Page 147 - The Tribology Handbook

Flexures and knife edges A25

Table 25.4 Important material properties for various applications of knife edges and pivots

Application requirement Desired mated prwp

1. High load capacity for a given bearing geometry hardness'
High
modulus of elnstici@
2. Ability to tolerate overload, impact or rough treatment A measure of ductility in compression, so that overload can be accom-
generally modated by plastic deformation rather than chipping or fracture
3. Requirements I and 2 together (for example for weigh- High hardness together with some ductility. In practice various metallic
bridges, strength-testing machines, etc.) materials with hardnesses greater than 60 Rc (690 Knoop) are usually
specified
4. Very low friction with useful load capacity where freedom hardness'
from impact and overloading can be expected (for example high modulus of elasta'city using various brittle materials having ex-
in sensitive force balances and other delicate equipment) ception~ly high hadness
-~~
~
5. High wear resistance High hardness is generally beneficial
- -
6. Little indentation of block by knife edge or pivot Hardness of block > hardness of knife edge or pivot. (This is nearly always
desirable; the differential should be at least 5y0)
7. The two members of the bearing have to slide relative to one Low tendency to adhesion to avoid high sliding friction and wear; in
another at the contact (see examples (b)) and must be practice it is often sufficient to avoid using identical materials
metallic to withstand impact, etc.
8. Bearing to be used in a sensitive force balance Non magnetic; should not absorb moisture or be subject to any other
weight variation. (Agate, for example, is unsatisfactory in the latter
respect since it is hygroscopic)
9. Bearing to be used in a potentially corrosive environment Good corrosion resistance, especially if requirement 8 has to be met
(includes 'normal' atmospheres)

Table 25.5 Relevant properties of some knife edge and pivot materials

Approximate maximum
Modulur of elastic+, E Load capakp continuous operating
Hardness
Material factor, H'/E DuctiliQ tempaature in air ~~~~~~
H, KnOOp (N/m' x 10") (lbf/in2 x IO6) (arbitrary units)
"C "F
High carbon stemel to 690 21 30 2.3 Some 250 500 Poor
Tool steels to850 21 30 3.4 Some 650 I200 Poor-Good
Stainless steel (440C) 660 21 30 2.1 Some 430 800 Moderate

Agate 730 7.2 10.4 7.4 None 5752 1070' Excellent
Synthetic corundum 2100 38 55 11.6 None 1500 2700 Excellent
(AI2 03)
Boron carbide 2800 45 65 17.4 None 540 1000 Excellent
Siiicon carbide 2600 41 60 16.4 None 800 1470 Excellent
~
Hot pressed siIicon nitride 2000 31 45 13 None 1300 2400 Excellent
Notes I 1. Materials with pr corrosion resistance can often be protected by grease, oil bath or surface treatments (such aschromising ofsteels) .
2. Phase change temperature.

A25.4