Richard Kennison, senior design engineer at Blue Diamond, explains how the latest computer based technologies can be used to engineer a seal that is not only energy efficient but has an extended operating life
The humble shaft seal is frequently an undervalued component in an engineering system. Yet whilst a well-engineered seal is often unrecognised, a poorly designed – or poorly specified – seal, is one of the most common causes of unnecessary power losses, increased maintenance or, in extreme cases, total mechanical failure.
At Blue Diamond, however, we believe that even the simplest product should be engineered to suit the intended application, which is why we use Finite Element Analysis (FEA) software. FEA allows design engineers to apply all constraints on a seal in a computer environment, including factors such as speed, friction, pressure, temperature and the simulation of non-linear and linear materials. The software can also take into account the tolerances in the assembly with any play or movement being accommodated within the design.
For some years, our motorsport business has successfully been using MSC Marc Advanced Nonlinear & Multiphysics package to allow the simulation of high performance low friction shaft seals and bespoke elastomeric components. It quickly became apparent that the same methodology could be applied to improve the performance of ‘Catalogue’ seals. For the customer it requires a few more targeted questions up front to allow an accurate simulation of the behaviour of the seal to be carried out, but better that than the searching questions that occur when the engineering system has failed.
The macroscopic behaviour of elastomers used in shaft seals is complex and typically depends upon time, operating temperature and load history from both damage and fatigue. All elastomers have a spectrum of behaviour, from brittle or glassy at low temperatures, to rubbery at high temperatures. The effect on material extension deformation occurs abruptly as the operating temperature increases – see Figure 1, below.
Temperature and time effects derive from the molecules of the elastomer sliding around during deformation. Due to material viscoelasticity this leads to a reduction in lip force and sealing function over time.
By creating an initial 2D section of the seal, the engineers at Blue Diamond can apply a quadratic mesh which allows the elastomers material properties to be applied. Typical meshes have between 2000 and 3000 elements, a proportion of which are allocated to the elastomer with the remainder allocated to the Polytetrafluoroethylene (PTFE) counterface. Each element contains the material properties of the elastomer or polymer material, allowing an accurate simulation to be made.
By combining the inherent flexibility of the elastomer and the low friction capability of the PTFE material, we achieve greatly improved performance over that of a ‘Catalogue’ part.
Figure 2, shows a typical example of a simulated seal section reacting to the application of pressure. By using material data collected at the expected operating temperature, the simulation allows Blue Diamond to calculate the sealing lip force allowing for predictions to be made on the friction performance (Power loss), sealing ability and life of the seal through a bespoke wear sub routine.
The nonlinear FE package MSC Marc automatically handles the changing contact conditions that occur during service including self-contact, separation, sliding and friction. Once the initial FE model has been created the seal lip can be tuned to meet the exact operating criteria by selective adjustments of the contact interference, elastomeric material and lip geometry.
Producing high performance seals
When the engineers are satisfied with the performance of the seal using the computer simulation, samples can be manufactured and the seal characteristics confirmed using Blue Diamond’s in-house test and development facility. Here, seals can be subjected to harsh environments – usually well beyond the expected performance envelope of even the most arduous field conditions. The results are used to assist in the development of advanced designs and to help solve engineering problems more efficiently.
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.
A manufacturing facility is adjacent to the test area, which helps to minimise development times. After all, correctly engineered seals can make a significant contribution to reducing losses and extending operational life of many products.
Blue Diamond Technologies
T: 023 8025 8966