Ch 42

Spur Gear

A common type of gear that has straight teeth and are used to connect parallel shafts, they are easily made and the cheapest of all types of gears.

Rack

A gear with infinite radius, having teeth that lie on a straight line on a plane. The teeth may be normal to the axis of the rack of helical as to mate with spur or helical gears, respectively.

Hob

A long rack tooth that has been wrapped around a cylinder in the form of a helix and fluted at intervals to provide a number of cutting edges.

Crown Gear

A special form of bevel gear having a 180 degree cone apex angle. It is a disk with the teeth on the side of the disk. Seldom used, but they have the important quality that they will mesh properly with a bevel gear of any cone angle.

Involute Curve

A tooth form generated by a point on a straight line when the line rolls around a base circle in order to achieve a condition of providing uniform pressure and motion and to minimize friction and wear.

Plug Tap

After the taper tap has the thread started in proper alignment, a plug tap, which has only a few tapered threads to provide gradual cutting of the threads to depth, is used to cut the threads as deep into the hole as its shape will permit.

Diametrical Pitch

An expression of dimensions, it is the number of teeth (n) per unit of pitch diameter (DP= N/PD). The important tooth elements can be specified in terms of the diametrical pitch.

Broaching

Another way to form cut teeth; all the tooth spaces are cut simultaneously and the tooth is formed progressively.

Pitch Circle

Corresponds to the diameter of the wheel. Thus the angular velocity of a gear is determined by the diameter of this imaginary pitch circle.

Form Milling

Either a single of multiple form cutter may be used. A single form cutter has a single singular row of teeth and is tilted at an angle equal to the helix angle of the thread and is fed inward radically to full depth while the work is stationary.

Cold roll forming

Gears with accuracy sufficient for most applications can be made in very large numbers by cold ion forming (but only on ductile metals).

Bottoming Tap

It has no tapered threads and is used to finish the few remaining threads at the bottom of the hole to full depth.

Herringbone Gear

Neutralize side thus by having, two helical gear halves, one having a right hand and the other a left hand helix.

Self opening die heads

Not suitable for use on high-speed, production-type machines. They are used on turret lathes, screw machines, NC lathes and special threading machines for cutting external threads.

Pitch

P, refers to the distance from a point on one screw thread to the corresponding point on the next thread measured parallel to the length axis of the part.

Gear Grinding

Precision grinding operations are used on external and internal bearing diameters, critical length dimensions, and fine surface finishes after heat treatment.

Hobbing

Process that uses hobs.

Thread Cutting

Process used to cut thread and refers to the manufacture of threads on external diameters lathes are sometimes used to cut the threads.

Blanking

Refers to the initial forming or machining operations that produce a semi finished part ready fr gear cutting, starting from a piece of raw material. Turning on chuckers or lathes, face, and centering of shafts, milling, and sometimes grinding fall into this category of operations.

Threading

Refers to the manufacture of threads on external diameters.

Worm Gear

Similar to a screw gear, having multiple thread type being very common. Worm gears have non intersecting axes and usually are at right angles.

Bevel Gear

Sometimes called teeth on a cone, are used to transmit motion between intersecting shafts. The teeth are cut into the surface of a truncated cone.

Taper Tap

Tap that has a tapered end that will enter the hole a sufficient distance to help align the tap; also. the threads increase gradually fall depth, so this tap requires less torque to use. Only a through hole can be tapped completely with a taper tap, because it acts to full depth only behind the tapered portion.

Gear Teeth

Teeth added to prevent slipping and to assure that their relative motions are constant.

Pitch Diameter

The angular velocity of a gear is determined by the diameter of this imaginary pitch circle. All design calculations relating to gear performance are based on the pitch-circle diameter or, more simply, the pitch diameter.

Gear Hobbing

The cutting of a gear by means of a hob. It is a continuous action.

Thread Tapping

The cutting of internal thread by means of a multiple-point tool.

Circular Pitch

The distance between the corresponding points on adjacent teeth, measuring along the pitch circular with (pi/ diam pitch).

Gear Roll Forming

The manufacturing of gears by cold roll forming has been highly developed and widely adopted in recent years. Many of the gears in automobile transmissions are made this way. The hardened forming rods are very accurately made, and the roll formed gear teeth usually have excellent accuracy. They also seldom require further finishing and have excellent wear characteristics.

Addendum

The radial distance from the pitch circle to the outside diameter.

Dedendum

The radial distance from the pitch circle to the root circle. It is equal to the addendum plus the clearance.

Collapsible Tap

These taps are similar to self opening die heads in that the cutting elements collapse inward automatically when the thread is completed.

Gear Shaping

To carry out the shaping process, the cutter and the gear blank must be attached rigidly to their respective shafts, and the two shafts must be interconnected by suitable gearing so that the cutter and the blank rotate positively with respect to each other and have the same pitch line velocities.

Gear Finishing

To operate efficiently and have satisfactory life, gears must be accurate tooth profiles and the faces of the teeth must be smooth and hard. These qualities are particularly important when gears must operate quietly at high speeds.

Thread Rolling

Today this process produces the largest number of threads. Can produce both internal and external threads but the material must be ductile. Rolling is simple and is restricted to standardized and simple parts.

Fluteless Tap

Used to cold form many internal threads (.5 in diameter) into holes in ductile metals. Forming action is essentially the same as rolling external threads. It produces somewhat better accuracy than cutting taps and tap life is often greater than that of HHS machines. Especially suitable for forming threads in dead end holes because no chips are produced.

Planetary Gearing

is a gear system that consists of one or more outer gears, or planet gears, rotating about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier which itself may rotate relative to the sun gear. Epicyclic gearing systems may also incorporate the use of an outer right gear or annulus, which meshes with the planet gears.

Worm

is similar to a screw.