An overlooked area for savings
May 1st 2005 Fans, pumps and HVAC systems receive an efficiency boost from belt drives
With the advent of higher energy costs and Enhanced Capital Allowances, energy efficiency has been a hot topic. Many examples are given where small percentage increases in efficiency can reduce long term energy usage and provide substantial financial savings. Fans, pumps and HVAC systems are obvious targets since they tend to be run constantly and so provide the most visible savings. When all the focus is on equipment such as inverter drives and motors however, it is easy to overlook other elements in a power transmission system and their contribution towards energy efficiency.
Belt drives are a prime example of an overlooked area where other gains in efficiency can either be wiped out or enhanced. Bryan Burton, power transmission specialist at WYKO explains how the right specification for belt drive systems can deliver a similar gain in efficiency as upgrading to an EFF1 electric motor from an EFF2. Site-wide, the difference can amount to thousands of pounds each year in energy savings.
“Why look at belt drives in typical fan and pump applications? Well, simply because they are still the most commonly used form of power transmission for these machines, despite the rise of variable speed inverter controls.
The speed ratio capability of a belt drive allows the electric motor to operate at or near its optimum (most efficient) speed. Belt drives have low lifetime costs, are versatile and easy to maintain, but can also contribute significantly to energy savings if correctly designed and installed”.
Specifying the right belt type, correct pulley size and the optimum number of pulley grooves/belts is critical to achieving both full service life and maximum efficiency.
A simple wedge belt drive system, when correctly specified, can be expected to deliver 25 000 hours of service (~3years continuous, or 5years normal use) before belt replacement is required, and the good news is that it could achieve a power transmission efficiency of over 95%.
Without expert knowledge however it is easy to specify a system that does not immediately look wrong, but can be running at as low as 80% efficiency. The resultant costs are sobering: Using a typical air handling unit as an example; the energy cost for a fan powered by a 37 kW 4-pole electric motor running 24hrs/day for 1 year (assuming an average electrical energy cost of £0.06 / kW.hr) would be around £20 000 per annum. The difference between a well specified system and a poorly specified one therefore is £2917 per annum for just one fan.
How to maximise belt efficiency
- Which Type of Belt to Use
Historically, the wedge belt is a derivative of the vee belt, but with a deeper trapezoidal cross section compared with a vee belt of similar top width. This gives greater power transmission capability but at some expense in efficiency.
The Cogged Raw Edge (CRE) wedge belt has no fabric jacket material on the working flanks (hence Raw Edge) and has moulded ‘corrugations’ (Cogs) in the belt base, the combined effect of which is to dramatically reduce the bending stiffness of a wedge belt section, compared with conventional wrapped/jacketed belts.
CRE wedge belts can also transmit more power than conventional wrapped wedge belts. This is due to:
- Greater tensile strength - derived from greater overall load carrying cord section.
- Lower mass/unit length - which reduces centrifugal tension loss.
- Greater bending fatigue resistance - due to reduced bending stiffness.
The latter two attributes also contribute to increased drive efficiency.
- Contributing factors for calculating efficiency
Efficiency losses in friction belt drives come from two main sources:
- Creep – is an inescapable issue in friction belt drives.
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. Research data indicates that a value of 0.5% is appropriate for an optimum design of drive, correctly installed and maintained. An average value for commercial installations, however, would be nearer 1.0%. This creep, or speed loss, translates directly into an efficiency loss in terms of actual power transmitted.
- Hysteresis and friction energy loss – is present wherever belt bending and belt ‘entry-to’ and ‘exit-from’ pulley grooves occurs.
The textile filled elastomeric 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.
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.
Hysteresis and frictional energy losses can account for as little as 3.0% transmission loss in an optimum design of drive, correctly installed and maintained using CRE wedge belts. The value for commercial installations is, however, anywhere between 4.0% and 10%, depending on drive design and conditions.
Other factors such as pulley misalignment, worn pulleys or incorrect belt tension can push this figure significantly higher.
It is important to remember that these percentage losses apply to the drive capacity, not just the power transmitted – accentuating the
- Drive Design
The effects of creep, hysteresis and friction energy loss help to promote CRE wedge belts over their wrapped counterparts. Wrapped belt drives may also use more belts on similar pulley diameters (due to their lesser power capacity), which will further reduce efficiency. The net result is an average 1.5% greater loss in any given drive conditions, using wrapped belts.
Service factor
When calculating a drive solution in terms of pulley diameters, groove numbers and belt size, a Service Factor is used to help ensure consistent belt life is achieved.
Service Factors are a ’leveller’ which help achieve this consistency for differing prime mover & driven machine characteristics, and various duty cycles. Drive performance, therefore, depends on an adequate Service Factor, but drive efficiency is only achieved by not “over-factoring”.
Energy losses are therefore minimised by using drives with Cogged Raw Edge belts and Appropriate Service Factors – which help optimise the drive capacity.
[The formula above shows how excessive drive capacity is punitive on efficiency, without contributing to the effectiveness of the drive].
Other contributing factors for efficiency: When designing a belt drive system it is worth considering using larger pulleys to reduce the degree of belt bending and lower trip rates to reduce the frequency of bending, both of which have a positive effect on reducing transmission losses.
Lower drive ratios are also desirable as the high wrap angle on the smaller pulley has a positive effect on the available tension ratio.
There is a significant difference between common practice and best practice in the design and application of wedge belt drives.
The cumulative percentage losses of a poor specification can reduce the efficiency of a belt drive system substantially and cost thousands in wasted energy costs. On the positive side, a belt drive system can also provide a long lasting, efficient and cost effective solution on applications such as fans and pumps when specified correctly.
The potential savings on even a single application make it eminently worthwhile to spend just a little time and effort seeking expert advice. More articles from ERIKS UK: | 
