Knife Edges And Pivots

Middle-aged readers may remember expensive watches with the words "17 Sapphires" printed on the face, roughly where the word "Quartz" now appears. The message was that the mechanism had bearings made of sapphire (Al2O3) and that that was good. A really expensive watch had, not sapphires, but diamonds.

They are examples of good materials for knife edges and pivots. These are bearings in which two members are loaded together in nominal line or point contact, and can tilt relative to one another, or rotate freely about the load axis (Figure 13.1). The main requirements of materials for such bearings are high hardness (to carry the contact pressures) and high modulus (to give positional precision and to minimize frictional losses). But in what combination? The answer depends on the objective: maximum precision or maximum load-capacity. Table 13.1 summarizes the requirements.

Read on to view a worked solution or get CES Selector to load this case study automatically.

knife edge and pivot

Figure 13.1 A knife edge and a pivot. They have small contact area, for precision, and high strength, to avoid damage by contact stresses.

FUNCTION

Knife edges and pivots

OBJECTIVE

(a) Maximize positional precision for given load

(b) Maximize load capacity for given geometry

CONSTRAINTS

(a) Contact stress must not cause damage to either surface

(b) Adequate toughness (for pivots exposed to shock loading)

(c) Low thermal expansion (for high precision pivots)

Table 13.1 The design requirements

The Model

The Selection

Postscript

Further Reading

Related Case Studies