Guest Post by John Ayers (first posted on CERM ® RISK INSIGHTS – reposted here with permission)
Reliability is designed into a product. Poor reliability is long term, difficult and expensive to rectify because it is woven into the fabric of the product. That is why high reliability products require complete and accurate analyses, simulations, and models to be successful. High reliable components are essential as well. Single point failures are taboo but, in some cases, there is no way to avoid them.
This paper presents two examples of how to design single point failures for high reliability.
I was the project manager for a 4 M Lb. servo operated antenna mount. It was part of a national defense system. The reliability (availability) requirement for the mount was extremely high due to the importance of the mission. The mount had to rotate in azimuth +/- 270 degrees. The radar required coolant water. This meant a high reliable azimuth bearing and rotary joint to carry coolant water from the ship to the rotating mount was necessary. If one or both of these components failed, then the vital radar system would have to go to a dry dock to be replaced. Provision for removeable structural panels were built into the structure to minimize the down time of the radar which was anticipated to be a year or more.
Both of these components were single point failures because it was not practical to add redundancy for them.
The American Bearing Manufacturers Association (ABMA), formerly the AFBMA defines the Basic Rating Life, L10 as the bearing life associated with a 90% reliability when operating under conventional conditions. In other words, how many hours will the bearing last under the applied load and speed. Failure is defined as the beginning of the bearing raceway surface metal fatigue failure which can be checked visually periodically through a port in the bearing.
The dynamic load (determined by bearing manufacturer), applied load (defined by a finite element analysis), and number of rotations required for life of bearing (determined by converting the life calculation in meters to rotations) are the primary parameters that determine the bearing L10 life. The bearing discussed herein, was a large diameter (over ten feet), four row, roller bearing. This design reduced the applied loads and spread them more evenly over the bearing raceways. The other action taken was to select the best manufacture that made large roller bearings. Once the bearing leaves the factory, there is nothing that can be done for the L10 life because the bearing is closed, preloaded, and tested. In addition, the design of the bearing support structure on the mount must be sufficiently stiff to meet the assumptions made during bearing design, fabrication and assembly. In-process bearing fabrication and assembly require multiple inspections to ensure quality is met before closing and pre-loading the bearing. The amount of preload applied is important. It must be sufficient to reduce the excess play, but care must be taken not to apply too much preload.
A rotary joint is a rotary sealing device that connects rotating equipment to fixed piping for the transfer of coolant water in this case. The O-ring seals were the major wearing components of the rotary joint. There are a number of reasons O-ring seals fail but the most common are due to excessive friction between sealing faces and improper lubrication or surface finish of the metalwork. A classic example of where a rotary union can be found is in large hydraulic excavators or backhoes. The union permits hydraulic fluid to be transferred to the track drive system while allowing the cab and excavator arm to pivot 360° about the base.
The rotary joint used on the rotating mount was very large, customed designed and built. It was over six feet in height and greater than five feet in diameter. Multiple O-rings were pre-placed on the rotary joint that allowed seal replacement to be done quickly with minimal impact on the radar operation.
This design approach greatly increased the reliability of the rotary joint and greatly reduced the risk of not meeting the availability requirement to a very low acceptable level.
In spite of the engineering and analyst efforts to design a high reliable product, in some cases it is not practical to avoid single point failures. This paper describes two examples of how to design a reliable solution for a single point failure.
In the case of the azimuth bearing, the choice of using a four-row, roller type allowing lower applied loads to be distributed more evenly over the raceways, combined with very conservative bearing loads and stiff supporting structure increased the L10 life beyond that needed to meet the radar availability requirement.
Pre-placing multiple O-rings on the rotary joint allowed quick seal replacement and minimal interruption of radar operation. Thorough testing of the rotary joint including demonstrating the quick replacement of an O-ring seal in the factory before shipment was also crucial to verifying the design met all requirements.
If you have a single point failure in your design that cannot be resolved, keep in mind there are ways to reliably design it to meet the reliability requirement
Currently John Ayers is an author, writer, and consultant. He authored a book entitled Project Risk Management. It went on sale on Amazon in August 2019. He authored a second book entitled How to Get a Project Management Job: Future of Work. It is on sale on Amazon. The first is a text book that includes all of the technical information you will need to become a Project Manager (PM). The second book shows you how to get a PM job. Between the two, you have the secret sauce to succeed. There are links to both books on his website. https://projectriskmanagement.info/He has presented numerous Webinars on project risk management to PMI. He writes columns on project risk management for CERM (certified enterprise risk management). John also writes blogs for Association for Project Management (APM) in the UK. He has conducted a podcast on project risk management. John has published numerous papers on project risk management and project management on LinkedIn.
John earned a BS in Mechanical Engineering and MS in Engineering Management from Northeastern University. He has extensive experience with commercial and U.S. DOD companies. He is a member of the Project Management Institute (PMI. John has managed numerous large high technical development programs worth in excessive of $100M. He has extensive subcontract management experience domestically and foreign. John has held a number of positions over his career including: Director of Programs; Director of Operations; Program Manager; Project Engineer; Engineering Manager; and Design Engineer. He has experience with: design; manufacturing; test; integration; subcontract management; contracts; project management; risk management; and quality control. John is a certified six sigma specialist, and certified to level 2 Earned Value Management (EVM). Go to his website above to find links to his books on Amazon and numerous papers.
As a project manager, I was responsible for a major subcontract That was under contract to design, fabricate, install and test a 4 M lb, servo-controlled antenna mount.