Intermittent mechanisms are those that perform a cycle of movement then return to a “resting state” for a time before repeating the movement cycle. In the valve train of a four-stroke internal-combustion piston engine, intake and exhaust valves must be opened and closed once at appropriate times during four piston strokes (intake, compression, power, exhaust). During compression and power strokes, the valves are closed and the system “rests.” As the late, great airflow wizard Kenny Augustine put it, “The only time your engine can make any power is when the valves are closed.”
In a classic movie camera such as the hefty “blimp 35s” that used to populate movie sets, the operating sequence begins with a frame of 35mm film in the gate, held immobile by four tapered registration pins that pass through the perforations at the edges of the film. Light from the lens briefly throws an image of whatever the camera is pointed at onto the film and then a shutter closes. The registration pins are withdrawn and a pair of film-advance claws engages the perforations to pull the next frame into the gate. To ensure that the advance claws don’t tear the perforations, there are 180-degree loops of free film above and below the gate, so only a very small mass of film is moved (film is fed from the supply spool by a sprocket drive). Now the registration pins are again driven into the film perforations, making sure that this next frame is held exactly as was the previous one. If not, eventual viewers of the film will see jitter as the image jumps around on the screen randomly. Good registration is essential to the illusion of reality!
One the film has advanced and the new frame is registered, the shutter opens, allowing the image produced by the lens to reach the film, being stored there as chemical changes in the film’s emulsion.
And repeat. For normal movie use, the frame rate was 24 frames per second but special cameras for recording high-speed events were made to perform the above sequence of events at up to 500 frames per second. Beyond that, intermittent motion had to be abandoned, so the remarkable films of nuclear bomb tests showing the fireball (sometimes with spooky “eyes” in it) coming into being and expanding at Mach 25 were made by rotating prism systems, throwing images onto a strip of film motionless on the inside of an arc-shaped drum.
To get an idea of how conventional camera intermittent motion was achieved, google “Geneva motion.” It is wonderfully clever and was once considered as a means of driving the intake valves of two-stroke Grand Prix racing motorcycle engines.
The valves of four-stroke engines are shaped like a disc on the end of a stick. The edge of the disc is ground at an angle (usually approximately 45 degrees) to seal against and self-center upon a similarly ground valve-seat ring that is part of an intake or exhaust port in the cylinder head. The age-old task of “grinding the valves” was the act of placing abrasive paste between valve and seat and rotating the valve back and forth against the seat to produce a good seal.
The “stick” is the valve’s stem, more slender than a pencil, which fits closely into a guide that is also part of the head. Holding the valve against its seat are one or more helical wire valve springs, pushing up against a spring retainer that is colleted to the stem near its upper end. The valve port curves in from the side under the spring. Pushing on the end of the stem lifts the valve off its seat, opening a flow path between port and cylinder.
In most motorcycle engines today, the valves are moved by a rotating cam that is cylindrical save for a smoothly ground eccentric and vaguely ovoid projection called the cam lobe. As the cam rotates, the lobe, acting through mechanism, pushes the valve open and then, thanks to spring pressure, closes it. To operate its valves once in the two revolutions of the crank required to complete the four piston strokes of the four-stroke cycle, the cam or cams rotate at one-half of crank speed.
Between the cam and the flat tip of the valve stem is the valve tappet or “lifter.” This may take the form either of a little “bucket” moving in a cylindrical guide, its flat bottom facing the cam lobe while its open end installs over the valve stem and spring(s), or it may have the form of a pivoted rocker, one side of which is driven by the cam lobe while the other nearly touches the stem of the valve when it is closed.
There is always a small clearance between the cam/tappet combination and the end of the valve stem because as a cold-started engine warms up its valves (whose heads are exposed to combustion heat) thermally expand considerably. If there were no clearance, valve expansion would push the valves slightly open when they should be closed—can’t have that because any leakage destroys cylinder compression (this is why buyers of older cars often perform a compression test to measure the state of valves and piston rings). Valve clearance, as it is called, is just enough to accommodate valve expansion, plus a little extra. Because the exhaust valve is exposed to hot exhaust gas on both sides of its head, it runs hotter than the intake valve and so is typically given more valve clearance.
In engines with self-clearancing hydraulic valve lifters, no valve clearance is necessary. Why don’t all engines have such lifters? Although much improvement in high-speed operation of hydraulic lifters has been achieved, there can be irregular operation at high revs, so sportbike engines continue to have mechanical lifters.
