Figure 7-1. Balance Versus Unbalance Condition.

TM 55-4920-201-14

7-6. It is evident that the amount of unbalance

the assembly. (Refer to appendix A). The static

force will remain the same if the typical values

balance tolerance may be expressed directly in

ounce-inch units; it may also, however, be ex-

shown are reversed. That is, a weight of 6 ounces at

a distance of 2 inches produces the same amount of

pressed in several other ways, any of which can be

tilting force as a weight of 2 ounces at a distance of

converted to ounce-inch units for easy measurement

6 inches. Similarly, any other combination, such as

and direction comparison. For example, propeller

4 ounces x 3 inches, whose product is the same, will

balance tolerances are sometimes expressed as

produce the same magnitude of tilting force. In

"0.001 inch x propeller weight." Since 0.001 is

comparing amounts of unbalance, the product of w

already in inches, it can be substituted directly for d

x d is always the significant figure. It follows,

in the general formula, m = w x d. If the weight of

naturally, that a 2-ounce weight at a distance of 12

the propeller, in ounces, is substituted for w in the

inches will produce twice as much tilting force as

formula, the product will be the propeller balance

the same 2-ounce weight at a distance of 6 inches.

tolerance in ounce-inches. Example: Propeller

weight = 200 pounds (3,200 ounces). Propeller

7-7. If, as sometimes happens, weights are ex-

balance tolerance = 0.001 x propeller weight.

pressed in pounds instead of ounces, the products

Substituting specific values in general formula m=

will be in pound-inches instead of ounce-inches.

w x d:

While the operation of the general formula is

m = 3,200 (ounces x 0.001 (inch)

identical in either case, it must be remembered that

m = 3.20 ounce-inches = propeller balance

one pound-inch represents 16 times as much tilting

tolerance.

force as one ounce-inch. Similarly, since one ounce

equals 28 grams (approximately), one ounce-inch

7-11. In other cases, the balance tolerance may not

represents 28 times as much tilting force as one

be expressed in any numerical units at all, but

gram-inch. These units must, of course, be con-

directly in terms of a specific part (Example: AN-

verted to ounce-inches before their tilting force can

XXX washer) at a specific location on the

be directly compared with other tilting forces

assembly. Since, in these cases, the weight (w) of

expressed in ounce-inches.

the specified part is constant and its location on the

assembly (d) is also constant, the limiting amount

7-8. BALANCE TOLERANCE.

of permissible unbalance is controlled just as ef-

7-9. Balance tolerance is the amount of unbalance

fectively as if the balance tolerance had been ex-

established as the maximum that can be tolerated

pressed in ounce-inch units. If, for comparison

in specific propeller or helicopter components. The

purposes, it is desired to express such a balance

specified balance tolerance means, in effect, that

tolerance in terms of ounce-inches, it is only

the assembly need not be perfectly balanced, which

necessary to multiply. the weight of the specified

would be an ideal but impractical condition. Since,

part (in ounces) by its distance (in inches) from the

however, the balance tolerance does limit the

effective pivot point.

amount of unbalance which can be tolerated, `a

7-12. "GO - NO GO" BALANCE INDI-

method is needed to determine whether the amount

CATIONS.

of unbalance actually present in a specific assembly

is smaller or greater than this limiting amount.

7-13. Sensitivity of the suspension balancers can be

readily adjusted so that a specific deflection of the

(Refer to paragraph 7-12).

black indicator disc with respect to the indicator

7-10. Static balance tolerance for the specific

bushing (tangency of the indicator circles) will

assembly being balanced is usually specified in the

occur when the limiting amount of permissible

applicable maintenance or overhaul instructions for

7-2