Machine design and equipment design come in many different shapes and sizes. While some are more akin to flavor-of-the-month approaches, there are design philosophies that are very useful in the appropriate environment.
For a high volume consumer product, especially one with “no user serviceable parts inside,” the philosophy of “design for assembly” might be useful in simplifying the assembly process with snap-together parts, reducing overall part count and meeting cost targets.
Processes that include casting, fabrication, heat treatment, machining and finishing would call for a different design approach. “Design for manufacturability” would fit the bill.
On the other hand, many projects require matching the equipment and system performance to the operating needs, which provides the long-term, most cost-effective system to meet the defined mission. We call this “design for reliability.”
According to the Center for System Reliability (CSR), “Reliability should be designed and built into products at the earliest stages of product development. As most of a product’s lifecycle cost has been locked in by the time its design is complete, design for reliability is the most economically sound approach to take.”
Reliability doesn’t just happen. It’s the result of careful planning and effective execution. We can measure it using overall equipment effectiveness (OEE), or the combination of quality, performance and availability. To reach target production, a machine must be operating (available), must be operating within specification (quality) and must be operating at the required cycle rate (performance). The goal of our design effort is simply to meet those requirements and maximize OEE.
The mission must be fully defined to achieve maximum performance and efficiency upon completion. Variables, including throughput, power consumption, environment, materials being handled and required lifetime, must be accounted for. If the lifetime is relatively short, a sealed bearing requiring no maintenance might be selected. If a piece of machinery must last for 20 years, the maintenance and replacement must be accounted for, as well as the relative life of alternate materials. A schedule for inspection, measurement and a feedback loop must be defined to ensure adequate warning of developing problems. There are situations in which a unit that fails predictably but can be repaired quickly with standard parts might be more effective than a unit that fails less often but needs special skills and parts to repair. The question is how we define the reliability goal. We might be able to deal with one hour of downtime a week to replace a worn impeller, but not two days every three months to replace a more complex and reliable part.
The design and review process helps ensure the following questions are answered:
- Can the system be constructed?
- Can the system be operated?
- Can the system be maintained?
- Can the system be updated for changes in requirements?
- Can this all be accomplished within the budget constraints?
Designing-in inspection capabilities and indicators promotes reliability. When an operator or maintenance technician can’t easily and safely access the equipment, a check mark for completion is likely to appear, whether the task is completed or not.
An example is the need to do routine inspection on a belt drive. The old design used a sheet metal shroud over the belt pulleys. The machine had to be stopped and the guard removed to do this simple check. Because operations didn’t want to shut the system down, the check was often skipped. Belt failure then occurred at inconvenient times, resulting in conflicts with operation and maintenance.
Using design-for-reliability principles, a new shroud was made using an expanded metal front. This was painted black, and the belt and pulleys can now be inspected with the system running using a strobe lamp. The inspections are completed every time, and belt changes are scheduled before failure occurs. Because the system doesn’t have to be stopped to be checked, operations now actually encourage inspections. Reliability happens.
If you can’t measure it, you can’t control it
Designing systems having feedback mechanisms is a critical element for long-term success. Regardless of the information source — failure logs, failure analyses or a computerized maintenance management system (CMMS) — this organization can compare reliability of the same or similar equipment to a performance standard. Early warnings of problems or confirmations of successes are equally important. We don’t need to mess with perfection.
The earlier in a project we start to design for reliability, the easier the process will be. This includes operation, inspection and maintenance practices, not just the hardware/software. By considering how a system is to be operated and maintained, we can eliminate problems such as inaccessible control positions, confusing controls and hidden maintenance tasks. An oil reservoir might need to be moved so the level is visible or the order of a row of switches rearranged so they are sequential. Small changes can avoid large problems later.
For example, because of bad design, the controls for seal water valves at a water treatment plant were located 25 ft from the pump/motor starter panel. In addition, the path to reach the seal water controls wasn’t direct. Although the operators had been told the importance of seal water flow, no one had actually told them why seal water flow had to be started before the pump. Only after a series of costly seal failures did the training get changed. One could call this “design for unreliability.”