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Slick shaft seals are savers

01 June 2009

Reduce friction between shaft and seal and you are well on the way to saving a lot of energy over the lifetime of the equipment on which they are installed. But how do you achieve this without compromising seal performance? Blue Diamond believes it’s a matter of precise computer simulation and rigorous physical testing

In today’s competitive markets, there is a much greater awareness of energy efficiency, and engineers are looking closely at every aspect of their product design to see how the lifetime energy consumption can be improved. For example, products such as domestic appliances, lighting and motor vehicles have to carry an energy rating which makes it easier for the customer to compare the performance of similar products.

One way that the energy consumption can be reduced is by maximising efficiency, for example by reducing energy losses. Most items of rotating equipment feature seals, which make contact with a rotating shaft and prevent media getting either in or out, for example, lubricating oil, refrigerant, gas or water. When the lip of the seal bears down on the shaft there is friction and stiction to be overcome. Over the life of the machine this energy requirement can be significant – and may increase as the seal ages or if the machinery is used infrequently allowing dirt and debris to build up. So what can be done to minimise these energy losses whilst still maintaining an effective seal?

The traditional approach to selecting a seal for a particular application simply means looking through an engineering supplies catalogue and selecting a seal that is a close match to the requirements. However, because these standard seals are manufactured to perform across a wide range of operating conditions, their design and construction has to be a compromise to cover the myriad of possible uses.

As efficiency has become more important, a modest improvement in seal design could significantly reduce losses. This could translate into savings of thousands of pounds for a manufacturer of electric motors by enabling them to offer a smaller and more efficient motor for a given application. Other examples where improved efficiencies provide significant savings include washing machines, refrigerators panning CCTV cameras – the list is endless.

Blissfully unaware
But how does an engineer obtain a seal that is custom engineered to an individual application and which is expected to maximise efficiency? Many may be blissfully unaware of the technology that goes into the design and production of a modern, high-performance seal. The geometry of the lip, for example, the radial load and materials all affect the performance of a seal and, with so many different applications, materials and design options, designing the correct seal can be a daunting task.

Fortunately there are companies that have the knowledge and experience to design and manufacture seals to meet individual customer’s requirements. Using powerful software, CAD and parametrics packages, the physical details of the seal - for example, inner and outer dimensions, materials and seal style - can be entered into the program.

Finite Element Analysis (FEA) allows design engineers to apply all constraints on a seal in a computer environment. This includes factors such as speed, friction, pressure, temperature, bolt pre-tensions, and to simulate non-linear and linear materials. The software can also take account of the tolerances in the assembly with any play or movement being accommodated within the design.

Blue Diamond is a company that uses FEA techniques during the design process. Its seals are available in a range of standard materials including Viton or FKN, silicone, nitrile, NBR or ACN. An algorithm will show whether a standard seal is suitable or whether a special material such as PTFE is required. Should this be the case, the PTFE is chemically bonded to the substrate to give better performance and durability than a wafer type seal.

By changing different elements of the design, the application engineer can then achieve the optimum result. For example, the material, lip interference and spring loads can all be instantly varied and their effect on the performance of the seal observed. All this is possible without having to manufacture any prototype products, with their inherent implications of time and cost.

When the design team is satisfied with the performance of the seal using the computer simulation, samples are manufactured and the characteristics confirmed using Blue Diamond’s in-house test and development facility. This allows seals to be subjected to harsh environments — usually well beyond the expected performance envelope of the seal duty. The results are used to assist in the development of advanced designs and so help solve engineering problems more efficiently.

Scope of tests
The test rigs can control important parameters such as speed and direction of rotation, temperature and pressure. The settings are programmed with ramp or step functions to simulate proposed running conditions as closely as possible, and for life testing, multiple repetitions of test cycles and sub-test routines can be performed to produce the cumulative test duration desired.

Blue Diamond’s rigs have the capacity to subject seals to shaft speeds up to 25,000 rpm and eccentricity of housings and shafts can be set significantly higher than would be the case in a duty environment. Test fluids are heated to a maximum of 150oC or cooled to sub-zero temperatures by a low volume, closed loop, indirect system to ensure rapid response to any control set point. Meanwhile, pressures can be controlled from vacuum at one extreme to160bar at the other.

Handling seals of up to 200mm shaft diameter, the rigs can run at 3,000rpm under pressurised conditions. Each test seal position is naturally monitored for leakage and there is also continuous recording of seal friction. Dimensional checks can be made using the latest sub-micron multi-sensing vision systems, which allow even tiny amounts of wear to be accurately detected. As a measure of the success of this engineering and development programme, high performance seals for many of the world’s leading racing car teams have been developed using these facilities, which Blue Diamond believes represent the current state of the art.

Sited adjacent to the test area is a well-equipped manufacturing facility, and because the product design and manufacturing takes place under the same roof, customers can be confident of achieving a rapid development from concept to usable prototype.

So, why make do with a seal that over its lifetime will cause significantly more energy to be consumed than a custom seal that has been carefully designed and rigorously tested? By investing in the right seal for the job the competitive status of your product can be significantly improved, resulting in increased sales and improved customer satisfaction.

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