Installation of balance weights to address "spikes" problem

P/001/26/RJSG (1999)

Justin Greenhalgh, JAC, May 1999

Introduction

In earlier papers (P/001/19, eg) the theory was set out that the "spikes" were caused by a combination of high stiffness in the structure, unevenness in the surface of the track, and temperature effects. It was observed that the simple solution, to decrease the load on the central bearing so that the wheel loads would increase, was not workable. This is because the antenna is heavier at the back, and any uniform increase to give adequate weight at the front would overload the back. After extensive discussions and thought (P/001/21) it has been agreed to implement the most obvious solution: to add weight to the front of the antenna.

Weights

Lead was chosen as the best material because of its high density. We elected to put about three tonnes onto the antenna. Referring to figure 2 of P/001/19, the effect will be to raise the lower two traces by 1.5 tonnes each. The lead was obtained from Hilo Hospital, who receive radioactive samples in lead containers and release them for sale once the radiation levels are below background. Most of the lead is in the form of the containers which weigh about 34 lb each, and part of it was delivered as small scraps and we melted these down into 30 lb ingots.

Platform

A platform has been constructed upon which weights may be placed. This is a substantial structure and weighs about 0.5 tonnes. It was done without any formal design documentation. Once the lead was put onto the platform, a kick-board was added to stop the weights falling off in case of earthquake (not shown in this picture).

Strain gauges

A permanent strain gauge system has been installed with which it is possible to see the effect of the extra weights, and to track changes in wheel loading over time. This will enable a better understanding of the temperature effects and hopefully a more rational approach to central bearing adjustments. The system consists of two pairs of strain gauges, one on the inside and one on the outside of the track. In each pair, one gauge is strained, and the other is positioned to be strain-free. They are wired into a bridge to ameliorate temperature effects, however at the time of writing they seemed to be suffering from drifts and it may be that the system design needs revisiting. See P/001/25 for details.

Installation of weights

The weights were installed on May 11th (see photos). Inclinomtery was taken before and after, including strain gauge data - it is reported in the usual inclinometry report series. The bottom line was that the weights seemed to have increased the load on the front wheels in the manner we had intended. One by-product of the installation was to slightly lower the plastic castors that fit under the apron and which are normally out of contact with the track. We adjusted them to put them about 3mm out of contact.