Portion of Stroke at which the Steam is cat off.

Lap.

1/3

7/24

¼

5/24

1/6

1/8

1/12

1/24

A

1/8

•178

•101

•143

•126

•109

.093

•074

068

1/16

•18

•118

•1

.085

•071

068

•043

•027

1/32

•118

•101

066

•069

.053

•043

088

•024

0

•092

•082

•067

066

041

086

•011

B

1/8

•033

•026

•019

•012

•008

•004

•001

•001

1/16

•06

•062

•04

•03

•022

•015

.008

•002

1/32

•073

•066

•051

•042

066

098

013

•004

0

•092

.082

067

066

•044

•033

•022

.011

The units in the columns of the table marked A express the distance of the piston, in parts of its stroke, from the end of the stroke when the exhaust port in advance of it is closed ; and those in the columns of the table marked B express the distance of the piston, in parts of its stroke, from the end of its stroke when the exhaust port behind it is opened.

Illustration

A slide-valve is to cut off at 1/6 from the end of the stroke of the piston, the lap on the exhaust side is 1/32 of the stroke of the valve (16 inches), and the stroke of the piston is 60 inches. At what point of the stroke of the piston will the exhaust port in advance of it be closed, and the one behind it opened?

Under 1/6 in table A, opposite to 1/32. is .053, which X 60, the length of the stroke = 3 18 inches; and under 1/6 in table B, opposite to 1/32 is .033, which x 60 = 1.98 inches.

If the lap on the exhaust side of this valve was increased, the effect would be to cause the port in advance of the valve to be closed sooner, and the port behind it opened later. And if the lap on the exhaust side was removed entirely, the port in advance of the piston would be shut, and the one behind it open, at the same time.

The lap on the steam side should always be greater than that on the exhaust side, and the difference greater the higher the velocity of the piston.

In fast-running engines alike to locomotives, it is necessary to open the exhaust valve before the end of the stroke of the piston, in order to give more time for the escape of the steam.