has been studied by varying the tension from 30 to 90kg, at 4 as well as 6kW loads. In all the tests, the belt was allowed to run after applying the tension, and the slippage was recorded after 30 minutes. The results are depicted in figure below. From the graph it is clear that the wrapped V belt experiences higher slippage than the REC belt at any given running condition. The power rating in general is decided based on the maximum power at which the belt slippage does not exceed one percent. Under this laboratory test condition, the power rating of the wrapped belt is around 5kW and that of the REC belt is 8kW. This means that the power rating of the REC belt is 60% higher than that of the wrapped belt.
Conclusion
It has been observed from this study that the REC belt gives an energy saving of 3.68 to 3.98% over the wrapped V belt under laboratory testing conditions. The cost economic analysis based on these data indicates an attractive return on investment on an REC belt. Benefits like less tension on shaft and associated bearings, less down time and better reliability add up with a considerable energy saving potential.
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