2231. Measuring Small Losses in Rotating Assemblies

Paper

Abstract

When a mechanical assembly is exposed to a rotational environment, the gravitational attraction of the earth causes the structure to flex. This results in a small hysteresis loss. This loss is dramatically increased if relative motion occurs between parts in the assembly. Such motion may occur from slip at bolted joints or sloshing of fluids present in the body. In the context of mechanical vibration, these sorts of losses are characterized as ”damping” and limit response at resonant frequencies of the body. In a rotational environment, mechanical losses cause a decrease in the spin rate of the body. Small losses result in a gradual decrease in the roll rate over a long period of time. In 1993, Los Alamos National Laboratories signed a contract with Space Electronics for the development of a simulator which would rotate a test assembly such as a satellite on a frictionless gas bearing and measure the decay in spin rate resulting from the mechanical losses described above. Losses corresponding to retarding torques as small as 0.001 lb-inch were considered significant. The test machine also had to determine the moment of inertia of the payload and correct for static and dynamic unbalance of the rotating assembly. Since windage losses would obscure the data we were trying to measure, a helium chamber was built into the machine. The chamber used with this gas bearing system uses a novel fabric membrane concept which allows the air to be thoroughly expelled before the helium is introduced. The paper gives a brief overview of the losses which occur in rotating bodies and the consequences of those losses. The unique measuring machine is described in detail. Test data is given on the losses in the machine itself using a smooth solid cylinder as a payload. Although it wasn’t designed for this purpose, this machine is an extremely sensitive device for determining the windage losses for various airfoil shapes. We first established a baseline windage loss using a smooth cylinder spinning in a helium atmosphere. Various ”paddles” were attached to the cylinder and the decay rate measured in both air and helium at different speeds. The machine reliability measured the drag due to projections as small as the head of a 1/4 inch socket head cap screw located at a radius of 4 inches rotating at speeds below two resolutions per second. This paper is divided into two sections. The first section, written by Richard Boynton and Kurt Wiener of Space Electronics. describes the construction of the gas bearing machine . The second section, written by Norman Hunter and Richard Harlow of Los Alamos National Labs, gives specific data from some very interesting tests which were run on the gas bearing machine.