Advanced Bearing Technology Simplifies Pump Designs

Bearings are a key component in process pumps, their reliability and operating life having a significant effect in ensuring that operating costs are minimised. As the pace of advanced bearing technology accelerates, opportunities for technology transfer mean that bearings can play a wider role in reducing the whole life costs of pumps. This is achieved not by supplying cheaper components, but by applying the latest materials, sealing and lubrication technology to 1) improve the pump design and reduce its manufacturing cost; 2) reduce pump maintenance costs; 3) increase pump availability, or "up time," and 4) reduce individual process times.

The potential benefits of Barden's technology transfer initiative to the UK process industry are immense. Currently the UK market for pumps is worth about £980M a year. Centrifugal pumps account for 80% of this total, each being equipped with between 4 and 5 bearings - 2 or 3 in the pump and at least two in the motor. The typical L10 life specified for these bearings is three years, but when properly installed, lubricated and maintained this should rise to 12 or even 15 years. In practice, however, average-bearing lives can be as low as just one year, accounting for 60% to 70% of all process pump failures.

Even making a generous allowance for both incorrect assembly and poor maintenance, then undoubtedly some of these failures are due to the process industries historical insistence that bearings should cost very little. What is becoming increasingly obvious, however, is that the decision in favour of a specific bearing technology solution should always be taken after analysing the overall cost / benefit issue and not merely on the basis of purchase price. Buttressing this argument is the fact that today's high technology bearings offer improved features which enable whole life cost reductions to be achieved, providing benefits to both pump manufacturers and users - despite an overall higher bearing price.

Reducing Manufacturing Costs

The process of whole life cost reduction begins at the design stage of the pump, and exploits the possibilities for simplifying the mechanical design. This in turn simplifies the pump assembly and the subsequent maintenance task, resulting in both direct and indirect cost savings. One major way in which design simplification can be achieved is through the replacement of oil lubrication with grease. Only a few years ago, oil lubrication was necessary to achieve speed indices (n.dm = speed in rpm x the ball PCD in mm) above one million. Nowadays, bearings with greases such as FAG/Barden's high-speed grease ARCANOL L75 can be operated reliably at significantly higher speeds, enabling grease lubrication to become the rule for most high-speed applications. This trend is also supported by additional benefits of the lubrication medium, including lower temperature operation and appreciably longer life.

The savings potential resulting from the change from oil lubrication to grease lubrication is quantified. Substantial cost reductions are effected because an oil lubricating system is no longer required neither are injection rings or machining for the bearing lubricating holes. When compared to oil throwaway lubrication, the savings per pump more than compensate for the additional cost of the new bearing arrangement. Furthermore, the reliability of the pump is increased since the lubrication is virtually fail-proof.

Reducing Maintenance Costs

With average bearing lives in the worst of cases being as low as one year, maintenance costs for many pump users are unacceptably high, leading to higher whole life costs. One possibility for a reducing these costs is replacement of the existing bearings with sealed, grease- lubricated bearings. While lifetime lubrication with grease combined with a seal has been available as a standard for high-speed bearings for quite some time, angular contact bearings have only been available as open versions, due to a number of reasons, not least their thinner cross section. FAG/Barden has solved this problem with a revolutionary seal and recess design that enables seals to be fitted to retain grease and exclude contamination from both sides of the bearing.

The Barden RSD seals have retention characteristics equal to standard seal designs, but are fitted in such a way that there is no contact with the bearing inner rings. As a result, friction is minimised and there is no rise in the bearing operating temperature. The seals perform two main two functions. Firstly, they provide protection against external contamination. The airflow that may drive the grease out of the bearings and may lead to an accumulation of contaminants and drying of the grease is largely reduced. Secondly, the seals keeps the base oil of the grease in the bearing, which is especially advantageous with a vertical bearing arrangement as found in many types of pumps.

