Energy saving potential
An extensive study by PIX Transmission has examined the energy saving potential of raw edged cogged (REC) transmission belts over wrapped belts, when operating under similar conditions in HVAC environments.
In these days of rising energy prices and more stringent regulations, it is important to consider all options for reducing the energy profile of all machinery. This is something our customers are asking us for, and was a primary reason for us to carry out the study.
So PIX, who supply both types of belts, carried out the research, to quantify the comparative energy efficiency of Raw Edge Cogged against traditional Wrapped construction. The results have proved conclusively that REC belts can benefit HVAC customers with potential energy savings of up to 3.8% over wrapped belts.
Any machinery that requires high power transmission coupled with small pulley diameters and high speeds can benefit from REC belts, and ventilation manufacturers are an obvious place to spread the word of their energy saving potential. REC belts with their preformed cogged structure provide extra flexibility and smooth running characteristics, so are ideal for smaller pulleys. They also transmit significantly more torque than their wrapped counterparts.
Efficiency losses in friction belt drives come from two main sources – creep and friction. When belts are installed on pulleys, their static tension is fairly equally distributed between the two ‘strands’ of the belt stretched between the pulleys. When the drive is set in motion, torque transmission is achieved by the installed tension being redistributed, forming a tight side and a slack side, the change in length between tight and slack side strands can only be accommodated by ‘creeping’ of the belt surface over the pulley. The net result of this creep is an effective speed loss at the driven pulley
Friction energy loss is present wherever belt bending and belt ‘entry-to’ and ‘exit-from’ pulley grooves occurs. The textile filled compounds used in wedge belt manufacture are intended primarily to support the load carrying cords of the belts effectively, whilst having good wear properties. A cogged raw edge belt construction minimises belt bending-stiffness, but the materials do have a finite damping energy ratio, which means that hysteresis energy is converted into heat during the constant bending and straightening of the belt in operation.
Hysteresis and friction losses occur as the belt slips across the groove flanks each time the belt enters and leaves the grooves. Raw edge belts have a lesser coefficient of friction, hence suffer less friction loss. The effects of creep, friction energy loss help to promote REC wedge belts over their wrapped counterparts. Wrapped belt drives may also utilise more belts on similar pulley diameters (due to their lesser power capacity), which will further reduce efficiency.
These advantages are well recognised by engineers in flagship OEM producers. Paul Wenden, Engineering Director of Fläkt Woods said: “This is an important and valuable investigation, and one that endorses what we at Fläkt Woods have accepted for some time now. The REC system has measurable advantages over the wrapped belt – and when we are all looking for ways to demonstrate how to maximise energy efficiency, it serves as an important signpost towards best practice.”
The investigation 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.
A new belt drive with proper installation will initially give 95 to 97% efficiency. Over time energy is lost due to slippage, flex bending of 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.
PIX 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, with tensioner pulleys applied to reach the required tension – this was done by moving the tensioner pulley upward until the specified tension is reached. The belt was allowed to run for one minute without applying the load on the generator so that the belt was able to seat properly into 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.
The energy saving study was calculated by applying a 6kW load on the generator and the cumulative power consumed by the motor with wrapped belt A42 was recorded in the data acquisition system for every 30 seconds, up to 150 hours. Then a REC belt AX 42 was tested under similar conditions and the data recorded in the system.
Study on belt tension decay with time
In this study wrapped V belt was mounted on the fixture, 50 kg of tension was applied and it was allowed to run under 6 kW load in the generator.
The tension decay with time was recorded for every 30 seconds using the data acquisition system. The testing condition is set in such a way that the rig gets tripped whenever the belt slippage exceeds the preset limit of 4% and in such cases the belt was allowed to cool down to 30 minutes, the belt tension was re-adjusted to the original level and then the testing restarted until the belt failed to transmit power.
The tension decay study for REC belt under similar conditions as that of wrapped belt was done until the belt failure.
From the graph the following inferences can be drawn;
A significant portion of tension decay takes place within 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 grooves
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 counteract the tension decay by working against the belt growth.
The performance of 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 of wrapped belt drive in comparison with REC belt has been studied by varying the tension from 30 to 90kg, at 4 as well as 6kW loads. In all the tests, the belt after applying the tension was allowed to run and only the slippage recorded after 30 minutes was considered for the comparison.
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 1%. Under this laboratory test condition, the power rating of wrapped belt is around 5kW and that of REC is 8kW which means that under this running condition the power rating of REC belt is 60% higher than that of wrapped belt.
The calculations clearly demonstrate that REC belts give energy savings of 3.68 to 3.98%, over the wrapped V belt equivalent, under laboratory testing conditions. The cost economic analysis based on the field test data indicates an attractive return on investment of REC belt for the HVAC industry. Apart from intangible benefits like less tension on shaft and associated bearings, less down-time, and better reliability, with high energy saving potential; the pay back period, on investment of REC belts, can be as short as three to four weeks, depending on the drive.