Practical Reliability Engineering, 5th Edition

8

Reliability of Mechanical Components and Systems

8.1 Introduction

Mechanical components can fail if they break as a result of applied mechanical stresses. Such failures occur primarily due to two causes:

1. Overstress leading to fracture. Stresses may be tension, compression or shear. Bending stresses cause tensile and compressive forces, but fracture usually occurs in tension.
2. Degradation of strength, so that working stresses cause fracture after a period of time.

For example, a pressure vessel will burst if the pressure exceeds its design burst strength, or if a crack or other defect has developed to weaken it sufficiently.

Mechanical components and systems can also fail for many other reasons, such as (though this list is by no means exhaustive):

• Backlash in controls, linkages and gears, due to wear, excessive tolerances, or incorrect assembly or maintenance.
• Incorrect adjustments on valves, metering devices, and so on.
• Seizing of moving parts in contact, such as bearings or slides, due to contamination, corrosion, or surface damage.
• Leaking of seals, due to wear or damage.
• Loose fasteners, due to incorrect tightening, wear, or incorrect locking.
• Excessive vibration or noise, due to wear, out-of-balance rotating components, or resonance.

Designers must be aware of these and other potential causes of failure, and must design to prevent or minimize their occurrence. Appreciation of ‘Murphy's Law’ (‘if a thing can go wrong, it will’) is essential, particularly in relation to ...