Whilst the use of sealed bearings is initially more expensive, savings are made due to the fact that maintenance intervals are longer and stocking of the grease and subsequent greasing of the bearings is no longer necessary. Also, since the grease stays in the bearing during operation, the cleaning of the bearing environment requires less work.

Another key point is the fact that the cleanliness of the bearing no longer depends on careful handling by the fitter, which means that there is an increased amount of safety with regard to mounting errors.

Increased Bearing Service Life.

In the overall drive to reduce whole life costs of pumps another key objective has to be increasing the bearing service life. Incurred wear is one decisive factor adversely affecting service life - and hence the efficiency of the pump. However, wear can be reduced, and also the tendency towards mixed friction during pump start/stop operation, by the use of Barden's X-Life Ultra bearings. A synergy of materials, surface engineering technology, sealing and lubrication technology, these bearings achieve a service life, which is up to three times the service life of conventional bearings.

The cost savings, which can be achieved using X-Life Ultra series bearings, are ably demonstrated using the example of a hypothetical screw pump, equipped with a bearing arrangement consisting of two 60mm bore angular contact bearings in a back-to-back formation.

Assuming the pump price when new is £900, and the costs for a new bearing arrangement for the pump, including bearings, labour and production loss, amount to £350. Out of this figure the bearing cost is £50, giving a ratio of bearing cost to the other costs for exchanging the bearings of 1 to 6 (£300/£50). The pump operates on a three-shift system and, due to its start-stop procedures during operation, exhausts the life of its bearing lubricant in one year, necessitating a bearing exchange. The yearly maintenance costs are therefore £350.

Replacing the existing conventional bearings with hybrid (ceramic balls & steel rings) and sealed bearings of the same series, the average time until new bearings are required is increased from one to four years, due to the reduced tendency towards mixed friction during start-stop operations. While the bearing costs are 8 times higher (£400 as opposed to £50 see Figure 2), yearly maintenance costs are reduced to £175, corresponding to savings of 50%. Moreover, even if the life of the new bearings is only 3 times longer, the saving still amounts to 33.33%.

Increasing Pump Availability

The benefits of increasing bearing technology life also translate into increased availability, or "up-time" leading to a reduction of the pump or process rate per hour. As an example of this: if the availability of a process pump with a rate per hour of £67 increases from 80 to 90%, the process rate per hour decreases to £59.

In addition to its function in increasing pump availability, bearing technology can also play a major role in reducing overall process times. An increase of the permissible speed of a bearing arrangement can be achieved by changing from grease to oil lubrication. However, This entails a change in the pump design and additional expenditure for mating components and auxiliary devices. Another possibility is to change the bearing while maintaining the boundary dimensions.

Taking the example of rigidly preloaded 70mm bore diameter bearings in a tandem-O-tandem arrangement: a change from standard high speed (HS) angular contact bearings to Barden's X-Life Ultra units filled with Arcanol L75 grease, realizes a speed increase from approx.14500 to 19000 rpm. This means that increased capacity is achieved without the need to revert to oil and it's associated additional costs.

Extreme Example Proves Value of Advancements In The Technology Of Bearings

If one example proves conclusively the role of technology in reducing the overall life costs of pumps it is the NASA space shuttle. The bearings supplied for use on the main engine fuel pumps of the shuttle initially had to be replaced after each flight, at a cost of £100,000. Changing these bearings for FAG/Barden's X-Life Ultra Bearings means that now six flights are undertaken before the bearings are replaced, reducing the pump maintenance costs by nearly 84%.

In addition to underlining technology benefits, what the above example also underlines is that a decision in favour of a specific bearing solution should only be taken after examining the whole life cost and not merely on the basis of the bearing purchase price. Furthermore, if technology is employed correctly, cost reductions may be achieved in spite of a higher bearing price, which, in turn, offers advantages for both the pump user and pump manufacturer.

About The Author:

Mark Harris is a successful author and regular contributor to http://www.bearing-n-bearings.com. A one-stop resource for the bearing industry featuring articles, comparisons and reviews of bearing related products and manufacturers.