Maltese cross


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Maltese cross

(in a film projector) a cam mechanism of this shape that produces intermittent motion

Maltese cross

A cross that is formed by four equal triangles or arrowheads joined at their points; the outer edge of each arm is indented with an angle.

Maltese Cross

 

a device for the conversion of continuous rotation into intermittent rotation. It is a component of the mechanical systems of automatic machine tools, motion-picture projectors, and devices in which periodic interruptions of motion are necessary (for example, for performing a particular manufacturing operation or for exposing a frame of a motion-picture film). Maltese crosses with internal and external coupling are used.

Upon rotation of the driving element, or crankshaft (1), in a Maltese cross with external coupling (Figure 1), its pin (2) enters the slot (3) of the driven element, or cross (4), at point A; the pin slides within the slot and turns the cross, and then leaves the slot at point B. The cross stops and remains motionless until the crankshaft pin, which continues in motion, returns to point A, where it enters the next slot in the cross, and so on. To fix the cross—that is, to prevent its spontaneous rotation during its stationary phase—the crankshaft is equipped with a cylindrical locking projection (5) with a recess, and the cross is circumscribed by arcs (this makes it resemble a Maltese cross, the emblem of the Maltese order, which gave the mechanism its name). Rotation of the cross is possible only when its limb coincides with the recess in the projection.

Figure 1. Maltese cross with external coupling: (ω1,) and (ω2) angular velocities of the driving and driven elements

The number of slots z in a cross usually ranges from three to 12. One revolution of the crankshaft is accompanied by 1/z revolution of the cross. During revolution of the crankshaft with a constant angular velocity, the ratio of the time the cross spends in motion to the time it spends at rest is equal to the ratio of the lengths of the arcs ∪ ACB to ∪ AC’B, or (z — 2)/(z + 2). To increase the durations of the stops, the crankshaft is rotated at a variable speed, which is high while the cross is turning and low until it stops. If the interruptions in motion must be reduced, the crankshaft is equipped with several pins. Maltese crosses with internal coupling (Figure 2) are distinguished by smooth rotation of the cross and by small dimensions.

Figure 2. Maltese cross with internal coupling: (ωl1) and (ω2) angular velocities of the driving and driven elements

Elements of metalworking lathes use three-dimensional Maltese crosses, which are designed to transmit rotation to a shaft that usually crosses the drive shaft at an angle of 90°.

REFERENCES

Artobolevskii, I. I. Mekhanizmy v sovremennoi tekhnike, vol. 1. Moscow, 1970.
Mashinostroenie: Entsiklopedicheskii spravochnik, vol. 9. Moscow, 1949. Pages 95-98.

I. G. GERTSKIS

Maltese cross

A cross formed by four equal triangles or arrowheads joined at their points.
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