A simple crank. This basic design has been and still is the staple of machine design. Since the line of action on the sliding member is not always parallel with the motion, a lateral force is developed. A guide must therefore be provided that can bear this lateral force. Often, a connected piston can be designed with an elongated "skirt" to carry this load without twisting (typical automotive pistons are a good example). However, in some applications a separate load bearing member must be provided, and is then attached to the piston via another link. Such an arrangement is called a "crosshead."
	A Scotch Yoke. By allowing the crank pin to slide perpendicular to the desired linear motion, the lateral force is functionally eliminated. However, some lateral load is still present since the force of the crank pin is applied off-center to the sliding member. This creates a torsion on the sliding member that must be countered.
	A simple lobe cam. The sliding member rides on the edge of an irregularly shaped portion of the shaft. The part that is directly in contact with the cam lobe is called the "follower" for obvious reasons, and may or may not include a rolling element to reduce friction. The cam profile (shape) shown here is a half sine wave, and the follower moves according to the formula: x = sin(α/2) In practice, almost any motion profile can be created, which is a great advantage to this mechanism. Since the line of force from the cam to the follower is not parallel with the follower's motion (it is perpendicular to the cam face at the point of contact) a torsion is created in the follower that must be countered.
	A cylindrical cam. The sliding member rides in a groove cut into a cylinder. Note the interesting property that the axis of rotation and direction of sliding motion are parallel; a unique feature of this mechanism. Also, the follower is "positively driven" in both directions: unlike the lobe cam, the follower is trapped in the groove and can not "jump off" the cam profile. The cam profile shown here is a half sine wave, and the follower moves according to the formula: x = sin(α/2) In practice, almost any motion profile can be created, which is a great advantage to this mechanism. Since the line of force from the cam to the follower is not parallel with the follower's motion (it is perpendicular to the angle of the groove wall at the point of contact) a torsion is created in the follower that must be countered.
	A gear-crank mechanism, formally known as a hypocycloid gear drive. The planetary gear is connected to a crank (not shown) and is rotated inside the stationary ring gear. If the pitch diameter of the planet gear is exactly one half that of the ring gear, a point on the pitch circle of the planet gear then follows a straight line as shown in the animation. An enlarged gear tooth at this point allows the mounting of a pin to which a sliding member can be attached. I do not remember exactly where this design came from. I remember seeing it used on a model Stirling engine on a website (which I can't find anymore!). This arrangement was used to drive the displacer piston, which does not tolerate lateral loads in most Stirling engine designs. This also eliminated the need for a crosshead and reduces friction at a cost of complexity in manufacture. It's documented existence is much older, though. Note that the travel of the sliding member is twice the crank diameter, allowing the design to be relatively compact. This arrangement produces no lateral or torsional forces on the sliding member. To view an improved, more detailed and much larger version, click here. (250KB, 935px square)
	This linkage arrangement is known as a Trammel Crank. Two sliding members, connected by a rigid link, ride in intersecting guides in a stationary plate. The slots are at 90 degrees as shown here, however this is not a strict requirement. The midpoint of the connecting link describes a perfect circular path as the sliders are moved through their cycle. A crank pin could therefore be attached to this point, and the sliding members then produce straight-line motion as a result of their constraining guides. Note that if you remove one of the sliders, the mechanism becomes almost identical to a standard cranks and crosshead arrangement. The important difference is the total travel of the slider is twice that of the crank's diameter, whereas a standard crosshead would only give a travel equal to the crank diameter. Another interesting property is a point on the link that is not on-center will describe an ellipse. The link arm can be extended beyond the sliding members to scribe an elliptical shape of almost any desired size and ratio.
	Sometimes people come up with very... unique solutions. A stationary pinion gear drives an oval internal gear that is free to slide along two axes, one of which (but possibly both) is used to drive a connected sliding member. In the animation, the central pinion rotates 2.6 times for each oscillation. Obviously the track length and pinion size both factor into this ratio. Even though the track is sliding, the line of action from pinion to slider is parallel and almost coincident, so lateral forces are very minimal. A unique advantage to this arrangement is the stroke length can be made very long without the device getting any wider. Any of the other arrangements shown here will grow in width in order to accommodate longer stroke lengths. Also worth noting is that the motion is not sinusoidal, but rather constant velocity with sinusoidal motion only at the end of the strokes when the pinion rides in the semicircular portion. Using a perfectly round track will yield sinusoidal motion.
	A pair of gears and linkages of equal length connect to a beam which them moves in a straight line. Because the mechanism is symmetrical, lateral forces from the two linkages are canceled and are not transmitted to the sliding member. In the animation, the gears appear to be rotating only slightly compared to the cranks. This is because each frame advances the gears 20 degrees, but the angle between teeth is 18 degrees... causing an illusion of only 2 degrees of movement per frame. You'll have to trust me that I did rotate both gears 20 degrees for each frame. :)