EMC & Thermal Management


Flat EMC gaskets, compression stops and Monte Carlo analysis


By Gerry Young, applications engineering team leader, Parker Chomerics - a division of Parker Hannifin Corporation


W


hen designing an EMC (electromagnetic compatibility) sealing arrangement, the best course of action is


generally to specify a gasket in a groove, typically an O-ring seal or another standard profile. However, in some applications, the use of this seal type is not possible and flat gaskets instead become the preferred option. In such cases, the correct management of gasket deflection under compression is vital to maximise its effectiveness and operating life. Flat gaskets come in a variety of materials that offer different properties and performance. On the face of it this is simple: manufacture the gasket, apply the fasteners and assemble. Unfortunately, it is not quite that easy, especially when the gasket is primarily rubber and some areas of it are relatively small in cross section. Take the common example of small shell size MIL-DTL-38999 gaskets, which are typically 0.81mm thick and have a large central hole of 15-25mm diameter and four fixings screw holes of around 3mm diameter. Many factors require careful scrutiny to ensure optimal application, not least achieving the correct level of deflection/compression. The Chomerics conductive elastomer handbook indicates that a CHO-SEAL 1285 flat gasket should be deflected between 5 per cent and 15 per cent of its initial thickness, which in this case is 0.81mm with a tolerance of ±0.13mm. Aside from this somewhat large tolerance, which results from the sheet moulding process, another critical consideration is the elasticity modulus for these products, which is generally around 1-2MPa. Furthermore, the compression modulus of the gasket relates to its shape factor and is generally in the region of 30- 50MPa. This converts to a spring rate using the formula K =AEc/t, which for the connector gasket in Figure 1 is around 10,500N/mm. Where K is the spring rate in N/mm, A is the area in mm², Ec is the compression modulus


42 June 2025 Monte Carlo analysis


As with practically all indeterminate problems, turning to statistics and a spreadsheet will prove helpful, in this particular case through the application of a Monte Carlo analysis. By calculating the mean and standard deviation of the compression stop and gasket thickness, and then running a few hundred simulations using random combinations of the compression stop thickness and gasket thickness, it is possible to see the likelihood of over- or under-compression.


Fig 1: Typical connector gasket


in N/mm² and t is the thickness in mm. (Gent, Engineering with Rubber, chapter 8, 2012) Considering the area of the gasket is around 250mm² and the screws used to secure the connector are likely to be M2.5, the torque for fastening such a screw will be around 0.6Nm. Using the approximation T=0.2Fd, where F=1,200N, four M2.5 screws provide a load of approximately 4,800N. Given the calculated compression modulus, this load will deflect the gasket by around 0.45mm, which is way in excess of what is required to compress the gasket by a maximum of 15 per cent.


Pull out all the stops


It is therefore necessary to consider the use of compression stops, which will shoulder the excess load upon reaching 15 per cent deflection. These stops, typically manufactured from 5000 or 6000 series aluminium, have a compression modulus of around 70GPa or 70,000N²/mm. So even with just four 2mm diameter compression stops, there is more than enough area to provide a solid method of preventing gasket over-deflection. A nominal 10 per cent deflection on a 0.81mm gasket equates to a stop height of 0.729mm ±0.05mm. However, there is another issue: the tolerance on the sheet of rubber material is ±0.13mm, which could result in a deflection range of -14 per cent to +27.5 per cent in the worst cases. So, how is


Components in Electronics


it possible to overcome this problem? Well, in all honesty the maximum and minimum sheet thickness scenario practically never occurs, so what is required is a predictive tool that minimises the likelihood of gasket over- or under-deflection in most cases.


Sheet thickness/mm Sheet tolerance ± mm


0.51 0.81 1.14 1.57 2.36 3.18


Stop thickness


0.4 0.7 1


1.3 1.6 1.9 2.2 2.5 2.8 3.1


0.1


0.13 0.15 0.18 0.25 0.25


Sheet tolerance ± mm


0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05


Fig 3: Compression stop thickness tolerances www.cieonline.co.uk


The table below (Figure 2) is based on standard Chomerics sheet tolerances from the company’s catalogue. As shown in Figure 2, aiming to deflect a 0.81mm thick gasket between 5-15 per cent of its thickness harbours a problem since the tolerance is ±0.13mm, or ±16 per cent. A similar issue is evident with compression stops, but thanks


Tol as% of sheet thickness


19.61% 16.05% 13.16% 11.46% 10.59% 7.86%


Fig 2: Chomerics standard CHO-SEAL sheet thickness tolerances


Tol as% of Stop thickness


12.50% 7.14% 5.0%


3.85% 3.13% 2.63% 2.27% 2.00% 1.79% 1.61%


SD


0.0083 0.0083 0.0083 0.0083 0.0083 0.0083 0.0083 0.0083 0.0083 0.0083


Standard deviation


0.017 0.022 0.025 0.030 0.042 0.042


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56