PACKINGr.

163

box (hemp packing being used) has been found to ränge from 3to 8 per cent. at 700 lbs. pressure. The difference of pressureduring the rise and fall of the accumulator represents from 1to 2 per cent. of the power.

Experiments made by Mr. John Hiek have alforded dataas to friction, from which useful rules have been deduced.He found that friction increases directly vvith pressure. Withleather packing for rams of different diameters, if the pressureper unit of area be the same, friction varies directly as thediameters, or as the square roots of the gross loads. Neitherthe depth of the leather nor the length of the ram affects thetotal friction. With hydraulic machines in good Order, theamount of friction may be taken to be 1 per cent. for rams of 4inches diameter, and 1^ per cent. for rams of 8 inches diameter.

Erom these experiinents the following formula is deduced :—F = D x P x 0

"VVhere F = total friction of leather packing.

D — diameter of ram in inches.

P = pressure per square inch.

C = coefficient.

C = '0471 with new, or badly rubrieated, leathers.

G = '0314 witla leathers in good condition, and well lubricated.

The following table of Mr. Hick shows the frietionalresistance in percentage of the total hydraulic pressure forrams from 2 inches up to 20 inches in diameter:—

D inches.

Fper cent.

D inches.-

F per cent.

2

2'00

12

0-33

3

1-33

13

0-30

4

1-00

14

0-28

5

0-80

15

0-26

6

0'66

16

0-25

7

0-57

17

023

8

0'5@

18

0-22

9

0'44

19

0-21

10

0.40

20

0-20

11

0'38

F = P j 0-09347 - 0-00000019291 P j

Where F = friction in lbs.

D = diameter of piston in feet.

P = gross pressure in lbs. per square foot of piston area(correct up to about 1 ton per square incli).