Valve mechanism is simple but the complexity is in the shape of the cam lobes. Before valve opening can begin, a so-called “clearance ramp” of gradual lift takes up the valve clearance. An audible click results, so the sewing-machine sound of a mechanical-lifter engine idling is the sum of all clicks. Parts are fatigued and destroyed by suddenly applied stress, so the cam lobe cannot bang the valve open. Instead, it must move the valve with acceleration that increases at a tolerable rate, loading the parts at a rate that their materials can survive. When the valve reaches approximately a quarter of its full lift, this cam-driven acceleration phase ends. From now on, for the other three-quarter of the lift and for the first three-quarter of valve closing, the valve spring must keep the valve train firmly against the cam lobe. If rpm is too high, the valve train too heavy, or the spring too weak, there may not be enough spring pressure to keep the valve train against the cam lobe and the result is valve “float,” the valve train leaving contact with the cam lobe with the possible results being: 1) the exhaust valve(s) hitting the piston just before TDC on the exhaust stroke; and/or 2) the spring later smacking the valve back into contact with the cam in a damaging mechanical impact.
But with a properly shaped cam lobe and enough spring to maintain valve-train contact with it at all times, valve motion is completely controlled and will decelerate the valve during closing at a rate it can survive for thousands of miles of operation. All of the grace necessary to this desired result is in the subtle shape of the cam lobe.
And what about desmodromic valves, which have a second cam profile to close the valve, making springs unnecessary? In Ducati’s system, the closing lever pulls up on the valve stem to close the valve, driven by the closing cam. Think of that closing lever as Ducati’s “valve spring” because there is no way that L-shaped lever, loaded as it is in bending, can be made completely rigid. Its degree of flexure in operation sets a practical rpm and endurance limit. In Ducati’s older air-cooled engines, the levers have a simple shape appropriate to their relatively low-stress operating conditions, but in that company’s MotoGP engines they become hefty I-beam structures that are products of intense stress analysis. In the 800cc era of MotoGP, 20,000 rpm was a normal occurrence. By 1964, Honda had operated a special research engine at up to 27,000 rpm in combustion studies with teensy-weensy valves and springs.
A third intermittent motion of interest is that of automatic weapons, which must perform the materials-handling job of moving a fresh round from its supply system (magazine, belt, ammo feed chute), position it on the axis of the gun barrel, then chamber it by pushing it into that part of the barrel called the chamber to a specified depth. Then some form of closure must prevent the firing of the round from blowing part of the round (the powder case) back out of the chamber. This can take various forms: a block, sliding at right angles to the barrel in a machined slot, a lockable bolt, sliding axially back after firing to make room for the next round ahead of it, or a “blowback” action, in which only the inertia of the bolt, shoved rapidly forward by a spring after being blown back by the previous firing, provides the momentary force to hold the round in place during the brief instant of firing. Once the round is in place, it is fired by a pin in the bolt or block, striking the impact-ignition primer in the base of the powder case.
The 40mm Bofors autocannon, seen in film of kamikaze attacks on US ships in the Pacific War, is an example of the sliding block, while the lockable bolt is present in most automatic military rifles, and the blowback action is used on many submachine guns and on the classic Oerlikon 20mm antiaircraft cannon. In the last case, the ignition of the round takes place before the bolt has fully rammed it into the chamber. As the firing pin indents the primer, there is some delay as the flash ignites nearby powder grains and combustion pressure begins to rise inside the powder case.
In a way, this is no different from the fact that spark-ignition internal-combustion engines typically ignite their fuel-air mixture well before piston TDC. This is necessary because ignition, as in a round of ammunition, takes time to develop significant combustion pressure. In piston engines, the ignition spark is typically timed to occur somewhere in the range of 30 to 40 degrees BTDC.
Typically, the larger the round, the slower the action, something that is familiar in industrial-materials handling. The power to operate gun mechanism can be taken from recoil, by driving a piston with combustion gas tapped from the barrel, or by external electrical power. The roller-locked bolt of the respected German MG42 machine gun cycled at 1,200 rounds per minute. Its barrel had to be exchanged for a cool one after 250 rounds to prevent “cooking off” before bolt closure. The blowback Oerlikon 20mm fired at roughly 600 rounds per minute and the Bofors 40 at 120.
Extreme rates of fire are achieved by multi-barrel rotary cannon or revolver guns. In their case, the several stages of bolt-opening, ejection of the empty case, chambering of the next round, bolt closure, and firing-pin operation take place “around the circle” as the other barrels are fired in sequence. Such guns were forced into being by the higher speeds of jet aircraft, which greatly reduced the time during which a target could be held in the sight picture. The answer was higher rates of fire in hope of putting seven rounds into an aircraft crossing your line of flight (GE’s 1946 design, the M61 Vulcan 20mm gun at 6,000 rounds per minute) or to “dig a hole” through tank armor by repeated hits on the same spot (by GE’s 4,000-rpm GAU-8 30mm rotary, designed after 1970 and installed on A-10 aircraft).
Engineers are people who may be startled into wakefulness at 4 a.m. by a sudden idea. Brains never sleep. They constantly search for useful patterns in all the information they contain.