Page 143 - The Tribology Handbook

A25 Flexures and knife edges

MATERIALS FOR FLEXURE HINGES AND TORSION SUSPENSIONS

I
EXAMPLE OF A FLEXURE HINGE EXAMPLE OF A TORSION SUSPENSION

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Flexure hinges and torsion suspensions are devices which Selection of the most suitable material from which to
connect or transmit load between two components while make the elastic member will depend on the various
allowing limited relative movement between them by requirements of the application and their relative import-
deflecting elastically. ance. Common application requirements and the corres-
ponding desired properties of the elastic member are listed
in Table 25.1.

Table 25.7 Important material properties for various applications of flexure hinges
and torsion suspensions

Application requirement Desired material properg Application requirement Desired material property
1. Small size High maximum permissible stress, 6. Elastic component has High electrical conductivity, k,
to carry an electric
fmax = yield strength, fy
current
unless the application involves
a sufficiently large number of 7. Elastic component has High thermal conductivity, k,
stress cycles for fatigue to be to provide a heat path
the critical condition, in which
case : 8. Elastic component has Negligible hysteresis and elastic
to provide the main after-effect; non magnetic
f,. = fatigue strength, fF
reactive force in a sen-
sitive measurement or
2. Flexure hinge with High f,,,/E: control system
maximum movement
for a given size E = Young’s Modulus
9. As 8 and may be sub- Low temperature coefficient of
ject to temperature thermal expansion and elastic
3. Flexure hinge with the High fLa,/EZ modulus (E or G)
maximum load capa- fluctuations
city for a given size and 10. As 6 but current has Low thermoelectric e.m.f. against
movement
to be measured accur- copper (or other circuit conduc-
ately by system of tor) and low temperature coeffi-
4. Flexure hinge with High 1/E: note that stiffness can be which elastic com- cient of electrical conductivity
minimum stiffness (for made zero or negative by suitable ponent is a part
a given pivot geo- pivot geometry design
metry) 11. Elastic component has As for 1-10 above, but properties,
~~
5. Torsion suspension High fiaX = shear to operate at high or for example strength, must be
U/G:
G
x
with minimum stiff- modulus, U = aspect ratio low temperature those at the operating tempera-
ness for a given sus- (width/thickness) of suspension ture
pended load cross-section. U is not a material 12. Elastic component has Appropriate, good, corrosion re-
property but emphasises the value to operate in a potenti- sistance, especiallyifrequirements
of being able to manufacture the ally corrosive environ- 8 or 10 have to be met
suspension material as thin flat
strip ment (includes
‘normal’ atmospheres)
A25.1