IN QUESTIONS AND ANSWERS For Machinists, Firemen, Electricians and Steam Engineers

Zwicker's Revised Practical Instructor

| July/August 1996

VALVE MOTION.

Q. What is an eccentric?
A. An eccentric is a subterfuge for a crank; it is anything out of center.

Q. How would you find the throw or stroke of an eccentric?
A. By measuring the heavy and the light side; the difference between the two is the stroke or throw.

Q. What throw should a common slide valve engine eccentric have?
A. Generally double the width of the entry or steam ports.

Q. If you changed the size of the eccentric would it alter the throw of the valve?
A. No, it would not, but if you changed the position of the eccentric on the shaft it would.



Q. What is a cam?
A. A cam has no definite meaning; it has 1, 2, 3 or 4 motions; they are used on puppet valve engines, such as are in use on high pressure river steamboats.

Q. How would you measure your valve and eccentric rods?
A. By placing the crank-pin at its dead-center, the center of the eccentric straight or plumb above the center of the shaft, the rocker-arm perpendicular, and the valve covering both ports equally; then take a tram and measure from the center of the eccentric to the center of the pin where the eccentric rod hooks on (generally the lower pin) for the eccentric rod, and from the outside center of the pin where the valve rod is attached to the furthermost end of the valve, allowing for two nuts at each end of the valve, called adjusting and jamb nuts.