When taking steps to improve one bearing characteristic, it is important to weigh any measure taken against how it may affect the others.
In low-power applications, the resulting friction may be more than the power source can provide. For example, bearings are often preloaded to improve stiffness and reduce deflection, but preload increases starting and running torque. Such applications can actually be quite challenging when stiffness, accuracy and low torque become critical…in fact, these are often conflicting requirements. For the preloaded bearing, S ax = 80/0.000275 = 290,909 lb/in.Įven applications that seem simple, such as low-speed/low-load applications, can benefit from acceptance testing. In this graph, the deflection of the non-preloaded bearing is S ax = 80/0.000585 = 136,752 lb/in. In an acceptance test, this is usually a static axial load approximating the operational load. Preloading the rolling elements lowers deflection when the operating load is applied. How bearing stiffness improves with preload Their small envelope and low mass make them appealing, but since the races and rolling elements can weigh as little as 10% of what similar elements of standard-section bearings weigh, it is best to confirm their suitability. These small, lightweight packages often operate at low power, with mechanisms that rotate slowly and may be highly loaded. Thin section bearings, which are often specified for weight-critical or space-critical applications, can often benefit from acceptance testing.
repair would be very expensive, difficult or impossible (e.g., a satellite).the supplier must provide detailed accountability or an audit trail.failure may result in a life-threatening situation (e.g., medical equipment).
comprehensive documentation is required (e.g., military applications).high reliability is demanded of the end product (e.g., space-based applications).Generally, acceptance testing should be done when: For the supplier, acceptance testing can increase product quality, document that design requirements have been met, and ensure user satisfaction. It also reduces unexpected downtime to repair or replace components that fail to meet system requirements. For the user, it adds a dimension of quality control and performance reliability (often supported by formal documentation). Incidents like these demonstrate how acceptance testing can benefit both the user and supplier. Acceptance testing could have identified the high torque values and triggered a design review before this embarrassing discovery. This necessitated costly rework and required a redesign of the lubrication and preload requirements, delaying the project. Once the system was built and tested, it became clear that the bearing's starting torque exceeded the power available to rotate it. angular contact bearings for the gimbal without specifying a friction-torque requirement, and the vendor supplied a standard bearing that met the lubrication and preload requirements. Relying strictly on catalog values can be a gamble, as demonstrated in the design of a fire control system for a U.S. Should that not be practical, other bearing options can be investigated at an early stage of the design process. If not, components can be reworked to bring the assembled bearing within required specifications. and confirms whether or not these values are inside expected limits. Acceptance testing measures torque, preload, runout, stiffness, etc. Furthermore, performance variation from unit to unit is common in bearings. Acceptance testing (or end-of-line verification) provides it.Ĭatalog performance values are not actual characteristics, but averages with a tolerance attached.
But in critical applications with a narrow performance window, designers need a greater safety margin. Kaydon white papers Trust but verify: the value of acceptance testingįor many applications, the values and engineering data in bearing manufacturers' catalogs-typically derived from calculations based on nominal dimensions-provide all the information needed for bearing selection.
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