Reducing Aircraft Brake Squeal with a Damped Brake-Rod

Abstract

In multi-brake aircraft landing gear systems, it is common for brake rod bending to participate in brake squeal vibration modes. Experiments have proven that damping the bending mode of the rod can reduce brake vibration. A patented method of doing so with a simple, lightweight, split-tube insert for tubular brake rods is described. Equations for sizing the insert for a given interference pressure are presented, results of bench tests are summarized, and further development is discussed. Incorporating damping mechanisms in the load path of the brake rod (which must operate in either tension or compression) is complex, and tends to lead to a much heavier assembly. One such rod [2], found from bench tests [1] to have very little longitudinal hysteresis, was nonetheless found to reduce brake squeal by 50 to 75% by virtue of the bending motion of the rod. Although such designs are of doubtful value because of complexity and weight, other designs specifically addressing the bending mode alone have been found to be even more effective. The evidence is strong that suggests any technique that enhances brake rod bending-mode damping will reduce brake squeal. This paper discusses such a method [3]. INTRODUCTION BASIC PRINCIPLES OF THE SLEEVE DAMPER Brake squeal is well known as one of the dominant modes of aircraft braking system vibration. It can be excited, aggravated, or destabilized by system characteristics such as a negative friction-speed coefficient, or structural feedback. The dominant motion is pitch-plane torsional oscillation of the nonrotating brake structure at frequencies near 200 Hz. in large brakes. There are several squeal modes of a multi-brake landing gear system, differentiated by the phase of one brake with respect to the others, but all are system modes characterized by the participation of nonbrake structural components and motions, and include some amount of brake rod bending. The amplitude of vibration can be reduced by introducing damping in any component that participates. Historically, most designers have focused on modifications of the brake itself. While many design techniques have been successful, the high temperature environment of the brake limits the potential for others. In recent years, some designers have focused on the potential of incorporating damping mechanisms in the brake rod itself, since it is clearly the elastic structural component of the system which governs the squeal modes of vibration, and does not operate over wide temperature ranges. Typically, these efforts have been directed towards means to increase the rods longitudinal energy absorption. A tubular rod acting in the presence of oscillating tension or compression loads will react by bending about its neutral axis, which is coincident with the geometric axis of symmetry of its cross-section. A thin-wall sleeve inserted into the rod bends about the same neutral axis. There is no relative motion between the sleeve and rod anywhere. If the sleeve is split longitudinally at one location, however, and a finite gap exists along the split, the neutral axis of bending of the sleeve will be displaced radially away from the split, and bending in the plane of the split will cause relative motion at the interface between sleeve and rod (as shown in Figure 1) because the sections of the nested beams do not have the same neutral axis. It is analogous to a laminated beam or leaf spring. If there is an interference pressure distribution between the members, this relative motion will be resisted by additional longitudinal friction shear forces between them. The amount of relative motion is proportional to the displacement of the neutral axis. Bending in the plane perpendicular to the split is resisted by a similar mechanism (also shown in Figure 1). Introducing a split moves the shear, or flexure, center to a point outside of the sleeve and opposite the location of the split. Bending of the rod in this plane causes the sleeve to twist about its longitudinal axis. The twist is equal and in the same direction at both ends of the sleeve, and in the opposite direction at the sleeve (c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. center. Twisting a split sleeve also causes longitudinal displacements: the edges of the sleeve gap move longitudinally opposite each other. As shown in figure 1, the shear center (SC) is much further from the rod center than the neutral axis (NA) is, implying that the relative motion is much greater for this direction of bending.