Elsevier

Sealing Technology

Volume 2012, Issue 11, November 2012, Pages 9-13
Sealing Technology

Feature
Dynamic behaviour and sealing performance of piston rings used in squeeze-film-dampers

https://doi.org/10.1016/S1350-4789(12)70485-3Get rights and content

This article discusses the initial stages of research that is investigating the behaviour of piston rings used as seals in squeeze film dampers – devices that cap rotor oscillations and reduce dynamic forces in jet engines. It covers an essential system analysis of seal contact surfaces and provides an evaluation of the dynamic parameters describing the system and their effect, leading to hypotheses that aim to describe the piston ring rotation phenomenon, as observed using a test rig, and the sealing effect. It also offers an overview of the results as part of the experimental validation.

Section snippets

Piston rings

The piston rings (PR) used in a SFD are static contact seals with a rectangular cross-section and butt joint. Figure 3 (on page 10) shows a mounted (right) and unmounted (left) PR.

The purpose of the PR is to seal the radial oil clearance and reduce the oil flow towards the side surfaces. The inner ring of the SFD has two grooves which hold the PR in place and ensure that it functions properly. The PR running-face is in contact with the bore of the outer ring of the SFD.

For many years detailed

Motivation

The behaviour of the SFD can be only partially described and predicted by equations based on plain bearing theories. So only a few factors that influence the damping behaviour (limited to the geometry) have been described analytically.

As yet, the influence of lateral seals has not been researched fully and mathematically described, and the detailed functionality of the PR in the SFD has been only marginally covered.

The function and functionality of piston rings used in internal combustion

System analysis

For this study, the structure in which the SFD is integrated is not of interest and it is not necessary to analyse both PRs. As a matter of fact both PRs are identical, but only one is exposed to high loads. Because of the asymmetric structure of this SFD (that is, the oil-feeding groove is not centred) the PR in question is the furthest from the oil-feeding groove.

A sealing contact is formed between the PR-running face and the SFD outer ring. The spring load of the PR ensures that a steady

Piston-ring motion hypothesis

The pendulum motion described earlier can lead to circumferential motion of the PR in its groove. To evaluate this potential motion, the frictional contact between the PR flank and its groove flank facing the radial oil clearance of the SFD (Figure 4 bottom left or upper right) will be analysed.

Figure 6 illustrates the equilibrium of the forces involved. This figure shows the frictional contact between the PR flank and the flank of its groove (as viewed in the direction from the radial oil

Factors influencing sealing behaviour

A seal is basically an obstacle which forces oncoming oil to change its direction of flow.

A slow down in the oil flow through a change in direction, and pressure and speed results in a change in momentum. A momentum balance at the seal enables sealing resistance to be introduced, including factors such as oil density, leakage area and leakage flow.

Leakage can be caused by geometric misalignments, and dynamic analysis reveals that a leakage spot is often related to the orbital excitation of the

Experimental validation

To validate the above described hypotheses on the sealing effect and circumferential movement of the piston rings, the following investigations were accomplished.

The test rig's special design enabled the macroscopic motion of the piston rings to be observed. Figure 8 (on page 11) shows that the side surface of the SFD (centre) is completely enclosed by a transparent plastic cover. This cover enables the oil to be collected while the movement is clearly observed.

To determine the influence of

Conclusion

The analysis of the sealing surfaces of the PR, which has been presented, together the study of existing forces, has led to a hypothesis for the sealing effect and the movement of the PR. The initial steps taken to confirm the hypothesis were discussed.

The dynamic experiments, using circular excitation of the inner ring of the SFD, reveal that there is oil flow not only through the PR closed gap, but also at the circumference of the PR. Different PR configurations cause significant

Outlook

The results presented in the section covering the experimental validation are only the first steps taken to validate the hypothesis discussed in this article.

In further studies the determination of the different oil flows is an important validation component. These can be determined analytically using the identified leaking spots and pressure. In particular, the difference between the two PRs (opposite and located close the oil-feeding grove) can provide valuable data – especially showing that

Acknowledgement

This article is based on research work carried out in the OPTITHECK project, which is funded by the German Federal Ministry of Economics and Technology under grant number 20T0812 and is run in cooperation with Rolls-Royce Deutschland Ltd & Co Kg.

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(This article is based on a paper, submitted by the authors, entitled ‘Study of dynamic piston-ring behavior and sealing performance of real-size squeeze-film-damper and experimental validation.’)

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