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UNIVERSAL JOINT

To couple two shafts in different planes, you need to use a universal joint. Universal joints have various

Figure 11-21.-Universal joint (Hooke type).

Figure 11-22.-Ring-and-trunnion universal joint.

forms. They are used in nearly all types and classes of machinery. An elementary universal joint, sometimes called a Hooke joint (fig. 11-21), consists of two U-shaped yokes fastened to the ends of the shafts to be connected. Within these yokes is a cross-shaped part that holds the yokes together and allows each yoke to bend, or pivot, in relation to the other. With this arrangement, one shaft can drive the other even though the angle between the two is as great as 25 from alignment.

Figure 11-22 shows a ring-and-trunnion universal joint. It is merely a slight modification of the old Hooke joint. Automobile drive shaft systems use two, and sometimes three, of these joints. You will read more about these in chapter 13 of this book.

The Bendix-Weiss universal joint (fig. 11-23) provides smoother torque transmission but less structural strength. In this type of joint, four large balls transmit the rotary force, with a smaller ball as a spacer. With the Hooke type universal joint, a whipping motion occurs as the shafts rotate. The amount of whip depends on the degree of shaft misalignment. The Bendix-Weiss joint does not have this disadvantage; it transmits rotary motion with a constant angular velocity. However, this type of joint is both more expensive to manufacture and of less strength than the Hooke type.

CAMS

A cam is a rotating or sliding piece of machinery (as a wheel or a projection on a wheel). A cam transfers motion to a roller moving against its edge or to a pin free to move in a groove on its face. A cam may also receive motion from such a roller or pin. Some cams do not move at all, but cause a change of motion in the contacting part. Cams are not ordinarily used to transmit power in the sense that gear trains are used. They are used to modify mechanical movement, the power for which is furnished through other means. They may control other mechanical units, or they may synchronize or lock together two or more engaging units.

Cams are of many shapes and sizes and are widely used in machines and machine tools (fig. 11-24). We classify cams as

    1. radial or plate cams,

    2. cylindrical or barrel cams, and

    3. pivoted beams.

A similar type of cam includes drum or barrel cams, edge cams, and face cams.

The drum or barrel cam has a path cut around its outside edge in which the roller or follower fits. It imparts a to-and-from motion to a slide or lever in a plane parallel to the axis of the cam. Sometimes we build these cams upon a plain drum with cam plates attached.

Plate cams are used in 5"/38 and 3"/50 guns to open the breechblock during counter-recoil.

Edge or peripheral cams, also called disc cams, operate a mechanism in one direction only. They rely on gravity or a spring to hold the roller in contact with the edge of the cam. The shape of the cam suits the action required.

Figure 11-23.-Bendix-Weiss universal joint.

Figure 11-24 - Classes of cams

Face cams have a groove or slot cut in the face to provide a path for the roller. They operate a lever or other mechanism positively in both directions. The roller is guided by the sides of the slot. Such a groove can be seen on top of the bolt of the Browning .30-caliber machine gun or in fire control cams. The shape of the groove determines the name of the cam, for example, the square cam.



 


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