07-05-2013, 04:16 PM
MANUFACTURING OF WORM GEAR
MANUFACTURING OF WORM.docx (Size: 4.61 MB / Downloads: 27)
Introduction
The worm gears are widely used for transmitting power at high velocity ratios between non intersecting shafts that are generally, but not necessarily, at right angles.It can give velocity ratios as high as 300 1 or more in asingle step in a minimum of space, but it has a lower efficiency. The worm gearing is mostly used as a speedreducer, which consists of worm and a worm wheel orgear. The worm (which is the driving member) is usuallyof a cylindrical form having threads of the same shape asthat of an involute rack. The threads of the worm maybe lefthanded or right handed and single or multiple threads.The worm wheel or gear (which is the driven member) is similar to a helical gear with a face curved to conformtothe shape of the worm. The worm is generally made ofsteel while the worm gear is made of bronze or cast ironfor light service.
Types of Worm Gears
The following three types of worm gears are important from the subject point of view :
1. Straight face worm gear, as shown in Fig. 31.2 (a),
2. Hobbed straight face worm gear, as shown in Fig. 31.2 (b), and
3. Concave face worm gear, as shown in Fig. 31.2 ©.
The straight face worm gear is like a helical gear in which the straight teeth are cut with a form
cutter. Since it has only point contact with the worm thread, therefore it is used for light service.
The hobbed straight face worm gear is also used for light service but its teeth are cut with a
hob, after which the outer surface is turned.
Strength of Worm Gear Teeth
In finding the tooth size and strength, it is safe to assume that the teeth of worm gear are always
weaker than the threads of the worm. In worm gearing, two or more teeth are usually in contact, butdue to uncertainty of load distribution among themselves it is assumed that the load is transmitted byone tooth only. We know that according to Lewis equation,
Wear Tooth Load for Worm Gear
The limiting or maximum load for wear (WW) isgivenbyWW = DG .b .KwhereDG = Pitch circle diameterof the worm gear, Worm gear assemblyb= Face width of the worm gear, and
K = Load stress factor (also known as material combination factor).The load stress factor depends upon the combination of materials used for the worm and wormgear. The following table shows the values of load stress factor for different combination of worm and worm gear materials.
Design of Worm Gearing
In designing a worm and worm gear, the quantities like the power transmitted, speed, velocity
ratio and the centre distance between the shafts are usually given and the quantities such as lead
angle, lead and number of threads on the worm are to be determined. In order to determine the
satisfactory combination of lead angle, lead and centre distance, the following method may be used:
In this part of the book, we consider those manufacturing processes in which the starting work material is either a liquid or is in a highly plastic condition, and a part is created through solidification of the material. Casting and molding processes dominate this category of shaping operations. With reference to Figure 10.1, the solidification processes can be classified according to the engineering material that is processed: metals, ceramics, specifically glasses, and polymers and polymer matrix composites (PMCs). Casting of metals is covered in this and the following chapter. Glass working is covered in Chapter 12, and polymer and PMC processing is
treated in Chapters 13, 14, and 15.Casting is a process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity. The term casting is also applied to the part that is made by this process.
OVERVIEW OF CASTING TECHNOLOGY
As a production process, casting is usually carried out in a foundry. A foundry is a factory equipped for making molds, melting and handling metal in molten form, performing the casting process, and cleaning the finished casting. The workers who perform the casting operations in
1)Casting Process:
Discussion of casting logically begins with the mold. The mold contains a cavity whose geometry determines the shape of the cast part.The actual size and shape of the cavity mustbe slightly oversized to allowfor shrinkage that occurs in the metal during solidification and cooling. Different metals undergo different amounts of shrinkage, so the mold cavity must be designed for the particular metal to be cast if dimensional accuracy is critical. Molds are made of a variety of materials, including sand, plaster, ceramic, and metal. The various casting processes are often classified according to these different types of molds.