Mobile Roof Project

Northern Arizona University College of Engineering
Capstone Design

j eff Smith

j osh Platt              =         j 3 Designs

j oe Bast


Executive Summary

The objective of this project is to minimize the heat transfer between rooms and reduce maintenance required by the cable pulley system that operates the Mobile roofs at the NPOI (Navy Prototype Optical Interferometer). An AutoCAD drawing of the building is shown below. Primary goals of the project are to use as many of the current parts as possible and to have minimal impact on the buildings current configuration.  This project was proposed by Dr. Nathaniel White of Lowell Observatory, and chosen by the j 3 Design team. 

The Mobile roof is driven with a friction drive cable-pulley system; two cables run through all of the interior rooms in the building and along the length of the roof on the exterior. One turnbuckle per cable is used to ensure that each cable tension is correct.  Holes for cable travel are cut in the electronics-siderostat wall. These holes allow the heat generated in electronics room to flow into the siderostat room. 

Several design solutions were considered, it was necessary to compare these solutions using a design matrix and several criteria to choose a design. 

The design chosen is the Cable Isolation System. This system fulfills the goal of minimizing the heat transfer between the rooms of the building by implementing PVC pipes and seals to thermally isolate the electronics room from the siderostat room. The maintenance requirements of the current system will be handled with the use of a constant rate tensioner pulley system that is mounted on the outside of the building.

Design Philosophy

The design philosophy for the j 3 design team is one that emphasizes innovation and simplicity.  This philosophy encompasses goals such as: effective system design, trouble-free implementation and operation, minimal maintenance, and an economical solution.

Analysis of Current System

After our first site visit to NPOI it was apparent that there were some current conditions that needed to be determined.  Such as cable tension, thermal expansion of the cable, and the heat transfer between the electronics room and the siderostat room. The tension in each cable was measured at 150 lbs. For a worst case scenario we assumed a foot of snow on the roof and found the tension to be 2200 lbs in each cable. The thermal expansion of the cable was found to be 17.5 mm (0.69 in). And the heat transfer rate was found to be 0.31 Watts.



Now that we understand how the cable behaves under changing temperatures, we were able to propose a system that would eliminate the need to manually adjust the cable tension. Our solution to eliminate the maintenance requirement of the current system is to implement cable tensioners.

These tensioners will be mounted to the mobile roof rails as shown in the figure above. They will tension the cable via a pulley and spring mechanism. Once setup, there will be no maintenance associated with this design. A stress analysis was performed on the tensioner assembly and the findings show that the tensioner will not fail. Factors of safety ranged from 95 to 1700.


Heat transfer

The heat transfer problem was solved using PVC pipes and seals. As shown below the PVC pipe will be suspended, beginning in the siderostat room. Where the pipe bisects the electronics room, a seal will be constructed of PVC sheet to cover the large hole left by the gutter.

A hole will be cut into the seal the size of the pipe’s outside diameter and will be bonded to the seal, thus eliminating airflow into the siderostat room at that juncture. The heat transfer was found to be reduced by five times

Both of the designs: the cable enclosure system, and the constant rate tensioner bracket, consist mostly of parts that are locally found and in some cases are used on a regular basis at Lowell Observatory. The tensioner was the only part that we had to order and was an off the shelf from McMaster Carr catalogue found on the world wide web at This means not only an economical design but also an easily attainable one as well. 


The cable enclosure system meets the requirements of the project, it is cost effective costing only $300. The actual cost will be around $100 because most of the parts our client had on hand with the exception of the tensioner. The system utilizes all of the current roof opening system. It decreases heat transfer into the siderostat room by 80% and it reduces maintenance by implementing cable tensioners. 



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