SLIPPED ECCENTRIC-RODS.
Where slotted rods are used, they frequently slip, making the engine lame. The cause of trouble in such a case can be identified by moving the engine slowly, with the cylinder-cocks open. The disturbance to the regularity of the valve’s motion, caused by a slipped rod, will admit steam prematurely on one end of the cylinder, while it delays the admission on the other end. The valve is made to travel more on one side of the exhaust center than on the other. Lengthening or shortening the valve-stem has a similar effect, but this makes the engine lame in both gears; while the slipping of an eccentric-rod only makes the engine lame in the motion that the rod belongs to. This is subject to a slight modification, however; for the back-motion eccentric being badly out of square, will affect the correctness of the forward motion, when the engine is working close hooked up. But in full motion it will not be perceptible.
DETECTING THE CAUSE OF A LAME EXHAUST.
If in moving the engine ahead slowly, with the cylinder-cocks open, it is found that steam is admitted to the cylinder before the piston has nearly reached the center or dead point, or that the back cylinder-cock does not show steam till after the piston has passed the back center, the eccentric-rod is too long. The rod being too short produces precisely an opposite effect. The steam arrives late on the back stroke, and ahead of time on the forward stroke. This is different from the action of the steam where an eccentric has slipped. In that case, there will be pre-admission of steam before the beginning of both strokes, or post-admission, that is, late arrival of steam, for both strokes. Take a go-ahead eccentric for example. If it slips backward on the shaft, its effect will be to delay the admission of steam till after the beginning of each stroke; and, if it slips forward, the result will be to accelerate the lead of the valve opening the steam-port before the piston has reached the commencement of each stroke.
WHAT TO DO WHEN ECCENTRICS, STRAPS, OR RODS BREAK.
When either of these accidents happens, the safest plan is to take down both straps and rods on the defective side. Some engineers leave the back-up eccentric strap and rod on, when the forward strap or rod has broken; but it is a little risky under certain conditions. After getting the eccentric straps and rods down, drop the link-hanger away from the tumbling-shaft, disconnect the valve-stem, and tie the valve-rod to the hand-rail. Then set the valve in the middle of the seat, so that it will cover both the steam-ports, and hold it in that position by pinching the stem with the gland, which is done by screwing up the gland obliquely. Take down the main rod, and block the cross-head securely at the back end of the guides. Good hardwood blocking prepared beforehand should be used for this purpose, and it ought to be fastened with a rope or marline. A neater plan for holding the cross-head in place is described by Frank C. Smith, in the Torch. He says, “Have the blacksmith make a hook out of a piece of inch and a half round iron; also a piece about fifteen inches long by one and a half thick, and four inches wide, with a hole through the center for the shank of the hook to pass through. This shank is threaded for a nut. Now, when it is necessary to block a piston, get it to the back end, pass the hook around the wrist of the cross-head, and the other end through the straight piece which bears against the yoke supporting the back end of the guides; run up a nut on the shank of the hook, hard against the cross-piece, and the piston is secured.” The piston being properly fastened, it is a wise supplement to the work to tie the cylinder-cocks open, or to take them out altogether. The engine is now ready to proceed on one side.
Young engineers can not be too strongly impressed with the necessity for having the cross-head properly secured before trying to move the engine. I have repeatedly known of serious damage being caused by placing too much confidence in weak blocking. Taking out the cylinder-cocks is a wise security against accidents of this kind; for, should a little steam be passing through the valve, it has a port of escape without putting heavy pressure on the piston.
DIFFERENT WAYS OF SECURING THE CROSS-HEAD.
In regard to the method of securing the piston when one side of an engine is taken down, there is considerable diversity of opinion among engineers. Some men maintain that the proper and quick plan is, merely to move the piston to one end of the cylinder, pushing the valve in the same direction, so that the steam-port will be open at the end away from the piston. This will keep the cylinder full of steam, and hold the piston from moving. But, if by any accident the valve should be moved to the opposite end of the seat, steam would get to the wrong end of the cylinder, and the piston would certainly smash out the head. Another risky plan, practiced by men economical of work, is to place the valve on the center of the seat, and let the piston go without fastening. These slipshod methods do not pay.