V-belts – an overlooked improvement?
Published: 08 July, 2011
Engineers could be missing an easy win when it comes to energy and cost savings. Chris Rice at Optibelt reports.
V-belts are widely used in many factories and, as the first or only link in a drive chain, are responsible for transmitting a significant percentage of the total power consumed by a factory’s drives. Whether part of the building infrastructure or as part of a production process, a medium sized or large facility may have tens or even hundreds of V-belt drives, some of which may run 24/7, transmitting large amounts of power. A percentage of this will be lost due to inefficiencies – some of which are easily avoidable.
An interesting exercise is to perform a rough calculation of how many kWh of power are transmitted by V-belts across all drives within your factory in one year. The answer can be surprising, particularly for larger companies with multiple sites. Then consider that as much as ten percent of this can be un-necessarily wasted due to inefficiencies and it becomes clear that the potential savings can easily run into the tens or even hundreds of thousands of pounds per year depending on the level of V-belt usage.
Many users have low expectations of V-belts and so have overlooked this as an area for improvement. To put this into perspective consider the following points:
The design life of a good quality V-belt is around 25,000 hrs. That’s nearly three years of continuous 24/7 operation or eight years at 12hrs/ day, 5 days per week. Very few belt drives come anywhere near to reaching this potential and more than half will achieve less than one third of this. Many maintenance engineers believe that six months is a reasonable belt life.
A good quality, properly installed and maintained V-belt can run at up to 97% efficiency. Yet typically most will run at below 94%.
A large portion of these inefficiencies can be eliminated easily and with little or no capital cost by addressing a few points of maintenance good practice. In fact substantial cost savings in other areas along with productivity improvements can also be achieved.
However, before exploring the maintenance solution the question should be asked, and answered honestly: Will we/ can we maintain our drives?
To maintain or not to maintain?
Ensuring that belts are correctly fitted and, that they are regularly checked and
maintained at the correct tension is, in theory, a straight forward and low cost process. Yet the majority of belts out there on the UK’s drives are rarely or even never checked or adjusted. Everyday, Optibelt engineers are called to look at belts which are squealing, slipping or flapping – obvious signs of a problem, yet many more will go unnoticed, wasting energy and heading for premature failure.
There are many reasons why belts are neglected. They are often seen as low cost, consumable items and as changing belts is often quite a quick process it is easy to take the attitude – “just sling on another set”. Other components in the drive train such as motors, bearings, gearboxes etc. require comparatively little maintenance and as higher value items, preventative maintenance schedules tend to be designed around them.
In many cases, despite the best intentions, it is just not possible or economical to shut down plant in order to perform maintenance.
Whatever the reasons, if the answer to the above question is “No. We can’t maintain our belts as often as we should”, then serious consideration should be given to the use of “service-free” V-belts. These are designed to operate at optimum efficiency without the need for servicing throughout their working life and are a readily available, drop-in replacement for standard belts.
Whether using standard or service-free belts, a belt drive will not operate efficiently or achieve its potential life unless it is installed correctly in the first place.
Installation
There are a few basic points of good practice to address when installing a belt drive, the first of which relate to the pulley which should be checked for wear and alignment. Wear tends to be most prevalent on the smaller pulley and can be checked in seconds using a pulley gauge. These cost only a few pounds or are sometimes given away free by belt manufacturers. Worn pulleys are likely to result in reduced belt life and increased slip. In most cases replacements are low cost, off the shelf items and the cost of replacing will usually be outweighed by the savings achieved. Worn pulleys can also be indicative of other problems with the drive.
Pulley alignment is important as misalignment will cause excessive wear of both the belts and pulleys, a reduction in drive efficiency and other problems such as increased noise. It is one of the most common causes of problems yet it is usually very easy to detect and correct.
Alignment can be checked simply by placing a straight edge across the two pulley faces. Alternatively there are a number of tools available which make the job quicker and easier. For example, laser alignment tools, which magnetically clamp onto one pulley and project a line across targets on the other pulley allow an engineer to make adjustments while viewing alignment. These are available from under £300.
Correct belt installation tension is critical and is closely linked to drive efficiency. The optimum tension is a product of several factors including transmitted power, pulley size, centre distance etc. and is therefore unique to each individual drive. The widely practised procedure of checking tension by feel does not generally take this into account and often leads to incorrectly tensioned drives. Belts, which are too tight will operate less efficiently due to increased friction and internal material stress which can also generate heat. The belts are also likely to fail prematurely and there is a risk of overloading bearings. Belts, which are too slack will slip, perform inefficiently, overheat and fail prematurely.
Good quality belt manufacturers provide information and tools to assist with correct belt tensioning and the procedure need not take any longer than the checking by thumb method. Tools range from simple spring gauges, which are available for a few pounds to acoustic tension metres, which are more accurate and measure the “note” in Hz when the belt is plucked like a guitar string. These are available from a few hundred pounds. Some of these can allow an engineer to input the drive parameters and calculate the optimum tension. Others rely on separate software, which can be downloaded from the belt manufacturer.
Once belts have been installed correctly it is important to ensure that this tension is maintained. Most manufacturers recommend that this is checked shortly after fitting, once the drive has been run for a few hours, because there is always an initial drop in tension as the belts “bed-in” to the pulleys. This is usually accounted for in the recommended installation tension, which is typically around 30% higher than the optimum running tension. It may not be necessary to make any adjustments but it is good practise to check this - if practical.
Unless “Service-free” belts are used, belts will loose tension over time. How quickly this happens depends on several factors including belt quality, power transmitted and number of hours operation per day. The optimum preventative maintenance schedule therefore can only be established for each drive by trial and error but a good starting point would be monthly.
Maintenance inspections need not take more than a few minutes. Once the drive is stopped and isolated, a visual inspection of the belt surface to check for wear or other damage and a check of the belt tension using one of the methods previously discussed is all that is required. Correct belt tensions can be written on or near the drive or incorporated into the maintenance plan documentation. If re-tensioning is required then this should be to the “Running” tension rather than the initial “Installation” tension.
Consideration should be given to the quality of the belts installed. There is a whole spectrum of belts of varying quality on the market but unfortunately in the UK an increasing number are from the lower end as distributors source cheap belts from low-tech manufacturers and sell them under their “own brand” range. Others are sold under well-known and trusted brand names making it harder for users to establish the true quality of their belts.
The use of low quality belts can render good maintenance practise almost impossible and pointless and will almost invariably cost the user more. Large length variances can make the installation of sets of belts at the correct tension impossible while rapid and inconsistent stretch between consecutive sets of belts means that it is not possible to establish an effective maintenance schedule.
Engineers are advised to ask their supplier for details of the company who manufactures their belts as this may differ significantly from their expectations.
The fact that V-belts are so extensively used combined with the ease with which substantial improvements are immediately achievable for little or no cost means that optimising your V-belts can yield very significant returns in terms of the ratio of Savings versus Cost & Time to implement. You will find that a little attention to this area is well worthwhile.
For further information please visit: http://www.optibelt.co.uk/
